[2p+2s] Type Cycloaddition Reactions of Naphtho[b]cyclopropene with Trithiocarbonyl Compounds to Form 1,3-Dihydronaphtho[2,3-c]thiophene Derivatives

Reactions of naphtho[b]cyclopropene with trithiocarbonyl derivatives in chloroform afforded 1,3-dihydrobenzo[2,3-c]thiophene derivatives in moderate yields through a [2p + 2s] type cycloaddition process.The similar reaction with a dithiocarbonate compound gave a recovery of the starting material.A reaction with thiotropone resulted in a [8p + 2s] type adduct in 5% yield.Solvent effects on the product yields revealed that the reactions proceeded via ionic intermediates.Naphtho[b]cyclopropene ( 1) is a stable colorless crystalline material in spite of its large strain energy resulted from the distorted bond angles of the bridge-head sp 2 hybridized carbon atoms.However, it is known that compound 1 is highly reactive as a 2p component in cycloaddition reactions owing to the strained structure and a deficient aromaticity. 1 Recently, the carbenic behavior of 1 was also proposed. 2 The reactivities of cyclopropene derivatives attached to aromatic rings such as benzocyclopropene, 3 5Hcycloprop[f]isobenzofuran, 4 or 1 have been researched.Recently, [4p + 2s] and [8p + 2s] types cycloadduct were obtained in reactions of 1 with nitrones and tropones, respectively. 5 As a series of our research, reactions of 1 with trithiocarbonyl compounds were performed to give 1,3dihydronaphtho[2,3-c]thiophene derivatives.We report here the results of the reactions.Compound (1) and an equimolar amount of dimethyltrithiocarbonate (2) in chloroform were refluxed for