388075-89-6Relevant articles and documents
The organoalkali route to vitamin A and β-carotene
Rauchschwalbe, Guenter,Zellner, Armin,Schlosser, Manfred
, p. 3903 - 3909 (2007/10/03)
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.
