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ARTICLE Received 18 Apr 2012 | Accepted 23 Oct 2012 | Published 20 Nov 2012 DOI: 10.1038/ncomms2216 Scalable organocatalytic asymmetric Strecker reactions catalysed by a chiral cyanide generator Hailong Yan1, Joong Suk Oh1, Ji-Woong Lee1,w & Choong Eui Song1 The Strecker synthesis is one of the most facile methods to access racemic a-amino acids. However, feasible catalytic asymmetric Strecker reactions for the large-scale production of enantioenriched a-amino acids are rare. Here we report a scalable catalytic asymmetric Strecker reaction that uses an accessible chiral variant of oligoethylene glycol as the catalyst and KCN to generate a chiral cyanide anion. Various a-amido sulphone substrates (alkyl, aryl and heteroaryl) can be transformed into the optically enriched Strecker products, a-amino- nitriles, with excellent yields and enantioselectivities. Moreover, the robust nature of the catalyst enables a ‘one-pot’ synthesis of enantiomerically pure a-amino acids starting from a-amido sulphones and simple catalyst recycling. These features can make this protocol easily adaptable to the practical synthesis of unnatural a-amino acids. 1 Department of Chemistry, Sungkyunkwan University, 300 Cheoncheon, Jangan, Suwon, Gyeonggi 440-746, Korea. w Present address: Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim an der Ruhr, Germany. Correspondence and requests for materials should be addressed to C.E.S. (email: [email protected]). NATURE COMMUNICATIONS | 3:1212 | DOI: 10.1038/ncomms2216 | www.nature.com/naturecommunications 1 & 2012 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2216 -Amino acids are the building blocks of proteins and are robustness of the catalyst enables a ‘one-pot’ synthesis of widely used as components of pharmaceutically enantiomerically pure non-natural a-amino acids and easy aactive molecules and chiral catalysts1. In parti- catalyst recycling. cular, synthetic unnatural a-amino acids have had a significant role in the area of peptide research. They have Results been used extensively in peptide analogues to limit Reaction design. To extend the application of our polyether- conformational flexibility, enhance enzymatic stability, and tethered chiral catalyst (Fig. 1), we first examined a Strecker improve pharmacodynamics and bioavailability2. The Strecker reaction of the Boc (tert-butyloxycarbonyl)-protected imine 2a synthesis3—the reaction of an imine or imine equivalent with (ref. 25) with KCN in the presence of 10 mol% of (R)-1a in toluene hydrogen cyanide, followed by nitrile hydrolysis—is a standard at 0 1C (conditions (a) in Fig. 2). However, the reaction proceeded method for producing a-amino acids4–7. Although there is very sluggishly to afford the desired a-amino nitrile product (R)-3a already a number of useful catalytic asymmetric variants showing with disappointing enantioselectivity (69% enantiomeric excess high catalyst efficiency8–17, the limited utility of existing catalytic (ee), 24% yield). However, interestingly, when using the a-amido methods may be due to several important factors, including the sulphone 4a, which is a stable synthetic precursor of imine 2a,the relatively complex and precious nature of the catalysts, and the same reaction proceeded smoothly, affording the product with an use of hazardous cyanide sources (for example, HCN, acetone excellent yield and enantioselectivity (93% ee, conditions (c) in cyanohydrin or trimethylsilyl cyanide) in super-stoichiometric Fig. 2). It should be noted here that the preparation of amounts, preventing their practical application to large-scale a-aminonitriles from a-amido sulphones has recently been processes. Moreover, it has often been observed that direct reported26–29. However, the enantioselective versions using hydrolysis of optically active a-aminonitriles can result in asymmetric phase transfer catalysts27,28 and quinine as a chiral deterioration of optical purity of the amino acid products18. base29 showed a limited substrate scope and/or only moderate Therefore, it is still highly desirable to develop a more practical enantioselectivity. We next conducted the Strecker reaction of the and general catalytic system, which includes a convenient cyanide imine 2a with HCN as a cyanide source in the presence of source (for example, KCN), a broader scope of substrates, easily the catalyst (R)-1a. Even after 60 h, we could obtain only a trace available and recyclable catalysts, and mild reaction conditions amount of a-amino nitrile (R)-3a ( 5%) with a very low ee value and racemization-free hydrolysis conditions. o (34% ee; conditions (b) in Fig. 2). These results suggested that the Recently we have developed a highly efficient methodology for interaction between the sulphinate group ( À SO Ph) and nucleophilic substitution reactions with various potassium salts, 2 the catalyst is very important for catalyst performance. using oligoethylene glycols as a new type of a multifunctional promoter19,20. The multifunctional activation mode of a polyethylene glycol renders unprecedented selectivity, activity Optimization of the reaction conditions. Next, we screened the and efficiency to various inorganic salts. Further investigation catalytic efficiency of a variety of chiral BINOL-based bis into the asymmetric application of this protocol revealed that a (hydroxy) polyethers 1 to shed light on the relationship between chiral variant of oligoethylene glycols could be applied as a new the structures of catalysts and reaction outcomes. As shown in structural motif of a chiral promoter system, which is highly Fig. 3, the observed relationship between the structure and cat- accessible from an axially chiral 1,10-binaphthol backbone and alytic performance was almost the same as that observed pre- polyethylene glycols (Fig. 1)19,21. Using this catalytic system, we viously in the desilylative kinetic resolution of racemic alcohols have reported a highly enantioselective desilylative kinetic with KF (ref. 21). Thus, only those catalysts having halogen resolution of a variety of silyl-protected secondary alcohols with substituents at the 3,30-position on the BINOL group exhibited potassium fluoride21. Under the reaction conditions, this type of a catalytic activity (Fig. 3a). When catalysts (R)-1b and (R)-1c promoter brings insoluble potassium salts into the organic having no halogen substituent at the 3,30-position (R ¼ H or Ph) solution by forming a chiral ion pair. This resulting chiral ion were employed, only a very low level of conversion and enantio- pair can then mediate an asymmetric reaction. Therefore, we selectivity were observed. Moreover, a dramatic enhancement in presumed that our chiral anion generator can be applied to both activity and enantioselectivity was observed when switching catalytic enantioselective Strecker synthesis by generating a chiral from chlorine to iodine (from 38 to 93% ee). Thus, the best cyanide anion to provide a more general and practical method to enantioselectivity was obtained when the 3,30-diiodo-substituted access enantiomerically pure non-natural amino acids. chiral BINOL catalyst 1a was used. Higher activity and greater Here we report a new organocatalytic, highly enantioselective stereoselectivity of 3,30-diiodo-substituted catalyst 1a than those Strecker protocol, using 3,30-diiodine-substituted BINOL (1,1’-bi- having chlorine or bromine substituents at 3,30-position can be 2,2’-naphthol)-based catalyst 1a, which enables the direct use of a explained by the greater polarizability of the iodine atoms com- broad scope of a-amido sulphones22–24, as stable precursors of pared with those of the chlorine and bromine atoms, which the corresponding sensitive and unstable N-carbamoyl aldimines, allows it to more strongly coordinate to the potassium cation (see and potassium cyanide as an inexpensive, safe and convenient ref. 21 for X-ray crystal structure of the catalyst–KF complex). In cyanide source in an organic medium. In particular, the structural addition to this strong coordination of the iodine atom to the X X O O O O O O OH K OH O K O O HO H H H A A X X R LG A– : F–, Cl–, Br–, I– Chiral bis-hydroxy polyether – – – O OAc, CN, SCN chiral anion generator HO OH n Figure 1 | Chiral bis-hydroxy polyether as a chiral anion generator. LG, leaving group; X, halogen atom. 2 NATURE COMMUNICATIONS | 3:1212 | DOI: 10.1038/ncomms2216 | www.nature.com/naturecommunications & 2012 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms2216 ARTICLE I I O O O O OH HO I I (R )–1a (10 mol%) NBoc Conditions (a) NHBoc Conditions (b) NBoc KCN (1.05 equiv) HCN (1 equiv) CN Toluene, 0 oC, 60 h Toluene , 0 oC, 60 h 24%, 69% ee <5%, 34% ee 2a (R )–3a 2a Conditions (c) KCN (1.05 equiv) 90% Yield Toluene, 0 oC, 60 h 93% ee NHBoc SO2Ph 4a Figure 2 | Strecker synthesis using the chiral anion generator 1a. Boc, tert-butyloxycarbonyl. NHBoc NHBoc Catalyst (10 mol%) SO Ph CN 2 KCN (1.05 equiv.) Toluene 0 oC, 60 h 4a (R )–3a R R X X O O O O O O O O OH HO OH HO R R X X (R )–1b (R = H): 32%, 15% ee (R )–1d (X = Cl): 91%, 38% ee (R )–1c (R = Ph): 38%, 11% ee (R )–1e (X = Br): 91%, 43% ee (R )–1a (X = I): 90%, 93% ee Br Br Br Br O O O O O O O O OH HO OH HO Br Br Br Br (R )–1f: 44%, 9% ee (R )–1g: 65%, 0% ee I I I I O O O O O O O O OMe HO OMe MeO I I I I (R )–1h: 46% yield, 51% ee* (R )–1i: 0% yield* Figure 3 | Influence of the catalyst structure on the reaction. (a) Effects of the substituents at the 3,30-position on the BINOL group for the catalyst performance. (b) Effects of the ether chain length for the catalyst performance. (c) Effects of phenolic protons for catalyst performance. *The reactions were carried out at room temperature for 36 h.