The organoalkali route to vitamin A and beta-carotene

The reductive cleavage of methyl vinyl-beta -ionyl ether (1) or the deproto nation of 3,2',6'-tetramethyl-5-(1-cyclohexenyl)-1,3-pentadiene (2) gives r ise to an organometallic C-15 species that combines selectively with a vari ety of electrophiles at the terminal chain position, Its reaction with alde hydes, however, is less clean. In particular, (E)-beta -formyl-2-butenyl ac etate gives the expected adduct 7a and, after dehydration, vitamin A acetat e only in poor yield, The same is true for the analogous reaction with 2,7- dimethyl-2,4,6-octatriendial, which ultimately affords beta -carotene. Vita min A acetate can also be prepared, this time in moderate yield, by functio nalization through consecutive deprotonation, borylation, oxidation and ace tylation of a C-20 pentaene hydrocarbon having the required skeleton. Both the C-15 and the C-20 organometallic key intermediates adopt spontaneously a zigzag-like outstretched conformation which, upon electrophilic trapping, directly and exclusively leads to the all-(E) configuration.