A HIGHLY DIASTEREOSELECTIVE AND ENANTIOSELECTIVE SYNTHESIS OF THE TOP HALF OF KIJANOLIDE

A highly diastereo- and enantioselective synthesis of spirotetronate 4 corresponding to the top half of kijanolide is reported. This synthesis features the novel exo-selective Diels-Alder reaction of triene 6 and the chiral, nonracemic dienophiles (R)-7 and (R)-8. The reaction of 6 and (R)-7 produced a mixture of the desired exo cycloadduct 28, the unexpected exo diastereofacial isomer 29, and a minor amount of the endo cycloadduct 30. However, the Diels-Alder reaction of 6 and dienophile (R)-8 with the more sterically demanding tert-butyl substituent provided a 13-14:1 mixture of exo cycloadduct 38 and endo isomer 39; the exo diastereofacial isomer corresponding to 29 was not observed. Elaboration of 28 and 38 to spirotetronate 4 proceeded by way of the Dieckmann cyclization of alpha-acetoxy ester 5.