Preparation of key intermediates for the asymmetric synthesis of oxygenated elemanoids

Abstract

In connection with search for chiral key intermediates useful for the synthesis of elemanoids, (3R,4S)-3-acetoxymethyl- and 3-tert-butyldimethylsiloxymethyl-4-isopropenyl-3-vinylcyclohexanone, (12a, R = Ac and TBDMS) and (1S,4S,5R)-4-isopropenyl-1-methoxy-5-vinyl-7-oxabicyclo[3.2.1]octane (24, R = Me) have been synthesised in good overall yields from (1R,5S)nopinone 5via(1R,5S)-4-hydroxymethyl-6,6-dimethylbicyclo[3.1.1]hept-3-en-2-one, 17a, (1R,4R,5S)-4-acetoxymethyl-6,6-dimethyl-4-vinylbicyclo[3.1.1]heptan-2-one 20, and its cyclobutanering opened product, (4S,5R)-1-acetoxy-5-acetoxymethyl-4-isopropenyl-5-vinylcyclohex-1-ene 21. The enantiomer of 17a, (1S,5R)-17b, was prepared from (1R,5S)-5 as the common starting material. The above compounds, (3R,4S)-12a as well as 24, are useful for the asymmetric synthesis of (5S,10R)-elemanoid with oxygen functions at the C–8 and C-14 positions, while the compounds (3S,4R)-12b, enantiomers of 12a which could be derived from 17b according to a sequence of reactions for the preparation of 12a from 17a are useful for the synthesis of (5R,10S)-oxygenated elemanoid.