Portentol: an unusual polypropionate from the lichen Roccella portentosa

Abstract

Portentol, from R. portentosa, is shown by chemical and physical means to possess the spiro-(2-oxabicyclo-[2,2,2]octane-5,2′-1′-oxacyclohexane) structure (1). Decarboxyportentone (6) has been aromatised to give the 3-methyl-2-oxopent-3-enylbenzene derivative (10), the methyl ether of which on stepwise degradation, led to the acetophenone derivative (16). The proposed structure is well supported by extensive mass spectrometry. Portentol is formally a polypropionate, the polyketide (24) being a reasonable biosynthetic intermediate.