Synthesis and Ring-Opening of a Fused Bicyclo[2.1.0] pentan-5-one Acetal

Abstract

Dimethoxycarbene, generated from 2,2-dimethoxy-5,5-dimethyl-D³ -1,3,4-oxadiazoline (8), adds to the electron-deficient double bond of ring-fused cyclobutene diesters (11), (12) and (15) from the face remote from the methano bridge to give the respective bicyclo[2.1.0]pentanone acetals (13), (14) and (16). In two cases, dimethylcyclopropanes (17) and (24) also formed as by-products in these reactions. Mechanisms for the formation of all products and by-products are proposed, especially in regard to an unusual 1,3-dipolar cycloaddition product (23) isolated from the reaction of (8) with (11). Hydrolysis of acetal (13) to the ring-fused cyclopropanone (27) is thwarted by preferential release of ring strain of an external cyclopropyl carbon–carbon bond. In acid media this results in the formation of a benzonorbornane-fused cyclobutane triester (26) with proton capture from the same face, likely from concerted methoxy group-assisted opening of the three-membered ring. Kinetic product (26) is isomerized to the thermodynamic isomer (30) with sodium methoxide. Attempted deacetalization of (13) with iodotrimethylsilane gives (26) when traces of acid are present, but provides the thermodynamic product (30) directly in the presence of pyridine.