Wittig reaction exocyclic olefin

The B-alkyl-9-BBN undergoes an interesting reverse reaction to afford the parent alkene when treated with benzaldehyde. Consequently, the reaction is uniquely employed for the synthesis of exocyclic olefins (Chart 24.3). The hy-droboration of cyclic olefins with an internal double bond, followed by homologation with carbon monoxide in the presence of lithium trimethoxyaluminum hydride afford B-(cycloalkylmethyl)-9-BBN. This intermediate on treatment with benzaldehyde leads to an exocyclic methylene compound (Chart 24.3) [16]. Since the synthesis proceeds from the cycloalkene, thus it provides a valuable alternative to the customary methylenation of carbonyl compounds by Wittig and related procedures. The method also provides a clean synthesis of deuterium-labeled compounds (Eq. 24.10) [16], without positional scrambling or loss of label. Consequently, methylmethylene-d -cyclopentane in 52% isolated yield is obtained. 

For introduction of the exocyclic double bond two methodologies are particularly useful. The first one is based on the reaction of the aldehydes (ketones) with the Wittig reagents route a) [6]. In the second one route b), an acetylenic functionality is introduced [25], which may be further converted either into the Z- or -olefins by proper reducing reagents (H2/ Lindlar catalyst or e. g. L1A1H4, respectively) (O Scheme 5) [6]. 

A synthesis of the related sesquiterpenoid pentalene, 26, also began with the Weiss-Cook reaction product 12 (R = R" = H). Wittig olefination was carried out on the mono-ketal of 12 followed by hydrolysis of the ketal. The resultant ketone was reduced to the corresponding alcohol and the exocyclic alkene underwent cyclopropanation to produce 23. Oxidation of the alcohol generated a ketone that could be converted to the enone using the... 

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