Improved synthetic route to enantiomerically pure samples of the tetrahydropyran-2-ylacetic acid core associated with the phytotoxic polyketide herboxidiene

The phosphine oxide (2), which embodies the tetrahydropyran-2-ylacetic acid core associated with the phytotoxic polyketide herboxidiene (1) and which is a key intermediate in a projected synthesis of this natural product, has been prepared in a highly enantio- and diastereo-selective manner. The pivotal steps in this new and improved synthesis of compound (2) involve Katsuki-Sharpless asymmetric epoxidation of the allylic alcohol (4) to give epoxide (7) and subsequent ring-cleavage of the latter compound with trimethylaluminium to give diol (9). The derived acetate (10) is then readily ozonolysed to give the previously reported aldehyde (11), although now in high enantiomeric excess. Compound (11) can be elaborated, by established chemistry, to the target oxide (2).