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Both enantiomers of trans-2,3-cis-3,4-dihydroxyproline, 4 and 5, have been stereoselectively synthesized from 2,3-O- isopropylidene- d -glyceraldehyde 1, by taking advantage of a divergent and parallel synthetic strategy, utilizing N-(tert-butoxycarbonyl)-2-(tert-butyldimethylsiloxy)pyrrole (TBSOP) as the common four-carbon synthon.

Exploiting novel 2-(tert-butyldimethylsiloxy)thiophene (1, TBSOT) as versatile carbon nucleophile and both enantiomers of glyceraldehyde acetonide as chiral sources, an entry to both enantiomers of anti HIV-active 2′,3′-dideoxy-4′-thiocytidine 10 and ent- 10 was devised and executed.

The present article mainly highlights recent approaches to biofunctional carbohydrates and hydroxylated alkaloids, including our own achievement which utilize enantiopure or enantioenriched precursors or templates derived from the chiral pool exploiting stereoselective totally chemical homologative techniques.

The embodiment of 4-aminoproline residues (Amp) into the arginine-glycine-aspartate (RGD) sequence led to the discovery of a novel class of high-affinity alpha Vbeta 3/alpha Vbeta 5 integrin binders [IC 50 h (alpha Vbeta 3) 0.03-5.12 nM; IC 50 h (alpha Vbeta 5) 0.88-154 nM]. A total of eight cyclopeptides of type cyclo-[-Arg-Gly-Asp-Amp-], 5- 12, were assembled by a standard solid-phase peptide synthesis protocol that involved the C2-carboxyl and C4-amino functionalities of the proline scaffolds, leaving the N (alpha)-nuclear site untouched. Functionalization of this vacant proline site with either alkyl or acyl substituents proved feasible, with significant benefit to the integrin binding capabilities of the ligands. Notably, six out of eight cyclopeptide inhibitors, 5- 7 and 9- 11, showed moderate yet significant selectivity toward the alpha Vbeta 3 receptor. The three-dimensional structure in water was determined by NMR techniques and molecular dynamics calculations. Docking studies to the X-ray crystal structure of the extracellular segment of integrin alpha Vbeta 3 complexed with reference compound 1 were also performed on selected analogues to highlight the structural features required for potent ligand binding affinity.

The syntheses of racemic thiazolidinyl thymine and cytosine derivatives 6a and 6b, and 10a and 10b, prototypes of a novel family of heterosubstituted nucleosides, have been achieved from rhodanine (3) or thiazolidine (7) by a short sequence of steps. The key reaction involved efficient coupling of silylated thymine and cytosine to N-Boc-protected 4- and 5-acetoxythiazolidines, 5 and 9, assisted by terr-butyldimethylsilyl trifluoromethanesulfonate.

The structure of 2,3-dideoxy-4-thio-d-arabino-heptonic acid 1,4-lactone has been determined by X-ray diffraction. Crystals of the compound are orthorhombic, space groupP212121, with cell dimensionsa=11.555(5),b=10.451(5),c=9.604(5) A, andZ=4. The diffraction experiment has allowed to assign the absolute configuration (4R, 5R, 6R) to the chiral centers of the molecule.

γ-Substituted siloxypyrrole derivatives 5–7 have been synthesized by direct alkylation of N-(tert-butoxycarbonyl)-2-(tert-butyldimethylsiloxy)pyrrole 1. These underwent subsequent alkylation with alkyl halides or aldehydes to produce γ,γ-disubstituted α,β-unsaturated lactam intermediates in good yields. Oxidative cleavage of the C(3)–C(4) bond within the lactam moiety gave rise to a number of α-substituted α-amino acids. These include racemic α-methylphenylalanine 14, α-benzylphenylalanine 15, α-benzylserine 18 and α-methylthreonine 21.

Molecular superposition models guided the design of novel melatonin receptor ligands characterized by a 2-acylaminomethyltetrahydroquinoline scaffold. Starting from the structure of N-anilinoethylamide ligands, the flexible chain was conformationally constrained to reproduce the bioactive conformation of melatonin. Structure-activity relationships were investigated, focusing on the substituent at the nitrogen atom, the position of the methoxy group, and the replacement of the amide side chain by urea and thiourea groups. The compounds were tested for binding affinity and intrinsic activity at human MT1 and MT2 receptors. Structural optimization resulted in N-[(1-benzyl-1,2,3,4-tetrahydro-5-methoxyquinolin-2-yl)methyl]propionamide (UCM1014), with picomolar MT2 binding affinity (K(i) = 0.001 nM), more than 10000-fold selectivity over the MT1 receptor, and a full agonist profile (GTPγS test), being the most potent MT2-selective full agonist reported to date. Molecular dynamics simulations provided a rationale for high binding affinity, stereoselectivity, and agonist behavior of these novel melatonin receptor ligands based on superposition models and conformational preference.