Synthesis and structural revision of naturally occurring ayapin derivatives

The synthesis of three highly oxygenated naturally occurring coumarins, 8-methoxy-6,7-methylenedioxycoumarin, 5-methoxy-6,7-methylenedioxycoumarin and 5,8-dimethoxy-6,7-methylenedioxycoumarin is described for the first time, together with a new method for the preparation of ayapin (6,7-methylenedioxycoumarin). Comparison of the spectroscopic data of the synthetic tetraoxygenated coumarin 5,8-dimethoxy-6,7-methylenedioxycoumarin with literature reports resulted in the structural revision of several natural coumarins. Two coumarins, both identified as 5,8-dimethoxy-6,7-methylenedioxycoumarin must have other structures, while the structure of another coumarin, described as the isomeric 7,8-dimethoxy-5,6-methylenedioxycoumarin has to be revised to 5,8-dimethoxy-6,7-methylenedioxy-coumarin. (C) 2005 Elsevier Ltd. All rights reserved.