Synthesis of the C(2)−C(13) Fragment (The A−B Spiroketal Unit) of Spongistatin 1 (Altohyrtin A):  Use of a Common Intermediate for the Synthesis of Both Spongistatin Spiroketals

[GRAPHIC] A convergent synthesis of 14 corresponding to the A-B spiroketal core of spongistatin 1 has been accomplished via an iodo-spiroketalization reaction of glycal 9, which was synthesized in three steps from a late-stage intermediate used in our synthesis of the C-D spiroketal fragment of spongistatin 1. Elaboration of 14 to the A-B spiroketal 15 was accomplished in three steps.