Ring-Opening/Ring-Closing Protocols from Nitrothiophenes: Six-Membered versus Unusual Eight-Membered Sulfur Heterocycles through Michael-Type Addition on Nitrobutadienes

When Ar is a low-aromaticity homo- or heterosystem. the sulfonyl-stabilized anion of nitrobutadienes 4 (which derive from the initial ring opening, of 3-nitrothiophene) undergoes a rather surprising addition onto the aromatic ring itself, thereby leading to the construction of an unusual eight-membered sulfur heterocycle condensed with the original Ar ring. The competitiveness of such a pathway with respect to the formation of the thiopyran ring (i.e., addition onto the nitrovinyl moiety) is favored at low temperatures, thus revealing its nature as a kinetically controlled process.