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.