Furan ring opening ‐ indole ring closure: Synthesis of furo[2′,3′:3,4]‐cyclohepta[1,2‐b]indolium chlorides

Abstract

A new synthetic approach to furo[2′,3′:3,4]cyclohepta[1,2‐b]indolium chlorides is elaborated starting from 2‐acetylaminoaryldifurylmethanes or 2‐aminoaryldifurylmethanes under treatment with methanolic HCl solution. The reaction proceeds in three steps: recyclization, intramolecular cyclization, and disproportionation. In this case the furan ring takes part in building up both pyrrole and seven‐membered rings. The same salts can be obtained directly from 2‐acetylaminobenzaldehydes and 2‐methylfuran under similar conditions without isolation of corresponding 2‐acetylaminoaryldifurylmethanes.

Phenylfurylthieno[2,3-b]pyridyl-methanes. synthesis and reactions of the furan ring transformation

“…In difference to compounds 4b-d,g,h acetylamino derivative 4i under the same conditions is smoothly converted into the corresponding recyclization and deacetylation product, a derivative of pyrrolothienopyridine 7a (Scheme 8). We note that a similar transformation, including the two reactions, was observed by us previously for aromatic analogs [7,16,17]. The conversion 4i → 7a occurs in ethanol in a shorter time and with a higher yield than in acetic acid.…”

Synthesis of tetracyclic system of 2,4‐di(tert‐butyl)‐6,7‐dihydrofuro[2′,3′:3,4]cyclohepta[1,2‐b]indole

“…At the same time, it was found earlier that reactivity of (2-hydroxyphenyl)-bis(5-methyl-2-furyl)methanes toward acids is similar to that of (2-aminophenyl)bis(5-methyl-2-furyl)methanes [8,9]. Therefore, it would be interesting to study POCl 3 -induced recyclization of (2-hydroxyphenyl)bis(5-tertbutyl-2-furyl)methanes for comparison with the results obtained for recyclizations of 12 and 13.…”

“…In contrast, similar benzofuran-and indole-based tetracyclic compounds are unstable under the reaction conditions. These substances disproportionate resulting in the corresponding tropylium salts 4 easily (Scheme 1) [8,9].…”

First example of a synthesis of the tetracyclic 2,4-di(tert-butyl)- 6,7-dihydrofuro[2',3':3,4]cyclohepta- [1,2-b]indole system

“…In these reactions, the furan ring acts as a formal equivalent of a 1,4-diketone: one carbonyl group participates in the formation of a new heterocycle, while the other is liberated. In the case of arylbis(5-methyl-2-furyl)methanes, the recyclization may be accompanied by a secondary cyclization involving the liberated carbonyl group [1][2][3][4][5], which leads to fused tetracyclic heterocycles. In contrast, the secondary cyclization does not occur when a bulky tert-butyl group is present at C-5.…”