A new stereocontrolled synthesis of uncommon tripeptides derived from 2,6-diaminopimelic acid (2,6-DAP)

Expeditious synthesis of enantiopure, orthogonally protected bis-α-amino acids (OPBAAs) and their use in a study of Nod1 stimulation

A convenient approach towards the synthesis of orthogonally protected chiral bis-α-amino acids (OPBAAs) is described. The key transformations include: (1) a highly stereoselective conjugation (alkylation) of the Schöllkopf bis-lactim ethers and oxazolidinyl alkyl halides to build a backbone skeleton; and (2) our orthogonal protection strategy. A series of enantiopure OPBAAs bearing a variety of alkyl chain as a spacer; two stereogenic centers; and three protecting groups were prepared as examples. These versatile molecules were applied to the synthesis of biologically interesting di- or tri-peptide analogues, including chiral iE-meso-DAP and A-iE-meso-DAP, for the study of Nod1 activation in the innate immune response.

The synthesis and biological testing of bacilysin analogues

A series of compounds based on the structure of bacilysin were synthesised and tested for antibacterial activity. The key steps in the syntheses are the coupling of an iodide to a diketopiperazine (DKP) and mono-lactim ether scaffold, respectively. The diastereoselectivity of the coupling reactions was dependant on the scaffold, with selectivity for DKP of about 4:1 and mono-lactim ether exceeding 98:2. Subsequent elaboration of the compounds to give open chain dipeptides and DKPs that mimic the structure of bacilysin but substitute the epoxy ketone for a saturated or unsaturated ketone is described. Overall yield from coupling to final product was between 5 and 21 %, with the yield of the saturated products notably higher. The open chain dipeptides demonstrated moderate antibacterial and antifungal activity.

Engineered dehydrogenase biocatalysts for non-natural amino acids: efficient isolation of the D-enantiomer from racemic mixtures

With a view to their use in the kinetic resolution of racemic non-natural amino acids, five variants of the enzyme L-phenylalanine dehydrogenase, the wild-type enzyme from Bacillus sphaericus and four active-site mutants, have been tested with a range of amino acids. In each case, the rates of reaction with 0.2 mM L-amino acid and with the racemic mixture at 0.4 mM were compared, so that the starting concentration of the active substrate was kept constant. Although the D-amino acids are not substrates, they were inhibitory in all cases. The extent of inhibition, however, varied greatly from compound to compound and among the mutants. With the N145L mutant and DL 4-O-Me-Phe, the equimolar D-enantiomer gave 83.2% inhibition, and with the wild-type enzyme there was 86.7% inhibition with racemic norleucine. By contrast, with these same substrates the N145V mutant showed less than 9% and 24% inhibition respectively. The N145A mutant was selected for use with DL-4-Cl-Phe. The pH was decreased from the enzyme's optimum of 10.4 to 9.5 to minimise breakdown of the coenzyme NAD(+), and the coenzyme was recycled by molecular oxygen with the assistance of a commercial diaphorase. Reaction on a 200 micromole scale in 20 ml ethanolamine HCl buffer, pH 9.5, with 25 microg N145A enzyme and 100 microg diaphorase, was monitored by chiral HPLC. The L-isomer was removed to an extent of >99% after 40 h, with the D-isomer peak undiminished. The pure D-isomer was isolated from the reaction mixture in 85% overall yield after ion-exchange chromatography.

Diastereoselective synthesis of quaternary α-amino acids from diketopiperazine templates

Sequential enolate alkylations of (S)-N(1)-methyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one and (S)-N(1)-p-methoxybenzyl-5-methoxy-6-isopropyl-3,6-dihydropyrazin-2-one proceed with excellent levels of diastereoselectivity (>90% de) affording quaternary alpha-amino acids in high enantiomeric excess (>98% ee) after deprotection and hydrolysis.

Synthesis of Functionalized, Unsymmetrical 1,3,4,6-Tetrasubstituted 2,5-Diketopiperazines

A general and efficient method for the synthesis of unsymmetrical 1,3,4,6-tetrasubstituted 2,5-diketo- piperazines (DKPs) is described. Cyclization of N-amide alkylated dipeptide methyl esters, followed by alkylation, furnished the corresponding tetrasubstituted DKPs in good overall yields. The influence of steric hindrance in the alkylation reactions appeared to be of lesser importance as long as reactive alkylating agents were used. Furthermore, we have demonstrated the use of tetrasubstituted DKPs as a scaffold for further chemical manipulations to produce novel DKPs with desired properties.

Molecular modeling directed synthesis of a bicyclic analogue of the δ opioid receptor agonist SNC 80

In order to find novel delta opioid receptor agonists, the pharmacophoric benzhydryl moiety of the lead compound SNC 80 (1) was dissected and the phenyl residues were attached to different positions of the 6,8-diazabicyclo[3.2.2]nonane core system (4). The position of the carboxamido group, the stereochemistry, the C3/C4 bond order and the kind and length of the spacer X were considered. The resulting compounds were compared with the four energetically most favourable conformations of SNC 80 by a multifit analysis. These calculations led to the structures 5-10, which fit best to SNC 80. Herein the synthesis of one of these compounds (9) is described. Starting from (S)-glutamate two alternative routes are detailed to obtain the key intermediate 14. A variation of the Dieckmann cyclization, which uses trapping of the first cyclization product with ClSiMe(3) provided the mixed acetal 20, which was carefully hydrolyzed to yield the bicyclic ketone 17. Stereoselective addition of phenylmagnesium bromide, dehydration, LiAlH(4) reduction and exchange of the N-6 residue afforded the designed compound 9. The affinities of 9 towards delta, mu, kappa and ORL1 receptors were determined in receptor binding studies with radioligands. Only moderate receptor affinity was found.