Synaxial 1,3-interactions

In conformationally rigid systems, steric factors play an important role in determining the facial selectivity of alkylations of enolates. Whereas alkylation of enolate A proceeds via a chairlike transition state, attack of the electrophile on enolate B in an antiparallel manner is subject to a synaxial interaction with the axial Me group. Hence, parallel approach of the electrophile through a twist boat transition state prevails. 

In the case of cw-4a,5,6,7,8,8a-hexahydro-4/f-l,3-benzoxazines, the heterocycle is flattened out compared to the perhydro-l,3-benzoxazines, and there can be no 1,3-synaxial interaction of a hydrogen atom at C-2 with the carbocycle. The axial preference is insensitive to the configuration... 

Enamines are ambident nucleophiles giving C- and N-alkylated products. Acceptable yields of C-alkylated products are obtained by using reactive alkyl halides such as CH3I, ally lie and benzylic halides, and a-halocarbonyl compounds. The resultant iminium ion intermediates no longer behave as a enolates, thus dialkylation is avoided. The stereochemical course of alkylation of the enamine derived from 2-methylcy-clohexanone is depicted below. The reason for the preferred parallel alkylation via a boat-like transition state over antiparallel alkylation via a chair-like transition state is the synaxial RX // CH3 interaction in the latter case. ... 

In the absence of strong steric interactions (synaxial), the Michael addition proceeds via an antiparallel approach of the electrophilic vinyl ketone to the enolate anion. 

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