A TANDEM RADICAL MACROCYCLIZATION-TRANSANNULAR CYCLIZATION APPROACH TOWARDS THE TAXANES

Separate treatment of the iodotrienedione 19 and the iododienynedione 38 with Bu3SnH-AIBN produces the corresponding taxane ring systems 25 (25-30%) and 56 (50-60%) respectively by way of tandem radical macrocy clisation-radical transannular cyclisations. By contrast the analogous iodopolyenones 61, 63, 65a, 66a, 86a and 87a, together with the corre sponding iodoenynones 39a and 59 failed to undergo similar tandem radi cal reactions, and instead gave products resulting from direct reducti on of the carbon-iodine bonds in these substrates. The structures of t he taxane analogues 25 and 56 followed from analysis of their NMR spec troscopic data and comparison with similar NMR data for related taxoid s described in the literature. The stereochemistry of 56 was secured f rom an X-ray crystal structure determination of the 1,5-diol 57 produc ed from reduction of 56 with DIBAL.