Acid-catalysed cyclisation reactions of the lignan cubebin

Abstract

SzcmnzaryThe structures of the acid-catalysed reaction products from cubebin have been elucidated. CUBEEIN (I)f*2 is reported to be dehydrated to 'cubebinic ether' (11)2 when treated with halogen acids in acetic acid; this ether is said to be reduced to 'cubebinol' (111) by sodium and alcoh01.2~~ On the other hand, cubebin gives a compound of unknown structure 'isocubebinic ether' when treated with sulphuric acid-acetic acid.3 Surprisingly, this gives 'cubebinol' with methanolic hydrochloric acid.3 ReimFestigation now shows that all three named products are in fact cyclisation products, and the accepted chemistry4 of this lignan must be superseded.N.m.r. and other spectral data for cubebin are compatible with structure (I) and show that it exists, in chloroform, almost entirely as the hemiacetal. 'Isocubebinic ether', C,,H,,O,, m.p. 156-157"' [a]? + 26.9" (CHCl,) was chromophorically similar to cubebin in the U.V. region,