SYNTHESIS AND X-RAY-ANALYSIS OF DIHYDRO-1,2,4,5-TRIOXAZINE - EVIDENCE OF A STEPWISE MECHANISM FOR THE [3 + 3] CYCLOADDITION OF CARBONYL OXIDES WITH NITRONES

Carbonyl oxides, derived by ozonolysis of vinyl ethers, readily undergo [3 + 3] cycloaddition reactions with nitrones affording dihydro-1,2,4,5-trioxazines in fair to excellent yield. The structures of dihydro-3,5,6-triphenyl-1,2,4,5-trioxazine (5f) and dihydro-3-cyclohexyl-5-methyl-6,6-diphenyl-1,2,4,5-trioxazine (5t) were unambiguously determined by X-ray analysis. Ozonolysis of 1-cyclohexyl-2-methoxyethene in the presence of either (E)- or (Z)-alpha-(4-methylphenyl)-alpha-phenyl-N-methylnitrone gave a 1:1 mixture of two stereoisomeric cycloadducts. This result, in conjunction with the structure of the relevant 5t, suggests that the [3 + 3] cycloaddition proceeds by a stepwise mechanism.