Carotenoids and related compounds. Part XVIII. Synthesis of cis- and di-cis-polyenes by reactions of the Wittig type

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Abstract
Some claims to have achieved “cis-olefination” have been re-examined. Attempts to obtain a number of cis-allylic Wittig reagents were unsuccessful. cis-Diethyl 3-methoxycarbonyl-2-methylprop-2-enylphosphonate has been prepared, but its condensation with aldehydes was accompanied by extensive stereomutation. Evidence was obtained for retention of configuration during Wittig reactions with cis- and trans-citral. Methyl diethylphosphonoacetate condensed with 4-hydroxybut-2-enolide and with its 2- and 3-methyl analogues to give the half esters of the corresponding cis,trans-muconates exclusively. Reaction of 4-hydroxy-3-methylbut-2-enolide with allylic Wittig reagents gave polyene acids with complete retention of (cis-) stereochemistry about the αβ-double bond, and a high proportion of cis-configuration about the newly formed carbon–carbon double bonds. The cis-11,cis-13 isomer of vitamin A acid has been characterised.

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