Transformations of the tetracyclic skeleton of eudistomins by neighboring‐group participation in the Mitsunobu reaction

Mitsunobu reaction mediated introduction of nitrogen nucleophiles into trans 1-hydroxy eudistomins afforded in high yields rearranged products instead of the anticipated cis eudistomins due to neighboring group participations. Transannular interaction in the 6,5,6,7-membered tetracyclic ring system of the sulfur atom in the 1,6,2-oxathiazepine ring, to give a thiiranium ion containing intermediate, was followed by intermolecular attack of the present nucleophiles to give a 6,5,6,6-membered tetracyclic ring system, now with a 1,5,2-oxathiazine ring. Substitution of the hydroxyl group in a trans 1-hydroxy dethia carba eudistomin derivative (i.e. with a 1,2-oxazepine ring) also gave rearranged products due to transannular interaction from the bridgehead nitrogen atom, to give via an aziridinium ion intermediate, products with a 6,5,7,6-membered tetracyclic ring system containing an 1,2-oxazine ring.