Substitution on the A-Ring Confers to Bryopyran Analogues the Unique Biological Activity Characteristic of Bryostatins and Distinct From That of the Phorbol Esters

A close structural analogue of bryostatin 1, which differs from bryostatin 1 only by the absence of the C<sub>30</sub> carbomethoxy group (on the C<sub>13</sub> enoate of the B-ring), has been prepared by total synthesis. Biological assays reveal a crucial role for substitution in the bryostatin 1 A-ring in conferring those responses which are characteristic of bryostatin 1 and distinct from those observed with PMA.