Asymmetric synthesis of 4-substituted prolines as conformationally constrained amino acid analogues

Treatment of readily available chiral building block 1 with (2R)-2,3-O-isopropylideneglyceraldehyde (5) provides a new route for asymmetric synthesis of 2,4-disubstituted pyrrolidines. Several proline-amino acid chimeras: proline-leucine, proline-lysine, proline-arginine and proline-glutamic acid, are synthesized in highly diastereomerically pure forms.     

Synthesis of conformationally constrained lysine analogues

The synthesis of two conformationally constrained lysine analogues is reported. The synthesis of the novel analogue 1 based on the 3-aza-bicyclo[3.1.0]hexane system is accomplished from the known tricycle 3 in eight steps. The synthesis of the analogue 2 is accomplished in eight steps from 4-hydroxy proline. Both analogues are synthesized appropriately protected for Fmoc/Boc solid-phase peptide synthesis.     

Stereoselective synthesis of conformationally constrained glycosylated amino acids using an enzyme-catalyzed desymmetrization

As part of an effort to probe the mechanism by which glycosyltransferases recognize glycoproteins and assemble the core structures of O-linked oligosaccharides, constrained glycopeptides, compounds 2 and 3, based on the alpha-N-acetylgalactosaminyl serine substructure 1, were designed. In this paper we describe a stereoselective preparation of protected versions of these compounds. A pig liver esterase-catalyzed enzymatic desymmetrization of a diacetate substrate, 10, was employed as a key component in the synthesis.     

Chemoenzymatic synthesis of conformationally rigid glutamic acid analogues

All stereomers of cyclohexane and cyclopentane-derived analogues of glutamic acid have been synthesized from the corresponding 3-keto-cycloalkyl carboxylic acid esters by a combination of microbial steps and standard chemical methods.     

Facile syntheses of conformationally constrained analogues of lysine and homoglutamic acid

A facile divergent synthesis of the novel amino acid trans-4-aminoethyl-l-proline and trans-4-carboxymethyl-l-proline from commercially available trans-4-hydroxy-l-proline was developed. These conformationally constrained analogues of l-lysine and l-homoglutamic acid are useful proline templated amino acids (PTAAs) with potential applications in protein engineering and de novo protein design.     

2-Oxopiperazine-based gamma-turn conformationally constrained peptides: synthesis of CCK-4 analogues

2-Oxopiperazine derivatives 1 have been designed as mimetics of gamma-turn conformationally constrained tripeptides. The synthetic pathway devised for the preparation of both epimers of 1 at C(5) involves a reductive amination of cyanomethyleneamino pseudopeptides with amino acid derivatives, followed by regiospecific lactamization of the resulting C-backbone branched pseudopeptides. The versatility of this methodology is illustrated in the synthesis of analogues of the tetrapeptides Boc-[Nle(31)]-CCK-4 and Boc-[Lys(o-tolylaminocarbonyl)(31)]-CCK-4. The introduction of the new conformational restriction into these Boc-CCK-4 analogues led to a loss of 2 or 3 orders of magnitude in the affinity at CCK receptors. These results suggest the absence of a gamma-turn in the bioactive conformation of the C-terminal tripeptide of CCK-4.     

Stereoselective synthesis of new conformationally restricted analogues of a potent CGRP receptor antagonist

[structures: see text] A stereocontrolled racemic synthesis of conformationally restricted analogues 2a and 2b of a potent CGRP receptor antagonist 1 by novel functionalization of 2-substituted octahydropyrido[1,2-a]pyrazin-6-ones is described. The new diastereoselective LDA-promoted alpha-nitration of intermediate lactams established the required trans-configuration in the desired products.     

Synthesis of conformationally constrained spirodihydrofuropyridine analogues of epibatidine

Conformationally constrained spirofuropyridine analogues of epibatidine, syn-2 and anti-2, in which the 7-azabicyclo[2.2.1]heptane system and the 2-chloropyridine ring are held rigidly with the shorter and longer N-N distances, respectively, were synthesized from N-Boc-7-azabicyclo[2.2.1]heptan-2-one. The preliminary binding studies suggested that syn-2 has stronger binding affinity for nAChRs than anti-2.     

Synthesis of conformationally constrained analogues of RGD tripeptide

Convergent syntheses of N-functionalized (3S,4S)-3-amino-4-vinyl-piperidin-2-one 10, trans-3-substituted proline 18 and (3S)-3-amino-piperidin-2-one 28 are developed. By incorporating these building blocks to an appropriate position, conformationally constrained analogues of RGD tripeptide (Arg-Gly-Asp) 2, 3 and 4 are designed and synthesized.     

Stereoselective synthesis of conformationally constrained cyclohexanediols: a set of molecular scaffolds for the synthesis of glycomimetics

The practical, stereoselective synthesis of the three diastereoisomeric 1,2-trans-dicarboxy-4,5-cyclohexanediols 1-3 (DCCHDs) is described, starting from a common precursor, easily available in both enantiomeric forms. The regioselective derivatization of all functional groups of 1 is also reported. The three DCCHDs are locked in a single chair conformation and thus can be used to mimic vicinally disubstituted monosaccharides of any relative configuration.     

Chemo-enzymatic synthesis of conformationally constrained oligosaccharides

N-Acetyllactosamine derivative 4, which has a methylene amide tether between C-6 and C-2', was enzymatically glycosylated using rat liver alpha-2,6-sialyltransferase (ST6GalI) or recombinant human fucosyltransferase V (FucT-V) to give conformationally constrained trisaccharides 5 and 6, respectively. The methylene amide linker of 4 was installed by a two-step procedure, which involved acylation of a C-6 amino function of a LacNAc derivative with chloroacetic anhydride followed by macrocyclization by nucleophilic displacement of the chloride by a C-2' hydroxyl. The conformational properties of 4 were determined by a combination of NOE and trans-glycosidic heteronuclear coupling constant measurements and molecular mechanics simulations and these studies established that the glycosidic linkage of 4 is conformationally constrained and resides in only one of the several energy minima accessible to LacNAc. The apparent kinetic parameters of transfer to LacNAc and conformationally constrained saccharides 3 and 4 indicates that fucosyltransferase V recognize LacNAc in its A-conformer whereas alpha-2,6-sialyltransferase recognizes the B-conformer of LacNAc.     

Stereoselective synthesis of conformationally constrained (2S,3S)-3-hydroxyornithine

A convenient and efficient route is described for the highly stereoselective synthesis of δ-amino protected and conformationally restricted (2S,3S)-3-hydroxyornithine through the N-benzylnitrone adduct to the α,β-unsaturated bicyclic lactam 2 derived from (S)-pyroglutaminol.     

Synthesis of jasplakinolide analogues containing a novel omega-amino acid

The synthesis of the omega-amino acid 4 is described utilizing a two-dimensional synthesis strategy combined with an enzymatic differentiation of homotopic ester groups. The amino acid 4 features two non-bonded interactions that result in conformational constraints on a cyclic construct. This amino acid was incorporated into the four macrolactams 17, 22, 31, and 37. The ring in 17 and 22 is 18-membered, whereas 31 and 37 have a 19-membered ring. The pairs with the same ring size differ in a N-methyl group. For the larger macrolactams (31 and 37) conformational analysis showed that the macrocyclic rings are somewhat more rigid than in the natural lead, the depsipeptide jasplakinolide. Nevertheless, their conformations are comparable to the natural product. There are no intramolecular hydrogen bonds, neither is the cis-rotamer populated in the N-methyl compound 37. Due to the increased flexibility of the smaller macrolactams 17 and 22 and signal overlap, a distinct solution structure could not be obtained for these compounds. The amino acid 4 should be useful for restricting the conformation of other small peptides.     

Synthesis and chromatographic resolution of conformationally constrained analogues of homotaurine

The first series of conformationally constrained analogues of homotaurine is reported. The partial constriction of the skeleton was realized through the insertion of a cyclopropyl ring, between the α,β- and β,γ-positions, thus affording, respectively, trans- and cis-2-aminomethylcyclopropane-1-sulfonic acids and trans- and cis-(2-aminocyclopropyl)methanesulfonic acids. The resolution of all four racemic mixtures was accomplished using HPLC system carrying the polysaccharide-based Chiralpak^® IB^® column as the chiral stationary phase. The coupling with an ‘Evaporative Light Scattering Detector (ELSD)’ has been particularly valuable during the chromatographic study.     

Synthesis of 5-substituted pipecolic acid derivatives as new conformationally constrained ornithine and arginine analogues

Two, orthogonally protected, constrained analogues of arginine have been synthesised in a diastereodivergent manner. The key step involved an electrophilic or radical functionalisation of methyl N-Boc-5,6-dehydropipecolate.     

Design and synthesis of conformationally constrained glycosylated amino acids

To probe the conformational requirements of O-linked glycoproteins for binding to various enzymes and receptors, two conformationally constrained glycosylated amino acids, 2 and 3, were designed. The analogues were found to represent two potential low energy conformations of the parent conjugate, 1, by molecular modeling. A convergent synthesis of both 2 and 3 from D-galactose and L-methionine is presented.     

Total synthesis of a conformationally constrained didemnin B analog

The total synthesis of a didemnin B analogue containing a conformationally constrained replacement for the isostatine moiety is reported. Synthetic highlights include an improved preparation of 2-hydroxy-3-cyclohexenecarboxylic acid and a new strategy for accessing the macrocycle.     

Enantioselective synthesis of constrained phenylalanine analogues

Constrained phenylalanine derivatives containing hydrophobic groups and hydrogen bond acceptor and/or donor functionalities were synthesized through a tandem palladium-mediated Heck reaction followed by a rhodium(II)-catalyzed asymmetric hydrogenation. Aryl bromides were found to be better substrates in providing products with higher purity and in good yield. The cesium carbonate-mediated cyclization proceeded smoothly in good yield and optical purity. Aryl iodides reacted selectively over bromides under Jeffery-type conditions (Pd(OAc)₂, Bu₄NCl, Et₃N) providing an opportunity for further metal-mediated functionalization.     

Synthesis of two conformationally constrained analogues of the minor tobacco alkaloid anabasine

The anabasine analogues spiro[4-azaindan-1,2'-piperidine] (7) and spiro[6-azaindan-1,2'-piperidine] (8) have been prepared. A series of palladium-catalyzed reactions, where an intramolecular cyclization constituted a key reaction, were utilized for the preparation of the two target compounds.     

Stereoselective synthesis of multisubstituted alkenes via conformationally labile alkenyllithium species

Stereoselective synthesis of tri- and tetrasubstituted alkenylsilanes has been realized by the selective intramolecular silicon migration in the rapidly equilibrating alkenyllithium species. Subsequent copper- and palladium-mediated coupling with allyl and aryl halides provides tri- and tetrasubstituted alkenes possessing all different carbon-substituents with complete stereoselectivity. [structure: see text]