Chiroptical properties of 1-carbapenam and orbital mixing in nonplanar amides

Abstract

Optical rotatory properties of β-lactam structures are calculated using semiempirical Extended Hückel and CNDO wavefunctions. Possible interpretations of CD and UV absorption data for a recently reported 5-epi-1-carbapenam, (5S)-1-azabicyclo-[3.2.0] heptan-7-one, are proposed. This compound lacks the sulfur of a penicillin nucleus, but still shows a Cotton effect at 231 nm. Analysis is aided by calculations on some monocyclic β-lactams, N-methyl-3-aminoazetidin-2-ones, where the Me carbon is incrementally bent out of the plane of the 4-membered ring. When N is pyramidal, the n and π orbitals of planar amides become mixed into two orbitals which correctly yield opposite signs for the Cotton effects at 231 nm and at just below 200 nm. A correspondence is proposed between the CD bands of 5-epi-1-carbapenam and those observed for another carbocyclic, nonplanar lactam, 4-azatricyclo(4.4.0.0 3,8] decan-5-one. The origin of the spectroscopically relevant orbitals of penams, penam sulfoxides, and other β-lactams is shown in a correlation diagram. In the spectral calculations, shortcomings of the semiempirical methods are encountered and discussed.