A synthetic study of some triquinane natural products and microbial reduction of prochiral spirodiketones

Zhu, Yanyi (1993) A synthetic study of some triquinane natural products and microbial reduction of prochiral spirodiketones. Masters thesis, Memorial University of Newfoundland.

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Abstract

The first half of this thesis describes an approach to the synthesis of a group of triquinane sesquiterpenes, with (±) deoxypentalenic acid as the particular target. The synthesis started from readily available isophorone. The central quaternary center of the natural product was established in an early step by a double acylation reaction on a cyclohexene derivative. Cleavage of the double bond was followed by reclosure to a five-membered ring. The relative stereochemistry at a stereogenic methine was controlled by catalytic hydrogenation. An intramolecular aldol reaction was used to cyclize the third ring of the triquinane moiety. The remaining steps to (±)-deoxypentalenic acid were modeled in reactions leading to β-keto esters and in reductions of the ketone moiety of a β-keto ester. -- The second part of the thesis provides the results of chiral reductions of 1,3-cyclopentanedione derivatives using baker's yeast. The series of diketones that was examined included 7,9,9-trimethylspiro[4.5|dec-7-ene-1,4-dione. The yeast reduction of this compound to (4S,5R)-4-hydroxy-7,9,9-trimethylspiro[4.5]dec-7-en-l-one (123) proceeded with high facial selectivity and enantioselectivity, as determined by an analysis of the corresponding Mosher's ester [(+)-α-methoxy-α-trifluoromethylphenyl-acetate derivative]. The facial selectivity was compared with that of the chemical reduction using sodium borohydride. The absolute stereochemistry was established from CD spectra and an X-ray structure. The transformation of compound 123 into (4R,5S)-4-hydroxy-4,7,9,9-tetramethylspiro[4.5]dec-7-en-l-one, an optically active form of an intermediate in the triquinane synthesis, required four steps.

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