3 Determination of Absolute Configuration by Synthesis : (+)-Abscisic Acid and (-)-14-Methylhexadec-8-cis-en-1-ol
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Synthetic methods have long been used to solve stereochemical problems in natural products chemistry. Herein are described recent two examples. Absolute Configuration of (-)-14-methylhexadec-8-cis-en-l-ol (I) and methyl (-)-14-methylhexadec-8-cis-enoate (II). These were isolated from a female dermestid beetle (Trogoderma inclusum) as the components of the sex attractant. The (S)-pheromones (I, II) have been synthesized from (S)-(-)-2-methylbutan-l-ol and shown to be dextrorotatory. Therefore the natural and levorotatory pheromones possess the 14R stereochemistry. Absolute configuration of (+)-abscisic acid (XIV). Our startegy to attack this problem was to secure an optically active ketone (XV) as a common intermediate from which both the natural grasshopper ketone (XVI) and dehydrovomifoliol (XVII) could be synthesized employing reactions with known steric courses. Since the absolute configuration of the grasshopper ketone had been established by X-ray analysis, this would enable us to assign the absolute stereochemistry of (+)-dehydrovomifoliol (XVII) which had been converted by others to (+)-abscisic acid (XIV). The present synthesis yielded (+)- and (-)-dehydrovomifoliols and (-)-trans-epoxy ketone (XXXIIIa). The last compound was indistinguishable from an sample derived from violaxanthin. Thus the stereochemistries of (+)-abscisic acid and related carotenoids were supported by synthesis.
