Primary 2-alkenyl carbamates

The lithium-(—)-sparteine complexes, derived from primary 2-alkenyl carbamates, are usually configurationally labile even at —78 °C. During the investigation of the (ii)-crotyl carbamate 301, the (—)-sparteine complex (5 )-302 crystallized in a dynamic thermodynamic resolution process (equation 76) and stereospecific substitutions could be performed with the slurry An incorrect assignment of the configuration of the lithium inter-... 

The regioselectivity in the reaction of enantioenriched lithioallyl carbamates with chlorotributyl- or chlorotrimethyltin depends largely on the substitution pattern of the alkenyl residue. Those derived from primary 2-alkenyl carbamates 302 provide exclusively or with large excess the 1-substitution products 313 with inversion of the configuration (equation No (E)-ent-314 was observed. 

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. ... 

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