Detailed analysis of the proton and carbon-13 NMR spectra of cyclo(L-prolyl-L-N-methylphenylalanyl) in chloroform and methanol in relation to its nonmethylated analog provided information on the conformation of the title compound in solution as well as on the effect of N-methylation and solvation. The X-ray structure of the title compound in the crystalline state showed the same conformational features as the solution structure. The phenyl group folds over the diketopiperazine ring which resembles a flattened half-chair. Both amide bonds are considerably nonplanar. The pyrrolidine ring of proline shows a strong pucker at the ring junction with the largest chi 5 value hitherto observed.

Institute of Organic Chemistry and Biochemistry Czechoslovak Academy of Sciences Prague

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