Identification of thioether intermediates in the reductive transformation of gonyautoxins into saxitoxins by thiols.

O-Sulfate group of gonyautoxin I and IV is transformed into methylene to form neosaxitoxin by thiols such as glutathione, a common cellular scavenger, in mild conditions. We isolated the intermediate of this conversion and propose that this reaction proceeds through formation of thiohemiketal, 1,2 shift to form stable thioether intermediate, and then redox exchange at sulfur atom to form the final product.