Michael-type conjugate addition, studies

Studies of the nucleophilic substitution of 2//-benzimidazole-2-spirocyclohexane (106) indicate that sulfur nucleophiles attack via an initial Michael-type 1,4-conjugate addition, followed by a prototropic shift in the adduct (107). When the nucleophile is electron withdrawing (e.g., phenyl-... 

Mechanistically, enamine and Lewis-acid-mediated conjugate additions are complex. The opportunity exists for the product-determining step to occur at a number of points and, without further study, the precise nature of the manifold is not entirely clear. In some enamine cases where the stereoselectivity likely results from the conjugate addition, a synclinal type transition state seems to be involved. With the Mukaiyama-Michael addition, some processes implicate an open-extended pathway. Despite the mechanistic uncertainties that remain, sufficient data are now available so that the stereochemistry in many cases can be anticipated by extrapolation. 

The reactivity of conjugated double bonds is well known [126] and we were able to perform Michael-type additions on the double bond of the dehydroproline residue. Computer graphics studies of pristinamycin 11 confirmed the accessibility of this functionality (see Sect. 5.1.4). 

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