Monoamines and Derivatives

Chiral amines have been transformed into chiral imines RCH=NG, which are usually in equilibrium with the tautomeric enamines. These enamines undergo asymmetric alkylations, and the best results are often obtained with ethers 1.58 or with valine derivatives 1.59 (R = i-Pr, R = tert-Bu) [169, 173,253] in the presence of bases. Enamines, lithioenamines and zinc enamines derived from imines are very potent Michael donors that often participate in highly stereoselective reactions [161, 162, 169, 173, 254, 257, 260, 262, 267], Chiral imines can suffer very selective addition reactions of organomagnesium reagents [139, 253, 254] and allyl-metals [154, 258]. They also suffer stereoselective Ti-catalyzed silylcyanation [268], Strecker reaction [266], and [2+2] or [4+2] cydoadditions [131, 256, 263], When the reaction produces an imine product, the chiral auxiliary is recovered after acidic hydrolysis. However, when an amine is obtained as the product, as is often the case from phenethylamine derivatives, the chiral residue is cleaved by hy-drogenolysis. In such cases, the chiral amine is not, strictly speaking, a chiral auxiliary. But these processes will be discussed anyway because of their importance in asymmetric synthesis. 

Reactions of enamine derivatives of auxiliary 1.64 [162, 252, 262] are often disappointing, except when R = CH20SiMe3 [162], However, the alkylation of enamines derived from 1.65 (R = Me) gives satisfactory results [253, 275], as do the Michael additions of enamines of bicyclic pyrrolidine 1.68 [294], 

Kurth and coworkers [299] have proposed the use of (2R, 67 )-2,6-bis-ben-zyloxymethylpiperidine 1.69 as an auxiliary for alkylation of a,P-unsaturated amides. 

Chiral aminonitriles such as 1.70 and 1.71 have been prepared either from the corresponding amino-1,3-diol [300] or from (LR, 2S)-ephedrine 1.61 (R = Me) [301], Carbanion derivatives of these reagents undergo stereoselective alkylations, carbonyl additions [301] and Michael reactions [300], Since the reaction products are aminonitriles, the chiral auxiliary is easily cleaved by mild hydrolysis. 

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