Model Studies Directed Towards the Synthesis of the Oxetane D-Ring of Paclitaxel: Assessment of the Oxyanion-Assisted Retro-Diels-Alder Reaction as a Means for Generating Oxete

Abstract

In connection with attempts to generate oxete (8) via anionically assisted retro-Diels–Alder reaction, the oxetanes (11) and (12) have been prepared together with their acyclic analogues (35) and (36). The X-ray crystal structure of compound (11) has been determined. Each of the substrates (11), (12), (35) and (36) was treated with potassium hydride in the presence of 18-crown-6, conditions known to promote retro-Diels–Alder reactions in other systems. However, the first three substrates failed to engage in any such reaction even at temperatures up to 100˚C. In contrast, at 18˚C compound (36) reacted to give anthrone (13) in quantitative yield. These results indicate, inter alia, that regioelectronic effects play a crucial role in determining the success, or otherwise, of anionically-assisted retro-Diels–Alder reactions leading to polarized alkenes.