Flexibility of supposed "rigid" molecules: substituted 2,5-piperazinediones (diketopiperazines)

Conformation of cyclolinopeptide a observed by circular dichroism

A stereochemical investigation, by circular dichroism, of a synthetic nonapeptide (cyclolinopeptide A) in several organic and organic-sulfuric acid solvents is presented. From this examination, and results found for a conformationally rigid model compound, 1,7,7-trimethyl-3-azabicyclo [2.2.1] heptan-2-one(camphorolactam), it is concluded that cyclolinopeptide A may exist in several conformations in solution. None of these conformations is believed to be stabilized by intramolecular hydrogen bonds. Some details on an x-ray analysis of the cyclic nonapeptide are also presented.

The crystal structures of 2,5-piperazinediones having epipolysulfide bridges between c3 and c6: the structures of n,n'-dimethyl-3,6-epitetrathio-2,5-piperazinedione

The crystal and molecular structure of N,N'-dimethyl-3,6-epitetrathio-2,5-piperazinedione (C(6)H(8)O(2)N(2)S(4)) has been determined from three-dimensional x-ray diffraction data collected by counter techniques. The substance crystallizes in the orthorhombic space group Fdd2, with a = 15.352(5), b = 20.432(7), and c = 6.635(2) A; V = 2081.2(8) A(3), D(meas) = 1.72(2) g/cm(3), and z = 8 molecules per unit cell. The molecule lies on a crystallographic 2-fold axis, the piperazinedione ring is in the boat conformation, and the deviation from planarity is 18 degrees . The bonds of the tetrasulfide chain alternate in length so that the S-S distances are 2.0244(9), 2.076(1), and 2.0244(9) A. The structural data were refined by least-squares methods to an R(F) of 2.5% by use of the 1089 independent reflections (2theta </= 71 degrees ; MoKalpha) for which F(0) (2) >/= 3sigma (F(0) (2)). Since this molecule is chemically identical with the active center of the recently isolated natural product sporidesmin G, our structural study constitutes a description of the epitetrathio-2,5-piperazinedione fragment of that molecule.

Diketopiperazines in peptide and combinatorial chemistry

Diketopiperazines (DKPs), the smallest cyclic peptides, represent an important class of biologically active natural products and their research has been fundamental to many aspects of peptide chemistry. The advent of combinatorial chemistry has revived interest in DKPs for two reasons: firstly, they are simple heterocyclic scaffolds in which diversity can be introduced and stereochemically controlled at up to four positions; secondly, they can be prepared from readily available alpha-amino acids using very robust chemistry. Here synthetic methods, conformation, as well as applications of DKPs are summarized and discussed critically.