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The Organoalkali Route to Vitamin A and β‐Carotene
Author(s) -
Rauchschwalbe Günter,
Zellner Armin,
Schlosser Manfred
Publication year - 2001
Publication title -
european journal of organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.825
H-Index - 155
eISSN - 1099-0690
pISSN - 1434-193X
DOI - 10.1002/1099-0690(200110)2001:20<3903::aid-ejoc3903>3.0.co;2-r
Subject(s) - chemistry , deprotonation , electrophile , yield (engineering) , adduct , medicinal chemistry , stereochemistry , organic chemistry , catalysis , ion , materials science , metallurgy
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 C 15 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 β‐carotene. Vitamin A acetate can also be prepared, this time in moderate yield, by functionalization through consecutive deprotonation, borylation, oxidation and acetylation 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.