ROLE OF CH/O AND CH/π INTERACTIONS IN STRUCTURAL STABILITY : EFFECT ON ENDO SELECTIVITY IN 1,3-DIPOLAR REACTION OF ELECTRON-DEFICIENT 4-OXO-4H-PYRAZOLE 1,2-DIOXIDES WITH N-SUBSTITUTED MALEIMIDES

The molecular structure of the 1:2 cycloadduct derived from 1,3-dipolar reaction of 3,5-bis(methoxycarbonyl)-4-oxo-4H-pyrazole 1,2-dioxide (1a) with N-phenyl maleimide (2a) was established to be an anti-endo-endo form by the single crystal X-ray analysis. The weak attractive CH/O interaction between la and 2a is conjectured to play an important role in the preferential formation of the endo transition-state. The formation mechanism of the adduct is discussed on the basis of transition-state structures optimized at the B3LYP/6-31G(d) level of theory.