1.5- Dienes phosphonium ylide alkylation

Axelrod et alP have describe the synthesis of 1,5-dienes by alkylation of an allylic phosphonium ylide with an allylic bromide. The method was used to synthesize pure all-( )-squalene from ( , )-fame-syl bromide and its tributylphosphonium salt. No isomerization occurred during the reaction and reduction of the resulting phosphonium salt with lithium in ethylamine gave squalene in 65% yield. 

Similar types of alkylation have been applied in syntheses of other steroidal side-chains, including that of 22-trans-26,27-dinorergosta-5,22-dien-3j8-ol (387), a novel marine sterol. In addition, the Wittig reaction has been used to prepare various possible polyene intermediates in phytosterol biosynthesis. The aldehydes (389) and (390) were prepared (Scheme 26) from stigmasterol acetate (388b) by modification of a known procedure. These aldehydes were then alkylated with a variety of ylides derived from phosphonium salts, leading to a series of polyenes (391) and (392). ... 

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