Structure and unexpected chiroptical properties of chiral 4‐pyrrolidinyl substituted 2(5 H )‐furanones

Planar 2(5 H )‐furanones substituted at C4 with a chiral pyrrolidinyl group show CD spectra which are apparently due to the distortion of the C4‐N1 bond of sp 2 character from the plane defined by the 2(5 H )‐furanone ring atoms and/or due to the presence of substituents in the pyrrolidine ring. This is a new, previously not encountered structural factor determining the chiroptical properties of 2(5 H )‐furanones and emerging from the analysis of X‐ray diffraction data and quantum mechanical DFT computations. In the presence of a C5 pseudoaxial substituent in the furanone ring, the sign of the furanone n–π* and π–π* transition Cotton effects is determined primarily by the previously postulated allylic helicity rule. Chirality 16:405–413, 2004. © 2004 Wiley‐Liss, Inc.