TANDEM THERMAL CLAISEN-COPE REARRANGEMENTS OF COUMARATE DERIVATIVES - TOTAL SYNTHESES OF THE NATURALLY-OCCURRING COUMARINS - SUBEROSIN, DEMETHYLSUBEROSIN, OSTRUTHIN, BALSAMIFERONE AND GRAVELLIFERONE

Thermal Claisen rearrangements of derivatives of 4'-O-methyl and 4'-O-benzyl methyl coumarates 3, prepared from the corresponding umbelliferone derivatives, have been investigated. Simple allyl derivatives rearrange predominantly to the vacant ortho-position while prenyl derivatives undergo a sterically driven tandem para-Claisen rearrangement. After thermal rearrangement, the resulting 2'-hydroxycinnamate esters readily relactonise to yield 3-, 6- and 8-allylated coumarin derivatives in synthetically useful yields.