A Synthetic Strategy for Saxitoxin Skeleton by a Cascade Bromocyclization: Total Synthesis of (+)-Decarbamoyl-α-saxitoxinol.

A new synthetic strategy for the formation of the ABC tricyclic framework of saxitoxin was developed. The BC ring moiety, including a spiro-aminal structure, was first constructed stereoselectively by a newly designed cascade bromocyclization of a readily available internal alkyne bearing guanidine and urea. The A ring was then synthesized by a guanylation of a cyclic urea, easily prepared via the oxidative cleavage of the diol of the cascade product, followed by addition of cyanide. This strategy enables the concise stereocontrolled total synthesis of (+)-decarbamoyl-α-saxitoxinol, which is a naturally occurring saxitoxin analogue.