Synthesis of polyunsaturated constituents of phenolic lipids

Abstract

The diene, ( ZZ)-[(8,11)-pentadecadienyl]salicylic acid, (or 2-hydroxy-6-[( ZZ)-pentadeca-8,11-dienyl]benzoic acid), has been synthesised by two routes. In the first, the key intermediate methyl or ethyl 2-hydroxy-6-(7-bromoheptyl)benzoate has been converted to methyl or ethyl 2-hydroxy-6-(10-hydroxydec-8-ynyl)benzoate and thence by reaction of the corresponding bromide with 1-pentynylmagnesium bromide to methyl or ethyl 2-hydroxy-6-(pentadeca-8,11-diynyl)benzoate. Selective reduction afforded methyl or ethyl 2-hydroxy-6-[( ZZ)-pentadeca-8,11-dienyl]benzoate. An attempt to employ the Grignard reagent from methyl 2-methoxy-6-(non-8-ynyl)benzoate and reaction with 1-bromohex-2-yne was ineffective because of a side reaction of the former with ethylmagnesium bromide to give a ketone. In the third approach ethyl 2-methoxy-6-methylbenzoate was alkylated with 1-iodotetradeca-7,10-diyne and the product selectively reduced as before to the O-methyl ether ethyl ester. A variety of C 14 intermediates has been prepared for the derivation of the 8( E),11( E), 8( E),11( Z) and 8( Z),11( E) stereoisomers by the alkylation procedure. A similar methodology of alkylation can be adopted for obtaining corresponding trienes in which some progress has been made commencing with the synthesis of the 8( Z),11( Z),14 compound.