Ring Transformation of Annulated Benzofuran Derivatives to Medium-Sized Lactones

Dedicated to Prof. Karl Heinz Dötz on the occasion of his 80^th birthday

Abstract

Benzannulated eight- and nine-membered-ring lactones were prepared by ring transformation of annulated benzofuran derivatives. The reaction sequence starts with a Brønsted acid catalyzed hydration of the enol ether moiety of the starting materials furnishing hemiacetals as intermediate products which underwent retro-Claisen-type C–C bond cleavage under the reaction conditions. The use of 15 mol% TfOH and two equivalents of water in CHCl₃ at 85 °C turned out to be optimal. Products with exocyclic carboxylate function could be further derivatized by amide formation. In total, eleven medium-sized lactones were prepared. Furthermore, three benzannulated spirolactones were obtained from starting materials with exocyclic carboxamide moiety.

Key words

benzofuran - lactone - medium-sized rings - ring expansion - ring transformation

Publication History

Received: 11 September 2023

Accepted after revision: 16 October 2023

Accepted Manuscript online:
16 October 2023

Article published online:
22 November 2023

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