Syntheses of the AB and EFGH ring segments of gambierol

Stereocontrolled syntheses of the AB and EFGH ring systems of gambierol (1) are described. The two key intermediates 3 and 55, representing the AB and EFGH ring frameworks, were prepared from 2-deoxy-D-ribose via linear sequences. Brown's asymmetric allylboration and the intramolecular hetero-Michael reaction were successfully applied to the construction of the A ring moiety. Synthesis of the EFGH ring segment 55 was achieved by the SmI₂ mediated reductive cyclization, constructing the EF ring bearing two 1,3-diaxial methyl groups, and the palladium catalyzed coupling of enol triflate and zinc bishomoenolate, making the GH ring moiety. Attempted convergent approaches toward the EFGH ring framework are also described.