Intramolecular reactions of dialkoxycarbenes with a carbonyl group

Abstract

Thermolysis of 5,5-dimethyl-2-methoxy-2-(2-oxocyclohexylmethoxy)-Δ3-1,3,4-oxadiazoline (3a) in benzene at 110°C generated a carbonyl ylide intermediate that gave, in a minor side reaction, a product of 1,3-dipolar cycloaddition to the carbonyl group. The major fate of the ylide was fragmentation to acetone and a dialkoxycarbene, MeO (RCH2O)C:, where R = 2-oxocyclohexyl. The carbene, in turn, underwent overall [2 + 1] cycloaddition to the carbonyl group, presumably to afford diastereomeric dialkoxyoxiranes that could not be isolated. A product of methanolysis of the presumed oxiranes was isolated by GC and its structure was determined by means of X-ray diffraction. Methanol is believed to result from reaction of an intermediate or product with adventitious water. Deliberate hydrolysis of the product of methanolysis gave a hydroxy lactone, the structure of which was also secured by means of single crystal X-ray diffraction. Those structures indicated the likely structures of the oxirane precursors. A second, and major, product from the carbene was a bicyclic ketone. Similar products were obtained from intramolecular reactions of a carbene tethered to cyclopentanone through a two-carbon or a one-carbon tether. Some compounds that are apparently "dimers" of the oxiranes were also isolated and identified by means of X-ray diffraction. These dimers are thought to originate from the reaction of a ring-opened oxirane (a dipole or a cation from electrophilic opening of the oxirane) with a molecule of the oxirane rather than from bimolecular reaction between two oxirane molecules. The oxiranes open by cleavage of the (RO)2C—O bond rather than the oxirane C—C bond, the normally observed sense of thermal ring opening. The properties of the carbenes and the presumed oxiranes serve to point out the options available to such intermediates and suggest that some new, synthetically useful reactions may become possible.Key words: bicyclic ketone, dialkoxycarbene, dialkoxyoxirane, hydroxylactone, "oxirane dimers".