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

The chiroptical properties of the title compounds bearing various substitue nts at the N(1) and N(2) nitrogen atoms are discussed. It was found that th e sign of the n-pi* Cotton effect centered at about the 230-250 nm region c an 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 predictab le by Weigang's lactam sector rule. MNIX calculations, supported by X-ray m easurements, showed that substituents at the nitrogen atoms significantly a ffect the conformation of the five-membered ring. It was additionally concl uded that the conformation of the five-membered ring in 5-substituted pyraz olidin-3-ones is the sign-determining factor for an n-pi* transition.