Enantioselective total synthesis of the di-O-methyl ethers of (–)-agatharesinol, (+)-hinokiresinol and (–)-sugiresinol, characteristic norlignans of Coniferae

Abstract

Facile enantioselective syntheses of the di-O-methyl ethers of the norlignans, (–)-agatharesinol (–)-1a, (+)-hinokiresinol (+)-2a and (–)-sugiresinol (–)-3a are described. Grignard addition of vinylmagnesium bromide to an aldimine (–)-13, prepared from the tert-butyl ester 11 and 4-methoxycinnamaldehyde 12, afforded a homochiral vinyl aldehyde, (–)-3-(4-methoxyphenyl)pent-4-enal (–)-14 in >95% ee, which was converted into a diastereoisomeric mixture of 1,3-bis(4-methoxyphenyl)pent-4-en-1-ols (3R)-6 by a second Grignard reaction with 4-methoxyphenylmagnesium bromide. Sharpless' asymmetric dihydroxylation of the vinyl alcohols (3R)-6 proceeded diastereoselectively to give the triol of desired relative stereochemistry (2S,3S)-7. This, upon dehydration, afforded (–)-di-O-methylsugiresinol (–)-3b, the subsequent acid-catalysed cyclization of which gave (–)-di-O-methyl agatharesinol (–)-1b. (+)-Di-O-methylhinokiresinol (+)-2b was readily obtained by the dehydration of the vinyl alcohols (3R)-6.