1- Alkenyl carbamates, asymmetric deprotonation

On the basis that a wide variety of (S)-configurated (a-carbamoyloxy)alkyllithium derivatives are accessible by (—)-sparteine-mediated deprotonation30, Hoppe and coworkers have described the synthesis of enantiomerically and diastereomerically pure cyclopen-tanols 38 by asymmetric cyclocarbolithiation reaction of 5-alkenyl carbamates like 36. Its deprotonation with s-BuLi/(—)-sparteine gives a chiral organolithium which cyclizes to benzyllithium 37 via 5-exo-trig and again with retention of configuration at the carbanionic... 

It is especially remarkable that optically active homoaldoi adducts can be obtained when enantiomeri-cally pure 2-alkenyl carbamates (47 R = alkyl) are employed. Apparently the deprotonation occurs with retention of configuration and leads to configurationally stable liAium derivatives, which, after metal exchange with Ti(OPr )4, again with retention, add to aldehydes with efficient 1,3-chirality transfer coupled with enantiofacial differentiation at the carbonyl group, indicating a rigid six-membered transition state. Recently even an asymmetric homoaldoi reaction by enantioselective lithiation of prochiral primary alkenyl carbamates in the presence of (-)-sparteine was reported. ... 

The first asymmetric lithium-ene reaction, based on the enantioselective (-)-sparteine-induced deprotonation of 2-alkenyl carbamates, provides a route to enan-tiomerically enriched carbocycles. " ... 

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