Easy access to 4-nitrothiochroman S,S-dioxides via ring-enlargement from 3-nitrobenzo[b]thiophene

The (E)-2-aryl-1-[2-(methylthio)phenyl]-1-nitroethylenes 5 can easily be oxidized to the relevant sulfones 6 and effectively subjected to cyclization via an intramolecular Michael addition after metallation with lithium bis(trimethylsilyl)amide in THF. After quenching with ammonium chloride the 3-aryl-4-nitrothiochroman S,S-dioxides 2 are obtained as diastereomeric mixtures in good to excellent yields. Both yields and stereochemistry of the ring-closure step appear to be influenced by steric effects of the 3-aryl moiety. As sulfides 5 derive from an initial ring opening of 3-nitrobenzo[b]thiophene (1), the overall 1 to 2 process can be considered as an effective 5 to 6 ring enlargement of the sulfur heterocycle. A conformational 1H NMR and molecular-mechanics investigation on the isolated diastereomeric 2 has also been accomplished.