Formation and Rearrangement of a 2,2-Dimethoxyoxirane from Dimethoxycarbene and Fluorenone

Thermolysis of 2,2-dimethoxy-5,5-dimethyl-Δ 3 -1,3,4-oxadiazoline ( 1 ) at 110 °C in the presence of 9-fluorenone ( 3 ) yielded 9-(dimethoxymethylene)fluorene oxide ( 4 ) and methyl 9-methoxyfluorene-9-carboxylate ( 5 ), which are the result of the addition of dimethoxycarbene ( 2 ) to the carbonyl group of 9-fluorenone. While the epoxide 4 is most likely generated from direct [2 + 1] cycloaddition, the ester 5 is the result of subsequent rearrangement of the oxirane. Thermolysis of the trideuteriomethoxy oxadiazoline 1 - d 3 in the presence of fluorenone showed that 5 is not formed by an intermolecular process because evidence for crossover of the label in the product 5 was not found. The rearrangement of 4 to 5 does not proceed by ring opening and intramolecular methyl transfer either but, instead, by ring opening and intramolecular methoxy transfer. Thus, as predicted by the Baldwin rules, the strained intramolecular 5-endo-tet methyl transfer is avoided in favor of another process.