Terpenoid chemistry. XXV. The absolute configuration of (–)-ngaione, (–)-epingaione and related substances

Abstract

The absolute configuration of (-)-ngaione has been confirmed as (2'S,5'R)-4-methyl-1-(5'-methyl-2',3',4',5'-tetrahydro[2',3''-bifuran]-5'-yl)pentan-2-one as was proposed by Hegarty, Kelly, Park and Sutherland in 1970, by converting it into (–)-ngaiane, (2S,5S)-5-methyl-5-(4''-methylpentyl)-2,3,4,5-tetrahydro[2,3'-bifuran], which is shown to resist ring opening by nucleophiles. Further confirmation was obtained by the ozonolysis of (–)-ngaiane to (–)-(4R)-4,8-dimethylnonan-4-olide comparable in molecular rotation with (–)-(4S)-4-methylhexan-4-olide which has been correlated with (–)-(R)-linalool. Wolff-Kishner reduction of the hydrazone or semicarbazone of (–)-ngaione yields mainly an E,Z(1S,4ξ)-1-(furan-3'-yl)-4,8-dimethylnon-5-en-1-ol with some ngaiane. Being (+)-ngaione, ipomeamarone does not have the absolute configuration assigned by Kubota, Nakajima and Matsuura in 1964. The name ipomeamarone is redundant, implies an incorrect stereochemistry and should be abandoned.