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

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.