55497-47-7

The reductive cleavage of methyl vinyl-β-ionyl ether (1) or the deprotonation of 3,2′,6′,6′-tetramethyl-5-(1-cyclohexenyl)-1, 3-pentadiene (2) 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)-β-formyl-2-butenyl acetate gives the expected adduct 7a 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 βcarotene. Vitamin A acetate can also be prepared, this time in moderate yield, by functionalization through consecutive deprotonation, borylation, oxidation 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.

Various 1,2-disubstituted-1,3-dienes have been synthesised through the alkylation of 3-methyl-3-sulfolene (1) followed by thermolysis of the resulting 2-alkyl-3-methyl-3-sulfolene (3,7). The alkylations with bulky iodides, particularly, containing ethylene ketal or phenyl sulfide moiety yield considerable amounts of rearranged 2-alkyl-3-methyl-2-sulfolene (4). The sulfolenes 3, 4 and 7 have been desulfonylated under mild conditions to yield corresponding 1,3-dienes.