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 100degreesC. In contrast, at 18degreesC 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.