Strategies for Accessing cis-1-Amino-2-Indanol

cis-1-amino-2-indanol is an important building block in many areas of chemistry. Indeed, this molecule is currently used as skeleton in many ligands (BOX, PyBOX…), catalysts and chiral auxiliaries. Moreover, it has been incorporated in numerous bioactive structures. The major issues during its synthesis are the control of cis-selectivity, for which various strategies have been devised, and the enantioselectivity of the reaction. This review highlights the various methodologies implemented over the last few decades to access cis-1-amino-2-indanol in racemic and enantioselective manners. In addition, the various substitution patterns on the aromatic ring and their preparations are listed.


Introduction
1-Amino-2-alcohols are crucial building blocks in many fields, including medicinal and organic chemistry.Consequently, both their synthesis and the development of new methodologies to obtain them constitute an important area of organic chemistry [1][2][3][4].More particularly, cis-1-amino-2-indanol (1) plays a central role in organic synthesis as a ligand or chiral auxiliary due to its rigid cyclic skeleton (Scheme 1).This structure is the key moiety of BOX and PyBOX ligands widely used in asymmetric catalysis [5][6][7].Notably, oxazaborilidine catalysts derived from cis-1-amino-2-indanol are often more efficient than other chiral 1-amino-2-alcohol structures in the enantioselective reduction in carbonyl compounds [8].In addition, cis-1-amino-2-indanol is an important derivative when used as a chiral auxiliary in several asymmetric transformations, such as diastereoselective enolate alkylation or diastereoselective reduction [9].This compound, like other chiral amines, also has applications in the resolution of racemic carboxylic acid bearing a chiral carbon at position α [9].It is also an interesting substructure for drug design.It is present in Indinavir sulfate (Crixivan ® ), an HIV protease inhibitor for the treatment of acquired immunodeficiency syndrome (AIDS) developed by Merck (Rahway, NJ, USA) [10,11], or in KNI-10006 for anti-malarial treatment [12].
Due to its wide range of applications, the synthesis of cis-1-amino-2-indanol has attracted the interest of numerous academic and industrial researchers, leading to the development of various synthetic pathways (Scheme 2) [9,13].Most of these strategies involve indane skeleton as the starting point, but total syntheses from other building blocks have also been reported.Whatever the starting material, the same synthetic difficulties have to be overcome.One of the first points to control is the selective introduction of nitrogen and oxygen atoms in the -C1 and C2 positions, respectively.Afterward, the main challenge is to control the cis relationship between the oxygen and nitrogen atoms and to carry out the process enantioselectively.
From the indane skeleton, the main strategy for obtaining the cis relationship between nitrogen and oxygen atoms involves the key intramolecular formation of a cis-5-membered ring (Scheme 2, (A)).This approach requires a final hydrolysis step to deliver the desired cis-1-amino-2-indanol.Alternative methodologies have been developed relying on the epimerization of C 1 or C 2 centers, or diastereoselective reduction (Scheme 2, (B) and (C)).Interestingly, other syntheses have been developed from non-indanic precursors, allowing the use of the chiral pool to introduce the right configurations (Scheme 2, (D)).
Scheme 2. Main strategies developed for the synthesis of cis-1-amino-2-indanol.the desired cis-1-amino-2-indanol.Alternative methodologies have been developed relying on the epimerization of C1 or C2 centers, or diastereoselective reduction (Scheme 2, (B) and (C)).Interestingly, other syntheses have been developed from non-indanic precursors, allowing the use of the chiral pool to introduce the right configurations (Scheme 2, (D)).Ritter type reaction on indene oxide is a practical route for the synthesis of enantiopure cis-1-amino-2-indanol.This strategy was reported by Senanayake in 1995, first starting from indene oxide (2) prepared via the reaction of indene with hydrogen peroxide.This allowed the isolation of cis-1-amino-2-indanol (−/+)-1 under acid conditions after hydrolysis of oxazoline intermediate 3 (Scheme 3) [14].The first attempts using 2.0 equivalent of 97% sulfuric acid in acetonitrile from −40 °C to RT gave the desired compound with 55-60% yield.Interestingly, when the reaction was performed with fuming sulfuric acid (21% SO3), the formation of indanone 4 was suppressed, and the yield improved to 78-80%.

Syntheses from
Scheme 3. Ritter type reaction from indene oxide.
From a mechanistic point of view, the authors proposed that in the presence of H2SO4 alone, the epoxide 2 is in equilibrium with the opened form 6 (Scheme 5).This carbocation could be reversibly trapped by the counter anion (HSO4 − ), giving 7, which was observed via NMR analysis, or evolve toward the formation of the undesired ketone 4 via 1,2 hydride shift.However, the major pathway consists of its trapping by acetonitrile, leading to the corresponding nitrilium ions cis-8 and trans-8 as a mixture of diastereomers.The success of this reaction is due to the equilibrium between the cis-and trans-nitrilium 8, Scheme 4. Enantioselective synthesis of (1S,2R)-1-amino-2-indanol via the Ritter process.
From a mechanistic point of view, the authors proposed that in the presence of H 2 SO 4 alone, the epoxide 2 is in equilibrium with the opened form 6 (Scheme 5).This carbocation could be reversibly trapped by the counter anion (HSO 4 − ), giving 7, which was observed via NMR analysis, or evolve toward the formation of the undesired ketone 4 via 1,2 hydride shift.However, the major pathway consists of its trapping by acetonitrile, leading to the corresponding nitrilium ions cis-8 and trans-8 as a mixture of diastereomers.The success of this reaction is due to the equilibrium between the cisand trans-nitrilium 8, which is displaced by the fast cyclization of the cis-nitrilium, leading to 3. In addition, this mechanism explains the transfer of chiral information from the C 2 carbon, since no epimerization via the formation of a carbocation occurs at this position.When the reaction is performed in the presence of SO 3 , epoxide 2 is proposed to evolve toward the formation of cyclic sulfate 9 observed at −40 • C via NMR.This intermediate is in equilibrium with the corresponding carbocation 10 through regioselective ring-opening.Suppression of the indanone by-product is likely to result from this fast equilibrium, which probably enables the 1,2 hydride shift process.The trapping of 10 by acetonitrile renders a mixture of cis and trans-nitrilium 11, which cyclizes while releasing SO 3 to finally deliver the desired cis-1-amino-2-indanol with 78-80% yield after hydrolysis.
Molecules 2024, 29, 2442 4 of 28 which is displaced by the fast cyclization of the cis-nitrilium, leading to 3. In addition, this mechanism explains the transfer of chiral information from the C2 carbon, since no epimerization via the formation of a carbocation occurs at this position.When the reaction is performed in the presence of SO3, epoxide 2 is proposed to evolve toward the formation of cyclic sulfate 9 observed at −40 °C via NMR.This intermediate is in equilibrium with the corresponding carbocation 10 through regioselective ring-opening.Suppression of the indanone by-product is likely to result from this fast equilibrium, which probably enables the 1,2 hydride shift process.The trapping of 10 by acetonitrile renders a mixture of cis and trans-nitrilium 11, which cyclizes while releasing SO3 to finally deliver the desired cis-1-amino-2-indanol with 78-80% yield after hydrolysis.
Scheme 5. Proposed mechanism for Ritter type reaction from epoxide.
Interestingly, and in accordance with the mechanism, stereochemistry is controlled by the configuration of the C2 center; consequently, the reaction works with enantiopure diol 12 as the starting material (Scheme 6) [14,16].Indeed, when the reaction was performed with cis-1,2 diol or trans-1,2 diol, the product was obtained with high yield and without loss of chiral information.It is worthwhile noting that a better yield was obtained when cis-diol was used as the substrate instead of epoxide in the presence of H2SO4 (81% (Scheme 6) vs. 55-60% (Scheme 3)).Interestingly, and in accordance with the mechanism, stereochemistry is controlled by the configuration of the C 2 center; consequently, the reaction works with enantiopure diol 12 as the starting material (Scheme 6) [14,16].Indeed, when the reaction was performed with cis-1,2 diol or trans-1,2 diol, the product was obtained with high yield and without loss of chiral information.It is worthwhile noting that a better yield was obtained when cis-diol was used as the substrate instead of epoxide in the presence of H 2 SO 4 (81% (Scheme 6) vs. 55-60% (Scheme 3)).which is displaced by the fast cyclization of the cis-nitrilium, leading to 3. In addition, this mechanism explains the transfer of chiral information from the C2 carbon, since no epimerization via the formation of a carbocation occurs at this position.When the reaction is performed in the presence of SO3, epoxide 2 is proposed to evolve toward the formation of cyclic sulfate 9 observed at −40 °C via NMR.This intermediate is in equilibrium with the corresponding carbocation 10 through regioselective ring-opening.Suppression of the indanone by-product is likely to result from this fast equilibrium, which probably enables the 1,2 hydride shift process.The trapping of 10 by acetonitrile renders a mixture of cis and trans-nitrilium 11, which cyclizes while releasing SO3 to finally deliver the desired cis-1-amino-2-indanol with 78-80% yield after hydrolysis.
Scheme 5. Proposed mechanism for Ritter type reaction from epoxide.
Interestingly, and in accordance with the mechanism, stereochemistry is controlled by the configuration of the C2 center; consequently, the reaction works with enantiopure diol 12 as the starting material (Scheme 6) [14,16].Indeed, when the reaction was performed with cis-1,2 diol or trans-1,2 diol, the product was obtained with high yield and without loss of chiral information.It is worthwhile noting that a better yield was obtained when cis-diol was used as the substrate instead of epoxide in the presence of H2SO4 (81% (Scheme 6) vs. 55-60% (Scheme 3)).Scheme 6. Ritter type reaction from diols.Scheme 6. Ritter type reaction from diols.
More recently, Lambert's group described an electrophotocatalytic amino-oxygenation reaction of aryl olefins (Scheme 7) [32].The reaction used a trisaminocyclopropenium (TAC) ion catalyst under electrochemical conditions and compact fluorescent lamp (CFL) irradiation in the presence of acetonitrile and 5.0 equivalent of water.This enabled the formation of oxazolines 3 and 14 with 55% and 43% yield, respectively, from the corresponding indene derivatives.
More recently, Lambert's group described an electrophotocatalytic amino-oxygenation reaction of aryl olefins (Scheme 7) [32].The reaction used a trisaminocyclopropenium (TAC) ion catalyst under electrochemical conditions and compact fluorescent lamp (CFL) irradiation in the presence of acetonitrile and 5.0 equivalent of water.This enabled the formation of oxazolines 3 and 14 with 55% and 43% yield, respectively, from the corresponding indene derivatives.Regarding the mechanism (Scheme 8), the authors first proposed an electrochemical oxidation of the photocatalyst 15 to generate the corresponding radical dication 16.Its photoexcitation renders 17, which shows high oxidizing properties (E * red = 3.3 V vs. SCE) and can thus oxidize indene to furnish the corresponding radical cation 18.The regioselective trapping of this intermediate by water at C2 position leads to benzylic radical intermediate 19, which in turn could be oxidized by 16 or directly by the anode.This reaction furnishes the benzylic carbocation 6, which is subsequently reversibly trapped by an equivalent of acetonitrile.The highly favored cis-cyclization delivers the final oxazoline 3. The authors stressed the importance of controlling the amount of water used in the reaction (5.00 equiv.), as a large excess could lead to the trapping of highly electrophilic intermediates generated during the process.Indeed, during the optimization course, when water was used in excess (50.00 equiv.), the authors isolated an aldehyde derivative resulting from probable oxidative cleavage of 1,2 diol intermediates.Regarding the mechanism (Scheme 8), the authors first proposed an electrochemical oxidation of the photocatalyst 15 to generate the corresponding radical dication 16.Its photoexcitation renders 17, which shows high oxidizing properties (E * red = 3.3 V vs. SCE) and can thus oxidize indene to furnish the corresponding radical cation 18.The regioselective trapping of this intermediate by water at C 2 position leads to benzylic radical intermediate 19, which in turn could be oxidized by 16 or directly by the anode.This reaction furnishes the benzylic carbocation 6, which is subsequently reversibly trapped by an equivalent of acetonitrile.The highly favored cis-cyclization delivers the final oxazoline 3. The authors stressed the importance of controlling the amount of water used in the reaction (5.00 equiv.), as a large excess could lead to the trapping of highly electrophilic intermediates generated during the process.Indeed, during the optimization course, when water was used in excess (50.00 equiv.), the authors isolated an aldehyde derivative resulting from probable oxidative cleavage of 1,2 diol intermediates.

Intramolecular Amide Cyclization
Intramolecular amide cyclization is an important class of reaction for the synthesis of cis-1-amino-2-indanol.The key point of this strategy is to form an amide/urethane derivative at C 1 position and a leaving group at C 2 position, which is engaged in a O-amide cyclization step to give the corresponding cis-5-membered ring.Two main approaches have been developed to access the key intermediate via either the N-opening of epoxide or direct electrophilic activation of indene.Scheme 8. Proposed mechanism for Ritter type reaction from indene under electrophotocatalytic conditions.

Intramolecular Amide Cyclization
Intramolecular amide cyclization is an important class of reaction for the synthesis of cis-1-amino-2-indanol.The key point of this strategy is to form an amide/urethane derivative at C1 position and a leaving group at C2 position, which is engaged in a O-amide cyclization step to give the corresponding cis-5-membered ring.Two main approaches have been developed to access the key intermediate via either the N-opening of epoxide or direct electrophilic activation of indene.
This strategy was first reported in 1951 by Lutz and Wayland based on the key formation of a cis-oxazoline obtained from the corresponding trans-1-amino-2-indanol (−/+)-1 ′ (Scheme 9) [33].The latter was obtained in two steps via addition of ammonia to 2-bromo-1-indanol (20), which probably proceeded through an epoxide intermediate to form 1, followed by quantitative amidation reaction rendering 21.The addition of thionyl chloride allowed an intramolecular cyclization with inversion of the configuration at C 2 , leading to the cis-oxazoline 22. Finally, acidic hydrolysis at reflux delivered the cis-1-amino-2-indanol (−/+)-1 with 68% yield.Scheme 8. Proposed mechanism for Ritter type reaction from indene under electrophotocatalytic conditions.

Intramolecular Amide Cyclization
Intramolecular amide cyclization is an important class of reaction for the synthesis of cis-1-amino-2-indanol.The key point of this strategy is to form an amide/urethane derivative at C1 position and a leaving group at C2 position, which is engaged in a O-amide cyclization step to give the corresponding cis-5-membered ring.Two main approaches have been developed to access the key intermediate via either the N-opening of epoxide or direct electrophilic activation of indene.
In 1997, Ghosh et al. reported the synthesis of both enantiomers of cis-1-amino-2-indanol (Scheme 11) [35].In this approach, racemic trans-1-azido-2-indanol (25) was obtained in two steps from indene via epoxidation, followed by opening with sodium azide.Enzymatic acylation proceeded in the presence of lipase PS 30 in a mixture of dimethoxyethane (DME) and isopropenyl acetate to give the unreactive alcohol (1S,2S)-25 (46% yield, ee > 96%) and acylated enantiomer (1R,2R)-26 (44% yield, ee > 96%).Once isolated, both enantiomers were then engaged in the same reaction sequence.The azido group was reduced via hydrogenation and in situ converted into carbamates 27 in the presence of diethylpyrocarbonate. Key cyclization with epimerization of the C 2 center occurred in SOCl 2 giving 28 with excellent yields, and a final basic hydrolysis delivered the enantiopure products.Didier proposed an alternative route for both enantiomers of 1 from β-ketoester 29 (Scheme 12) [36].Baker's yeast reduction of 29 yielded the enantiopure 2-hydroxy ester 30.The hydrolysis of 30 in the presence of NaOH led to a partial racemization at the C1 position, providing trans-isomer 31 after a selective crystallization.C1 epimerization was overcome by enzymatic hydrolysis, leading only to the cis isomer 31.The formation of the C-N bond at the C1 position occurred via a Curtius rearrangement triggered by diphenyl phosphorazidate (DPPA), rendering oxazolidinone 28 and carbamate 27 from cis and trans carboxylic acids, respectively.Basic hydrolysis of 28 led to (1R,2S)-1, whereas its enantiomer (1S,2R)-1 was obtained after inversion of the configuration at the C2 position thanks to an intramolecular cyclization of 27 followed by hydrolysis.
Didier proposed an alternative route for both enantiomers of 1 from β-ketoester 29 (Scheme 12) [36].Baker's yeast reduction of 29 yielded the enantiopure 2-hydroxy ester 30.The hydrolysis of 30 in the presence of NaOH led to a partial racemization at the C 1 position, providing trans-isomer 31 after a selective crystallization.C 1 epimerization was overcome by enzymatic hydrolysis, leading only to the cis isomer 31.The formation of the C-N bond at the C 1 position occurred via a Curtius rearrangement triggered by diphenyl phosphorazidate (DPPA), rendering oxazolidinone 28 and carbamate 27 from cis and trans carboxylic acids, respectively.Basic hydrolysis of 28 led to (1R,2S)-1, whereas its enantiomer (1S,2R)-1 was obtained after inversion of the configuration at the C2 position thanks to an intramolecular cyclization of 27 followed by hydrolysis.
Didier proposed an alternative route for both enantiomers of 1 from β-ketoester 29 (Scheme 12) [36].Baker's yeast reduction of 29 yielded the enantiopure 2-hydroxy ester 30.The hydrolysis of 30 in the presence of NaOH led to a partial racemization at the C1 position, providing trans-isomer 31 after a selective crystallization.C1 epimerization was overcome by enzymatic hydrolysis, leading only to the cis isomer 31.The formation of the C-N bond at the C1 position occurred via a Curtius rearrangement triggered by diphenyl phosphorazidate (DPPA), rendering oxazolidinone 28 and carbamate 27 from cis and trans carboxylic acids, respectively.Basic hydrolysis of 28 led to (1R,2S)-1, whereas its enantiomer (1S,2R)-1 was obtained after inversion of the configuration at the C2 position thanks to an intramolecular cyclization of 27 followed by hydrolysis.
In 1967, Heatchcok et al. proposed a more direct approach through iodoamidation of indene, leading to a trans iodocarbamate 32, which, under reflux in diglyme, yielded the cis-oxazolidinone 28 with 88% yield.A final hydrolysis under basic conditions in methanol at reflux afforded the desired racemic compound (−/+)-1 with 79% yield (Scheme 13) [37].In 1989, Ogura and coworkers developed a strategy for accessing oxazolidinone from alkene involving organotelluric compounds under Lewis acid catalysis (Scheme 14) [38,39].Under the best conditions, indene (5) was converted into oxazolidinone 28 with 79% yield in the presence of benzenetellurinyl trifluoroacetate, ethyl carbamate and boron trifluoride etherate under reflux of dichloroethane.In the mechanism proposed by the authors, the activation of the double bond enables its amidotellurinylation via intermediate 33.Under reflux of dichloroethane and assisted by BF3, cyclization of 34 with the inversion at C2 center occurred, leading to the corresponding oxazolidinone 28.Scheme 14. Telluric anhydride-induced formation of oxazolidinone.
Recently, there has been renewed interest in the amino-oxygenation of alkenes.In 2015, Tepe's group developed an approach with Br-N-(CO2Me)2 as the reagent (Scheme 15) [40].More particularly, indene 5 was transformed into the oxazolidinone 35 precursor of cis-1-amino-2-indanol with 48% yield.The authors proposed the conditions for hydrolysis for oxazolidinones (2M LiOH in THF); however, such conditions were not directly applied to 35.Li and coworkers reported a similar strategy using electrophilic iodine and urea as a partner, rendering N-substituted oxazoline 36 [41].Later, the use of benzamide as a partner was developed to isolate oxazoline 24 with 62% yield [42]. Finally, Hashimoto's group identified a new carbamate as a bifunctional N-and O-nucleophile, giv-Scheme 13. cis-1-amino-2-indanol from indene by iodoamidation.
In 1989, Ogura and coworkers developed a strategy for accessing oxazolidinone from alkene involving organotelluric compounds under Lewis acid catalysis (Scheme 14) [38,39].Under the best conditions, indene (5) was converted into oxazolidinone 28 with 79% yield in the presence of benzenetellurinyl trifluoroacetate, ethyl carbamate and boron trifluoride etherate under reflux of dichloroethane.In the mechanism proposed by the authors, the activation of the double bond enables its amidotellurinylation via intermediate 33.Under reflux of dichloroethane and assisted by BF 3 , cyclization of 34 with the inversion at C 2 center occurred, leading to the corresponding oxazolidinone 28.
In 1989, Ogura and coworkers developed a strategy for accessing oxazolidinone from alkene involving organotelluric compounds under Lewis acid catalysis (Scheme 14) [38,39].Under the best conditions, indene (5) was converted into oxazolidinone 28 with 79% yield in the presence of benzenetellurinyl trifluoroacetate, ethyl carbamate and boron trifluoride etherate under reflux of dichloroethane.In the mechanism proposed by the authors, the activation of the double bond enables its amidotellurinylation via intermediate 33.Under reflux of dichloroethane and assisted by BF3, cyclization of 34 with the inversion at C2 center occurred, leading to the corresponding oxazolidinone 28.Scheme 14. Telluric anhydride-induced formation of oxazolidinone.
Recently, there has been renewed interest in the amino-oxygenation of alkenes.In 2015, Tepe's group developed an approach with Br-N-(CO2Me)2 as the reagent (Scheme 15) [40].More particularly, indene 5 was transformed into the oxazolidinone 35 precursor of cis-1-amino-2-indanol with 48% yield.The authors proposed the conditions for hydrolysis for oxazolidinones (2M LiOH in THF); however, such conditions were not directly applied to 35.Li and coworkers reported a similar strategy using electrophilic iodine and urea as a partner, rendering N-substituted oxazoline 36 [41].Later, the use of benzamide as a partner was developed to isolate oxazoline 24 with 62% yield [42]. Finally, Hashimoto's group identified a new carbamate as a bifunctional N-and O-nucleophile, giv-Scheme 14.Telluric anhydride-induced formation of oxazolidinone.
Recently, there has been renewed interest in the amino-oxygenation of alkenes.In 2015, Tepe's group developed an approach with Br-N-(CO 2 Me) 2 as the reagent (Scheme 15) [40].More particularly, indene 5 was transformed into the oxazolidinone 35 precursor of cis-1amino-2-indanol with 48% yield.The authors proposed the conditions for hydrolysis for oxazolidinones (2M LiOH in THF); however, such conditions were not directly applied to 35.Li and coworkers reported a similar strategy using electrophilic iodine and urea as a partner, rendering N-substituted oxazoline 36 [41].Later, the use of benzamide as a partner was developed to isolate oxazoline 24 with 62% yield [42]. Finally, Hashimoto's group identified a new carbamate as a bifunctional Nand O-nucleophile, giving-in the presence of chiral hypervalent iodine-the corresponding oxazolidinone 37 with 75% yield and 50% ee [43].From a mechanistic point of view, this is an electrophilic activation of olefin by bromine-the iodine of hypervalent iodine-leading to intermediates 38 or 38′ (Scheme 16).Then, the N-nucleophile reacts with the electrophilic benzylic position of 38 or 38′, leading to the formation of trans amido product 39 or 39′, followed by an intramolecular cyclization with the nucleophilic oxygen, giving the desired compound 40.Scheme 16.General mechanism of indene amino-oxygenation.

•
Benzylic Csp 3 -H amination via radical pathway The use of trichloroacetamidates for radical β-amination reactions for the synthesis of cis-1-amino-2-indanol has been proposed by Nagib in 2017 (Scheme 17) [44].Indeed, From a mechanistic point of view, this is an electrophilic activation of olefin by bromine-the iodine of hypervalent iodine-leading to intermediates 38 or 38 ′ (Scheme 16).Then, the N-nucleophile reacts with the electrophilic benzylic position of 38 or 38 ′ , leading to the formation of trans amido product 39 or 39 ′ , followed by an intramolecular cyclization with the nucleophilic oxygen, giving the desired compound 40.From a mechanistic point of view, this is an electrophilic activation of olefin by bromine-the iodine of hypervalent iodine-leading to intermediates 38 or 38′ (Scheme 16).Then, the N-nucleophile reacts with the electrophilic benzylic position of 38 or 38′, leading to the formation of trans amido product 39 or 39′, followed by an intramolecular cyclization with the nucleophilic oxygen, giving the desired compound 40.Scheme 16.General mechanism of indene amino-oxygenation.

•
Benzylic Csp 3 -H amination via radical pathway The use of trichloroacetamidates for radical β-amination reactions for the synthesis of cis-1-amino-2-indanol has been proposed by Nagib in 2017 (Scheme 17) [44].Indeed, Scheme 16.General mechanism of indene amino-oxygenation.The use of trichloroacetamidates for radical β-amination reactions for the synthesis of cis-1-amino-2-indanol has been proposed by Nagib in 2017 (Scheme 17) [44].Indeed, when compound 41 was engaged in the reaction in the presence of PhI(OAc) 2 and NaI under light irradiation in acetonitrile, the desired compound was obtained with a high diastereomeric ratio (>20:1) and 81% yield after hydrolysis of the corresponding oxazoline 45.The reaction consisted of the formation of an imidate radical 42, which underwent 1,5-hydrogen atom transfer (HAT) to generate the corresponding benzylic radical 43.The trapping of the carbon-centered radical by iodine led to 44, followed by intramolecular cyclization.The resulting oxazoline 45 ring was hydrolyzed under acidic conditions, delivering cis-1-amino-2-indanol.A similar approach was developed in the meantime by He [45] using three equivalents of NIS in DCE under thermal conditions, and an approach using a catalytic amount of iodine was devised in 2019 by Nagib [46].Finally, Shi has proposed a new way of obtaining the imidate radical via direct abstraction of the hydrogen of the N-H bond hydrogen atom bond by the malonyl peroxide (MPO) radical [47].
Molecules 2024, 29, 2442 11 of 28 when compound 41 was engaged in the reaction in the presence of PhI(OAc)2 and NaI under light irradiation in acetonitrile, the desired compound was obtained with a high diastereomeric ratio (>20:1) and 81% yield after hydrolysis of the corresponding oxazoline 45.The reaction consisted of the formation of an imidate radical 42, which underwent 1,5hydrogen atom transfer (HAT) to generate the corresponding benzylic radical 43.The trapping of the carbon-centered radical by iodine led to 44, followed by intramolecular cyclization.The resulting oxazoline 45 ring was hydrolyzed under acidic conditions, delivering cis-1-amino-2-indanol.A similar approach was developed in the meantime by He [45] using three equivalents of NIS in DCE under thermal conditions, and an approach using a catalytic amount of iodine was devised in 2019 by Nagib [46].Finally, Shi has proposed a new way of obtaining the imidate radical via direct abstraction of the hydrogen of the N-H bond hydrogen atom bond by the malonyl peroxide (MPO) radical [47].
In a very well-designed study on the enantioselective synthesis of β-amino alcohols via radical amination, Nagib's group used the achiral 2-indanol imidate derivative 46 as a substrate for mechanistic studies (Scheme 18) [48].When placed under their best conditions, the corresponding oxazoline 49 was obtained with 67% ee and a diastereomeric ratio exceeding 20:1.However, the compound was not isolated, and low yield was obtained (12% NMR yield).The key point in the process is the enantioselective HAT via CuL*bound imidate 47, giving the corresponding chiral benzylic radical 48.It is important to note that the behavior of this substrate seemed to be different to others described in the publication, since better results were obtained.However, this result paved the way for further developments.In a very well-designed study on the enantioselective synthesis of β-amino alcohols via radical amination, Nagib's group used the achiral 2-indanol imidate derivative 46 as a substrate for mechanistic studies (Scheme 18) [48].When placed under their best conditions, the corresponding oxazoline 49 was obtained with 67% ee and a diastereomeric ratio exceeding 20:1.However, the compound was not isolated, and low yield was obtained (12% NMR yield).The key point in the process is the enantioselective HAT via CuL*bound imidate 47, giving the corresponding chiral benzylic radical 48.It is important to note that the behavior of this substrate seemed to be different to others described in the publication, since better results were obtained.However, this result paved the way for further developments.
Recently, He and coworkers have shown that binap-containing copper photocatalysts on pillar-layered MOF supports were interesting and stable photocatalysts for the conversion of N-acyloxy imidates 50 into oxazoline 24 (Scheme 19) [49].The oxazoline 24 precursor of cis-aminoindanol was obtained with 42% yield in the presence of a catalytic amount of the Cu photocatalyst and DABCO in N,N-dimethylacetamide (DMA) under blue LED irradiation.The reaction starts with the reduction of the N-acyloxy imidate 50 by the excited state of the photocatalyst, leading to the formation of a N-centered radical 51 and the oxidized photocatalyst (Scheme 19).In the following step, a 1,5 HAT occurs to furnish the benzylic radical 52, which reacts via single electron transfer (SET) with the oxidized photocatalyst to regenerate it at its ground state.On the other hand, this reaction enables the formation of the corresponding benzylic carbenium ion 53, which is trapped via intramolecular N-cyclization assisted by the base (DABCO).Recently, He and coworkers have shown that binap-containing copper photocatalysts on pillar-layered MOF supports were interesting and stable photocatalysts for the conversion of N-acyloxy imidates 50 into oxazoline 24 (Scheme 19) [49].The oxazoline 24 precursor of cis-aminoindanol was obtained with 42% yield in the presence of a catalytic amount of the Cu photocatalyst and DABCO in N,N-dimethylacetamide (DMA) under blue LED irradiation.The reaction starts with the reduction of the N-acyloxy imidate 50 by the excited state of the photocatalyst, leading to the formation of a N-centered radical 51 and the oxidized photocatalyst (Scheme 19).In the following step, a 1,5 HAT occurs to furnish the benzylic radical 52, which reacts via single electron transfer (SET) with the oxidized photocatalyst to regenerate it at its ground state.On the other hand, this reaction enables the formation of the corresponding benzylic carbenium ion 53, which is trapped via intramolecular N-cyclization assisted by the base (DABCO).Recently, He and coworkers have shown that binap-containing copper photocatalysts on pillar-layered MOF supports were interesting and stable photocatalysts for the conversion of N-acyloxy imidates 50 into oxazoline 24 (Scheme 19) [49].The oxazoline 24 precursor of cis-aminoindanol was obtained with 42% yield in the presence of a catalytic amount of the Cu photocatalyst and DABCO in N,N-dimethylacetamide (DMA) under blue LED irradiation.The reaction starts with the reduction of the N-acyloxy imidate 50 by the excited state of the photocatalyst, leading to the formation of a N-centered radical 51 and the oxidized photocatalyst (Scheme 19).In the following step, a 1,5 HAT occurs to furnish the benzylic radical 52, which reacts via single electron transfer (SET) with the oxidized photocatalyst to regenerate it at its ground state.On the other hand, this reaction enables the formation of the corresponding benzylic carbenium ion 53, which is trapped via intramolecular N-cyclization assisted by the base (DABCO).Carbamates 54 derived from 2-indanol are interesting substrates for the synthesis of cisaminoindanol via a radical pathway.Nicholas' group developed a copper-diimine catalyzed oxidative C-H insertion of carbamates (Scheme 20) [50].The reaction is proposed to work through the generation of a L-Cu-imido derivative behaving as a triplet-state diradical specie, in which the C-H amination is operated via a stepwise radical process [51].The authors developed an enantioselective version of this reaction, but moderate enantiomeric excesses were obtained (13-18% ee).The resulting carbamate can be hydrolyzed under several developed conditions [52].
cis-aminoindanol via a radical pathway.Nicholas' group developed a copper-diimine cat-alyzed oxidative C-H insertion of carbamates (Scheme 20) [50].The reaction is proposed to work through the generation of a L-Cu-imido derivative behaving as a triplet-state diradical specie, in which the C-H amination is operated via a stepwise radical process [51].The authors developed an enantioselective version of this reaction, but moderate enantiomeric excesses were obtained (13-18% ee).The resulting carbamate can be hydrolyzed under several developed conditions [52].
cis-aminoindanol via a radical pathway.Nicholas' group developed a copper-diimine cat-alyzed oxidative C-H insertion of carbamates (Scheme 20) [50].The reaction is proposed to work through the generation of a L-Cu-imido derivative behaving as a triplet-state diradical specie, in which the C-H amination is operated via a stepwise radical process [51].The authors developed an enantioselective version of this reaction, but moderate enantiomeric excesses were obtained (13-18% ee).The resulting carbamate can be hydrolyzed under several developed conditions [52].

)-H nitrene insertion catalyzed with the chiral complexes of Ru(II) associated with N-(2-pyridyl)-substituted N-heterocyclic carbine
In all these strategies, the protected amino alcohol must be released from the tricyclic compounds.Hydrolysis of oxazolidinone 28 under the basic condition, typically KOH, led to 1.For instance, this approach has been widely used in the synthesis of BOX ligands [71][72][73] or biologically active molecules [35].Recently, new conditions for the hydrolysis of cyclic carbamate in the presence of diethylenetriamine were developed and resulted in 1 with 86% yield [52]. Surprisingly, the conversion of 57a into (−/+)-1 was not described in the literature.However, it is well known that the reduction of cyclic sulfatame with LiAlH 4 yields the corresponding amino alcohol with retention of the config-uration [74,75], unlike hydrolysis, which would lead to an inversion of the configuration of the C-O bond and thus to the trans amino-indanol [76].No example of hydrolysis of a carbamimidate 57b leading to 1 has yet been reported.However, analogous structures were cleaved under acidic conditions (H 2 SO 4 ), releasing the amino-alcohol moiety with good yield [77,78].

Scheme 22. Meggers' approach to the synthesis of cis-1-amino-2-indanol.
In all these strategies, the protected amino alcohol must be released from the tricyclic compounds.Hydrolysis of oxazolidinone 28 under the basic condition, typically KOH, led to 1.For instance, this approach has been widely used in the synthesis of BOX ligands [71][72][73] or biologically active molecules [35].Recently, new conditions for the hydrolysis of cyclic carbamate in the presence of diethylenetriamine were developed and resulted in 1 with 86% yield [52]. Surprisingly, the conversion of 57a into (−/+)-1 was not described in the literature.However, it is well known that the reduction of cyclic sulfatame with LiAlH4 yields the corresponding amino alcohol with retention of the configuration [74,75], unlike hydrolysis, which would lead to an inversion of the configuration of the C-O bond and thus to the trans amino-indanol [76].No example of hydrolysis of a carbamimidate 57b leading to 1 has yet been reported.However, analogous structures were cleaved under acidic conditions (H2SO4), releasing the amino-alcohol moiety with good yield [77,78].

cis Stereochemistry Controlled by Epimerization 2.2.1. Epimerization via S N 2 at C1 Position
Resnick's group proposed a straightforward approach to the synthesis of 1 [79] from (1S,2R)-indanediol (12, ee 92%) prepared via dioxygenase hydroxylation of 2-indanol 55 (Scheme 23) [80].The carbon-nitrogen bond in 26 was formed through a two-step sequence, with an overall yield of 69%: (i) concomitant introduction of chlorine in the benzylic position and the acetylation of alcohol in position 2, delivering 59; and (ii) chlorine substitution by an azide.The aminolysis of 26 followed by the hydrogenation of azide 25 led to (1S,2R)-1.
In all these strategies, the protected amino alcohol must be released from the tricyclic compounds.Hydrolysis of oxazolidinone 28 under the basic condition, typically KOH, led to 1.For instance, this approach has been widely used in the synthesis of BOX ligands [71][72][73] or biologically active molecules [35].Recently, new conditions for the hydrolysis of cyclic carbamate in the presence of diethylenetriamine were developed and resulted in 1 with 86% yield [52]. Surprisingly, the conversion of 57a into (−/+)-1 was not described in the literature.However, it is well known that the reduction of cyclic sulfatame with LiAlH4 yields the corresponding amino alcohol with retention of the configuration [74,75], unlike hydrolysis, which would lead to an inversion of the configuration of the C-O bond and thus to the trans amino-indanol [76].No example of hydrolysis of a carbamimidate 57b leading to 1 has yet been reported.However, analogous structures were cleaved under acidic conditions (H2SO4), releasing the amino-alcohol moiety with good yield [77,78].

Epimerization via Mitsunobu Reaction at C 2 Position
In 1995, Ogasawara and Takahashi reported the synthesis of both enantiomers of cis-1amino-2-indanol via the resolution of trans-1-azido-2-indanol (25) (Scheme 24) [81].The racemic epoxide 2 was obtained in a two-step sequence via treatment of indene 5 with NBS to render trans-bromohydrin with 82% yield, followed by an epoxidation step mediated by NaOH.The opening of the epoxide by sodium azide rendered the racemic trans-1azido-2-indanol (25) with 93% yield.A screening of the conditions allowed the authors to identify Lipase PS (Pseudomonas sp.Amano) and vinyl acetate in tert-butyl methyl ether as the best conditions for alcohol resolution.Unreacted trans-(1S,2S)-azido-2-indanol (25) was recovered with 48% yield and 99% ee, whereas azido acetate (1R,2R)-26 was obtained with 49% yield and 98% ee.The latter was then converted into the corresponding alcohol via methanolysis with 92% yield without the erosion of chirality.On the other hand, Mitsunobu inversion of 25 was carried out in the presence of p-nitrobenzoic acid, diethyl azodicarboxylate (DEAD) and triphenylphosphine, rendering the corresponding cis-azido ester 60 with 75% yield.At this stage, two pathways were proposed to reach the enantiopure cis-1-amino-2-indanol (1S,2R)-1, either via concomitant reduction of the azido group and the ester with LiAlH 4 in THF (65% yield) or through a two-step sequence consisting of methanolysis of the ester followed by hydrogenation over palladium on carbon (98% yield).The same strategy was successfully applied to transform the other trans-azido-alcohol enantiomer into (1R,2S)-1.
ated by NaOH.The opening of the epoxide by sodium azide rendered the racemic trans-1-azido-2-indanol ( 25) with 93% yield.A screening of the conditions allowed the authors to identify Lipase PS (Pseudomonas sp.Amano) and vinyl acetate in tert-butyl methyl ether as the best conditions for alcohol resolution.Unreacted trans-(1S,2S)-azido-2-indanol (25) was recovered with 48% yield and 99% ee, whereas azido acetate (1R,2R)-26 was obtained with 49% yield and 98% ee.The latter was then converted into the corresponding alcohol via methanolysis with 92% yield without the erosion of chirality.On the other hand, Mitsunobu inversion of 25 was carried out in the presence of p-nitrobenzoic acid, diethyl azodicarboxylate (DEAD) and triphenylphosphine, rendering the corresponding cis-azido ester 60 with 75% yield.At this stage, two pathways were proposed to reach the enantiopure cis-1-amino-2-indanol (1S,2R)-1, either via concomitant reduction of the azido group and the ester with LiAlH4 in THF (65% yield) or through a two-step sequence consisting of methanolysis of the ester followed by hydrogenation over palladium on carbon (98% yield).The same strategy was successfully applied to transform the other trans-azido-alcohol enantiomer into (1R,2S)-1.
An analogous enantioselective approach was developed with sequential reductions (Scheme 26B).The first reduction with a biocatalyst-Daucus carota-led to alcohol 62, and then, the oxime function was hydrogenated to an amino group with H 2 in the presence of Pd/C with 99% ee and 95% conversion in two steps [86].
An analogous enantioselective approach was developed with sequential reductions (Scheme 26B).The first reduction with a biocatalyst-Daucus carota-led to alcohol 62, and then, the oxime function was hydrogenated to an amino group with H2 in the presence of Pd/C with 99% ee and 95% conversion in two steps [86].

Synthesis from (E)-Cinnamate Ethyl Ester
In 2006, Ko's group reported an eight-step enantioselective synthesis of cis-1-amino-2-indanol from (E)-cinnamate ethyl ester (68) (Scheme 27) [87].The synthesis started with a Sharpless asymmetric dihydroxylation, leading to the corresponding syn-diol 69 with 97% yield and 99% ee.The benzylic alcohol was then selectively substituted with inversion of the configuration under Mitsunobu conditions using as a nucleophile delivering 70 [88].After reduction of the azide 70 under a hydrogen atmosphere, the corresponding amino alcohol 71 was judiciously protected with 79% yield under oxazolidinone form 72 in the presence of triphosgene, allowing the protection and further deprotection of both alcohol and amine in a single step.The ethyl ester 72 is then engaged in a saponification reaction, in which the co-solvent (Et2O) plays an important role.Indeed, the choice of a Scheme 25.Synthesis of (1S,2R)-1 from 2-hydroxy-1-indanone. Scheme 25.Synthesis of (1S,2R)-1 from 2-hydroxy-1-indanone.

Synthesis from (E)-Cinnamate Ethyl Ester
In 2006, Ko's group reported an eight-step enantioselective synthesis of cis-1-amino-2-indanol from (E)-cinnamate ethyl ester (68) (Scheme 27) [87].The synthesis started with a Sharpless asymmetric dihydroxylation, leading to the corresponding syn-diol 69 with 97% yield and 99% ee.The benzylic alcohol was then selectively substituted with inversion of the configuration under Mitsunobu conditions using HN3 as a nucleophile delivering 70 [88].After reduction of the azide 70 under a hydrogen atmosphere, the corresponding amino alcohol 71 was judiciously protected with 79% yield under oxazolidinone form 72 in the presence of triphosgene, allowing the protection and further deprotection of both alcohol and amine in a single step.The ethyl ester 72 is then engaged in a saponification reaction, in which the co-solvent (Et2O) plays an important role.Indeed, the choice of a Scheme 26.Synthesis of 1 from 1,2-indanedion-1-oxime.

Synthesis from (E)-Cinnamate Ethyl Ester
In 2006, Ko's group reported an eight-step enantioselective synthesis of cis-1-amino-2indanol from (E)-cinnamate ethyl ester (68) (Scheme 27) [87].The synthesis started with a Sharpless asymmetric dihydroxylation, leading to the corresponding syn-diol 69 with 97% yield and 99% ee.The benzylic alcohol was then selectively substituted with inversion of the configuration under Mitsunobu conditions using HN 3 as a nucleophile delivering 70 [88].After reduction of the azide 70 under a hydrogen atmosphere, the corresponding amino alcohol 71 was judiciously protected with 79% yield under oxazolidinone form 72 in the presence of triphosgene, allowing the protection and further deprotection of both alcohol and amine in a single step.The ethyl ester 72 is then engaged in a saponification reaction, in which the co-solvent (Et 2 O) plays an important role.Indeed, the choice of a more polar solvent, such as THF, leads to detectable epimerization via 1 H NMR. The indane ring skeleton was introduced with 83% yield via a Friedel-Crafts acylation, first converting the carboxylic acid 73 into its acyl chloride derivative and then via addition of an excess of AlCl 3 .The corresponding indanone 74 was then reduced by an excess of silane catalyzed by BF 3 • Et 2 O under microwave irradiation, since low conversions were observed under thermal conditions (oil bath).The cis-1-amino-2-indanol was obtained after final deprotection of the oxazolidinone ring 28 under basic conditions.dane ring skeleton was introduced with 83% yield via a Friedel-Crafts acylation, first converting the carboxylic acid 73 into its acyl chloride derivative and then via addition of an excess of AlCl3.The corresponding indanone 74 was then reduced by an excess of silane catalyzed by BF3 • Et2O under microwave irradiation, since low conversions were observed under thermal conditions (oil bath).The cis-1-amino-2-indanol was obtained after final deprotection of the oxazolidinone ring 28 under basic conditions.Scheme 27.Ko's synthesis of cis-1-amino-2-indanol.

Synthesis from 7,3-Xylofuranose Derivative
In 2021, a successful enantioselective synthesis of cis-1-amino-2-indanol using the Diels-Alder reaction as a key step was reported (Scheme 28) [89].The chiral pool employed for the synthesis was the versatile chiron 7,3-xylofuranose derivative (7,3-LXF) prepared in two steps from diacetone-D-glucose [90].7,3-LXF was first transformed into diacetylated derivative 76 under acidic conditions.Then, diastereomeric allylation was achieved with 85% yield in presence of allylsilane and BF3 • Et2O, followed by Pd-cataβ-elimination, rendering 78 with 60% yield.Acidic deprotection allowed the formation of diol 79, followed by a selective tosylation reaction, enabling the introductionin the second sequence-of the amine group with an appropriate configuration.In order to avoid the formation of by-products during the Diels-Alder step, the amine 81 was first protected to render 82 and then refluxed in toluene at 150 °C, rendering-after CO2 extrusion from 83-compound 84.Rearomatization via DDQ led to 85 with 81% yield, and the final removal of the Boc group under acidic conditions afforded the desired cis-1-amino-2-indanol with 80% yield.Scheme 27.Ko's synthesis of cis-1-amino-2-indanol.

Synthesis from 7,3-Xylofuranose Derivative
In 2021, a successful enantioselective synthesis of cis-1-amino-2-indanol using the Diels-Alder reaction as a key step was reported (Scheme 28) [89].The chiral pool employed for the synthesis was the versatile chiron 7,3-xylofuranose derivative (7,3-LXF) prepared in two steps from diacetone-D-glucose [90].7,3-LXF was first transformed into diacetylated derivative 76 under acidic conditions.Then, diastereomeric allylation was achieved with 85% yield in the presence of allylsilane and BF 3 • Et 2 O, followed by Pd-catalyzed β-elimination, rendering 78 with 60% yield.Acidic deprotection allowed the formation of diol 79, followed by a tosylation reaction, enabling the introduction-in the second sequence-of the amine group with an appropriate configuration.In order to avoid the formation of by-products during the Diels-Alder step, the amine 81 was first protected to render 82 and then refluxed in toluene at 150 • C, rendering-after CO 2 extrusion from 83-compound 84.Rearomatization via DDQ led to 85 with 81% yield, and the final removal of the Boc group under acidic conditions afforded the desired cis-1-amino-2-indanol with 80% yield.

Synthesis from D-Phenylalanine
α-amino acids such as phenylalanine provide a readily available chiral pool for synthesis.Hiyama and coworkers reported a synthesis of enantiomerically pure (1S,2R)-1amino-2-indanol (1), starting from phenylalanine (Scheme 29) [91].The reaction began by the conversion of D-phenylalanine (64) into the corresponding optically pure hydroxylated compound 86 with 82% yield using a mixture of NaNO 2 -H 2 SO 4 [92].The latter was then protected and the carboxylic acid transformed in acyl chloride with thionyl chloride at 50 • C. The addition of AlCl 3 to 87 led to the cyclic structure 88 via Friedel-Crafts acylation without the loss of chiral information.A screening of several conditions for the hydrolysis of 88 without epimerization enabled the identification of the best conditions, i.e., Sc(OTf) 3 (20 mol%) in a mixture of H 2 O-MeOH (1:4) at room temperature, rendering 61 with 82% yield and >99% ee.The α-hydroxy ketone 61 was transformed into its oxime equivalent 62 obtained as a mixture of isomers.After optimization, the authors found out that diastereoselective hydrogenation using Pd black in MeOH/HBr afforded the final product (1S,2R)-1 with 66% yield.

Synthesis from D-Phenylalanine
α-amino acids such as phenylalanine provide a readily available chiral pool for synthesis.Hiyama and coworkers reported a synthesis of enantiomerically pure (1S,2R)-1amino-2-indanol (1), starting from phenylalanine (Scheme 29) [91].The reaction began by the conversion of D-phenylalanine (64) into the corresponding optically pure hydroxylated compound 86 with 82% yield using a mixture of NaNO2-H2SO4 [92].The latter was then protected and the carboxylic acid transformed in acyl chloride with thionyl chloride at 50 °C.The addition of AlCl3 to 87 led to the cyclic structure 88 via Friedel-Crafts acylation without the loss of chiral information.A screening of several conditions for the hydrolysis of 88 without epimerization enabled the identification of the best conditions, i.e., Sc(OTf)3 (20 mol%) in a mixture of H2O-MeOH (1:4) at room temperature, rendering 61 with 82% yield and >99% ee.The α-hydroxy ketone 61 was transformed into its oxime equivalent 62 obtained as a mixture of isomers.After optimization, the authors found out that diastereoselective hydrogenation using Pd black in MeOH/HBr afforded the final product (1S,2R)-1 with 66% yield.

Synthesis from D-Phenylalanine
α-amino acids such as phenylalanine provide a readily available chiral pool for synthesis.Hiyama and coworkers reported a synthesis of enantiomerically pure (1S,2R)-1amino-2-indanol (1), starting from phenylalanine (Scheme 29) [91].The reaction began by the conversion of D-phenylalanine (64) into the corresponding optically pure hydroxylated compound 86 with 82% yield using a mixture of NaNO2-H2SO4 [92].The latter was then protected and the carboxylic acid transformed in acyl chloride with thionyl chloride at 50 °C.The addition of AlCl3 to 87 led to the cyclic structure 88 via Friedel-Crafts acylation without the loss of chiral information.A screening of several conditions for the hydrolysis of 88 without epimerization enabled the identification of the best conditions, i.e., Sc(OTf)3 (20 mol%) in a mixture of H2O-MeOH (1:4) at room temperature, rendering 61 with 82% yield and >99% ee.The α-hydroxy ketone 61 was transformed into its oxime equivalent 62 obtained as a mixture of isomers.After optimization, the authors found out that diastereoselective hydrogenation using Pd black in MeOH/HBr afforded the final product (1S,2R)-1 with 66% yield.

Resolution
As previously mentioned, enantiopure cis-1-amino-2-indanol can be prepared from an enantiopure starting material, such as an epoxide, a diol or an azido-alcohol.However, a significant number of syntheses provide a racemic form of cis-1-amino-2-indanol.Various approaches have been proposed for resolving this racemic mixture.

Chemical Resolution
Interestingly, the resolution of racemic cis-1-amino-2-indanol was performed after its derivatization and chromatographic separation (Scheme 30) [93].After the preparation of racemic cis-1-amino-2-indanol, following the Lutz and Wayland strategy, the authors performed peptide coupling with Boc-Phe-OH followed by Boc deprotection, leading to the formation of a pair of diastereomers 89 and 90 separable via chromatography on silica gel.Both diastereomers were isolated, each with 40% yield, and enantioenriched cis-1-amino-2-indanol was isolated with 93% yield after cleavage of the amide bond.
derivatization and chromatographic separation (Scheme 30) [93].After the preparation of racemic cis-1-amino-2-indanol, following the Lutz and Wayland strategy, the authors performed peptide coupling with Boc-Phe-OH followed by Boc deprotection, leading to the formation of a pair of diastereomers 89 and 90 separable via chromatography on silica gel.Both diastereomers were isolated, each with 40% yield, and enantioenriched cis-1-amino-2-indanol was isolated with 93% yield after cleavage of the amide bond.Another general strategy for β-amino-alcohol resolution is to perform a kinetic resolution via enantioselective acylation of the alcohol in the presence of a chiral nucleophilic catalyst.Kawabata's group implemented this approach in cis-1-amino-2-indanol with a chiral aminopyridine as a catalyst (Scheme 31) [94].After a screening, 4-(dimethylamino)benzamide was determined to be the best protecting group for the amine function.In the presence of (i-PrCO)2O as the acylating agent and the chiral amino pyridine, the S alcohol from mixture 91 was preferentially acylated to 92, and the unconverted protected β-amido-alcohol (1S,2R)-91 could be recovered with an 99% ee after 64% conversion.A treatment with 6M HCl provided the targeted (1S,2R)-1-amino-2-indanol 1 with 68% yield.An analogous approach was proposed by Campbell with a N-4′-pyridinyl-α-methyl proline derivative as the catalyst and trifluoroacetyl as the nitrogen protecting group in order to facilitate its removal [95].Another general strategy for β-amino-alcohol resolution is to perform a kinetic resolution via enantioselective acylation of the alcohol in the presence of a chiral nucleophilic catalyst.Kawabata's group implemented this approach in cis-1-amino-2-indanol with a chiral aminopyridine as a catalyst (Scheme 31) [94].After a screening, 4-(dimethylamino)benzamide was determined to be the best protecting group for the amine function.In the presence of (i-PrCO) 2 O as the acylating agent and the chiral amino pyridine, the S alcohol from mixture 91 was preferentially acylated to 92, and the unconverted protected β-amido-alcohol (1S,2R)-91 could be recovered with an 99% after 64% conversion.A treatment with 6M HCl the targeted (1S,2R)-1-amino-2-indanol 1 with 68% yield.An analogous approach was proposed by Campbell a N-4 ′ -pyridinyl-α-methyl proline derivative as the catalyst and trifluoroacetyl as the nitrogen protecting group in order to facilitate its removal [95].Resolution of the racemic cis-1-amino-2-indanol can also be achieved via diastereomeric salt formation (Scheme 32).(S)-2-Phenylpropionic acid proved to be an efficient resolving agent, inducing a selective crystallization of the ammonium salt formed with (1R,2S)-1-amino-2-indanol (1).After filtration, the amino alcohol could be released under a basic work-up, leading to an enantiopure product with 35% yield, and the resolving agent recovered with 93% yield [96]. Similar results were obtained using tartaric acid as a resolving agent, either to perform enantioenrichment [15] or complete resolution [97].Resolution of the racemic cis-1-amino-2-indanol can also be achieved via diastereomeric salt formation (Scheme 32).(S)-2-Phenylpropionic acid proved to be an efficient resolving agent, inducing a selective crystallization of the ammonium salt formed with (1R,2S)-1-amino-2-indanol (1).After filtration, the amino alcohol could be released under a basic work-up, leading to an enantiopure product with 35% yield, and the resolving agent recovered with 93% yield [96]. Similar results were obtained using tartaric acid as a resolving agent, either to perform enantioenrichment [15] or complete resolution [97].
meric salt formation (Scheme 32).(S)-2-Phenylpropionic acid proved to be an efficient resolving agent, inducing a selective crystallization of the ammonium salt formed with (1R,2S)-1-amino-2-indanol (1).After filtration, the amino alcohol could be released under a basic work-up, leading to an enantiopure product with 35% yield, and the resolving agent recovered with 93% yield [96]. Similar results were obtained using tartaric acid as a resolving agent, either to perform enantioenrichment [15] or complete resolution [97].

Enzymatic Resolution
Enzymatic resolution is an interesting alternative.Various approaches have been devised involving the reactivity of alcohol or amine functions.Gotor's group proposed the acylation of N-Cbz protected racemic cis-1-amino-2-indanol with vinyl catalyzed by Pseudomonas cepacia lipase (PSL) (Scheme 33).These conditions enabled a R selective acylation of the alcohol 93 (43%, >99% ee) with 44% conversion [98].

Enzymatic Resolution
Enzymatic resolution is an interesting alternative.Various approaches have been devised involving the reactivity of alcohol or amine functions.Gotor's group proposed the acylation of N-Cbz protected racemic cis-1-amino-2-indanol 93 with vinyl acetate catalyzed by Pseudomonas cepacia lipase (PSL) (Scheme 33).These conditions enabled a R selective acylation of the alcohol 93 (43%, >99% ee) with 44% conversion [98].
meric salt formation (Scheme 32).(S)-2-Phenylpropionic acid proved to be an efficient resolving agent, inducing a selective crystallization of the ammonium salt formed with (1R,2S)-1-amino-2-indanol (1).After filtration, the amino alcohol could be released under a basic work-up, leading to an enantiopure product with 35% yield, and the resolving agent recovered with 93% yield [96]. Similar results were obtained using tartaric acid as a resolving agent, either to perform enantioenrichment [15] or complete resolution [97].

Enzymatic Resolution
Enzymatic resolution is an interesting alternative.Various approaches have been devised involving the reactivity of alcohol or amine functions.Gotor's group proposed the acylation of N-Cbz protected racemic cis-1-amino-2-indanol 93 with vinyl acetate catalyzed by Pseudomonas cepacia lipase (PSL) (Scheme 33).These conditions enabled a R selective acylation of the alcohol 93 (43%, >99% ee) with 44% conversion [98].The enzyme's ability to hydrolyze an ester was tested on N,O-diacetyl-cis-1-amino-2-indanol 96 (Scheme 34) [99].The alcoholysis of 96 was carried out in the presence of Candida antartica lipase B (CAL-B) and n-butanol.The very high enantiomeric ratio (E > 500) provided both the hydrolyzed ester 97 as well as the unreactive ester 98 with excellent ee and yields.The targeted amino-indanol was released under basic conditions without eroding the ee.The enzyme's ability to hydrolyze an ester was tested on N,O-diacetyl-cis-1-amino-2indanol 96 (Scheme 34) [99].The alcoholysis of 96 was carried out in the presence of Candida antartica lipase B (CAL-B) and n-butanol.The very high enantiomeric ratio (E > 500) provided both the hydrolyzed ester 97 as well as the unreactive ester 98 with excellent ee and yields.The targeted amino-indanol was released under basic conditions without eroding the ee.Interestingly, more recently, a continuous-flow resolution was directly performed on cis-1-amino-2-indanol with CAL-B immobilized on an acrylic resin (Novozym 435 ® (N435)) with EtOAc as the acyl donor (Scheme 35) [100].The flow system enabled selective acylation of the amino group of the (1S,2R) substrate, whereas analogous conditions in vials were much less effective, certainly because the controlled flow rate increased the local concentration of immobilized CAL-B, and consequently, the amino group acylation rate.The result was a conversion of 50% and an ee of 99% for the two alcohols recovered with E > 200.Interestingly, more recently, a continuous-flow resolution was directly performed on cis-1-amino-2-indanol with CAL-B immobilized on an acrylic resin (Novozym 435 ® (N435)) with EtOAc as the acyl donor (Scheme 35) [100].The flow system enabled selective acylation of the amino group of the (1S,2R) substrate, whereas analogous conditions in vials were much less effective, certainly because the controlled flow rate increased the local concentration of immobilized CAL-B, and consequently, the amino group acylation rate.The result was a conversion of 50% and an ee of 99% for the two alcohols recovered with E > 200.
Interestingly, more recently, a continuous-flow resolution was directly performed on cis-1-amino-2-indanol with CAL-B immobilized on an acrylic resin (Novozym 435 ® (N435)) with EtOAc as the acyl donor (Scheme 35) [100].The flow system enabled selective acylation of the amino group of the (1S,2R) substrate, whereas analogous conditions in vials were much less effective, certainly because the controlled flow rate increased the local concentration of immobilized CAL-B, and consequently, the amino group acylation rate.The result was a conversion of 50% and an ee of 99% for the two alcohols recovered with E > 200.

Substituted cis-1-Amino-2-Indanol
Due to the high interest in them for catalysis and for the development of drug candidates, numerous cis-1-amino-2-indanol derivatives with a substituent on the aromatic ring have been prepared.Two different strategies are implemented: (i) a post-functionalization approach, in which the cis-1-amino-2-indanol moiety is already formed, and substituents are introduced via coupling reactions; or (ii) a pre-functionalization approach, in which the introduction of the substituent to the aromatic ring is performed before the formation of the cis-1-amino-2-indanol skeleton with the strategies described above.This section presents an overview of the structure of substituted cis-1-amino-2-indanols and their preparation strategy.

Post-Functionalization
The post-functionalization strategy has been used in particular to prepare new catalysts based on cis-1-amino-2-indanol architecture (Scheme 36).This strategy offers the advantage of utilizing the commercially available enantiopure cis-1-amino-2-indanol, which is first protected under the carbamate form.At this stage, functionalization at position 6

Substituted cis-1-Amino-2-Indanol
Due to the high interest in them for catalysis and for the development of drug candidates, numerous cis-1-amino-2-indanol with a substituent on the aromatic ring have been prepared.Two different strategies are implemented: (i) a post-functionalization approach, in which the cis-1-amino-2-indanol moiety is formed, and substituents are introduced via coupling reactions; or (ii) a pre-functionalization approach, in which the introduction of the substituent to the aromatic ring is performed before the formation of the cis-1-amino-2-indanol skeleton with the strategies described above.This section presents an overview of the structure of substituted cis-1-amino-2-indanols and their preparation strategy.

Post-Functionalization
The post-functionalization strategy has been used in particular to prepare new catalysts based on cis-1-amino-2-indanol architecture (Scheme 36).This strategy offers the advantage of utilizing the commercially available enantiopure cis-1-amino-2-indanol, which is first protected under the carbamate form.At this stage, functionalization at position 6 can be performed through an electrophilic reaction, such as bromination [71,72,101], nitration [102] or Friedel-Crafts reaction [71].In addition to the Friedel-Crafts reaction, the C-C bond can be formed in two steps via a metal-catalyzed coupling reaction on the corresponding bromide [71][72][73]101]; an alkyl or aryl substituent can be introduced in this manner (Table 1).It should be pointed out that the regioselectivity of the electrophilic reaction mainly leads to substitution at position 6 (R 6 ), which limits the scope of this approach.
Molecules 2024, 29, 2442 22 of 28 can be performed through an electrophilic reaction, such as bromination [71,72,101], nitration [102] or Friedel-Crafts reaction [71].In addition to the Friedel-Crafts reaction, the C-C bond can be formed in two steps via a metal-catalyzed coupling reaction on the corresponding bromide [71][72][73]101]; an alkyl or aryl substituent can be introduced in this manner (Table 1).It should be pointed out that the regioselectivity of the electrophilic reaction mainly leads to substitution at position 6 (R6), which limits the scope of this approach.
Scheme 36.General strategy of post-functionalization.

Pre-Functionalization
The pre-functionalization approach implements the various strategies discussed earlier in this review, namely the Ritter reaction (Strategy A, Scheme 2), the Mitsunobu reaction (Strategy B, Scheme 2) or a reductive approach (Strategy C, Scheme 2).The difference lies in the starting substrate, indanone, which already carries substituents on the aromatic ring (Scheme 37).It offers the possibility of choosing different regioisomers, as well as poly-substituted products (Table 2).However, depending on the regioisomer desired, the Scheme 36.General strategy of post-functionalization.

Conclusions
cis-1-amino-2-indanol is a ubiquitous substructure in many areas of chemistry, and its synthesis has attracted the attention of many chemists in both academic and industrial fields for decades.A wide range of strategies have been implemented to prepare this molecule in order to overcome its main structural feature, namely the cis-introduction of an amino group and an alcohol group in positions 1 and 2, respectively.Most of the time, stereochemistry is controlled by forming a 5-membered intermediate ring.In this context, indene has proven to be a versatile starting material, either as a precursor of key functional groups, such as epoxide, diol, halohydrin in their racemic or enantiopure form, or directly functionalized via electrophilic activation of its double bond.Among the implemented strategies, direct C-H functionalization has recently attracted a great deal of interest.These promising approaches involve nitrene or radical intermediates.However, such strategies are mainly developed in their racemic versions, and the few attempts of enantioselective approaches have yielded low enantiomeric excesses.Moreover, the scope of all reactions is particularly limited in terms of the diversity of substitution on the aromatic ring.Position 6 offers the largest possibility of substitution by an electron-withdrawing or electron-donating group.The examples for positions 4, 5 and 7 are scarce and only concern alkyl or halogen groups.No electron-rich or electron-poor groups have been introduced on these positions, and consequently, their impact on the methods implemented for the introduction of the amino-alcohol moiety-in the pre-functionalization strategy-remains unknown.In this context, the development of functionalizations through Csp 3 -H activation provides interesting alternatives, as the reaction intermediates involved are different from those involved in older synthetic approaches.

Scheme 15 .
Scheme 15.Recent approaches to the amino-oxygenation of indene.

Table 1 .
Molecules obtained via the post-functionalization strategy.

Table 1 .
Molecules obtained via the post-functionalization strategy.

Table 2 .
Molecules obtained via the pre-functionalization strategy.