STRUCTURAL FEATURES OF STEREOISOMERIC ANALOGS OF CYCLOTETRADEPSIPEPTIDE [-(MeVal-Hyi)2-]. G. Tishchenko, Institute of Crystallography, Moscow Russia

DLLD(1),LDLD(2),DDDL(3),LDDD(4), DDDD(5), LLDD(6) stereoisomers of c[-(MeVal-Hyi)2-] were investi- gated by X-ray structure analysis. Cycles 1-5 have slightly elongated forms, asymmetric in 3,4, 5, centrosymmetric in 1, with C2 pseudoaxis in 2. The cycle 6 is square. Ester groups are trans in all cases, amide - cis in 1-5 and trans in 6. C=O groups are disposed in pairs under and above mean cycle plain in 1,6; on one side of it in 2,3. One C=O group (of Hyi2 in 4, of Val3 in 5)is directed inside the cycle, opposite to three others.In all molecules, exept 3, experimental conformations are close to calculated ones with minimal total energy. There is the conformation with energy slightly differing from minimal one in 3. The calculation was not carried out for the unique all-trans conformation 6.In crystal 5 there are two independent molecules with very close conformations. The most pronounced difference between these molecules is the side chains orientation of the residue 4 (trans and gauche). Experimental phi,psi-points are situated mainly near k and p-q minima on conformational maps for the model compounds Ac-L-MeVal-OMe and Ac-D-Hyi-NMe2. Exept the residues MeVal in 6, Hyi in 3, MeVal3 and Hyi2 in 5. This result agrees well with phi, psi-points arrangement near the other minima on conformational maps: l and r for 6 and 3, l and r simultaneously for 5 (minimum r is weak), as well as with yealds of cyclization reactions of the linear molecules. If for 1,2,4 yeald is 70-75%, for 3 and 6 40-45%, then for 5 it is only 8%. All structures were solved by direct method with full-matrix least squares refinement. R-factors are 0.066, 0.086, 0.11, 0.055, 0.051 and 0.049 for structures 1-6 respectively.