AN EXPLORATORY-STUDY ON THE OXO ENOL TAUTOMERIZATION OF SELECTED DIOXOPIPERAZINES AND THEIR SULFUR-CONTAINING ANALOGS

The ring structures of selected dioxopiperazines and their sulphur-con taining analogues undergo remarkable geometry changes during enolizati on as established by semiempirical (AM1) and non-empirical (HF/3-21G) MO computations. The dienols attempt to lose their ''pseudo-anti-aroma tic'' or ''anti-aromatoid'' character by three plausible mechanisms: ( i) changing the CX bond length; (ii) changing the NH out-of-plane angl e; (iii) changing the planarity/non-planarity of the six-membered ring . The effectiveness of these three mechanisms is discussed for cyclo(G ly-Gly), cyclo(Ala-Ala), cyclo(Pro-Pro) and their dithio derivatives c yclo(Glyt-Glyt), cyclo(Alat-Alat), cyclo(Prot-Prot).