Stereoselective Total Synthesis of Obolactones and 7',8'-Dihydroobolactones

A concise stereoselective total synthesis of two diastereomeric obolactones and 7',8'-dihydroobolactones has been achieved employing a metal-free catalytic δ-hydroxyalkynone rearrangement, which could set the required dihydro-γ-pyrone moiety. The desired first stereogenic center has been installed through the chiral pool material, L-aspartic acid. Next, the allylation reaction was strategically utilized to provide the requisite olefin bond for the intended ring-closing metathesis allowing the installation of remaining dihydro-α-pyrone moiety in the natural products. It also enabled targeting both dihydro-α-pyrone diastereomers. Thus, the first stereoselective total synthesis of (+)-7',8'-dihydroobolactone was accomplished establishing its structure and absolute configuration.