A sulfoxide-based ring annelation approach to fused, many-membered ring N,S-heterocycles

An approach to many-membered ring N,S-heterocycles involving sulfoxide electrophilic sulfenylation (SES followed by ring expansion of the derived sulfonium salt intermediate tin situ) is illustrated for 9- and 10-membered-ring compounds. Treatment of readily prepared sulfoxides 10a, 10b, 18a, 18b, 23a, and 23b with triflic anhydride (pyridine, CH2Cl2, 0 degrees C) provides heterocycles 13a (65%), 13b (60%), 19a (67%), 19b (67%), 24a (42%), and 24b (80%), respectively. Sulfoxides 5a and 5b, under several different conditions, give only the Pummerer dehydration products 6a and 6b, respectively. Diastereomeric sulfoxides 18a' and 18b', upon treatment with triflic anhydride, do not produce clean product mixtures or any of the desired heterocyclic products but, upon heating in toluene, are converted to the more stable isomers 18a and 18b, respectively. Conducting this isomerization in the presence of 2-mercaptobenzothiazole produces a disulfide indicative of the intermediacy of a sulfenic acid. However, the importance of sulfenic acid derivatives in the SES process leading to many-membered ring heterocycles remains to be determined.