Reactions of three [c]annelated 2-aminothiophenes with electron poor olefins

Viktor V. Zhdankin,D. Döpp,E. Sopbué Fondjo

doi:10.3998/ark.5550190.0007.a12

Abstract

A series of electron poor alkenes (maleic acid derivatives 3a-d, dimethyl maleate (4), ethyl acrylate (5a), phenyl vinyl sulfone (5b)) undergo (4+2)-cycloaddition to the thiophene moiety of benzopyrano(3,4-c)annelated 2-aminothiophenes 1a,b and 2. The primary adducts tend to release hydrogen sulfide, in this way the fused thiophene ring is replaced by a fused benzene ring bearing the amino group and any new substituents introduced by the dienophile. With maleic anhydride (3a) and dichloromaleic anhydride (3b) acylation of the amino group competes with the (4+2)-cycloaddition to the thiophene ring. The cycloaddition of the monosubstituted alkenes 5a,b follows a head-to-head regioselectivity as predicted from FMO-considerations.

Highlights

Among the three common five-membered fully unsaturated heterocycles, furan derivatives[1,2] have been shown to readily undergo (π4+π2)-Diels-Alder additions
A similar result obtained in refluxing glacial acetic acid was recently reported by Elnagdi et al.[12]
The solution was concentrated to one half of its volume under reduced pressure, and the solid precipitate was crystallized from ethyl acetate to give 422 mg (76%) of a yellow powder, mp 242-245°C

Summary

Introduction

Among the three common five-membered fully unsaturated heterocycles, furan derivatives[1,2] have been shown to readily undergo (π4+π2)-Diels-Alder additions. From the reaction of substrate 2 with 3a in refluxing toluene, the N-acylated product 7 and the mixture of the bicyclic adducts 8 and 9 (scheme 1), were obtained.

Results

Conclusion