Asymmetric Access of γ-Amino Acids and γ-Amino Phosphonic Acid Derivatives via Copper-Catalyzed Enantioselective and Regioselective Hydroamination

Abstract

Open AccessCCS ChemistryCOMMUNICATION6 Jun 2022Asymmetric Access of γ-Amino Acids and γ-Amino Phosphonic Acid Derivatives via Copper-Catalyzed Enantioselective and Regioselective Hydroamination Zhiping Yang†, Qingwei Du†, Yanxin Jiang and Jun (Joelle) Wang Zhiping Yang† Harbin Institute of Technology, Harbin 150001 Department of Chemistry, Southern University of Science and Technology (SUSTech)Shenzhen 518055 , Qingwei Du† Department of Chemistry, Southern University of Science and Technology (SUSTech)Shenzhen 518055 , Yanxin Jiang Department of Chemistry, Hong Kong Baptist University, Kowloon, Hong Kong 999077 and Jun (Joelle) Wang *Corresponding author: E-mail Address: [email protected] Department of Chemistry, Southern University of Science and Technology (SUSTech)Shenzhen 518055 Department of Chemistry, Hong Kong Baptist University, Kowloon, Hong Kong 999077 https://doi.org/10.31635/ccschem.021.202101128 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail γ-Aminobutyric acid is a major inhibitory neurotransmitter in the mammalian central nervous system that plays a substantial role in brain disorders. γ-Amino phosphonic acid is a unique surrogate of both natural and unnatural γ-amino acid. Because of their unique biological activity, γ-amino acid and γ-amino phosphonic acid derivatives have attracted considerable attention. However, an efficient and straightforward method for constructing chiral γ-substituted-γ-amino acid and γ-amino phosphonic acid derivatives remains a long-standing challenge. Herein, a highly efficient, versatile, and universal Cu-catalyzed asymmetric hydroamination of cinnamyl esters, cinnamyl phosphonates, and cinnamyl phosphine oxides is presented for accessing γ-amino acid and γ-amino phosphonic acid derivatives in good yields with high levels of enantiocontrol and regioselectivity. Download figure Download PowerPoint Introduction The exploration of efficient and general catalytic systems to construct optically pure functionalized compounds from readily accessible starting materials is significantly important in modern organic chemistry. It is well-known that γ-aminobutyric acid (GABA) is a major inhibitory neurotransmitter in the mammalian central nervous system (CNS) that plays a substantial role in brain disorders.1 There are many bioactive compounds and pharmaceutical products that contain this important structural motif, such as (R)-baclofen and (S)-pregabalin (Figure 1).2–4 Notably, different enantiomers of GABA derivatives have different biological activity. (R)-Baclofen is considerably more active than the corresponding (S)-enantiomer, which is used as an antispastic agent and muscle relaxant.2 The biological activity of pregabalin, a potent anticonvulsant related to the inhibitory neurotransmitter GABA, resides in the (S)-enantiomer.3,4 Thus, significant and numerous efforts have been devoted to developing catalytic asymmetric synthesis of γ-amino acid derivatives. Enantioselective Michael-type addition is the classical method to synthesize γ-amino acids.5–12 Other strategies to construct enantiopure GABA are the γ-amination of carbonyl compounds,13–19 three-component radical cascade reactions,20 and annulation reaction.21,22 Figure 1 | Representative γ-amino acids, γ-amino phosphonic acid derivatives, and aminophosphine ligands. TBS, tert-butyldimethylsilyl; TBDPS, tert-butyldiphenylsilyl; TIPS, triisopropylsily. Download figure Download PowerPoint γ-Amino phosphonic acid derivatives have also received considerable and continuous attention as the unique surrogates of both natural and unnatural amino acids due to their wide range of biological properties.23,24 The integration of the tetrahedral phosphonic acid group can imitate the tetrahedral transition state in peptide hydrolysis to enable these analogues to act as inhibitors of esterase or peptidase, for instance L-phosphinothricin and L-2-amino-4-phosphonobutanoic acid (L-AP4).25–27 Bialaphos is being used commercially as a herbicide.28 Tamiphosphor is an active molecule against oseltamivir resistant strains of the influenza virus.29–31 The biological activities of their phosphinic acids and phosphine oxide analogues have yet to be studied due to the lack of efficient methodologies for their construction. Furthermore, chiral aminophosphines are perceived as powerful ligands in transition-metal-catalyzed asymmetric reactions.32–36 Given the high demand of chiral amino phosphonic acid derivatives, researchers have been intensively focused on developing efficient methods to obtain these optically pure chiral aminophosphine derivatives. In general, chiral α-aminophosphine oxides and phosphonates are prepared by the Lewis acid and metal-catalyzed nucleophilic addition reaction of H-P(O)R2 with imines.37–46 Very recently, Huang et al.47 reported an elegant enantioselective reductive phosphonylation of secondary amides for the synthesis of enantioenriched chiral α-aminophosphonates by combining iridium with chiral thiourea catalysis. The most established synthetic protocol for β-aminophosphines is nucleophilic phosphide substitution of tosylates or mesylates, which are derived from natural or unnatural chiral aminoalcohols.34 Enantioselective phospha-Michael reaction to nitroalkenes for the synthesis of chiral β-aminophosphine oxides and β-aminophosphines was reported by Tan et al.48 and Duan et al.,49 respectively. Rh-catalyzed asymmetric hydrogenation of β-phosphorylated enamides was also an efficient method to access chiral β-aminophosphines.50 For the synthesis of chiral γ-amino phosphonic acid derivatives, most of the methodologies had been limited to the usage of chiral auxiliaries and α-amino acids, which greatly suffered from poor diversity, expensive chiral amino acid substrates, and multistep processes. In 2018, Zhou et al.51 developed asymmetric 1,4-hydrophosphonylation of benzofuran derived azadienes to synthesis γ-aminophosphonates. In this well-designed substrate, the aromatization driving force played a vital role, and therefore this system did not work with acyclic azadiene substrates. In response to the growing demand for chiral γ-amino acids and γ-amino phosphonic acid derivatives, it would be highly desirable to develop an efficient and universal asymmetric catalytic method for accessing them based on the transformation of readily available and bench stable starting materials. Cu-catalyzed enantioselective hydroamination of alkenes, pioneered by Buchwald, Hirano and Miura in 2013,52,53 is one of the most powerful tools for the preparation of chiral amine derivatives.54–64 Recently, Buchwald et al.65 developed a reversal Cu-catalyzed enantioselective hydroamination of α,β-unsaturated carbonyl compounds with 1,2-benzisoxazole, which readily gives primary amines (Scheme 1a). Simultaneously, Zhang et al.66 reported a similar regio-reversed hydroamination, capable of directly producing a series of β-amino acids, esters, amides, and nitriles. For phosphine substrates, the only example is Cu-catalyzed regioselective hydroamination of vinylphosphines reported by Miura et al.,67 providing a new approach to α-aminophosphines; the asymmetric version is still preliminary (Scheme 1b). Inspired by the previous elegant works65–67 and our continuous interest in constructing chiral organophosphorus compounds68,69 and amino esters,70,71 we herein present a general method to attain γ-substituted-γ-amino esters and γ-substituted-γ-amino phosphonic acid derivatives by diastereo- and enantioselective Cu-catalyzed electrophilic amination of cinnamyl derivatives (Scheme 1c). Scheme 1 | (a and b) Examples of Cu-catalyzed hydroamination in the synthesis of β-amino acid derivatives and α-aminophosphines. (c) Synthesis of γ-amino acids and γ-amino phosphine derivatives. Download figure Download PowerPoint Results and Discussion To begin the investigation, β,γ-unsaturated ester 1a and hydroxylamine ester 2a were chosen as the model substrates and dimethoxymethylsilane (DMMS) was chosen as the hydride source. An evaluation of ligands revealed (R)-DTBM-SegPhos ( L7) (DTBM, 3,5-tBu-4-MeO-C6H2) to be superior to all others tested (Table 1, entry 7 vs entries 1–6), which provided 3aa in 91% yield with 95% enantiomeric excess (ee) (Table 1, entry 7). When hydroxylamine esters with a para-Et2N phenyl group 2b was used as the amination reagent, both the yield and ee decreased (Table 1, entry 8 vs 7). The investigation of the solvent revealed tetrahydrofuran (THF) to be the best choice (Table 1, entries 8–10). Interestingly, adding PPh3 as a secondary ligand decreased the yield without loss of enantioselectivity (entry 11). When diethoxymethylsilane (DEMS) and polymethylhydrosiloxane (PMHS) were used instead of DMMS as hydride sources, both product yields and enantioselectivities decreased (entries 12 and 13). Table 1 | Optimization of Reaction Conditionsa Entry Ligand R Solvent Yield (%)b ee (%)c 1 L1 Me2N THF 35 45 2 L2 Me2N THF 53 85 3 L3 Me2N THF 27 45 4 L4 Me2N THF 13 35 5 L5 Me2N THF 10 - 6 L6 Me2N THF 65 90 7 L7 Me2N THF 91 95 8d L7 Et2N THF 70 90 9 L7 Me2N Toluene 83 82 10 L7 Me2N MTBE 53 98 11e L7 Me2N THF 79 95 12f L7 Me2N THF 73 93 13g L7 Me2N THF 80 91 aReaction conditions: 1a (0.15 mmol), 2a–2b (0.18 mmol), Cu(OAc)2 (5 mol %), ligand (6 mol %), DMMS (0.6 mmol), THF (0.75 mL). MTBE, methyl tert-butyl ether. bIsolated yields. cDetermined by HPLC analysis. dHydroxylamine ester 2b was used instead of 2a. e6 mol % PPh3 was added. fDiethoxymethylsilane (DEMS) was used instead of DMMS. gPolymethylhydrosiloxane (PMHS) was used instead of DMMS. With the optimized reaction conditions in hand, the substrate scope was then explored (Table 2). Generally, the ethyl (E)-4-phenylbut-3-enoate gave the corresponding product with the best yield and ee value ( 3aa vs 3ba, 3ca). β,γ-Unsaturated esters with substituents –Me, –F, –Cl, –Br, and –OMe on the aromatic rings also converted efficiently into chiral products 3da– 3ja with high yields and good enantioselectivities. Additionally, substrates with aromatic heterocycles, furan and thiophene, also gave the expected products in 90–91% yields with 91–95% ee. Unfortunately, alkyl-substituted β,γ-unsaturated esters were not suitable substrates. When γ-cyclohexyl β,γ-unsaturated esters reacted under standard conditions, only trace amounts of expected product was obtained with 40% ee. Furthermore, the scope of hydroxylamine esters was also investigated. Both the electron-rich or -deficient hydroxylamine esters, as well as heterocyclic hydroxylamine esters, were all applicable in this transformation, providing γ-amino esters 3ac– 3aj in good yields and with high levels of enantiocontrol. Table 2 | The Scope of β,γ-Unsaturated Esters and Hydroxylamine Estersa aReaction conditions: 1 (0.15 mmol), 2 (0.18 mmol), Cu(OAc)2 (5 mol %), (R)-DTBM-SegPhos (6 mol %), DMMS (0.6 mmol), THF (0.75 mL). Isolated yield. ee value was determined by HPLC analysis. Encouraged by the success of cinnamyl esters, we were next particularly interested in whether this strategy also works for cinnamyl phosphine derivatives. To address this question, we began our investigation using cinnamyl phosphine oxide 4a as the prototypical substrate (Table 3). Strikingly, the desired γ-amino phosphine oxide 5aa was obtained in 85% yield and 99% ee with slightly optimized reaction conditions. Several hydroxylamine esters with electron-rich or -deficient groups (–OMe and –Br) were screened and provided the expected hydroamination products 5ad– 5ah in moderate-to-good yields with excellent enantioselectivities. Generally, this catalyst system showed wide substrate scope and good functional group compatibility. Both electron-donating groups (–CH3, –OCH3, and –tBu) or electron-withdrawing groups (–F and –Cl) on the aromatic rings of phosphine oxide worked well, and the desired chiral γ-amino phosphine oxides 5ba– 5ha were obtained in good-to-excellent yields with high ee values in most cases. In addition, dibenzyl phosphine oxide also reacted smoothly to give the corresponding product 5ia in 80% yield with 93% ee. Various substituents including –Me, –OMe, and –Cl on the aromatic rings of cinnamyl phosphine oxide also transformed efficiently into the desired γ-amino phosphine oxides 5ja– 5oa with high levels of enantiocontrol. These promising results of cinnamyl phosphine oxide prompted us to continue examining cinnamyl phosphonates. To our delight, representative hydroxylamine esters were readily converted into γ-amino phosphonate 5pa– 5ph with over 90% ee and moderate-to-good yields. The absolute configuration of product 5aa was determined to be S by the comparison with the product (S)- 5aa obtained from a two steps synthesis from (S)-3-amino-3-phenylpropan-1-ol (Scheme 2a). Table 3 | The Scope of Hydroamination of Cinnamyl Phosphine Derivativesa aReaction conditions: 4 (0.2 mmol), 2 (0.3 mmol), Cu(OAc)2 (5 mol %), (R)-DTBM-SegPhos (6 mol %), DMMS (0.8 mmol), THF (2 mL). Isolated yields. ee value was determined by HPLC analysis. Scheme 2 | (a–c) Configuration determination of 5aa, application of chiral γ-amino esters, and γ-amino phosphine and synthesis of chiral δ-amino esters and δ-amino phosphine derivatives. Download figure Download PowerPoint To show the synthetic potential of the chiral-aminated products, the product (S)- 3ba can be further transformed to the corresponding alcohol 8 via a reported method.18 Alcohol 8 is the key intermediate for the synthesis of chiral drug cis-(+)-sertraline, an effective antidepressant (Scheme 2b).72 The resulting hydroamination product 5aa was further transformed by reduction with HSiCl3 to produce the desired product 9 in 82% yield with 99% ee. The Buchwald–Hartwig amination reaction with chiral γ-amino phosphine oxide 5na led to diphenylamine 10 without any loss of chirality (82% yield, 85% ee). In addition, we expanded the substrate scope to γ,δ-unsaturated amino esters and γ,δ-unsaturated amino phosphine derivatives. The chiral δ-amino esters and δ-amino phosphine derivatives were all obtained in moderate yields but maintain the high level of enantiocontrol under standard conditions (Scheme 2c). A protonation reaction using CD3OD instead of hydroxylamine ester was conducted to probe the regioselectivity of the generated organocopper species (Scheme 3). The only product 13 was labeled with 72% deuterium at the carbon adjacent to the phenyl ring (γ-position of the phosphine oxide). The regioselectivity of the hydrocupration is largely dictated by electronic properties, similar to alkenyl-arenes.73 By a detailed computational study of hydroamination of styrene with hydroxylamine esters, Tobisch confirmed migratory olefin insertion proceeds with high regioselectivity to afford an alkyl copper intermediate. The π-electron-withdrawing arene results in an effective depletion of electron density to the copper center, and also enhances the stability of the alkyl copper intermediate.74 According to these reports, we propose that insertion of an alkene into a chiral ligand-bound LCu(I)H species A would generate an alkyl copper complex B. Subsequent intramolecular SN2 cleavage of the hydroxylamine ester's N–O linkage followed by reductive elimination would form the C−N bond enantioselectively. The copper(I) species C would then undergo transmetalation with an external hydride-transfer reagent to reform the LCu(I)H species A. Scheme 3 | Deuterium-labeling experiment and proposed catalytic cycle. Download figure Download PowerPoint Conclusion Herein, we demonstrate an efficient, versatile, and unified hydroamination strategy of cinnamyl esters, cinnamyl phosphonates, and cinnamyl phosphine oxides in a regio- and enantioselective fashion, thus allowing highly enantioselective preparation of essential γ-amino acid and γ-amino phosphonic acid derivatives. We believe this universal protocol for accessing many optically enriched chiral γ-amino acid and γ-amino phosphonic acid derivatives will lay the foundation for further discovery of their unique bioactivity in medicinal chemistry and peptide synthesis. Supporting Information Supporting Information and chemical compound information are available in the online version of the paper. For nuclear magnetic resonance (NMR) spectra and high-performance liquid chromatography (HPLC) spectra of the compounds describe, see supplementary figures. Correspondence and requests for materials should be addressed to the corresponding author. Conflict of Interest The authors declare no competing financial interests. Funding Information The authors gratefully thank the National Natural Science Foundation of China (NSFC 21971102) and Guangdong Innovative Program (2019BT02Y335) for financial support. References 1. Purdy R. H.; Morrow A. L.; Moore P. H.; Paul S. M.Stress-Induced Elevations of γ-Aminobutyric Acid Type A Receptor-Active Steroids in the Rat Brain.Proc. Natl. Acad. Sci. U. S. A.1991, 88, 4553–4557. Google Scholar 2. Bowery N. G.; Hill D. R.; Hudson A. L.; Doble A.; Middlemiss D. N.; Shaw J.; Turnbull M. 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