An intramolecular nitrone-olefin dipolar cycloaddition-based approach to total synthesis of the cylindricine and lepadiformine marine alkaloids

A synthetic route to the cylindricine skeleton as well as to the reported structure of the marine alkaloid lepadiformine has been achieved using an intramolecular nitrone/1,3-diene dipolar cycloaddition as the key step. The synthesis began with sequential alkylations of acetone oxime to afford key intermediate oxime 30, which contains all of the carbons necessary to form the tricyclic skeleton of the alkaloids. Nitrone 40, available from oxime 30 by standard transformations, underwent an intramolecular 1,3-dipolar cycloaddition to provide isoxazolidine 43. Related 1,3-dipolar cycloadditions were also explored on two additional nitrone-olefin substrates 41 and 42, which were prepared in a manner similar to that of 40. The tricyclic alkaloid core 52 was formed stereoselectively by a tandem oxidation-Michael addition of amino alcohol 49 derived from isoxazolidine 43. Cleavage of the O-phenyl ether of 52 provided 2-epi-cylindricine C (53). Several unsuccessful attempts were made to convert 52 to cylindricine C by epimerization at C2. Tricyclic ketone 52 was deoxygenated to give amine 59, whose structure and relative stereochemistry were confirmed by single-crystal X-ray analysis of its picrate salt. Removal of the O-phenyl protecting group from 59 provided tricyclic amino alcohol 60 having the putative structure of lepadiformine, but whose NMR data did not correspond to those of the natural product.