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

The absolute configuration of (-)-ngaione has been confirmed as (2’S,5’R)-4-methyl-l-(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)-linalooI. Wolff-Kishner reduction of the hydrazone or semicarbazone of (-)-ngaione yields mainly an E, Z mixture of (lS,4ζ)-l-(furan-3’-yl)-4,8-dimethylnon-5-en-l-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.