Constrained Dipeptide Surrogates: 5- and 7-Hydroxy Indolizidin-2-one Amino Acid Synthesis from Iodolactonization of Dehydro-2,8-diamino Azelates

The constrained dipeptide surrogates 5- and 7-hydroxy indolizidin-2-one N-(Boc)amino acids have been synthesized from L-serine as a chiral educt. A linear precursor ∆4-unsaturated (2S,8S)-2,8-bis[N-(Boc)amino]azelic acid was prepared in five steps from L-serine. Although epoxidation and dihydroxylation pathways gave mixtures of hydroxy indolizidin-2-one diastereomers, iodolactonization of the ∆4-azelate stereoselectively delivered a lactone iodide from which separable (5S)- and (7S)-hydroxy indolizidin-2-one N-(Boc)amino esters were synthesized by sequences featuring intramolecular iodide displacement and lactam formation. X-ray analysis of the (7S)-hydroxy indolizidin-2-one N-(Boc)amino ester indicated that the backbone dihedral angles embedded in the bicyclic ring system resembled those of the central residues of an ideal type II’ β-turn indicating the potential for peptide mimicry.


Results and Discussion
Initially, 5-and 7-hydroxy indolizidine-2-one N-(Boc)amino esters 8 and 9 were pursued by pathways featuring a ring opening of 4-oxiranyl-2,8-diaminoazelates. Oxiranes 12a-c were synthesized by epoxidation of ∆ 4 -2,8-diaminoazelates 11a-c, which were respectively prepared from (E)-1,3-dichloroprop-1-ene by copper catalyzed S N 2 additions of zincates derived from methyl β-iodo alaninates 12a-c protected with Boc [25], Cbz [26], and Fmoc groups (Scheme 1) [27]. Although the 15 Hz coupling constant suggested the formation of the E-trans olefins 11a and 11b, without the corresponding Z-cis isomer, NOESY experiments were performed to confirm the double-bond geometry. The E-geometry of olefins 11a and 11b was ascertained by NOESY experiments in which the long-range through-space transfer of magnetization was observed, respectively, between the vinyl C4 (5.38 and 5.35 ppm) and allylic C6 protons (2.09 and 2.07 ppm) and between the vinyl C5 (5.51 and 5.48 ppm) and allylic C3 protons (2.47 and 2.50 ppm) (Scheme 1). No nuclear Overhauser effect was observed between the two vinyl protons nor between the two sets of allylic protons.
Molecules 2022, 26, x FOR PEER REVIEW 3 of 17 by way of diastereoselective iodolactonization chemistry inspired by the seminal research of the Bartlett laboratory [24].

Results and Discussion
Initially, 5-and 7-hydroxy indolizidine-2-one N-(Boc)amino esters 8 and 9 were pursued by pathways featuring a ring opening of 4-oxiranyl-2,8-diaminoazelates. Oxiranes 12a-c were synthesized by epoxidation of ∆ 4 -2,8-diaminoazelates 11a-c, which were respectively prepared from (E)-1,3-dichloroprop-1-ene by copper catalyzed SN2′ additions of zincates derived from methyl β-iodo alaninates 12a-c protected with Boc [25], Cbz [26], and Fmoc groups (Scheme 1) [27]. Although the 15 Hz coupling constant suggested the formation of the E-trans olefins 11a and 11b, without the corresponding Z-cis isomer, NO-ESY experiments were performed to confirm the double-bond geometry. The E-geometry of olefins 11a and 11b was ascertained by NOESY experiments in which the long-range through-space transfer of magnetization was observed, respectively, between the vinyl C4 (5.38 and 5.35 ppm) and allylic C6 protons (2.09 and 2.07 ppm) and between the vinyl C5 (5.51 and 5.48 ppm) and allylic C3 protons (2.47 and 2.50 ppm) (Scheme 1). No nuclear Overhauser effect was observed between the two vinyl protons nor between the two sets of allylic protons. Previously, epoxidations of N-Boc and N-Cbz allyl-and homoallyl-glycine esters with m-chloroperbenzoic acid (m-CPBA) in dichloromethane had given 1:1 diastereomeric mixtures of the corresponding oxiranes, which were inseparable by chromatography [28][29][30]. The C3-protons of benzyl (2S,4RS)-2-(Boc)amino-3-(2-oxiranyl)propionate was reported to exhibit a doubling of signals in the 1 H NMR spectrum [28]. The appearance of multiple sets of signals for the two possible isomers was similarly observed in the spectra of inseparable epoxide diastereomers 12a-c and validated by COSY spectra of the Cbz and Fmoc analogs 12b and 12c in which through-bond couplings between two sets of C3protons with two overlapping downfield α-(C2)-proton signals were observed. Oxiranes 12a-c were thus obtained as 1:1 diastereomeric mixtures, which were used in the subsequent chemistry. Previously, epoxidations of N-Boc and N-Cbz allyl-and homoallyl-glycine esters with m-chloroperbenzoic acid (m-CPBA) in dichloromethane had given 1:1 diastereomeric mixtures of the corresponding oxiranes, which were inseparable by chromatography [28][29][30]. The C3-protons of benzyl (2S,4RS)-2-(Boc)amino-3-(2-oxiranyl)propionate was reported to exhibit a doubling of signals in the 1 H NMR spectrum [28]. The appearance of multiple sets of signals for the two possible isomers was similarly observed in the spectra of inseparable epoxide diastereomers 12a-c and validated by COSY spectra of the Cbz and Fmoc analogs 12b and 12c in which through-bond couplings between two sets of C3-protons with two overlapping downfield α-(C2)-proton signals were observed. Oxiranes 12a-c were thus obtained as 1:1 diastereomeric mixtures, which were used in the subsequent chemistry.
Based on the successful synthesis of 6-hydroxymethyl I 2 aa diastereomers in which 5-hydroxymethyl prolines were prepared from a related C2 symmetric oxirane using Lewisacid activation with BF 3 ·Et 2 O in DCM at −78 • C [31], similar conditions were employed for the intramolecular ring-opening of epoxide 12a (Scheme 2). Multiple isomers of the material with a molecular ion corresponding to proline 13 and hydroxyproline 14 were obtained from oxirane 12a likely by endo and exo ring openings by the attack of the two different carbamate-protected nitrogen [28,32,33]. Considering that the isomeric mix could be due, in part, to carbocation intermediates formed under the Lewis acid conditions, a method to remove the Boc group without the ring opening of the epoxide was attempted featuring heating oxirane 12a in water at reflux [34]. Deprotection of the Boc group, intramolecular epoxide ring opening, and lactam formation all occurred upon treating 12a with boiling water. Amine protection with di-tert-butyl dicarbonate and triethyl amine in dichloromethane, however, afforded four isomers of 5-and 7-hydroxy I 2 aa esters 8 and 9, which were observed by LCMS in a 1:1:1:1 ratio. Employing Cbz-protected epoxide 12b, hydrogenolytic cleavage of the carbamate using hydrogen and palladium-on-carbon in ethanol commenced an epoxide ring opening and lactam formation sequence, which was followed by Boc protection as described above to afford four isomers of 8 and 9, which were observed in a 1:5:5:1 ratio by HPLC. The improvement in selectivity may be due to a favored exo-tet-like ring opening of the epoxide diastereomers by the free amine, which when generated at a lower temperature reacted to favor the proline instead of the hydroxyproline counterparts [32,33]. In spite the possibility of improved regioselectivity in the oxirane ring opening, the route (Scheme 2) was, however, deemed inefficient due to the complications engendered from the lack of diastereomeric selectivity in the epoxidation of olefins 11. Based on the successful synthesis of 6-hydroxymethyl I 2 aa diastereomers in which 5hydroxymethyl prolines were prepared from a related C2 symmetric oxirane using Lewisacid activation with BF3 . Et2O in DCM at −78 °C [31], similar conditions were employed for the intramolecular ring-opening of epoxide 12a (Scheme 2). Multiple isomers of the material with a molecular ion corresponding to proline 13 and hydroxyproline 14 were obtained from oxirane 12a likely by endo and exo ring openings by the attack of the two different carbamate-protected nitrogen [28,32,33]. Considering that the isomeric mix could be due, in part, to carbocation intermediates formed under the Lewis acid conditions, a method to remove the Boc group without the ring opening of the epoxide was attempted featuring heating oxirane 12a in water at reflux [34]. Deprotection of the Boc group, intramolecular epoxide ring opening, and lactam formation all occurred upon treating 12a with boiling water. Amine protection with di-tert-butyl dicarbonate and triethyl amine in dichloromethane, however, afforded four isomers of 5-and 7-hydroxy I 2 aa esters 8 and 9, which were observed by LCMS in a 1:1:1:1 ratio. Employing Cbz-protected epoxide 12b, hydrogenolytic cleavage of the carbamate using hydrogen and palladium-on-carbon in ethanol commenced an epoxide ring opening and lactam formation sequence, which was followed by Boc protection as described above to afford four isomers of 8 and 9, which were observed in a 1:5:5:1 ratio by HPLC. The improvement in selectivity may be due to a favored exo-tet-like ring opening of the epoxide diastereomers by the free amine, which when generated at a lower temperature reacted to favor the proline instead of the hydroxyproline counterparts [32,33]. In spite the possibility of improved regioselectivity in the oxirane ring opening, the route (Scheme 2) was, however, deemed inefficient due to the complications engendered from the lack of diastereomeric selectivity in the epoxidation of olefins 11. Scheme 2. Syntheses of 5-and 7-hydroxy Boc-I 2 aa-OMe 8 and 9 from epoxide 12.
Prompted by earlier success using transannular iodolactamization to prepare azabicyclo[X.Y.0]alkan-2-one ring systems [15,35], and related iodoamination protocols for preparing iodomethyl pyrrolidines and piperidines [36][37][38], ∆ 4 -diaminoazelate 11a was subjected to iodine and NaHCO3 at −20 °C (Scheme 3). The ring opening of the iodonium intermediate by one of the two carbamate-protected nitrogen appeared to be a method for selectively obtaining proline 15 instead of the azetidine counterpart; however, a mixture of diastereomeric iodolactones 16 was also produced as a competing side product. Considering the lactone as a potential means for differentiating between the two carboxylates, dihydroxylation of ∆ 4 -diaminoazelate 11a was performed using osmium tetroxide and Nmethylmorpholine N-oxide (NMO) in aqueous acetone to provide hydroxy lactone 17 as Prompted by earlier success using transannular iodolactamization to prepare azabicyclo[X.Y.0]alkan-2-one ring systems [15,35], and related iodoamination protocols for preparing iodomethyl pyrrolidines and piperidines [36][37][38], ∆ 4 -diaminoazelate 11a was subjected to iodine and NaHCO 3 at −20 • C (Scheme 3). The ring opening of the iodonium intermediate by one of the two carbamate-protected nitrogen appeared to be a method for selectively obtaining proline 15 instead of the azetidine counterpart; however, a mixture of diastereomeric iodolactones 16 was also produced as a competing side product. Considering the lactone as a potential means for differentiating between the two carboxylates, dihydroxylation of ∆ 4 -diaminoazelate 11a was performed using osmium tetroxide and N-methylmorpholine N-oxide (NMO) in aqueous acetone to provide hydroxy lactone 17 as a mixture of diasteromers [39]. Mesylate 18 was obtained by methanesulfonation of hydroxy lactone 17 using methanesulfonyl chloride and triethylamine in dichloromethane. Mesylate 18 was converted to hydroxy I 2 aa analogs 8 and 9 by a three-step sequence featuring proline formation after Boc group removal with HCl gas bubbles in dichloromethane, lactam cyclization upon treatment of the hydrochloride salt with triethylamine in methanol at reflux, and amine protection with di-tert-butyl decarbonate in dichloromethane. The HPLC chromatogram of the products from this sequence exhibited four peaks with molecular ions corresponding to 5-and 7-hydroxy Boc-I 2 aa-OMe isomers 8 and 9 (Scheme 3) in a 1:1:1:1 ratio. a mixture of diasteromers [39]. Mesylate 18 was obtained by methanesulfonation of hydroxy lactone 17 using methanesulfonyl chloride and triethylamine in dichloromethane. Mesylate 18 was converted to hydroxy I 2 aa analogs 8 and 9 by a three-step sequence featuring proline formation after Boc group removal with HCl gas bubbles in dichloromethane, lactam cyclization upon treatment of the hydrochloride salt with triethylamine in methanol at reflux, and amine protection with di-tert-butyl decarbonate in dichloromethane. The HPLC chromatogram of the products from this sequence exhibited four peaks with molecular ions corresponding to 5-and 7-hydroxy Boc-I 2 aa-OMe isomers 8 and 9 (Scheme 3) in a 1:1:1:1 ratio. Scheme 3. Strategies featuring iodoamination and dihydroxylation of ∆ 4 -diaminoazelate 11a.
Different mixtures of 5-and 7-hydroxy Boc-I 2 aa-OMe diastereomers 8 and 9 likely arose from a combination of a lack of facial selectivity in the epoxidation and the dihydroxylation of olefin 11 and competing nucleophilic attack from both nitrogen of diamino azelate epoxide 12 and methanesulfonate 18. The loss of stereochemical integrity may also arise from competing SN1 processes due to the epoxide ring opening prior to pyrrolidine formation. Intrigued by the production of iodolactone 16 as a side product from the iodoamination strategy, an iodolactonization approach was considered because of the high facial selectivity achieved on simpler γ,δ-unsaturated carboxylic acids [24,40,41].
After saponification of diester 11a with lithium hydroxide in aqueous dioxane, dicarboxylic acid 19 was treated with cesium carbonate and iodine in an ice-cold acetonitrile solution (Scheme 4). Analysis by LCMS demonstrated a major peak with a molecular ion corresponding to lactone 20. Subsequent treatment with iodomethane and potassium carbonate in DMF furnished the corresponding methyl ester tetrahydrofuran-2-one (1′R,5S)-16 after chromatography in 55% yield from diacid acid 19. Attempts to perform the iodolactonization without a base gave a product mostly from the loss of Boc protection. Employing the same three-step sequence described above to convert methane sulfonate 18 into esters 8 and 9, iodide (1′R,5S)-16 was transformed into separable 5-and 7-hydroxy I 2 aa esters (5S,6S)-8 and (6S,7S)-9 in 42% and 34% overall yields, respectively. Subsequent saponification of esters (5S,6S)-8 and (6S,7S)-9 gave, respectively, the acids (5S,6S)-2 and (6S,7S)-3 in 64% and 78% yields. Different mixtures of 5-and 7-hydroxy Boc-I 2 aa-OMe diastereomers 8 and 9 likely arose from a combination of a lack of facial selectivity in the epoxidation and the dihydroxylation of olefin 11 and competing nucleophilic attack from both nitrogen of diamino azelate epoxide 12 and methanesulfonate 18. The loss of stereochemical integrity may also arise from competing S N 1 processes due to the epoxide ring opening prior to pyrrolidine formation. Intrigued by the production of iodolactone 16 as a side product from the iodoamination strategy, an iodolactonization approach was considered because of the high facial selectivity achieved on simpler γ,δ-unsaturated carboxylic acids [24,40,41].

Assignment of Regio-Chemistry and Stereochemistry of 5-and 7-Hydroxy I 2 aa Esters
The configuration of the ring fusion and hydroxyl group carbons of the 5-and 7hydroxy I 2 aa esters 8 and 9, as well as the alcohol position on the ring system, were all assigned based on two-dimensional NMR spectroscopic experiments. The locations of the indolizidine-2-one ring protons were initially assigned by COSY experiments in which through-bond couplings were used to trace the sequence from the downfield shifted carbamate NH to the C9 hydrogen. Subsequently, heteronuclear single quantum coherence (HSQC) spectroscopy was used to correlate the protons linked to similar carbons. The βprotons on the same face as the C3 carbamate and C9 carboxylate appeared generally upfield of their α-counterparts due to anisotropic effects caused by the latter functional groups [42]. Finally, relative configurations were ascertained ( Figure 2) based on NOESY experiments in which the observed through-space transfers of magnetization were used to correlate the stereochemical assignments.

Assignment of Regio-Chemistry and Stereochemistry of 5-and 7-Hydroxy I 2 aa Esters
The configuration of the ring fusion and hydroxyl group carbons of the 5-and 7hydroxy I 2 aa esters 8 and 9, as well as the alcohol position on the ring system, were all assigned based on two-dimensional NMR spectroscopic experiments. The locations of the indolizidine-2-one ring protons were initially assigned by COSY experiments in which through-bond couplings were used to trace the sequence from the downfield shifted carbamate NH to the C9 hydrogen. Subsequently, heteronuclear single quantum coherence (HSQC) spectroscopy was used to correlate the protons linked to similar carbons. The β-protons on the same face as the C3 carbamate and C9 carboxylate appeared generally up-field of their α-counterparts due to anisotropic effects caused by the latter functional groups [42]. Finally, relative configurations were ascertained ( Figure 2) based on NOESY experiments in which the observed through-space transfers of magnetization were used to correlate the stereochemical assignments. The ring fusion protons (3.88 and 3.74 ppm) of 5-and 7-hydroxy Boc-I 2 aa-OMe (5S,6S)-8 and (6S,7S)-9 were respectively assigned the S stereochemistry based on nuclear Overhauser effects (nOe) with the C4β and C8β protons (1.99 and 1.84 ppm) and with the C3 proton (4.13 ppm, Figure 2). No long-range through-space transfer of magnetization was observed for the protons on the alcohol-bearing carbons. In the case of (6S,7S)-9, the relative nOe between the C7 proton was stronger for the C8α proton (2.35 ppm) compared to that of the C8β proton (2.15 ppm). The stereochemical assignments for Boc-(7-OH)I 2 aa-OMe (6S,7S)-9 were confirmed by X-ray analysis as discussed below.
The configurations of the hydroxyl group in Boc-(5-OH)I 2 aa-OMe (5S,6S)-8 and the iodolactone of tetrahydrofuran-2-one (1′R,5S)-16 were based on the latter serving as a common intermediate for both the former and Boc-(7-OH)I 2 aa-OMe (6S,7S)-9. The stereochemistry of the ring-fusion and alcohol carbons are respectively derived from the inversion on nitrogen attack of the iodide and retention on the lactone opening during synthesis of the bicycle. Although the order of attack of the iodine and carboxylate may proceed by a traditional iodonium intermediate (Scheme 4) [24], and by a more concerted nucleophile-assisted alkene activation mechanism [43], the stereochemical outcome of iodolactone (1′R,5S)-20 arises from the attack of iodine by the face of the olefin on the opposite side of the proximal carboxylate of ∆ 4 -azelate 19 (Scheme 4).
The configurations of the hydroxyl group in Boc-(5-OH)I 2 aa-OMe (5S,6S)-8 and the iodolactone of tetrahydrofuran-2-one (1 R,5S)-16 were based on the latter serving as a common intermediate for both the former and Boc-(7-OH)I 2 aa-OMe (6S,7S)-9. The stereochemistry of the ring-fusion and alcohol carbons are respectively derived from the inversion on nitrogen attack of the iodide and retention on the lactone opening during synthesis of the bicycle. Although the order of attack of the iodine and carboxylate may proceed by a traditional iodonium intermediate (Scheme 4) [24], and by a more concerted nucleophileassisted alkene activation mechanism [43], the stereochemical outcome of iodolactone (1 R,5S)-20 arises from the attack of iodine by the face of the olefin on the opposite side of the proximal carboxylate of ∆ 4 -azelate 19 (Scheme 4).

Materials and Methods
Anhydrous solvents (CH3CN, DMF, (CH3)2CO, CH2Cl2, and CH3OH) were obtained by passage through solvent filtration systems (GlassContour, Irvine, CA, USA). All reagents from commercial sources were used as received: Iodine was purchased from Aldrich (USA) and solvents were obtained from Fisher Chemical. The N-(Boc)-, (Cbz)-, and (Fmoc)-3-iodo-L-alanine methyl esters 10a-c were respectively prepared according to the literature methods reported in references [25][26][27]. Purification by silica gel chromatography was performed on 230−400 mesh silica gel; analytical thin-layer chromatography (TLC) was performed on silica gel 60 F254 (aluminum sheet) and visualized by UV absorbance or staining with KMnO4. Melting points are reported in degree Celsius (°C), uncorrected and obtained using a Mel-Temp melting point apparatus equipped with a thermometer on the sample that was placed in a capillary tube. Spectroscopic 1 H and 13 C NMR experiments were recorded at room temperature (298 K) in CDCl3 (7.26/77.16 ppm), DMSO-d6 (2.5/39.56), and CD3OD (3.31/49.0 ppm) on Bruker AV (500/125, and 700/175 MHz) instruments using an internal solvent as the reference. Spectra are presented in the Supplementary Materials. Chemical shifts are reported in parts per million (ppm), and coupling constant (J) values in Hertz (Hz). Abbreviations for peak multiplicities are s (singlet), d (doublet), t (triplet), q (quadruplet), q (quintuplet), m (multiplet), and br (broad). Certain 13 C NMR chemical shift values were extracted from HSQC spectra. High-resolution mass spectrometry (HRMS) data were obtained on an LC-MSD instrument in electrospray ionization (ESI-TOF) mode by the Centre Régional de Spectrométrie de Masse de l'Université de Montréal. Either protonated molecular ions [M + H] + or sodium adducts [M + Na] + were used for empirical formula confirmation. Infrared spectra were recorded in the neat on a Perkin Elmer Spectrometer FT-IR instrument, and are reported in reciprocal centimeters (cm -1 ). The X-ray structure was solved using a Bruker Venture Metaljet  .

Materials and Methods
Anhydrous solvents (CH3CN, DMF, (CH3)2CO, CH2Cl2, and CH3OH) were obtained by passage through solvent filtration systems (GlassContour, Irvine, CA, USA). All reagents from commercial sources were used as received: Iodine was purchased from Aldrich (USA) and solvents were obtained from Fisher Chemical. The N-(Boc)-, (Cbz)-, and (Fmoc)-3-iodo-L-alanine methyl esters 10a-c were respectively prepared according to the literature methods reported in references [25][26][27]. Purification by silica gel chromatography was performed on 230−400 mesh silica gel; analytical thin-layer chromatography (TLC) was performed on silica gel 60 F254 (aluminum sheet) and visualized by UV absorbance or staining with KMnO4. Melting points are reported in degree Celsius (°C), uncorrected and obtained using a Mel-Temp melting point apparatus equipped with a thermometer on the sample that was placed in a capillary tube. Spectroscopic 1 H and 13 C NMR experiments were recorded at room temperature (298 K) in CDCl3 (7.26/77.16 ppm), DMSO-d6 (2.5/39.56), and CD3OD (3.31/49.0 ppm) on Bruker AV (500/125, and 700/175 MHz) instruments using an internal solvent as the reference. Spectra are presented in the Supplementary Materials. Chemical shifts are reported in parts per million (ppm), and coupling constant (J) values in Hertz (Hz). Abbreviations for peak multiplicities are s (singlet), d (doublet), t (triplet), q (quadruplet), q (quintuplet), m (multiplet), and br (broad). Certain 13 C NMR chemical shift values were extracted from HSQC spectra. High-resolution mass spectrometry (HRMS) data were obtained on an LC-MSD instrument in electrospray ionization (ESI-TOF) mode by the Centre Régional de Spectrométrie de Masse de l'Université de Montréal. Either protonated molecular ions [M + H] + or sodium adducts [M + Na] + were used for empirical formula confirmation. Infrared spectra were recorded in the neat on a Perkin Elmer Spectrometer FT-IR instrument, and are reported in reciprocal centimeters (cm -1 ). The X-ray structure was solved using a Bruker Venture Metaljet

Materials and Methods
Anhydrous solvents (CH 3 CN, DMF, (CH 3 ) 2 CO, CH 2 Cl 2 , and CH 3 OH) were obtained by passage through solvent filtration systems (GlassContour, Irvine, CA, USA). All reagents from commercial sources were used as received: Iodine was purchased from Aldrich (USA) and solvents were obtained from Fisher Chemical. The N-(Boc)-, (Cbz)-, and (Fmoc)-3iodo-L-alanine methyl esters 10a-c were respectively prepared according to the literature methods reported in references [25][26][27]. Purification by silica gel chromatography was performed on 230−400 mesh silica gel; analytical thin-layer chromatography (TLC) was performed on silica gel 60 F254 (aluminum sheet) and visualized by UV absorbance or staining with KMnO 4 . Melting points are reported in degree Celsius ( • C), uncorrected and obtained using a Mel-Temp melting point apparatus equipped with a thermometer on the sample that was placed in a capillary tube. Spectroscopic 1 H and 13

Dimethyl (2S,4E,8S)-∆ 4 -2,8-(di-N-(Boc)amino)azelate (11a)
In a 250-mL round bottom flask, fitted with a three-way stopcock, CuBr•DMS (1.22 g, 0.006 mol, 0.13 equiv.) was weighed, dried gently with a heat gun under vacuum until the powder changed color from white to light green, placed under argon, treated with dry DMF (30 mL), followed by (E)-1,3-dichloroprop-1-ene (2.5 g, 0.023 mol, 0.5 equiv.). In a Schlenk tube, zinc (8.9 g, 0.14 mol, 3 equiv.) and iodine (0.35 g, 0.0014 mol, 0.03 equiv.) were mixed under an argon atmosphere, and thrice heated under vacuum with a heat gun for 10 min and cooled under a flush of argon. A solution of N-(Boc)-3-iodo-L-alanine methyl ester 10a (15 g, 0.046 mol) in dry DMF (30 mL) was added to the Schlenk tube and stirred for 1h, when TLC analysis confirmed the consumption of the iodide (R f = 0.7, 30% EtOAc in hexanes) and formation of the organozinc reagent (R f = 0.2, 30% EtOAc in hexanes). Stirring was stopped, the excess zinc powder was allowed to settle, and the supernatant was transferred dropwise via a syringe with care to minimize the transfer of zinc into the flask containing the copper catalyst. After stirring at rt overnight, TLC indicated a new spot (R f = 0.48, 40% EtOAc in hexanes) and the reaction mixture was diluted with ethyl acetate (150 mL), stirred for 15 min, and filtered through a silica gel pad. The filtrate was treated with water (100 mL), transferred into a separatory funnel, and diluted with ethyl acetate (50 mL). The organic phase was washed successively with 1 M Na 2 S 2 O 3 (2 × 100 mL), water (4 × 100 mL), and brine (2 × 100 mL), dried over Na 2 SO 4 , filtered, and evaporated. The volatiles were removed under reduced pressure to afford a residue that was purified by chromatography using 25-30% EtOAc in hexanes as the eluent. Evaporation of the collected fractions gave azelate 11a (11.4 g, 56%) as a colorless liquid:

Dimethyl (2S,4E,8S)-∆ 4 -2,8-(di-N-(Boc)amino)azelate (11a)
In a 250-mL round bottom flask, fitted with a three-way stopcock, CuBr•DMS (1.22 g, 0.006 mol, 0.13 equiv.) was weighed, dried gently with a heat gun under vacuum until the powder changed color from white to light green, placed under argon, treated with dry DMF (30 mL), followed by (E)-1,3-dichloroprop-1-ene (2.5 g, 0.023 mol, 0.5 equiv.). In a Schlenk tube, zinc (8.9 g, 0.14 mol, 3 equiv.) and iodine (0.35 g, 0.0014 mol, 0.03 equiv.) were mixed under an argon atmosphere, and thrice heated under vacuum with a heat gun for 10 min and cooled under a flush of argon. A solution of N-(Boc)-3-iodo-L-alanine methyl ester 10a (15 g, 0.046 mol) in dry DMF (30 mL) was added to the Schlenk tube and stirred for 1h, when TLC analysis confirmed the consumption of the iodide (Rf = 0.7, 30% EtOAc in hexanes) and formation of the organozinc reagent (Rf = 0.2, 30% EtOAc in hexanes). Stirring was stopped, the excess zinc powder was allowed to settle, and the supernatant was transferred dropwise via a syringe with care to minimize the transfer of zinc into the flask containing the copper catalyst. After stirring at rt overnight, TLC indicated a new spot (Rf = 0.48, 40% EtOAc in hexanes) and the reaction mixture was diluted with ethyl acetate (150 mL), stirred for 15 min, and filtered through a silica gel pad. The filtrate was treated with water (100 mL), transferred into a separatory funnel, and diluted with ethyl acetate (50 mL). The organic phase was washed successively with 1 M Na2S2O3 (2 × 100 mL), water (4 × 100 mL), and brine (2 × 100 mL), dried over Na2SO4, filtered, and evaporated. The volatiles were removed under reduced pressure to afford a residue that was purified by chromatography using 25-30% EtOAc in hexanes as the eluent. Evaporation of the collected fractions gave azelate 11a (11.4

Dimethyl (2S,4E,8S)-∆ 4 -2,8-(di-N-(Boc)amino)azelate (11a)
In a 250-mL round bottom flask, fitted with a three-way stopcock, CuBr•DMS (1.22 g, 0.006 mol, 0.13 equiv.) was weighed, dried gently with a heat gun under vacuum until the powder changed color from white to light green, placed under argon, treated with dry DMF (30 mL), followed by (E)-1,3-dichloroprop-1-ene (2.5 g, 0.023 mol, 0.5 equiv.). In a Schlenk tube, zinc (8.9 g, 0.14 mol, 3 equiv.) and iodine (0.35 g, 0.0014 mol, 0.03 equiv.) were mixed under an argon atmosphere, and thrice heated under vacuum with a heat gun for 10 min and cooled under a flush of argon. A solution of N-(Boc)-3-iodo-L-alanine methyl ester 10a (15 g, 0.046 mol) in dry DMF (30 mL) was added to the Schlenk tube and stirred for 1h, when TLC analysis confirmed the consumption of the iodide (Rf = 0.7, 30% EtOAc in hexanes) and formation of the organozinc reagent (Rf = 0.2, 30% EtOAc in hexanes). Stirring was stopped, the excess zinc powder was allowed to settle, and the supernatant was transferred dropwise via a syringe with care to minimize the transfer of zinc into the flask containing the copper catalyst. After stirring at rt overnight, TLC indicated a new spot (Rf = 0.48, 40% EtOAc in hexanes) and the reaction mixture was diluted with ethyl acetate (150 mL), stirred for 15 min, and filtered through a silica gel pad. The filtrate was treated with water (100 mL), transferred into a separatory funnel, and diluted with ethyl acetate (50 mL). The organic phase was washed successively with 1 M Na2S2O3 (2 × 100 mL), water (4 × 100 mL), and brine (2 × 100 mL), dried over Na2SO4, filtered, and evaporated. The volatiles were removed under reduced pressure to afford a residue that was purified by chromatography using 25-30% EtOAc in hexanes as the eluent. Evaporation of the collected fractions gave azelate 11a (11.4

Conclusions
The copper catalyzed SN2′ addition of zincate derived from methyl β-iodo alaninate onto (E)-1,3-dichloroprop-1-ene has given useful entry into a set of protected ∆ 4 -2,8-diaminoazelates (e.g., 11a-c). Attempts to fold the latter linear precursors into bicyclic 5-and 7-substituted indolizidin-2-one amino acid (I 2 aa) derivatives have, however, demonstrated the challenges of achieving diastereomeric and regioisomeric selectivity in the facial differentiation of the olefin. Epoxidation and dihydroxylation were unselective and gave oxiranes 12 and hydroxy lactone 17 as inseparable mixtures of diastereomers, which were shown by LCMS analyses to be convertible into mixtures of up to four hydroxy indolizidine-2-one isomers due in part to the inability to control the intramolecular cyclization of the respective nitrogen. Moreover, diastereomeric stereochemical integrity may have also been lost due to cyclization by way of planar SN1 intermediates.

Conclusions
The copper catalyzed SN2′ addition of zincate derived from methyl β-iodo alaninate onto (E)-1,3-dichloroprop-1-ene has given useful entry into a set of protected ∆ 4 -2,8-diaminoazelates (e.g., 11a-c). Attempts to fold the latter linear precursors into bicyclic 5-and 7-substituted indolizidin-2-one amino acid (I 2 aa) derivatives have, however, demonstrated the challenges of achieving diastereomeric and regioisomeric selectivity in the facial differentiation of the olefin. Epoxidation and dihydroxylation were unselective and gave oxiranes 12 and hydroxy lactone 17 as inseparable mixtures of diastereomers, which were shown by LCMS analyses to be convertible into mixtures of up to four hydroxy indolizidine-2-one isomers due in part to the inability to control the intramolecular cyclization of the respective nitrogen. Moreover, diastereomeric stereochemical integrity may have also been lost due to cyclization by way of planar SN1 intermediates.

Conclusions
The copper catalyzed S N 2 addition of zincate derived from methyl β-iodo alaninate onto (E)-1,3-dichloroprop-1-ene has given useful entry into a set of protected ∆ 4 -2,8diaminoazelates (e.g., 11a-c). Attempts to fold the latter linear precursors into bicyclic 5and 7-substituted indolizidin-2-one amino acid (I 2 aa) derivatives have, however, demonstrated the challenges of achieving diastereomeric and regioisomeric selectivity in the facial differentiation of the olefin. Epoxidation and dihydroxylation were unselective and gave oxiranes 12 and hydroxy lactone 17 as inseparable mixtures of diastereomers, which were shown by LCMS analyses to be convertible into mixtures of up to four hydroxy indolizidine-2-one isomers due in part to the inability to control the intramolecular cyclization of the respective nitrogen. Moreover, diastereomeric stereochemical integrity may have also been lost due to cyclization by way of planar S N 1 intermediates.
Supplementary Materials: The following supporting information can be downloaded, characterization data including 1 H and 13 C NMR spectra and X-ray coordinates.
Author Contributions: Conceptualization, methodology, validation, formal analysis, investigation, resources, writing-original draft preparation, writing-review and editing, all R.M. and W.D.L.; supervision, project administration, funding acquisition, all W.D.L. All authors have read and agreed to the published version of the manuscript.