Thieno[3,2-b]benzofuran - Synthesis and Reactions

Abstract

Thieno [3,2-b]benzofuran was synthesized starting from benzo [b]furan-3(2H)-one or benzo [b]furan-2-carbaldehyde. Electrophilic substitution reactions such as bromination, formylation, acetylation or nitration, take place in position 2. An electron donating group in position 2 directs further electrophilic substitution into positions 3 and 6, whereas compounds with an electron acceptor in position 2 are substituted exclusively in position 6. Metallation with butyllithium took place in position 2. The 1 H and 13 C NMR signals of the title compound were fully assigned.In our previous communications we discussed the synthesis 1 and reactivity 2 of a novel heterocyclic system, [1]benzothieno[3,2-b]furan (1). We observed its lower stability in acidic media, indicating an imperfect aromaticity and a weak enol ether character. Although the isomeric thieno[3,2-b]benzofuran (2) was prepared 3 , its synthesis has not been described in detail and no data on its reactivity are known. Neither compound 2 nor intermediates of its synthesis have been fully characterized by physicochemical methods. In this communication we describe novel synthetic approaches to compound 2 and discuss its reactivity in selected substitution reactions as compared with compound 1.In our search for an effective synthesis of heterocycle 2 we first tried the reaction sequence starting from benzo[b]furan-3(2H)-one (Scheme 1). This compound was converted into 3-chlorobenzo[b]furan-2-carbaldehyde (3) by the Vilsmeier-Haack formylation with N,N-dimethylformamide and phosphorus oxychloride. In contrast to the published data 3 , the nucleophilic substitution of chlorine atom with sulfanylacetic (thioglycolic) acid in an alkaline medium gave the desired [(2-formylbenzo[b]furan-3-yl)sulfanyl]acetic acid (4) only in a low yield (43%). Most probably, the starting S O O S 1 2 1468Vachal, Pihera, Svoboda:

Novel ferroelectric liquid crystals based on fused thieno[3,2‐b]furan and thieno[3,2‐b]thiophene cores

“…We previously studied extensively the chemistry of heterocyclic systems 1 and 2 and elaborated methods of synthesis of both the parent compounds and their 2,6-disubstitued derivatives [11,13,16]. Therefore, the design of the new materials is based on available methoxy esters 3/X (X5O, S) (scheme 2), i.e.…”

A Novel Synthesis of [1]Benzothieno[3,2-b][1]benzofuran

“…Synthesis and reactivity of [1]benzothieno [3,2-b]furan and thieno[3,2-b]- [1]benzofuran has been extensively investigated in the last years [1][2][3] . It was shown that [1]benzothieno [3,2-b]furan and its vinyl derivatives can be used 4,5 in Diels-Alder reactions for construction of a new benzofused heterocyclic system - [1]benzothieno [3,2-b] [1]benzofuran (1), the reactivity of which was also reported 6 .…”

A theoretical approach to the nucleophilic behavior of benzofused thieno[3,2-b]furans using DFT and HF based reactivity descriptors

“…There are many experimental results for benzothieno [3,2-b]furan 1 and thieno [3,2-b]benzofuran 2 ( Figure 1) showing their different reactivity and regioselective behavior in the electrophilic substitution reactions. 2,3 For example it was reported 2,3 that 2-position of benzofused thieno [3,2-b]furans 1 and 2 are most reactive to the attack of electrophilic reagents in the electrophilic substitution reactions such as chlorination, bromination, formylation, nitration, etc. When the substitution is continued, the 6-position of heterocycles undergoes substitution reaction.…”

“…Experimental and theoretical considerations on reaction mechanisms of benzofused heterocycles in the electrophilic substitution reactions point out a dual character in its reactivity. [2][3][4][5] One type of the reactions is the electrophilic substitution of benzofused thieno [3,2-b]furans as an aromatic compound, resulting in the substitution of 2-hydrogen via the aromatic electrophilic substitution reaction mechanism (Scheme 1).…”