The title compound represents the smallest member of cyclic proline peptides corresponding to the general formula c(DDLL-Pro4)n with a strictly D,D,L,L double-alternating sequence of the chiral amino acid residues. The cyclopeptides with n greater than or equal to 2 could be synthesized from both DDLL-Pro4 (1) and DLLD-Pro4 (2). The cyclic monomer (n = 1) resulted only from 2, whereas not even a trace could be found by cyclization of 1. The peptide exists in a strongly strained Ci symmetrical conformation (x-ray analysis) with alternating cis and trans peptide bonds (ctct form I). The cis peptide bonds deviate from planarity (omega = 22 degrees); two of the pyrrolidine rings show a "South" conformation (phi = -94 degrees), whereas the other residues exhibit C alpha-endo puckering (phi = -124 degrees). Two of the psi angles surprisingly occur at +41 degrees (anti-cis'), the others are located in the trans' region. A quantitative ring opening occurs with trifluoroacetic acid at room temperature. In solution the existence of an isomeric ctcc sequence (form Ia) is indicated. Dreiding model studies also suggested a favorable conformation with a tctc sequence (form II). Consequently, we performed molecular mechanics calculations, based on the CHARMM force field and semiempirical quantum mechanical AM1 calculations (MOPAC program). Pronounced differences in the backbone parameters were found using these two methods. However, the theoretical studies evidenced the experimentally obtained differences in the cyclization tendencies of the linear precursors.

The ionic compound [Si(acac)3][AgCl2] · CH2C12, (1) was synthesized by reaction of [Si(acac)3][HCl2] and silver acetate in dichloromethane. A singlecrystal X-ray analysis shows that the cation has an octahedral tris-chelate structure and that the anion is a polymeric chain, composed of edgesharing AgCl4 tetrahedra.

Cyclic tetrapeptides exclusively composed of L‐ and D‐Pro have been studied by theoretical means (conformational searches and molecular mechanics calculations using the CHARMM program) supported by 1H‐NMR spectroscopy, X‐ray analysis and chiroptical measurements. We explored the entire conformational space of the diastereomers cyclo(LLLL‐Pro4) (I), cyc1O(LDLD‐Pro4) (II) and CYClo(LLDD‐Pro4) (III) including the low‐energy conformations and the related interconversion paths. The conformational interconversions were found to be restricted to cis/trans isomerisations of the amide bonds. Owing to the polycyclic nature of cyclo(Pro4) most of the cis/trans transitions are hindered by energy barriers higher than 30 kcal/mol (up to 150–200 kcal/mol). A few transitions are characterized by computed energy barriers comparable to those found in linear ‐Xxx‐Pro‐ sequences (∼ 18 kcal/mol), and are therefore experimentally significant. Experimental evidence has been obtained in the case Of CyClo(LDLD‐Pro4), where two enantiomers are interconverted by a series of 4 cis/trans isomerisations ctct→cttt→tttt→tctt→tctc. The Eyring activation parameters of this reaction were determined in H2O and in DMF by chiroptical measurements (ΔH#= 44 and 28 kcal/mol; ΔS#=59 and 22 cal K −1 mol−1, respectively), and correlated with the calculated barriers. In I and III comparable series of four cis/trans isomerisations relate two main conformations with the peptide bond sequences ctct and tctc. In compound I pseudorotational images are interconverted via ctct→ccct→cctt→cctc→tctc. The pathway ctct→ccct→cctt→cctc→tctc. that relates diastereomeric main conformations of III involves exclusively low‐energy intermediates; however, the transitions leading to the all‐cis conformation are energetically unfavourable, and the conformational space is divided in three insulated domains.

The Structure of [Si(bzac)3]+[HC12]~ (bzac = 1-phenyl-1,3-butanedionato) U lf Thewalt* Sektion R öntgen-und E lektronenbeugung der U n iv ersität U lm , A lbert-Einstein-A llee 11, D-7900 Ulm Ulrich Link A bt. T heoretische Chemie der U niv ersität U lm , A lbert-E instein-A llee 11, D-7900 U lm Z. N aturforsch. 46b, 2 9 3 -2 9 6 (1 9 9 1 ); eingegangen am 18. O k to b e r 1990Silicon C om plex, 1-P henyl-1,3-butandionato, O ctah ed ral Silicon C om plexThe title com pound which w as prep ared by D ilthey already in 1903 has been investigated by X -ray crystallography. T he com plex cation has the expected o ctah e d ral tris-chelate structure w ith the ligands in a m eridional arrangem ent. T he com plex crystallizes as a racem ate. The Cl • • ■ Cl distance in the [HC12]~ anion is 3.145(4) A.