Pentanidium catalyzed enantioselective phase transfer reactions 1

Chapter Chapter Asymmetric Phase-Transfer Catalysis Catalyst and Reactions Chapter 1.1 Introduction to Asymmetric Phase-Transfer Catalysis Early in the mid to late 1960s, Starks together with Makosza and Brandstrom reported the catalytic activity of quaternary onium salts,1 which was the starting point of phase-transfer catalysis. Since then, phase-transfer catalysis went through an exponential growth as a practical methodology for organic synthesis. The advantages of this method lie in their simple experimental procedures, mild reaction conditions, inexpensive and environmentally reagents and solvents, and the possibility of conducting large-scale preparations.2 Nowadays, it becomes one of the most important synthetic methods used in various fields of organic chemistry, and also in many industrial applications. However, the development of asymmetric phase-transfer catalysis based on the use of structurally well-defined chiral, nonracemic catalysts had progressed rather slowly. Invention and development of novel chiral phase-transfer catalysts (PTCs) for suitable reactions are the driving force in phase-transfer catalysis, including chirarity installation and chiral modification to tetraalkylonium ions (Q+). Ever since chiral phase-transfer catalysts derived from cinchona alkaloids were uncovered in 1980s, asymmetric phase-transfer catalysis stepped into a new era. In particular, during the last two decades, asymmetric phase-transfer catalysis has drawn great scientific interest, and recent efforts have resulted in notable achievements. 1.1.1 General Mechanism of Asymmetric Phase-Transfer Catalysis In 1971, Starks proposed the term “phase-transfer catalysis” to explain the acceleration of reaction rate in the reaction between two substances located in two Chapter different immiscible phases in the presence of trtraalkylammonium or phosphonium salts.4 For example, the rate for the replacement reaction of 1-chlorooctane with aqueous sodium cyanide is accelerated by hexadecyltributylphosphonium bromide for thousands times. Phosphonium salts can transfer the cyanide anion from aqueous phase to organic phase by forming a quaternary phosphonium cyanide complex which makes the cyanide anion more soluble in organic solvents and sufficiently nucleophilic. Therefore, the reactivity of cyanide is tremendously enhanced in this process. The high rate of displacement is mainly due to the three characteristic features of the pairing cation (Q+): organic soluble, high lipophilicity and the large ionic radius. Generally, in asymmetric phase-transfer catalysis, two representative mechanisms were proposed for phase-transfer catalyzed reactions. First one is base catalyzed phase-transfer reactions. Typically, under basic condition, active methylene or methine groups could be easily functionalized by phase-transfer catalysis. Glycinate Schiff base is selected as the example to illustrate how the mechanism works (Figure 1.1). As shown in Figure 1.1, reaction proceeds from the interfacial deprotonation step. Substrate was deprotonated at the interface by the base from aqueous phase to give the corresponding metalenolate, which stays at the interface of the two layers. Subsequently, phase-transfer catalyst and metalenolate would undergo ion-exchange process to generate lipophilic chiral onium enolate, which could diffuse rapidly into Chapter Figure 1.1 General mechanisms for the asymmetric alkylation of glycine Schiff base the organic phase. Since enolate is in organic phase as homogeneous state, it would react with the electrophile RX much faster to afford the optically active monoalkylation product. Meanwhile, enolate generated at the interface shows low reactivity due to the less contact with electrophile. This type of reactions only works on two conditions: 1: ion-exchange step is sufficiently fast and chiral onium enolate is highly reactive (thousands times faster than the reaction between enolate and RX). 2: phase-transfer catalyst Q+ should provide effective shielding of one of the two enantiotopic faces of the enolate anion. The former minimizes the intervention of the direct alkylation of metal enolate to give racemic product, and the latter rigorously controls the absolute stereochemistry. In general, reaction variables (base, solvent, temperature, substrate concentration, and stirring rate) can be tuned to optimize the reactions. Another, relatively less-studied system is the nucleophilic addition of an organic or inorganic anion to prochiral electrophiles. Chiral phase-transfer catalyst undergoes ion-exchange process with the anion, which is usually used as aqueous Chapter solution of solid, to generate a chiral ion pair. Then the anion would attack a prochiral electrophile to create a new chiral center. The asymmetric epoxidation of chalcone using an aqueous solution of sodium hypochlorite is used as a typical example (Figure 1.2). Chiral onium hypochlorite (Q*+OCl-) is formed at the interface by ion-exchange, which would penetrate into the organic phase and react with the ennone.6 Differing from the previous example, chiral cation (Q+) should be designed to recognize enantiotopic face of the electrophilic reacting partner. Figure 1.2 General mechanism for the nucleophilic addition of anions to prochiral electrophiles 1.1.2 Mechanistic Consideration of Asymmetric Base Catalyzed Phase-Transfer reaction Generally, quaternary ammonium salt contains a sp3 quarternized nitrogen atom. Take cinchona alkaloid quaternary ammonium salt for example, the nitrogen atom takes the center of an imaginary tetrahedron composed of four carbons adjacent to the nitrogen Chapter (Figure 1.3).7 Three faces of this tetrahedron (F1.F2.F3) are efficiently blocked by steric groups, leaving one face (F4) sufficiently open to allow close contact between the enolate (of substrate) and the ammonium cation (of the catalyst). F1 was totally blocked by the ring system itself. F2 was blocked by the allyl or benzyl group of the secondary hydroxyl group. F3 was blocked by the substituent of the nitrogen. The extended π conjugation of the enolate and the imine adopt a face to face π interaction with the quinoline, which blocks one face of the E-enolate of imine. So the electrophile could only approach from the the other face to afford one enantiomer7. Figure 1.3 Interaction between glycine Schiff base enolate and ammonium cation 1.2 Asymmetric Phase-Transfer Catalysts - Chiral Quaternary Ammonium Salt. 1.2.1 Cinchona Alkaloid Based Phase-Transfer Catalysts 1.2.1.1 First generation Chapter In 1989, five years after the pioneering work by the Merck research group,8a Cinchona alkaloid derived ammonium was successfully utilized as catalyst for the asymmetric synthesis of -amino acids by O’Donnell et al.,8b,c by using glycinate Schiff base as a key substrate (Scheme 1.1). The asymmetric alkylation of glycinate Schiff base preceded smoothly under mild phase-transfer conditions, with N-(benzyl)cinchoninium chloride as catalyst, giving the alkylation product (R)-5 in good yield and moderate enantioselectivity (Scheme 1.1, eq 1). By simply switching to the cinchonidine derived catalyst 4, the product could be obtained with a similar degree of enantioselectivity, but with the opposite absolute configuration (S) (Scheme 1.1, eq 2). Scheme 1.1 Asymmetric alkylation of glycinate Schiff base with or 1.2.1.2 Second generation In the investigation of alkylation reaction of glycinate Schiff base 1, it was found that the O-alkylated ammonium salts show significant improvement.9 Ammonium bromide 6a-b (Figure 1.4), give 81% ee in the asymmetric phase-transfer catalyzed Chapter alkylation reactions. Figure 1.4 O-alkylated N-(benzyl)-cinchoninium and cinchonidinium bromide 1.2.1.3 Third generation Although asymmetric alkylation of the glycinate Schiff base can be achieved by using chiral phase-transfer catalysts derived from the relatively inexpensive, commercially available cinchona alkaloids, research in this area was rather slow, due to the low reactivity and enantioselectivity. However, a new class of cinchona alkaloid derived catalysts bearing an N-anthracenylmethyl group (third-generation catalysts) developed by two independent research groups have opened up a new era of asymmetric phase-transfer catalysis. In 1997, Lygo et al. developed the N-anthracenylmethylammonium salts and 8, 10 and applied them to the asymmetric phase-transfer alkylation of to synthesize -amino acids with much higher enantioselectivity (Scheme 1.2). At the same time, Corey et al.11 prepared O-allyl-nanthracenylmethyl cinchonidinium salt 11. By using solid cesium hydroxide monohydrate (CsOH·H2O) at very low temperature, they achieved a high asymmetric induction in the enantioselective alkylation of (Scheme 1.3). Chapter Scheme 1.2 Asymmetric alkylation of glycinate Schiff base with N-anthracenyl methyl ammonium salts and Scheme 1.3 Asymmetric alkylation of glycinate Schiff base with 11 1.2.1.4 Bis- and tri- ammonium salts During the development of the asymmetric sharpless dihydroxylation, it was found that ligand with two cinchona alkaloid unites attached to hetercyclic spacers led to a considerable increases in both the enantioselectivities and the scope of substrates. This effect had been utilized successfully by Jew, Park, and co-workers for the design of new chiral phase-transfer catalysts, with two and three cinchona alkaloid units, respectively.12 These catalysts substantially enhanced the enantioselectivities of the alkylation of and also expanded the range of alkyl halides. During the search for the Chapter ideal aromatic spacer, they found that catalyst 12 consisting of 2,7-bis(bromomethyl) -naphthalene and two cinchona alkaloid units exhibited remarkable catalytic activity and efficiency. Thus, mol% of 12 was sufficient for the asymmetric alkylation of with various alkylating agents (Scheme 1.4).12 Scheme 1.4 Asymmetric alkylation of glycinate Schiff base with 12 1.2.2 Binaphthyl Based Chiral Spiro Ammonium Salts 1.2.2.1 Bis-binaphthyl based ammonium salt In 1999, Maruoka’s group13 deleloped the structurally rigid, chiral spiro-ammonium salts 13 (Figure 1.5), which were derived from commercially available (S)-1,1’-bi-2naphthol, as new C2-symmetric phase-transfer catalysts and successfully applied them to the highly efficient, enantioselective alkylation of under mild phase-transfer conditions. The key finding was a significant effect of an aromatic substituent at the 3, 3’-position of one binaphthyl subunit of the catalyst (Ar) on the enantiofacial discrimination. (S, S)- 13e proved to be the catalyst of choice for the preparation of a variety of essentially enantiopure α-amino acids by this transformation. In general, 10 Chapter Scheme 1.9 Asymmetric alkylation of glycinate Schiff base with 19 Sasai’s group designed a bis(spiroammonium) salt 20 as a chiral phase-transfer catalyst, and successfully applied it to the alkylation reaction of with excellent yield and enantioseletivity (Scheme 1.10) 20. Scheme 1.10 Asymmetric alkylation of glycinate Schiff base with 20 Table summarizes asymmetric alkylation of glycinate Schiff base with various phase-transfer catalysts with different reactions conditions. Since this reaction is highly important in making amino acid. The investigation with new phase transfer catalyst is still a hot topic in organic catalytic chemistry. Table Phase-transfer catalyzed asymmetric alkylation of glycinate Schiff base 15 Chapter 16 Chapter 17 Chapter 1.3 Various Reactions Catalyzed by Phase-Transfer Catalysts. 1.3.1 Michael addition Asymmetric Michael additions of active methylene compounds to electron-deficient olefins are one of the useful approaches to form C-C bond. Plaquevent21 et al. developed the Michael addition of simple dimethyl malonate to 2-pentyl-2cyclopentenone utilizing K2CO3 as base and quinine- or quinidine-derived ammoniums as phase-transfer catalyst (Scheme 1.11). 21a Scheme 1.11 Enantioselective Michael addition of dimethyl malonate 23 to cyclopentenone By changing the acceptor to chalcone derivatives, Maruoka’s group accomplished the Michael addition with excellent results by binaphthyl based phase-transfer catalysts (Scheme 1.12).21b Catalyst 25a affords much better results than 25b. The additional hydroxyl groups in 25a provide extra hydrogen bondings, representing 25a a 18 Chapter dual-functioning chiral phase-transfer catalyst. Various enantioseletive conjugate additions of glycinate derivatives would be fully discussed in chapter 2. Scheme 1.12 Enantioselective Michael addition of diethyl malonate to chalcone 1.3.2 Aldol and Mannich reaction Enantioselective direct aldol reaction of glycinate Schiff base with aldehydes could afford β-hydroxy-α-amino acids, which are extremely important chiral units and building blocks in pharmaceuticals and drug. To date, the examples of phase-transfer catalyzed enantioselective direct aldol reactions are quite limited. Maruoka’s group developed an efficient, highly diastereo-and enantioselective direct aldol of glycinate Schiff base with various aliphatic aldehydes, utilizing 13f as phase-transfer catalyst (Scheme 1.13).22a After deprotection, chiral amino alcohol were obtained. By using the same series of catalyst 13e, Minnich reaction of glycinate Schiff base with α-imino ester 31 affords adduct with high enantioselectivity.22b Product could be easily converted into a precursor of streptolidine lactam (Scheme 1.14).22b 19 Chapter Scheme 1.13 Enantioselective direct Aldo reaction and synthesis of β-hydroxy-αamino acids Scheme 1.14 Enantioselective Mannich reaction between and 31 Scheme 1.15 Enantioselective Mannich reaction between and Boc-protect imine 34 20 Chapter Shibasaki and co-workers developed a tartrate-derived diammonium salt as phase-transfer catalyst for in the highly diastereoselective and enantioselective Mannich reaction of glycinate Schiff base to Boc-protect imine 34.22c The usefulness of the Mannich adduct 35 was further demonstrated by the straightforward synthesis of the optically pure tripeptide (Scheme 1.15). 1.3.3 Epoxidation Catalytic asymmetric epoxidation of electron deficient olefins has been investigated with various methods for a long period of time. Chiral phase-transfer catalysis has become one of the outstanding methods due to the easy operation and substantial practical saleable application. Arai and co-workers conducted the asymmetric epoxidation of chalcone with 30% hydrogen peroxide using chiral cinchona alkaloid derived ammonium salts 38 as phase-transfer catalysts (Scheme 1.16).24a Interestingly, enantioselectivity was found to be highly dependent on the para substituent in 38. Catalyst with Cl or I substitutent showed much better enantioselectivities. Scheme 1.16 Asymmetric epoxidation of chalcone with 38 by hydrogen peroxide Differing from the previous work, Corey’s group studied epoxidation of various -enone substrates with aqueous KClO as oxidant and dihydrocinchonidine-derived 21 Chapter ammonium salt 40 as catalyst, and achieved an excellent level of enantioselectivity (Scheme 1.17).24b Mechanism was already discussed in the previous section. Scheme 1.17 Asymmetric epoxidation of chalcone and derivatives with 40 by KClO Murphy and co-workers developed tetracyclic C2- symmetric guanidium salt 41 from (S)-malic acid, and successfully applied it to asymmetric epoxidation of chalcone (Scheme 1.18).24c Excellent yield and enantioselectivity were achieved. Scheme 1.18 Asymmetric epoxidation of chalcone with guanidine catalyst 41 Based on the binaphthyl chiral structure, Maruoka’s group developed several highly efficient chiral N-spiro ammonium salts 25 for the asymmetric epoxidation of various enone substrates.24d High ee should be ascribable to the extra hydrogen bonding provided by the hydroxyl group and the steric effect and electronic property of Ar and R group (Scheme 1.19). The catalyst shows excellent molecular-recognition ability toward enone substrates. As reference experiments to prove function of extra hydroxyl bonding, catalysts 25d-25f only provide 61%-66% ee for chalcone epoxidation. 22 Chapter Scheme 1.19 Bifunctional N-spiro catalyst 25 for asymmetric epoxidations Park, Jew, and co-workers applied chiral dimeric cinchona phase-transfer catalyst 45 for the catalytic asymmetric epoxidation of chalcone derivatives, providing the corresponding epoxide adducts in excellent yields and enantioselectivities.24e The essential key point was the introduction of surfactant, such as Span 20, to the reaction system, which dramatically improves yields and enantioselectivities (Scheme 1.20). Scheme 1.20 Asymmetric epoxidation of chalcone derivatives with 45 in the presence of Span-20 1.3.4 Fluorination Optically active organofluoride compounds play an important role in various fields of chemistry and catalytic enantioselective fluorination of carbonyl substrates has 23 Chapter appeared and become hot topic in recent years25a, 25b and ref relevant Asymmetric electrophilic fluorination of β-keto ester under phase-transfer condition is one of the elegant methods in this category. By using 49 N-fluorobenzenesulfonimide as a fluorinating agent, β-keto ester 47a would undergo asymmetric fluorination with 46, providing moderate enantioseletivity (Scheme 1.21).25a Scheme 1.21 Asymmetric fluorination of 47a with catalyst 46 Maruoka’s group improved the result by utilizing chiral bifunctional phase-transfer catalyst 50 (Scheme 1.22).25b Various cyclic ketones could also provide excellent yields and enantioselectivities, up to 99% yield, 98% ee. Notably, acyclic β-keto ester shows low enantioselectivities, only 5-20% ee. This result implies that, comparing to Z-enolates of cyclic β-keto esters, E-enolate may not be suitable for inducing additional ionic interactions between the ammonium cation and the ester carbonyl. Scheme 1.22 Asymmetric fluorination of 47b with phase-transfer catalyst 50 24 Chapter 1.3.5 Streck reaction Catalytic asymmetric Strecker reaction is one of the direct and viable strategies for the asymmetric synthesis of α-amino acids and their derivatives. Numerous recent efforts in this field achieved excellent results with alkylmetal cyanide or anhydrous hydrogen cyanide. In the regard of green chemistry and practical usage, phase-transfer catalyzed enantioselective Strecker reaction using KCN is highly desirable. Maruoka and co-workers reported the phase-transfer catalyzed asymmetric Strecker reaction by utilizing the tetranaphthyl backbone based ammonium salt 51 as catalyst (Scheme 1.23).26 Aqueous KCN solution was used as cyanide source and excellent yields and ee values were achieved. Scheme 1.23 Asymmetric Strecker reaction catalyzed by 51 using KCN 1.3.6 Other reactions. Phase-transfer catalysis has also shown many applications in many other reactions: such as: Darzen reaction;27 Neber reaction;28 Cyclopropanation reaction;29 Aziridination;30 Oxidation;31 Reduction32 and so on. Continuous efforts would uncover more reactions and give more comprehensive mechanistic understanding of 25 Chapter phase-transfer catalysis. Besides ammonium salts as phase-transfer catalyst, anionic phase-transfer catalysis has emerged as new phase-transfer concept33. Respectively, anionic phase-transfer catalyst transfer anionic anion to non-polar organic solvent to furnish organic reactions. 1.4 Summary Asymmetric phase-transfer catalysis based on chiral quaternary ammonium salts plays an important role in modern organic synthesis. Various reactions conducted under proper phase-transfer catalysts and conditions have achieved excellent reactivities and stereoselectivities. Asymmetric phase-transfer catalysis also provides new synthetic strategies to asymmetric synthesis. Development of new types of phase-transfer catalysts with general reactivity towards various reactions is still high desirable and important. 26 Chapter Reference 1. Makosza’s pioneering contributions are representative, see: (a) M. Makosza, Tetrahedron Lett. 1966, 4621; (b) M. Makosza, Tetrahedron Lett. 1966, 5489; (c) M. Makosza, Tetrahedron Lett. 1969, 673; (d) M. Makosza, Tetrahedron Lett. 1969, 677; 2. (a) Handbook of Phase Transfer Catalysis; Sasson. Y.; Neumann. 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Soc. 2012, 134, 8376-8379. 30 [...]... reaction is highly important in making amino acid The investigation with new phase transfer catalyst is still a hot topic in organic catalytic chemistry Table 1 Phase- transfer catalyzed asymmetric alkylation of glycinate Schiff base 1 15 Chapter 1 16 Chapter 1 17 Chapter 1 1.3 Various Reactions Catalyzed by Phase- Transfer Catalysts 1. 3 .1 Michael addition Asymmetric Michael additions of active methylene compounds... streptolidine lactam (Scheme 1. 14).22b 19 Chapter 1 Scheme 1. 13 Enantioselective direct Aldo reaction and synthesis of β-hydroxy-αamino acids Scheme 1. 14 Enantioselective Mannich reaction between 1 and 31 Scheme 1. 15 Enantioselective Mannich reaction between 1 and Boc-protect imine 34 20 Chapter 1 Shibasaki and co-workers developed a tartrate-derived diammonium salt as phase- transfer catalyst for in the... J Am Chem Soc 19 99, 12 1, 6 519 ; (b) Maruoka, K J Fluorine Chem 20 01, 11 2, 95; (c) Ooi, T.; Uematsu, Y.; Maruoka, K.; Adv Synth Catal 2002, 344, 288; (d) Ooi, T.; Uematsu, Y.; Maruoka, K J Org Chem 2003, 68, 4576; (e) Ooi, T.; Kameda, M.; Maruoka, K J Am Chem Soc 2003, 12 5, 513 9 14 Ooi, T.; Uematsu, Y.; Kameda, M.; Maruoka, K Angew Chem 2002, 11 4, 16 21; Angew Chem Int Ed 2002, 41, 15 51 15 Kitamura, M.;... Jersey, 2 010 3 (a) Dolling, U H.; Davis, P.; Grabowski, E J J J Am Chem Soc 19 84, 10 6, 446–447 (b) O'Donnell, M J.; William, D B.; Wu, S J Am Chem Soc 19 89, 11 1, 2353–2355; 4 Starks, C M J Am Chem Soc 19 71, 93, 19 5 5 O’Donnell, M J.; Bennett, W D.; Wu, S J Am Chem Soc 19 89, 11 1, 2353; 6 (a) Lygo, B.; Wainwright, P G Tetrahedron Lett 19 98, 39, 15 99; (b) Lygo, B.; Wainwright, P G Tetrahedron 19 99, 55,... D C M Tetrahedron Lett 20 01, 42, 13 43 7 (a) Corey, E J.; Xu, F.; Noe, M C J Am Chem Soc 19 97, 11 9, 12 414 12 415 (b) Andrus, M B.; Ye, Z.; Zhang,J Tetrahedron Lett 2005, 46, 3839-3842 (c) Jew, S.; Park, H Chem Commun 2009, 7090– 710 3 8 (a) Dolling, U.-H.; Davis, P.; Grabowski, E J J J Am Chem Soc 19 84, 10 6, 446 (b) O’Donnell, M J.; Bennett, W D.; Wu, S J Am Chem Soc 19 89, 11 1, 2353; (c) Lipkowitz, K B.;... ammonium salts as phase- transfer catalyst, anionic phase- transfer catalysis has emerged as new phase- transfer concept33 Respectively, anionic phase- transfer catalyst transfer anionic anion to non-polar organic solvent to furnish organic reactions 1. 4 Summary Asymmetric phase- transfer catalysis based on chiral quaternary ammonium salts plays an important role in modern organic synthesis Various reactions conducted... P G Tetrahedron 20 01, 57, 2403 11 Corey, E J.; Xu, F.; Noe, M C J Am Chem Soc 19 97, 11 9, 12 414 12 (a) Jew, S.; Jeong, B S.; Yoo, M.-S.; Huh, H.; Park, H.; Chem Commun 20 01, 12 44; (b) Park, H.; Jeong, B S.; Yoo, M.-S; Lee, J.-H.; Park, B; Kim M G.; Jew, S Tetrahedron Lett 2003, 44, 3497 (c) Park, H.; Jeong, B S.; Yoo, M.-S; Park, M.; Huh, H.; Jew, S Tetrahedron Lett 20 01, 42, 4645 13 (a) Ooi, T.; Kameda,... of glycinate Schiff base 1 with 15 12 Chapter 1 1.2.3 Other Types of Phase- Transfer Catalysts 1. 2.3 .1 C2-symmetric chiral pentacyclic guanidine Nagasawa and co-workers16 reported the asymmetric alkylation of 1 with the C2-symmetric chiral cyclic guanidines 16 The introduction of methyl substituents is crucially important to achieve high enantioselectivity The chiral catalyst 16 a results in the alkylation... by binaphthyl based phase- transfer catalysts (Scheme 1. 12).21b Catalyst 25a affords much better results than 25b The additional hydroxyl groups in 25a provide extra hydrogen bondings, representing 25a a 18 Chapter 1 dual-functioning chiral phase- transfer catalyst Various enantioseletive conjugate additions of glycinate derivatives would be fully discussed in chapter 2 Scheme 1. 12 Enantioselective Michael... bis(spiroammonium) salt 20 as a chiral phase- transfer catalyst, and successfully applied it to the alkylation reaction of 1 with excellent yield and enantioseletivity (Scheme 1. 10) 20 Scheme 1. 10 Asymmetric alkylation of glycinate Schiff base 1 with 20 Table 1 summarizes asymmetric alkylation of glycinate Schiff base 1 with various phase- transfer catalysts with different reactions conditions Since this reaction . chemistry. Table 1 Phase-transfer catalyzed asymmetric alkylation of glycinate Schiff base 1 Chapter 1 16 Chapter 1 17 Chapter 1 18 1. 3 Various Reactions Catalyzed by Phase-Transfer. Scheme 1. 13 Enantioselective direct Aldo reaction and synthesis of β-hydroxy-α- amino acids Scheme 1. 14 Enantioselective Mannich reaction between 1 and 31 Scheme 1. 15 Enantioselective. Thus, 1 mol% of 12 was sufficient for the asymmetric alkylation of 1 with various alkylating agents (Scheme 1. 4). 12 Scheme 1. 4 Asymmetric alkylation of glycinate Schiff base 1 with 12 1. 2.2