The organoalkali route to vitamin A and b-carotene

The reductive cleavage of Me vinyl-b-ionyl ether or the deprotonation of 3,2',6',6'-tetramethyl-5-(1-cyclohexenyl)-1,3-pentadiene gives rise to an organometallic C15 species that combines selectively with a variety of electrophiles at the terminal chain position. Its reaction with aldehydes, however, is less clean. In particular, (E)-b-formyl-2-butenyl acetate gives the expected adduct I and, after dehydration, vitamin A acetate only in poor yield. The same is true for the analogous reaction with 2,7-dimethyl-2,4,6-octatriendial, which ultimately affords b-carotene. Vitamin A acetate can also be prepd., this time in moderate yield, by functionalization through consecutive deprotonation, borylation, oxidn. and acetylation of a C20 pentaene hydrocarbon having the required skeleton. Both the C15 and the C20 organometallic key intermediates adopt spontaneously a zigzag-like outstretched conformation which, upon electrophilic trapping, directly and exclusively leads to the all-(E) configuration. [on SciFinder (R)]