A photoinduced two carbon ring expansion leading to an easy synthesis of γ-tropolone

Abstract

Irradiation of 3-methoxycyclopent-2-en-1-one (1) and an excess of acetylene in MeCN at 0 °C with Pyrex-filtered light from a Hg-lamp gives 5-methoxycyclohepta-2,4-dien-1-one (2), which undergoes SeO₂ oxidation to 4-methoxytropone (3), a known source of γ-tropolone (4); using dideuterioacetylene in place of acetylene, a 3:2 mixture of the isomeric 2,3- and 3,4-dideuterio analogues of (2) is obtained, suggesting a limitation of this method for substituted γ-tropolones.