Stereoselective syntheses of deuterium labelled marmesins 3a-3e as metabolic probes for biosynthetic studies are described. Starting from iodo-umbelliferone (5), the title compounds were readily available by copper catalysed alkylation-cyclisation and subsequent transfer- hydrogenation of the resulting furanocoumarins 6a-6e. The bioconversion of marmesin by a microsomal preparation of A. majus yielded psoralene (4) and acetone in a single step ...