Configurational assignment of 5-substituted pyrazolidin-3-ones using circular dichroism spectroscopy

The chiroptical properties of the title compounds bearing various substituents at the N(1) and N(2) nitrogen atoms are discussed. It was found that the sign of the n-pi* Cotton effect centered at about the 230-250 nm region can be correlated with the absolute configuration of the stereogenic center at C(5). It was also found that the sign of this Cotton effect is predictable by Weigang's lactam sector rule. MNIX calculations, supported by X-ray measurements, showed that substituents at the nitrogen atoms significantly affect the conformation of the five-membered ring. It was additionally concluded that the conformation of the five-membered ring in 5-substituted pyrazolidin-3-ones is the sign-determining factor for an n-pi* transition.