Chiral ammonia equivalent

As a potential approach towards enantiomerically pure amino acids containing a cyclopropane ring, Michael additions of enantiomerically pure chiral ammonia equivalents 95-100 have been examined (Fig. 5). 

A solution to this problem was found by using (4Ry 5S)-4,5-diphenyloxazoli-dine-2-one (100) [63] which turned out to be a suitable chiral ammonia equivalent permitting to achieve good diastereoselectivities with respect to the ste-reogenic center a to the alkoxycarbonyl group especially with 2 -substituted... 

An additional application of the observed trans diastereoselectivity in Michael reactions is demonstrated by performing this reaction with a c/5-4,5-diphenyloxazolidin-2-one as a chiral ammonia equivalent for the introduction of an amine functionality. This sequence, when carried out with enantiomerically pure (/ ,5)-4,5-diphenyloxazolidin-2-one, yielded diastereomerically and enantiomerially pure addition compounds which were separated by chromatography. ... 

Auxiliary methods can hold current value when the auxiliary is inexpensive, available in both enantiomeric forms, and, for amine synthesis, can be incorporated with concomitant generation of the new stereogenic a chiral amine center. Reductive amination with chiral ammonia equivalents not only holds this potential but is now a proven and established method that allows chiral primary amine synthesis in two reaction steps (reductive amination and hydrogenolysis) from prochiral ketones. The approach is of interest because of its overall reaction step efficiency. 

S) phenylethylamine (also referred to as a methylbenzylamine) and (R) and (S) tert butylsulfinylamide have gained widespread acceptance as useful chiral ammonia equivalents. The strategy is therefore to use these chiral amines to induce a new chiral amine stereogenic center on a ketone substrate. An alternative approach is to use a chiral ketone with an achiral source of nitrogen, for example, benzylamine. This latter approach is less often employed, but nonetheless equally important and discussed first. 

The method relies on the use of the chiral ammonia equivalents (R)- or (S)-N-tert-butanesulfl-namide, and Ellman has optimized their synthesis via the enantioselective catalytic oxidation of... 

Sigma-Aldrich Quote May 2006 for Two Common Chiral Ammonia Equivalents... 

The method may be of interest because it uses the least expensive chiral ammonia equivalent available (Table 8.2) in essentially equimolar quantities (1.05 equiv) as compared to the limiting reagent, the aldehyde. Additionally, this one-pot process has significant yield advantages over the two-step process (formation of the (R)- or (S)-N-a-methylbenzyl aldimine, isolation of this chiral aldimine, followed by carbanion addition). The reactions are fast (3 h at -78°C), but the low temperature requirements, low reactor volume to product ratio, and the need for 3 equiv of a cuprate (CuBr based) will be considered restrictive. Many times the cuprate can be reduced to 2.0 equiv... 

Trans-ACPC with (15,25) configuration was prepared in an elegantly short pathway (Scheme 13) by Enders et al. By addition of the chiral ammonia equivalent lithiated (5)-(-)-2-methoxymethyl-1 -trimethylsilylaminopyrro-lidine (TMS-SAMP) to co-halide-substituted enoate 84, followed by Michael-initiated ring closure (MIRC), 85 was formed with 96-98% diastereoselectivity (Scheme 13). After desilylation,... 

In the eyes of a synthetic chemist this might be called a nitrogen-Michael-ad-dition to the a,P-unsaturated acid moiety as the 1,4-acceptor. Thus, it occurred to us that our favourite auxiliary, the hydrazine SAMP, might be used as a chiral ammonia equivalent in asymmetric hetero Michael additions to enoates. 

Scheme 25. Efficient enantioselective synthesis of p-amino acids.Asynunetric hetero Michael additions using SAMP as a chiral ammonia equivalent...

C2-symmetrical secondary amines by applying the stereoselective reductive amination protocol using chiral amines as the source of chirality. In 2005, Nugent et al. published a novel method for the asymmetric reductive amination of prochiral aliphatic ketones116 where the (/ )- or (5)-a-methylbenzylamine (MBA) were employed as the cheap chiral ammonia equivalent. Ti(0 Pr)4/Raney Ni/H2 were employed as the catalyst system for the one-pot conversion of the prochiral ketones 116a-e to the corresponding chiral amines 117a-e with excellent dia-stereoselectivity (Scheme 39.33). 

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