The structure and chemistry of hallerin, a mixture of anomeric sesquiterpenoids from Laserpitium halleri Crantz subsp. halleri

Abstract

Hallerin (1), the major constituent of the roots of Laserpitium halleri Crantz subsp. halleri, has been shown to be a mixture of two anomeric sesquiterpene lactols having the (6S,7R,8S,11S)-trans,trans-8-angeloyloxy-6,12-epoxygermacra-1 (10),4-dien-12-ol structure. Hallerin (1) and its corresponding lactone (2) in solution adopt a conformation pseudoenantiomeric with that of germacrolides of the costunolide type; this gives rise to elemane and eudesmane derivatives of atypical steric series upon Cope rearrangement and acid-catalysed transannular cyclisation. Saponification of (1) affords (6S,7S,8S,11S,12R)-trans,trans-8,12-epoxygermacra-1 (10),4-diene-6,12 diol (17) as the sole product. The mechanism and the stereoselectivity of this reaction are discussed in terms of the formation of an intermediate hemiacylal and of the presence of a preferred reacting conformation for the hydroxyaldehyde (22) resulting from its deacylation.