Approaches to the Synthesis of Cyclophostin and Phosphonate Analog

Several approaches to the synthesis of a natural acetyl cholinesterase (AChE) inhibitor, cyclophostin and an isosteric endophosphonate analog were explored. These bicyclic molecules posses a novel seven membered cyclic enolphosphate/phosphonate ring fused to a butyrolactone. Two basic synthetic strategies: a) lactonization followed by the seven membered ring formation and b) the seven membered ring synthesis followed by lactonization, were applied in these syntheses. The synthesis of the endophosphonate analog, following strategy a, gave an unexpected enolether lactone molecule. Alternatively, strategy b successful produced the endophosphonate analog as a separable mixture of two diastereomers. Synthesis of cyclophostin was explored by strategy b. Attempted C-acetylation of an acyclic phosphate intermediate resulted in an unanticipated formation of cyclopropane ring. Following an alternate approach by strategy a, the natural product and its unnatural diastereoisomer were produced. Biological studies with cyclophostin, endophosphonate analog and various synthetic intermediates lead to the discovery of new class of AChE inhibitor.