Stereoselective Synthesis and Catalytical Application of Perillaldehyde-Based 3-Amino-1,2-diol Regioisomers

A library of regioisomeric monoterpene-based aminodiols was synthesised and applied as chiral catalysts in the addition of diethylzinc to benzaldehyde. The synthesis of the first type of aminodiols was achieved starting from (−)-8,9-dihydroperillaldehyde via reductive amination, followed by Boc protection and dihydroxylation with the OsO4/NMO system. Separation of formed stereoisomers resulted in a library of aminodiol diastereoisomers. The library of regioisomeric analogues was obtained starting from (−)-8,9-dihydroperillic alcohol, which was transformed into a mixture of allylic trichloroacetamides via Overman rearrangement. Changing the protecting group to a Boc function, the protected enamines were subjected to dihydroxylation with the OsO4/NMO system, leading to a 71:16:13 mixture of diastereoisomers, which were separated, affording the three isomers in isolated form. The obtained primary aminodiols were transformed into secondary derivatives. The regioselectivity of the ring closure of the N-benzyl-substituted aminodiols with formaldehyde was also investigated, resulting in 1,3-oxazines in an exclusive manner. To explain the stability difference between diastereoisomeric 1,3-oxazines, a series of comparative theoretical modelling studies was carried out. The obtained potential catalysts were applied in the reaction of aromatic aldehydes and diethylzinc with moderate to good enantioselectivities (up to 94% ee), whereas the opposite chiral selectivity was observed between secondary aminodiols and their ring-closed 1,3-oxazine analogues.


Introduction
Chiral catalysis, that is, the development and application of new chiral catalysts, is an evergreen topic in the field of organic, applied, and pharmaceutical chemistry.In recent years, aminodiols and their N-heterocyclic analogues have proved to be important building blocks of new chiral catalysts or even complex bioactive molecules with significant biological activities [1][2][3][4].Several aminodiol-based nucleoside analogues prepared recently have been shown to possess cardiovascular, cytostatic, and antiviral effects [5][6][7][8][9][10].The Abbott aminodiol, a useful building block for the synthesis of the potent renin inhibitors Zankiren ® and Enalkiren ® , was introduced into the therapy of hypertension [11,12].Aminodiols can also exert antidepressive activity.For example, (S,S)-reboxetine, a selective norepinephrine reuptake inhibitor, was approved in many countries for the treatment of unipolar depression [13].Other aminodiols may serve as starting materials for the synthesis of biologically active natural compounds.For example, cytoxazone, a microbial metabolite isolated from Streptomyces species, is a selective modulator of the secretion of TH2 cytokine [14,15].
In the present study, our aim was to prepare a library of diastereoisomeric and regioisomeric analogues of 3-amino-1,2-diols derived from commercially available (−)perillaldehyde as the chiral source.The study of the steric effect of both the aminodiol functionality and nitrogen substituents was also planned, applying the resulting trifunctional potential catalysts in the enantioselective addition of diethylzinc on benzaldehyde as a model reaction.

Synthesis of Aminodiols via Reductive Amination of Perillaldehyde
In our first approach, chiral aminodiols were synthesised from readily available (−)-(S)-peryllaldehyde 1, starting with a reduction according to a literature method using Pt/C catalyst in an n-hexane/EtOAc solvent system to obtain 2 [30]. 1 H-NMR monitoring was applied during the reaction to set up the appropriate conditions to afford the desired product.The resulting aldehyde was transformed into allylamines in two steps by reductive amination with (S)-and (R)-1-phenylethylamine 3a-c in good yield.The amine moieties were protected by the BOC (tert-butyloxycarbonyl) group, and 4a-c were hydroxylated in a stereoselective manner with the OsO 4 /NMO catalytic system to produce cis-vicinal aminodiols 5a-c (1S,2S) and 6a-c (1R,2R), whereas the obtained diastereomers in 1:1 mixture were successfully separated by column chromatography (Scheme 1).
treatment of unipolar depression [13].Other aminodiols may serve as starting materials for the synthesis of biologically active natural compounds.For example, cytoxazone, a microbial metabolite isolated from Streptomyces species, is a selective modulator of the secretion of TH2 cytokine [14,15].
In the present study, our aim was to prepare a library of diastereoisomeric and regioisomeric analogues of 3-amino-1,2-diols derived from commercially available (−)-perillaldehyde as the chiral source.The study of the steric effect of both the aminodiol functionality and nitrogen substituents was also planned, applying the resulting trifunctional potential catalysts in the enantioselective addition of diethylzinc on benzaldehyde as a model reaction.

Synthesis of Aminodiols via Reductive Amination of Perillaldehyde
In our first approach, chiral aminodiols were synthesised from readily available (−)-(S)-peryllaldehyde 1, starting with a reduction according to a literature method using Pt/C catalyst in an n-hexane/EtOAc solvent system to obtain 2 [30]. 1 H-NMR monitoring was applied during the reaction to set up the appropriate conditions to afford the desired product.The resulting aldehyde was transformed into allylamines in two steps by reductive amination with (S)-and (R)-1-phenylethylamine 3a-c in good yield.The amine moieties were protected by the BOC (tert-butyloxycarbonyl) group, and 4a-c were hydroxylated in a stereoselective manner with the OsO4/NMO catalytic system to produce cisvicinal aminodiols 5a-c (1S,2S) and 6a-c (1R,2R), whereas the obtained diastereomers in 1:1 mixture were successfully separated by column chromatography (Scheme 1).
The relative configuration of diastereoisomer 7a was determined by means of NOESY experiments: clear NOE signals were observed between the H-2 and H-4, H-2 and H-7, as well as the OH-1 and OH-2 protons (Figure 1).Debenzylation via hydrogenolysis of compounds 7a-7c over Pd/C in MeOH resulted in primary aminodiol 8 yielding the same compound with a moderate yield (Scheme 3).Besides NOESY experiments, the structure was also elucidated by X-ray crystallography (Figure 1).(ii) 10 % Pd/C, n-hexane/EtOAc 1:1 mixture, 1 atm, H2, rt, 5 h, 45%; (iii) R = Bn, 3 eq.LiAlH4, THF, reflux, 6 h, 58%; (iv) 35% aq.CH2O, Et2O, rt, 3 h-product could not be isolated.To compare the reactivity and ring closing ability of 7a-c with their diastereoisomers, the above-mentioned reactions were accomplished with the (1R,2R) 6a-c compounds as well.The deprotected 11a-c, the N-methyl-N-benzyl 13, and the primary aminodiol 12 products were obtained similarly to the diastereoisomeric analogues, while in the case of ring closure with formaldehyde, no ring-closed product could be isolated (Scheme 3).
To rationalise the spectacular difference observed in the tendency of 7a and 11a to undergo formaldehyde-mediated annulation to form 1,3-oxazine, we carried out a series of comparative theoretical modelling studies (details can be found in Section 4).Although fused 1,3-oxazine 14a could not be isolated, presumably due to its decomposition during To compare the reactivity and ring closing ability of 7a-c with their diastereoisomers, the above-mentioned reactions were accomplished with the (1R,2R) 6a-c compounds as well.The deprotected 11a-c, the N-methyl-N-benzyl 13, and the primary aminodiol 12 products were obtained similarly to the diastereoisomeric analogues, while in the case of ring closure with formaldehyde, no ring-closed product could be isolated (Scheme 3).
The relative configuration of diastereoisomer 7a was determined by means of NOESY experiments: clear NOE signals were observed between the H-2 and H-4, H-2 and H-7, as well as the OH-1 and OH-2 protons (Figure 1).Debenzylation via hydrogenolysis of compounds 7a-7c over Pd/C in MeOH resulted in primary aminodiol 8 yielding the same compound with a moderate yield (Scheme 3).Besides NOESY experiments, the structure was also elucidated by X-ray crystallography (Figure 1).To rationalise the spectacular difference observed in the tendency of 7a and 11a to undergo formaldehyde-mediated annulation to form 1,3-oxazine, we carried out a series of comparative theoretical modelling studies (details can be found in Section 4).Although fused 1,3-oxazine 14a could not be isolated, presumably due to its decomposition during chromatographic purification on silica, the calculated changes in Gibbs free energy unambiguously indicate that both cyclisations (7a to 10a and 11a to 14a) are thermodynamically feasible processes (Figure 2).It must be noted here that cyclisation of 11a to 14a is accompanied by the inversion of the fused cyclohexane ring, forcing the bulky isopropyl group into an axial position of the resulting trans-fused rigid bicyclic framework.(ii) 10 % Pd/C, n-hexane/EtOAc 1:1 mixture, 1 atm, H2, rt, 5 h, 45%; (iii) R = Bn, 3 eq.LiAlH4, THF, reflux, 6 h, 58%; (iv) 35% aq.CH2O, Et2O, rt, 3 h-product could not be isolated.To compare the reactivity and ring closing ability of 7a-c with their diastereoisomers, the above-mentioned reactions were accomplished with the (1R,2R) 6a-c compounds as well.The deprotected 11a-c, the N-methyl-N-benzyl 13, and the primary aminodiol 12 products were obtained similarly to the diastereoisomeric analogues, while in the case of ring closure with formaldehyde, no ring-closed product could be isolated (Scheme 3).
To rationalise the spectacular difference observed in the tendency of 7a and 11a to undergo formaldehyde-mediated annulation to form 1,3-oxazine, we carried out a series of comparative theoretical modelling studies (details can be found in Section 4).Although fused 1,3-oxazine 14a could not be isolated, presumably due to its decomposition during chromatographic purification on silica, the calculated changes in Gibbs free energy unambiguously indicate that both cyclisations (7a to 10a and 11a to 14a) are thermodynamically feasible processes (Figure 2).It must be noted here that cyclisation of 11a to 14a is accompanied by the inversion of the fused cyclohexane ring, forcing the bulky isopropyl group into an axial position of the resulting trans-fused rigid bicyclic framework.Since TLC monitoring indicated the transitional formation of 14a, we assume that its isolation was prevented by a hydrolytic decomposition taking place on the acidic silica surface during the chromatographic purification of the crude product.This view gains support from the structural characteristics and energetic data of further comparative DFT mod-elling studies on the iminium-generating ring opening of the diastereomeric O-protonated oxazines 10a/OH + → 10a/Im + and 14a/OH + → 14a/Im + (Figure 3).The energetic results unambiguously show that both ring fissions are thermodynamically feasible.However, the second process is accompanied by the inversion of the cyclohexane ring with the reorientation of the isopropyl group into equatorial position and a significant spatial separation of the iminium moiety and the axially oriented hydroxyl group in 11a/Im + .The latter is expected to undergo precursor-regenerating hydrolysis (11a/Im + + H 2 O → 11a + CH 2 O) rather than ring inversion-enabled recyclisation to 14a/OH + .On the other hand, upon neutralisation of the medium, sequential recyclisation and O-deprotonation of 7a/Im + can be considered as a smooth chemical route proceeding via the interaction of proximal nucleophilic and electrophilic functional groups on the highly rigid cyclohexane ring to reconstruct the isolable fused oxazine 10a.
Since TLC monitoring indicated the transitional formation of 14a, we assume that its isolation was prevented by a hydrolytic decomposition taking place on the acidic silica surface during the chromatographic purification of the crude product.This view gains support from the structural characteristics and energetic data of further comparative DFT modelling studies on the iminium-generating ring opening of the diastereomeric O-protonated oxazines 10a/OH + → 10a/Im + and 14a/OH + → 14a/Im + (Figure 3).The energetic results unambiguously show that both ring fissions are thermodynamically feasible.However, the second process is accompanied by the inversion of the cyclohexane ring with the reorientation of the isopropyl group into equatorial position and a significant spatial separation of the iminium moiety and the axially oriented hydroxyl group in 11a/Im + .The la er is expected to undergo precursor-regenerating hydrolysis (11a/Im + + H2O → 11a + CH2O) rather than ring inversion-enabled recyclisation to 14a/OH + .On the other hand, upon neutralisation of the medium, sequential recyclisation and O-deprotonation of 7a/Im + can be considered as a smooth chemical route proceeding via the interaction of proximal nucleophilic and electrophilic functional groups on the highly rigid cyclohexane ring to reconstruct the isolable fused oxazine 10a. Figure 3. Calculated thermodynamics of acid-promoted iminium-generating ring opening of O-protonated diastereomeric cyclohexane-fused 1,3-oxazines 10a/OH + and 14a/OH + , as assumed to be effected during chromatographic workup on silica.The highly exothermic energetics disclosed for both ring-opening reactions analysed here clearly indicate that formaldehyde-mediated annulations cannot proceed via iminium intermediates.
The aforementioned results, including the highly exothermic energetics disclosed for both ring-opening reactions analysed above, clearly indicate that formaldehyde-mediated annulations cannot proceed via iminium-type intermediates.Therefore, we propose here an alternative pathway for the formation of fused 1,3-oxazines as exemplified by theoretically modelling the one-step ring closure of the ring-inverted N-hydroxymethylated intermediate coupled to a four-membered water chain cluster (11a/hm/4H2O → 14a/4H2O, Figure 4).Accordingly, we assume that a H-bonded optimally arranged linear cluster of four water molecules strongly facilitates the kinetically and thermodynamically feasible one-step intramolecular SN2 reaction.It simultaneously activates the nucleophilic skeletal hydroxyl group and the leaving hydroxyl group in the pending hemiaminal residue by a concerted domino proton-migration along the H-bond-coupled chain of water molecules.The aforementioned results, including the highly exothermic energetics disclosed for both ring-opening reactions analysed above, clearly indicate that formaldehyde-mediated annulations cannot proceed via iminium-type intermediates.Therefore, we propose here an alternative pathway for the formation of fused 1,3-oxazines as exemplified by theoretically modelling the one-step ring closure of the ring-inverted N-hydroxymethylated intermediate coupled to a four-membered water chain cluster (11a/hm/4H 2 O → 14a/4H 2 O, Figure 4).Accordingly, we assume that a H-bonded optimally arranged linear cluster of four water molecules strongly facilitates the kinetically and thermodynamically feasible one-step intramolecular S N 2 reaction.It simultaneously activates the nucleophilic skeletal hydroxyl group and the leaving hydroxyl group in the pending hemiaminal residue by a concerted domino proton-migration along the H-bond-coupled chain of water molecules.As for the size of the linear H-bond transferring cluster, we found that it is the involvement of the four water molecules that allows the construction of such an ideal molecular architecture, which is suitable for undergoing ring closure via a concerted mechanism.It is also reasonable to rationalise the formation of 10a along this pathway.The validity of this and closely related reaction pathways is further supported by other well-documented examples [31,32].
ment of the four water molecules that allows the construction of such an ideal molecular architecture, which is suitable for undergoing ring closure via a concerted mechanism.It is also reasonable to rationalise the formation of 10a along this pathway.The validity of this and closely related reaction pathways is further supported by other well-documented examples [31,32].

Synthesis of Aminodiols via Overman Rearrangement of Perillyl Alcohol
(−)-(S)-Dihydroperyllaldehyde 2 proved to be an excellent starting material not only for the preparation of aminodiols 5-13 but also for the synthesis of regioisomeric 3-amino-1,2-diols, where the amino function is connected directly to the cyclohexane ring.Reduction of 2 served 8,9-dihydroperillic alcohol 15.The Overmann rearrangement of 15 led to the diastereoisomeric mixture of N-trichloroacetyl-protected allyl amines in a ratio of 1R:1S = 85:15 based on data determined by GC (Chirasil-DEX CB column) analysis of the crude product.The relative configuration of major product 18a, based on the NOESY spectra, was found to be similar to the literature example [20,33,34].
Since separation of the diastereomers was not feasible, the mixture of 16a and 16b was reacted in the following steps.The trichloroacetyl protecting group was exchanged for easily handlable Boc by alkaline hydrolysis followed by BOC2O treatment, affording 18a and 18b.Despite chromatographic efforts, the resulting diastereoisomers could not be separated.Therefore, the mixture of diastereoisomers was used in the next step (Scheme 4).The relative configuration of major diastereoisomer 18a was determined by means of NOESY experiments.Clear NOE signals were observed between the H-2 and the methylene Ha, between NH and H-3ax and H-5, as well as between the H-3eq and the methylene Hb protons (Figure 5).

Synthesis of Aminodiols via Overman Rearrangement of Perillyl Alcohol
(−)-(S)-Dihydroperyllaldehyde 2 proved to be an excellent starting material not only for the preparation of aminodiols 5-13 but also for the synthesis of regioisomeric 3-amino-1,2-diols, where the amino function is connected directly to the cyclohexane ring.Reduction of 2 served 8,9-dihydroperillic alcohol 15.The Overmann rearrangement of 15 led to the diastereoisomeric mixture of N-trichloroacetyl-protected allyl amines in a ratio of 1R:1S = 85:15 based on data determined by GC (Chirasil-DEX CB column) analysis of the crude product.The relative configuration of major product 18a, based on the NOESY spectra, was found to be similar to the literature example [20,33,34].
Since separation of the diastereomers was not feasible, the mixture of 16a and 16b was reacted in the following steps.The trichloroacetyl protecting group was exchanged for easily handlable Boc by alkaline hydrolysis followed by BOC 2 O treatment, affording 18a and 18b.Despite chromatographic efforts, the resulting diastereoisomers could not be separated.Therefore, the mixture of diastereoisomers was used in the next step (Scheme 4).
As for the size of the linear H-bond transferring cluster, we found that it is the involvement of the four water molecules that allows the construction of such an ideal molecular architecture, which is suitable for undergoing ring closure via a concerted mechanism.It is also reasonable to rationalise the formation of 10a along this pathway.The validity of this and closely related reaction pathways is further supported by other well-documented examples [31,32].

Synthesis of Aminodiols via Overman Rearrangement of Perillyl Alcohol
(−)-(S)-Dihydroperyllaldehyde 2 proved to be an excellent starting material not only for the preparation of aminodiols 5-13 but also for the synthesis of regioisomeric 3-amino-1,2-diols, where the amino function is connected directly to the cyclohexane ring.Reduction of 2 served 8,9-dihydroperillic alcohol 15.The Overmann rearrangement of 15 led to the diastereoisomeric mixture of N-trichloroacetyl-protected allyl amines in a ratio of 1R:1S = 85:15 based on data determined by GC (Chirasil-DEX CB column) analysis of the crude product.The relative configuration of major product 18a, based on the NOESY spectra, was found to be similar to the literature example [20,33,34].
Since separation of the diastereomers was not feasible, the mixture of 16a and 16b was reacted in the following steps.The trichloroacetyl protecting group was exchanged for easily handlable Boc by alkaline hydrolysis followed by BOC2O treatment, affording 18a and 18b.Despite chromatographic efforts, the resulting diastereoisomers could not be separated.Therefore, the mixture of diastereoisomers was used in the next step (Scheme 4).The relative configuration of major diastereoisomer 18a was determined by means of NOESY experiments.Clear NOE signals were observed between the H-2 and the methylene Ha, between NH and H-3ax and H-5, as well as between the H-3eq and the methylene Hb protons (Figure 5).The relative configuration of major diastereoisomer 18a was determined by means of NOESY experiments.Clear NOE signals were observed between the H-2 and the methylene H a , between NH and H-3 ax and H-5, as well as between the H-3 eq and the methylene H b protons (Figure 5).
The mixture of allyl amines was dihydroxylated with the OsO 4 /NMO oxidiser system, resulting in a mixture of diasteroisomers (19a-c) in a ratio 19a:19b:19c = 71:16:13.At this stage, isomer 19a could be isolated by column chromatography, while isomers 19b and 19c were obtained as an inseparable mixture (Scheme 5).The mixture of allyl amines was dihydroxylated with the OsO4/NMO oxidiser system, resulting in a mixture of diasteroisomers (19a-c) in a ratio 19a:19b:19c = 71:16:13.At this stage, isomer 19a could be isolated by column chromatography, while isomers 19b and 19c were obtained as an inseparable mixture (Scheme 5).
To isolate the other isomers too, acetal derivatives 20b and 20c were prepared with acetone from the two-component mixture, which allowed successful separation of 20b and 20c.BOC and acetal protective groups were removed in a single step under acidic conditions.The obtained relative configurations of epimers were determined via 2D NMR techniques (COSY, HMBC, HSQC, and NOESY) (Scheme 6 and Figure 6).To isolate the other isomers too, acetal derivatives 20b and 20c were prepared with acetone from the two-component mixture, which allowed successful separation of 20b and 20c.BOC and acetal protective groups were removed in a single step under acidic conditions.The obtained relative configurations of epimers were determined via 2D NMR techniques (COSY, HMBC, HSQC, and NOESY) (Scheme 6 and Figure 6).
NHBoc To isolate the other isomers too, acetal derivatives 20b and 20c were prepared with acetone from the two-component mixture, which allowed successful separation of 20b and 20c.BOC and acetal protective groups were removed in a single step under acidic conditions.The obtained relative configurations of epimers were determined via 2D NMR techniques (COSY, HMBC, HSQC, and NOESY) (Scheme 6 and Figure 6).The stereostructures of 21a, 21b, and 21c and the two diastereomeric components 18a (major isomer) and 18b (minor isomer) were determined by 1 H-and 13 C-NMR spectroscopy.The assignment of the 1 H-and 13 C-NMR signals was supported by 2D-COSY, 2D-HSQC, 2D-HMBC, and 2D-NOESY experiments.The stereostructure of 18b/minor was identified on the basis of its separated diagnostic signals and cross peaks discernible in the 1D and 2D spectra registered on the isolated ca.85/15 mixture ( 1 H-NMR) of 18a/major and 18b/minor (see later).In accordance with the general expectations, the isopropyl group was in the equatorial position in all of these cyclohexane derivatives, as evidenced by the NOE interactions detected between the axially positioned proton pair 4Hα/6Hα.A further interaction involving 4Hα and 2Hα was in accordance with the equatorial position of the NH3 + group on the rigid, six-membered skeleton in 21c.Providing additional evidence for their relative configuration, characteristic NOE interactions in 21a and 21b were disclosed between 4Hα and the NH3 + group.In 18a/major, the axial position of the Bocprotected amino group was confirmed by the NOE interaction between the NH-and 4Hα protons.Providing further evidence for its stereostructure with the axially positioned Bocprotected amino group, further interactions were also detected in the NOESY spectrum of 18a/major between proton pairs HA/1Hβ and HB/3Hβ (Figure 5).The axial position of the hydroxymethyl group in 21b and 21c was confirmed by the NOE interaction of its protons with 5Hβ and 3Hβ.In 18b/minor, the equatorial position of the Boc-protected amino group The stereostructures of 21a, 21b, and 21c and the two diastereomeric components 18a (major isomer) and 18b (minor isomer) were determined by 1 H-and 13 C-NMR spectroscopy.The assignment of the 1 H-and 13 C-NMR signals was supported by 2D-COSY, 2D-HSQC, 2D-HMBC, and 2D-NOESY experiments.The stereostructure of 18b/minor was identified on the basis of its separated diagnostic signals and cross peaks discernible in the 1D and 2D spectra registered on the isolated ca.85/15 mixture ( 1 H-NMR) of 18a/major and 18b/minor (see later).In accordance with the general expectations, the isopropyl group was in the equatorial position in all of these cyclohexane derivatives, as evidenced by the NOE interactions detected between the axially positioned proton pair 4H α /6H α .A further interaction involving 4H α and 2H α was in accordance with the equatorial position of the NH 3 + group on the rigid, six-membered skeleton in 21c.Providing additional evidence for their relative configuration, characteristic NOE interactions in 21a and 21b were disclosed between 4H α and the NH 3 + group.In 18a/major, the axial position of the Boc-protected amino group was confirmed by the NOE interaction between the NH-and 4H α protons.Providing further evidence for its stereostructure with the axially positioned Boc-protected amino group, further interactions were also detected in the NOESY spectrum of 18a/major between proton pairs H A /1H β and H B /3H β (Figure 5).The axial position of the hydroxymethyl group in 21b and 21c was confirmed by the NOE interaction of its protons with 5H β and 3H β .In 18b/minor, the equatorial position of the Boc-protected amino group was reflected in the NOE interaction detected between the axially positioned proton pair 1H α /3H α producing signals of well-resolved coupling patterns featuring significant splits due to characteristic 1,2-diaxial coupling interactions.Further supporting the relative configuration of the two diastereomers, the marked upfield shift of the 13 C signal of terminal methylene (=CH 2 ) in 18b/minor relative to that measured for 18a/major (104.9 ppm and 149.0 ppm, respectively) referred to a local steric congestion associated with the proximity of the bulky Boc-protected amino group and the exocyclic olefin residue on rigid ring system of 18b/minor (Figure 5).
Reduction of 19a, performed with LiAlH 4 , served the N-methylaminodiol 22, while deprotection of 19a and reductive alkylation of the formed 21a with benzaldehyde in the presence of NaBH 4 provided N-benzyl derivative 23a.Ring closure of aminodiols with a secondary amino function was carried out with formaldehyde.Interestingly, only the six-membered 1,3-oxazoline ring systems 24 and 25a were formed in both cases (Scheme 7).

Application of Aminodiol Libraries as Catalysts in the Addition Reaction of Diethyzinc to Benzaldehyde
The prepared aminodiol derivatives were applied as chiral catalysts in the enantiose lective addition of diethylzinc to benzaldehyde (26) as a model reaction, where (S)-and (R)-1-phenyl-1-propanol ((S)-27 and (R)-27) were formed as products (Scheme 9).Similar to the above process, starting from the liberated base 21b, N-benzyl aminodiol 23b and its ring-closed 1,3-oxazoline ring system derivative 25b were produced (Scheme 8).

Application of Aminodiol Libraries as Catalysts in the Addition Reaction of Diethyzinc to Benzaldehyde
The prepared aminodiol derivatives were applied as chiral catalysts in the enantiose lective addition of diethylzinc to benzaldehyde (26) as a model reaction, where (S)-and (R)-1-phenyl-1-propanol ((S)-27 and (R)-27) were formed as products (Scheme 9).

Application of Aminodiol Libraries as Catalysts in the Addition Reaction of Diethyzinc to Benzaldehyde
The prepared aminodiol derivatives were applied as chiral catalysts in the enantioselective addition of diethylzinc to benzaldehyde (26)  Our results are presented in Table 1.The enantiomeric ratio of the S and R was determined by a chiral GC CHIRASIL-DEX CB column according to literatur ods [35][36][37].Our results are presented in Table 1.The enantiomeric ratio of the S and R isomers was determined by a chiral GC CHIRASIL-DEX CB column according to literature methods [35][36][37].In these addition reactions, low to excellent catalytic activities were observed.Interestingly, aminodiol diastereoisomers provided opposite chiral induction, and 7a-c isomers proved better catalysts with S selectivity than 11a-c (best ee = 68% for 7a).The best catalytic activity was observed in the case of 10a, one of the 1,3-oxazoline N-benzyl derivatives, which afforded the best ee value (94% ee) with R selectivity (Table 1, entry 6, ee = 94%).Interestingly, oxazine 25a (ee = 62%, entry 21) promotes the R absolute configuration, whereas the diasteroisomer primer aminodiol 21b (ee = 60%, entry 15) provides the S configuration.Furthermore, all tested primer aminodiols (8, 12, 21a, 21b and 21c) promote the S configuration.
With the best catalyst for (S)-selectivity (7a) and for (R)-selectivity (10a), the diethylzinc addition reaction was extended to further aromatic aldehydes (Scheme 10).Our results are presented in Table 2.The enantiomeric purities of the 1-aryl-1-propanols obtained were determined by chiral HPLC analysis on a Chiralcel OD-H column (see Supporting Information Figures S147-S162), according to literature methods [23].
Int. J. Mol.Sci.2024, 25, x FOR PEER REVIEW sults are presented in Table 2.The enantiomeric purities of the 1-aryl-1-propan tained were determined by chiral HPLC analysis on a Chiralcel OD-H column (s porting information Figure S147 -S162), according to literature methods [23].

Discussion
Starting from natural (−)-(S)-peryllaldehyde (1), a monoterpene-based aminodiol library with five sub-libraries containing diastereo-and regioisomeric aminodiols was created, and the reaction of these aminodiols with formaldehyde resulted in bicyclic monoterpene-fused condensed 1,3-oxazines through stereochemistry-dependent ring closure.Molecular modelling was applied to explain this phenomenon.The aminodiols and their ring-closed derivatives were used as chiral catalysts in the enantioselective addition of diethylzinc to benzaldehyde from moderate to excellent activities.Comparing the regioisomers and diastereoisomers, aminodiols bearing vicinal hydroxyl groups on the cyclohexane skeleton with the (1S,2S,4S) configuration proved to be more efficient catalysts mainly with S selectivity.The ring closure of these aminodiols also showed the best but opposite (R) catalytic activity, when the obtained 1,3-oxazines 10a-c were applied in the addition of diethylzinc to benzaldehyde.The catalytic activity study was extended by applying the best catalyst for meta and para substituted aromatic aldehydes and, in the case of p-tolylaldehy aminodiol, 7a gave (R) selectivity.It is interesting to note that in the same enantiomeric library, we could find catalysts for both (S) and (R) selectivities.The 1,3-oxazines obtained proved to be excellent catalysts in the additions of diethylzinc to aromatic aldehydes, probably as a consequence of their conformationally constrained structures.Regioisomeric aminodiols and 1,3-oxazines proved less effective catalysts in the applied model reaction with similar selectivities.
X-Ray structural determination of compound 7a was determined by a Rigaku Oxford Diffraction Supernova diffractometer ((Rigaku Oxford Diffraction, Yarnton, UK) using Cu Kα radiation.The CrysAlisPro software package (version: 1.171.37.35) was used for cell refinement and data reduction (see Supporting Information part, Table S1).
General method for the reductive amination of 2 with amines The appropriate benzylamine (1.05 equiv, 27.6 mmol) was added to a stirred solution of compound 2 (4.00 g, 26.28 mmol) in dry EtOH (200 mL).The mixture was stirred at room temperature for 1 h.Afterwards, the solvent was evaporated, and the residue was redissolved in dry EtOH (200 mL) and then stirred for an additional hour.NaBH 4 (2.98 g, 78.84 mmol) was added in small portions to the reaction mixture, which was stirred at room temperature (for compound 3a) or under reflux (for compounds 3b and 3c) for 2 h.Next, the solvent was removed under vacuum, and the crude product was dissolved in ice-cold H 2 O (70 mL) and extracted with dichloromethane (DCM) (3 × 100 mL).The combined organic phase was dried (Na 2 SO 4 ), filtered, and evaporated.The crude product was purified via column chromatography on silica gel by applying toluene/EtOH 9:1, and then hydrochloride salts of the compounds were formed to yield 3a-c.

General procedure for tert-butyloxycarbonyl (BOC) protection of compounds 3a-c
To a stirred solution of the liberated bases of allylamines 3a-c (12 mmol) in dry THF (30 mL), di-tert-butyl dicarbonate (2.88 g, 13.2 mmol for 4a; 5.76 g, 26.4 mmol for 4b, and 3.56 g, 16.32 mmol for 4c), TEA (3.64 g 36 mmol), and a catalytic amount of DMAP (0.15 g, 1.2 mmol) were added.The mixture was stirred overnight at rt.After completion of the reaction, indicated by means of TLC, the solvent was evaporated.The crude product was purified by column chromatography on silica gel by using n-hexane/EtOAc 9:1 for 4a, n-hexane/EtOAc 19:1 for 4b, or n-hexane/Et 2 O 19:1 for 4c.

General method for dihydroxylation of 4a-c
To a solution of 4a-c (10 mmol) in acetone (50 mL), an aqueous solution of 4-methylmorpholine-4-oxide (NMO) (8.5 mL, 50% aqueous sol.) and a solution of OsO 4 in tert-BuOH (4.5 mL, 2% tert-BuOH solution) were added in one portion.The mixture was stirred at room temperature overnight, and then quenched by the addition of a saturated aqueous solution of Na 2 SO 3 (80 mL) and extracted with EtOAc (3 × 60 mL).The combined organic phase was dried, filtered, and evaporated.The products were purified by means of column chromatography on silica gel in n-hexane/EtOAc 4:1 mixture.

General procedure for the Boc deprotection of 5a-c and 6a-c
To a stirred solution of compounds 5a-c or 6a-c (3 mmol) in Et 2 O (25 mL), an 18% aqueous solution of HCl (70 mL) was added and the mixture was stirred overnight.After the reaction was complete (indicated by TLC), the solvent was removed by vacuum evaporation.The resulting hydrochloride salt was filtered off and washed with Et

General procedure for debenzylation of 7a and 11a
To a stirred suspension of palladium-on-carbon (10% Pd/C, 0.10 g) in a mixture of n-hexane/EtOAc (24 mL) (1:2 mixture for 7a, 1:1 mixture for 11a), 7a or 11a (1.8 mmol) was added and the reaction mixture was stirred under a H 2 atmosphere at room temperature and normal pressure.After the reaction was completed (monitored by TLC), the mixture was filtered through a short Celite pad, the solvent was concentrated, and the hydrochloride salts of compounds were formed.
(1S,2S,4S)-1-Aminomethyl-4-isopropylcyclohexane-1,2-diol hydrochloride 8 Prepared from 7a according to the general method.Yield: 0. General procedure used to form the N-methyl derivatives of compounds 9 and 13 A solution of appropriate compound (5a or 6a) (1.00 g, 2.65 mmol) in dry THF (12 mL) was added to a stirred suspension of LiAlH 4 (0.30 g, 7.95 mmol) in dry THF (15 mL) carefully at 0 • C. The mixture was stirred at reflux for 6 h.When the TLC indicated, a mixture of H 2 O (1 mL) and THF (16 mL) was added dropwise with cooling.The precipitated material was filtered off and washed with THF.The filtrate was dried (Na 2 SO 4 ), filtered, and concentrated in a vacuum.The crude products were purified via column chromatography on silica gel by applying DCM/MeOH 19:1 (for compound 9) n-hexane/EtOAc 2:1 (for compound 13).

General method for the preparation of 1,3 oxazines 10a-c
To a stirred solution of aminodiol (7a-c) (0.15 g) in Et 2 O (6 mL), a 35% aqueous solution of formaldehyde (4.5 mL) was added.The reaction mixture was stirred at room temperature for 3 h, then extracted with a 10% aqueous solution of KOH (10 mL).The aqueous layer was extracted with Et 2 O (3 × 25 mL), and then the combined organic phase was washed with brine (3 × 25 mL).The organic layer was dried (NaSO 4 ), filtered, and evaporated.The crude products were purified by column chromatography on silica gel by using an n-hexane/EtOAc 9:1 mixture for compound 10a and a 4:1 mixture for compounds 10b and 10c.To a solution of 2 (5.36 g, 35.2 mmol) in MeOH (110 mL), NaBH 4 (3.99 g, 105.6 mmol) was added in small portions at 0 • C. The reaction mixture was stirred at room temperature for 1 h.When the reaction was completed (indicated by TLC), the solvent was evaporated.The residue was redissolved in water (100 mL) and extracted with DCM (3 × 100 mL).The combined organic phase was dried (Na 2 SO 4 ), filtered, and concentrated to dryness.The crude product was used for the next step without further purification.Isolated product: 5.16 g (95%), light green transparent oil.All its spectroscopic data and physical properties agreed with those reported in the literature [30].To a solution of 15 (10.48 g, 67.9 mmol) in dry DCM (320 mL), 1,8-diazabicyclo [5.4.0]undec-7-ene (12.41 g, 81.5 mmol, 12.2 mL) was added applying an Ar atmosphere.Trichloroacetonitrile (10.3 mL, 97.4 mmol) was added in small portions to the reaction mixture, and it was stirred for 2 h at room temperature.Upon completion of the reaction (indicated by TLC), the solvent was evaporated.The crude product was purified via chromatography on silica gel by using DCM.The top one-third of the column was dry Na 2 SO 4, and the bottom two-thirds of the column were silica gel.The fractions that contained the imidate intermediate were collected, and the solvent was evaporated.The slightly yellow oil product was dissolved in dry toluene (300 mL) and reacted in an autoclave at 130 • C in an Ar atmosphere for 24 h.When the reaction was completed, the mixture was extracted with a cold aqueous solution of HCl (5%, 3 × 100 mL), and the organic layer was dried, filtered, and evaporated.Based on 1 H NMR measurements and GC determination (Chirasil-DEX CB column, 2 mL flow rate, 110 • C, Figure S145), the diastereomers formed a mixture with a ratio of 85:15.Although various column chromatography methods were carried out, the diastereomers were inseparable.Yield: 14.52 g (72%) (two diastereomers); orange oil; [α] D 20 = −33 (c 0.505, MeOH); 1 H NMR (CDCl 3 , 500.2MHz): δ = 0.87 (3H, d, J = 1.6 Hz, minor), 0.88 (3H, d, J = 1.6 Hz, minor), 0.90 (3H, d, J = 6.7 Hz, major), 0.92 (3H, d, J = 6.8 Hz, major), 0.97-1.08(1H, m), 1. 23 tert-Butyl ((1R,5S)-5-isopropyl-2-methylenecyclohexyl)carbamate 18a and tert-butyl ((1S,5S)-

5-isopropyl-2-methylenecyclohexyl)carbamate 18b
To a solution of 16a and 16b (11.11 g, 37.2 mmol) in EtOH/DCM 2:1 (66 mL), an aqueous solution of NaOH (5M solution, 525.6 mL) was added.The reaction was stirred at 50 • C for 15 h.As the TLC indicated, the reaction was cooled to room temperature, then extracted with DCM (3 × 150 mL).The combined organic phase was washed with brine (100 mL), dried, filtered, and evaporated.The residue was dissolved in dry THF (100 mL), and then triethylamine (11.29 g, 111.6 mmol), DMAP (0.45 g, 3.72 mmol), and di-tert-butyl dicarbonate (8.93 g, 40.9 mmol) were added to the reaction mixture.The mixture was stirred at room temperature for 12 h.After that, the solvent was evaporated, and the crude product was purified via column chromatography on silica gel by applying n-hexane/Et 2 O 9:1.The diastereomers were still inseparable in this step.The ratio of the formed diastereomers remained 85:15 based on the determination by GC (Chirasil-DEX CB column, 2 mL flow rate, 160 • C, Figure S146).

General procedure for preparation of N-benzyl derivatives 23a and 23b
To a solution of primer aminodiol base form 21a or 21b (0.19 g, 1.0 mmol) in dry EtOH (20 mL), 0.11 g (1.05 mmol) benzaldehyde was added, and the reaction mixture was stirred at room temperature.When the reaction was completed (monitored by means of TLC, 1 h), the solvent was concentrated and the residue was dissolved in dry EtOH (20 mL) and stirred for 1 h, and then NaBH 4 (0.11 g, 3.0 mmol) was added in small portions.When the reaction was completed (indicated by TLC), the solvent was concentrated to dryness; then, H 2 O (15 mL) was poured into the residue and extracted with DCM (3 × 25 mL).After drying (Na 2 SO 4 ), filtration, and solvent evaporation, the crude product was purified via column chromatography by applying a CHCl 3 /MeOH 9:1 mixture.To a suspension of LiAlH 4 (0.18 g, 4.7 mmol) in dry THF (5 mL), a solution of compound 19a (0.45 g, 1.57 mmol) in dry THF (5 mL) was added dropwise and stirred for 2 h, and then the excess of LiAlH 4 was quenched with a mixture of H 2 O (0.36 mL) and THF (5 mL) at 0 • C. The suspension was stirred for 1 h at room temperature and then filtered.The inorganic residue was washed with THF (3 × 30 mL), and then the organic layer was dried, filtered, and concentrated to dryness.As its hydrochloride salt, the crude product was crystallised using a solution of HCl (10%, in EtOH/Et 2 O).
Yield: 0.243 g (65%); white crystals m.General method for ring closure of compounds 22, 23a, and 23b with formaldehyde To a solution of aminodiol 22, 23a, and 23b (0.58 mmol) in Et 2 O (10 mL), an aqueous solution of formaldehyde (40%, 5 mL) was added, and the mixture was stirred at room temperature for 1 h.An aqueous solution of NaOH (5%) was added to the reaction mixture to make it alkaline and extracted with Et 2 O (3 × 20 mL).The combined organic phase was dried (Na 2 SO 4 ), filtered, and evaporated in vacuo.The crude products were purified by column chromatography (toluene/EtOH 4:1).General procedure for the reaction of benzaldehyde with diethylzinc in the presence of chiral catalysts.
A solution of Et 2 Zn in n-hexane (1M, 4.5 mL) was added to the appropriate catalyst (10 mol%) under an Ar atmosphere at room temperature.The reaction mixture was stirred for 20 min at room temperature, and then benzaldehyde (0.156 g, 153 µL, 1.5 mmol) was added.The mixture was stirred for a further 20 h at room temperature, then quenched with a saturated solution of NH 4 Cl (50 mL) and extracted with EtOAc (2 × 30 mL).The combined organic layer was dried (Na 2 SO 4 ), filtered, and evaporated.The ee values and absolute configurations of the obtained secondary alcohols were determined by chiralphase GC by using a CHIRASIL-DEX CB column, at 90 • C, after O-acetylation in an AcO 2 /4-dimethylaminopyridine/pyridine system.

DFT calculations on compounds 10a and 14a
All DFT calculations were carried out by Gaussian 09 Revision A.02 software (Gaussian Incorporation, Pittsburgh, PA, USA), package [Gaussian 09], using the M06-2X global hybrid DFT functional [38] and 6-31+G(d,p) basis set [39].Structural optimisations and subsequent frequency calculations were supported by the IEFPCM solvent model [40] parameterised with the dielectric constant of water (ε = 78.4) to represent the approximate polarity of the experimental reaction conditions.The Gibbs free-energy values of optimised structures were obtained by correcting the computed total energy with zero-point vibrational energy (ZPE) and the calculated thermal corrections.The optimised structures are available from the authors.

Figure 1 .
Figure 1.Structural determination of aminodiol 7a by X-ray crystallography and NOESY experiments.

Figure 1 .
Figure 1.Structural determination of aminodiol 7a by X-ray crystallography and NOESY experiments.

Figure 1 .
Figure 1.Structural determination of aminodiol 7a by X-ray crystallography and NOESY experiments.

Figure 3 .
Figure 3. Calculated thermodynamics of acid-promoted iminium-generating ring opening of Oprotonated diastereomeric cyclohexane-fused 1,3-oxazines 10a/OH + and 14a/OH + , as assumed to be effected during chromatographic workup on silica.The highly exothermic energetics disclosed for both ring-opening reactions analysed here clearly indicate that formaldehyde-mediated annulations cannot proceed via iminium intermediates.

Figure 4 .
Figure 4. Calculated kinetics and thermodynamics of the cyclisation of the ring-inverted N-hydroxymethyl derivative 11a taking place with the SN2 reaction promoted by a cascade of proton shifts along the H-bond-connected cluster chain of four water molecules.

Figure 4 .
Figure 4. Calculated kinetics and thermodynamics of the cyclisation of the ring-inverted Nhydroxymethyl derivative 11a taking place with the S N 2 reaction promoted by a cascade of proton shifts along the H-bond-connected cluster chain of four water molecules.

Figure 4 .
Figure 4. Calculated kinetics and thermodynamics of the cyclisation of the ring-inverted N-hydroxymethyl derivative 11a taking place with the SN2 reaction promoted by a cascade of proton shifts along the H-bond-connected cluster chain of four water molecules.

Figure 5 .
Figure 5. Structural determination of the diastereoisomers 18a and 18b by NOESY experiments.

Figure 5 .
Figure 5. Structural determination of the diastereoisomers 18a and 18b by NOESY experiments.

Figure 5 .
Figure 5. Structural determination of the diastereoisomers 18a and 18b by NOESY experiments.The mixture of allyl amines was dihydroxylated with the OsO4/NMO oxidiser system, resulting in a mixture of diasteroisomers (19a-c) in a ratio 19a:19b:19c = 71:16:13.At this stage, isomer 19a could be isolated by column chromatography, while isomers 19b and 19c were obtained as an inseparable mixture (Scheme 5).
a 10 mol%.b Yields were measured after silica column chromatography.c Determined on the crude product by GC (Chirasil-DEX CB column).d Determined by comparing the R t of the GC analysis and the optical rotations with literature data.

Table 2 .
[23]tion of diethylzinc to aromatic aldehydes 26b-d, catalysed by aminodiol derivatives.Yields were measured after silica column chromatography.cDeterminedon the crude product by chiral HPLC analysis on a Chiralcel OD-H column.dDeterminedby comparing the R t of the HPLC analysis and the optical rotations with literature data[23].