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

Abstract

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.

Conjugated Nitrodienes. Synthesis and Reactivity

“…These versatile compounds undergo 6π-electrocyclization in refluxing xylene to give 72–92% yields of nitrobenzene ring-fused derivatives 46 , whereas their reaction with LHMDS resulted in an intramolecular Michael addition to afford thiopyran S , S -dioxides 47 (Scheme ). In addition to these types of compounds, it should be noted that when the Ar group is a homo- or heterosystem with low aromaticity (2- or 3-thienyl, 1- or 2-naphthyl), anions 48 derived from 45 undergo addition onto the aromatic ring, thus leading to eight-membered sulfur heterocycles condensed with the original Ar ring 50 through nitronates 49 (Scheme ) …”

“…In addition to these types of compounds, it should be noted that when the Ar group is a homo-or heterosystem with low aromaticity (2-or 3thienyl, 1-or 2-naphthyl), anions 48 derived from 45 undergo addition onto the aromatic ring, thus leading to eightmembered sulfur heterocycles condensed with the original Ar ring 50 through nitronates 49 (Scheme 14). 45 Previously, 46 1,1-bis(methylthio)-3-nitro-4-pyrrolidino-1,3butadiene ( 41 The ring-opening reaction of 2-substituted 4-nitrothiophenes 54 with pyrrolidine/AgNO 3, followed by treatment with excess MeI, gave the polyfunctionalized 2-nitrodienes 55 (Scheme 16). 46,47 Similarly, 3-nitro-4-(phenylsulfonyl)thiophene (56) yielded compound 57, which, after replacement of the pyrrolidino moiety and oxidation of the methylthio group in 58, led to naphthalene, phenanthrene, and benzothiophene derivatives 59 (Scheme 17).…”