TCP Building Blocks for Oligosaccharide Synthesis: Progress Towards the Synthesis of Nodulation Factors

Abstract

The ability of tetrachlorphthaloyl (TCP) sugars to act as glycosyl acceptors as well as the viability of TCP as a global amine protecting group in the syntheis of polyglucosamine natural products such as N-methyl-N-lipid nodulation factors have been examined. Disaccharides corresponding to the reducing end segments and the core region of the target nodulation factors were assembled from n-pentenlyl glycosides. TCP acceptors were successfully coupled with a variety of pentenyl glycosyl donors to produce β-(1→4) oligosaccharides in good yields. Model coupling reactions to produce trisaccharides provided clear evidence for the disarming effect of an ester at O3 on a C4-OH in the glycosyl acceptor. Also, a unique pentenyl donor, which contained the desired N-metyl-N-lipid moiety of the non-reducing end segments of the target compounds, was synthesized and its efficacy in a coupling reaction was tested.     

Investigations on intramolecular 1,3-dipolar cycloadditions for the synthesis of isoxazolo-annulated pyrrolidines,piperidines and azepanes

Pyrrolidines, piperidines, and azepanes with annulated isoxazole, isoxazoline or isoxazolidine rings were prepared by intramolecular 1,3-dipolar cycloadditions of nitrones or nitrile oxides. The corresponding 1,3-dipoles were obtained from N-Boc-protected and N-allyl-, N-propargyl- or N-(3-butenyl)-substituted 2-aminoethanal or 3-aminopropanal.     

Synthesis of N-modified ganglioside GM3 derivatives

Ganglioside GM3 and its derivatives have many important biological functions. Using diethyl phosphite protected sialic acid as glycosyl donor and 3,2′,3′,4′-unprotected lactose as glycosyl acceptor, the sialic acid-containing trisaccharide was assembled with excellent anomeric stereoselectivity. The trisaccharide was further coupled with ceramide precursor to yield sphingosine 1. Based on this key intermediate, two different series of N-modified GM3 analogues with modifications on either the nitrogen of sialic acid residue or the nitrogen of ceramide moiety and GM3 itself were synthesized smoothly.     

N-Boc-Protected α-Amino Acids by 1,3-Migratory Nitrene C(sp3)−H Insertion

N-Boc-protected α-amino acids are synthesized in two steps from linear or branched carboxylic acid feedstocks. In the first step, the carboxylic acid is coupled with tert-butyl aminocarbonate (BocNHOH) to generate azanyl ester (acyloxycarbamate) RCO₂NHBoc. In the second step, this azanyl ester undergoes a stereocontrolled iron-catalyzed 1,3-nitrogen migration to generate the N-Boc-protected non-racemic α-amino acid. This straightforward protocol is applicable to the catalytic asymmetric synthesis of α-monosubstituted α-amino acids with aryl, alkenyl, and alkyl side chains. Furthermore, α,α-disubstituted α-amino acids are accessible in an enantioconvergent fashion from racemic carboxylic acids. The new method is also advantageous for the synthesis of α-deuterated α-amino acids. N-Boc-protected α-amino acids synthesized using this two-step protocol are ready-to-use building blocks.     

Highly enantioselective fluorescent recognition of amino acid derivatives by unsymmetrical salan sensors

Novel unsymmetrical salan fluorescent sensors 2a and 2b have been designed and synthesized. The chiral recognition of N-Boc-protected amino acids by 2a and 2b has been investigated. Sensor 2a possesses higher sensitivity and enantioselectivity than sensor 2b does. Job analysis and nonlinear regression results show that 2a can form a 1:1 stoichiometric complex with a N-Boc-protected amino acid. The obtained response selectivities and the association constants indicate that 2a is a highly enantioselective and sensitive fluorescent sensor toward N-Boc-protected amino acids.     

Collision-induced dissociation of alkali metal cationized and permethylated oligosaccharides: Influence of the collision energy and of the collision gas for the assignment of linkage position

Tandem mass spectrometry has been used to study the collision-induced decomposition of [M+Na]⁺ ions of permethylated oligosaccharides. It is shown that many linkage positions in one compound may be determined by the presence or absence, in a single spectrum, of specific fragment ions that arise from the cleavage of two ring bonds and that the yield of such ions depends strongly on the collision energy and nature of the collision gas. In contrast to the behavior of monolithiated native oligosaccharides, the collision-induced decomposition of the sodiated and permethylated oligosaccharide samples at low energy leads to preferential cleavage of glycosidic linkages. At high collision energies, the fragment ions formed by cleavage of more than one bond are greatly enhanced, especially when helium is replaced by argon as the collision gas. Furthermore, argon is the more efficient collision gas in inducing fragmentation of the precursor ions. As an example of the application of this method, the discrimination between the 1 → 3 and 1 → 6-linked mannose residues in the common core of N-glycans is described.     

Phosphorylation of betti base with diethyl chlorophosphate

Phosphoric acid derivatives containing chiral Betti base fragment were synthesized by reacting racemic and enantiopure N-Boc-protected 1-(α-aminobenzyl)-2-naphthols with diethyl chlorophosphate followed by deprotection.     

A concise synthesis of asymmetrical N,N′-disubstituted guanidines

We present a new and concise method for the preparation of asymmetrical N,N′-disubstituted guanidines starting from thiourea via the reaction of N-Boc-protected N′-alkyl/aryl substituted thioureas with an amine in the presence of mercury(II) chloride and triethylamine.     

In vitro antibacterial and antifungal assay of poly‐(ethylene oxamide‐N,N′‐diacetate) and its polymer–metal complexes

A new polyester, poly‐(ethylene oxamide‐N,N′‐diacetate) (PEODA), containing glycine moiety was synthesized by the reaction of oxamide‐N,N′‐diacetic acid and ethylene glycol and its polymer–metal complexes were synthesized with transition metal ions. The monomer oxamide‐N,N′‐diacetic acid was prepared by the reaction of glycine and diethyl oxalate. The polymer and its metal complexes were characterized by elemental analysis and other spectroscopic techniques. The in vitro antibacterial activities of all the synthesized polymers were investigated against some bacteria and fungi. The analytical data revealed that the coordination polymers of Mn(II), Co(II) and Ni(II) are coordinated with two water molecules, which are further supported by FTIR spectra and TGA data. The polymer–metal complexes showed excellent antibacterial activities against both types of microorganisms; the polymeric ligand was also found to be effective but less so than the polymer–metal complexes. On the basis of the antimicrobial behavior, these polymers may be used as antifungal and antifouling coating materials in fields like life‐saving medical devices and the bottoms of ships. Copyright © 2007 John Wiley & Sons, Ltd.     

Synthesis, characterisation and evaluation of poly[lactose acrylate-N-vinyl-2-pyrrolidinone] hydrogels for drug delivery

New biodegradable polymeric hydrogels based on biocompatible materials, lactose acrylate (LA) and N-vinyl-2-pyrrolidinone were designed and synthesized. LA was synthesized and characterized by Fourier transform infrared spectroscopy and proton nuclear magnetic resonance spectroscopy. Hydrogel synthesis was carried out by free-radical polymerization of the co-monomers using azobisisobutyronitrile as initiator and N,N^′-methylenebisacrylamide as crosslinker. These hydrogels were also characterized. Equilibrium swelling of the hydrogels was studied in phosphate buffer of physiological pH, 7.4 and at 37 °C. Propranolol hydrochloride was entrapped into these hydrogels and the in vitro release profile of this drug was established in phosphate buffer. The drug release followed a near zero-order fashion in the first 6 h and thereafter slowed down releasing more than 90% of the entrapped drug at the end of 48 h.     

Propargyl mediated intramolecular aglycon delivery (IAD): applications to the synthesis of core N-glycan oligosaccharides

The use of propargyl mediated intramolecular aglycon delivery (IAD) for the synthesis of the key Manβ(1→4)GlcNAc linkage of N-glycan oligosaccharides, including the core N-glycan pentasaccharide, is investigated. Isomerisation of a 2-O-progargyl group of manno thioglycoside donors to an allene is followed by iodonium ion mediated mixed acetal formation with the 4-OH of protected GlcNAc acceptors, and subsequent intramolecular glycosylation occurs with complete control of anomeric stereochemistry to form the Manβ(1→4)GlcNAc linkage. A variety of linear and convergent approaches (1+2, 3+1, 3+2) to the core pentasaccharide are investigated as means of probing the generality and limitations of this type of intramolecular aglycon delivery for the formation of β-mannoside linkages in complex oligosaccharides.     

Spirocyclic azetidines for drug discovery: Novel Boc-protected 7'H-spiro[azetidine-3,5'-furo[3,4-d ]pyrimidines]

A novel spirocyclic scaffold of 7'H-spiro[azetidine-3,5'-furo[3,4-d]pyrimidine] chemotype was synthesized in N-Boc-protected form. However, the scaffold was revealed to be unstable to storage when deprotected. The solution was found in the brief removal of the Boc protecting group and rapid acylation of the liberated NH-azetidine with a carboxylic acid imidazolide.     

Synthesis of three regioisomers of the pentasaccharide part of the Skp1 glycoprotein of Dictyostelium discoideum

Three regioisomers of the linear pentasaccharide part of the Skp1 glycoprotein, found in Dictyostelium discoideum, were prepared in the form of (2-trimethylsilyl)ethyl glycosides by means of 2+3 block syntheses using the disaccharide donor at the non-reducing end, and three different trisaccharide acceptors at the reducing end. Fucosylation of (2-trimethylsilyl)ethyl 3,4,6-tri-O-benzyl-β-d-galactopyranosyl-(1→3)-4,6-O-benzylidene-2-deoxy-2-NPhth-β-d-glucopyranoside with different fucosyl donors carrying an O-(2-naphthyl)methyl ether as a temporary-protecting group at positions C2, C3 or C4 gave rise to the protected core trisaccharides. After selective removal of the (2-naphthyl)methyl group, the resulting acceptors were glycosylated with the α(1→6) linked digalactosyl donor to yield the respective three regioisomeric pentasaccharides. Transformation of the phthalimido moiety into an N-acetyl group, followed by catalytic hydrogenation of the reducible-protecting groups furnished the free target pentasaccharides, which should be able to assist during the elucidation of the exact structure of the natural pentasaccharide.     

γ-Fluorophenyl-GABA derivatives from fluorobenzonitriles in high diastereomeric and enantiomeric excess

An enantioselective synthesis of α-fluoroaryl homoallylic amines in 52-71% yields and 76-93% enantioselectivities has been achieved via the allylboration of the corresponding fluorinated N-aluminobenzaldimines with B-allyldiisopinocampheylborane in the presence of methanol, followed by alkaline hydrogen peroxide workup. Crotylboration of these aluminobenzaldimines with potassium B-methoxy B-E- or -Z-crotyldiisopinocampheylborinate provided the corresponding β-anti- or -syn-methyl α-fluoroarylhomoallylamines, respectively in high de and ee. Similarly, alkoxyallylboration with lithium B-methoxy B-γ-OMEMallyldiisopinocampheylborinate provided the corresponding β-syn-alkoxyhomoallylamines in excellent de and ee. Representatives of these amino alkenes were converted to the corresponding optically active N-Boc-protected fluorinated amino alcohols via hydroboration-oxidation. Further chromium-mediated oxidation provided N-Boc-protected γ-fluorophenyl-γ-aminobutyric acids, which upon deprotection provided the corresponding γ-lactams.     

Synthesis of oligosaccharide fragments of the glycosylinositolphospholipid of Trypanosoma cruzi: a new selenoglycoside glycosyl donor for the preparation of 4-thiogalactofuranosyl analogues

A new selenoglycoside, phenyl 2,3,5,6-tetra-O-acetyl-4-thio-1-selenogalactofuranose, has been synthesized. This 4-thiogalactofuranosyl donor was used in the syntheses of heteroatom analogues of the di-, tri-, and tetrasaccharides corresponding to the oligosaccharide β-d-Galf-(1→3)-α-d-Manp-(1→2)-(β-d-Galf-(1→3))-α-d-Manp. These compounds represent fragments of the terminal end of the glycosylinositolphospholipid oligosaccharide found in the protozoan Trypanosoma cruzi, the causative agent of Chagas disease, and are intended for use as inhibitors of the enzymes that construct the native oligosaccharides. The syntheses employed the selective activation of a phenyl 4-thio-1-selenogalactofuranoside glycosyl donor over ethyl 1-thioglycoside glycosyl acceptors with NIS/TfOH.     

Straightforward, racemization-free synthesis of peptides with fairly to very bulky di- and trisubstituted glycines

Several fully protected tri- and pentapeptides containing a central symmetrical α,α-dialkyl glycine residue, with the alkyl group varying from methyl or ethyl to benzyl, were synthesized in good yields by a strategy based on the Ugi-Passerini reaction. Each Ugi-Passerini adduct was selectively cleaved and the product submitted to an assisted N,N′-dicyclohehylcarbodiimide coupling to an amino acid or dipeptide ester, respectively. Tripeptides as the above but containing a 4-methoxybenzyl group at the nitrogen atom of the central residue were also synthesized in fair to good yields by N-[(1H-benzotriazol-1-yl)-(dimethylamino)methylene]-N-methylmethanaminium hexafluorophosphate N-oxide assisted couplings. The results reported here show that our strategy is appropriate for routine synthesis of peptides incorporating these moieties.     

Conformational analysis of some mannose-containing and milk oligosaccharides: Correlation of their shape with inhibitory properties in antigen/antibody interactions

Possible conformations of two mannotetraoses and several milk oligosaccharides have been studied by an energy minimization procedure using semi-empirical potential functions. Changes in the terminal residue at the reducing end (cyclic to acyclic form) of these molecules do not affect the favored conformations of the remaining oligosaccharide moiety. However, differences in the overall shape of the native and reduced forms of the mannotetraose, mannose α(1–3) mannose α(1–2) mannose α(1–2) mannose are much less marked than between the native and reduced forms of lacto-N-tetraose. These differences are related to the effectiveness of the native forms as inhibitors of antibodies produced using synthetic antigens. Changes in the linkage of the residues at the nonreducing end of these molecules affect significantly the overall shape of these molecules. These differences also are related to their effectiveness as inhibitors. In the fucose-containing milk oligosaccharides the additional fucose residues only restrict the orientation of the backbone tetrasaccharide and do not push it into a totally new conformation. The fucose residues come on a surface of the molecule which is away from the region which may be important for binding. The present studies show that it is the overall shape of the molecule which is important in determining its inhibitory properties and give information as to how best to use the immune method for identification of unknown oligosaccharide sequences by specially prepared antibodies.     

Facile direct synthesis of unsymmetrical ureas from N-Alloc-, N-Cbz-, and N-Boc-protected amines using DABAL-Me3

A practical synthetic method for the direct synthesis of unsymmetrically substituted ureas from N-Alloc-, N-Cbz-, and N-Boc-protected amines is described. In this study, efficient direct conversion of the Alloc-, Cbz-, and Boc-carbamate compounds to ureas was achieved in the presence of DABAL-Me₃, an air stable and easily handled reagent. Using this reaction method, both protected aromatic and aliphatic amines were successfully transformed into various trisubstituted and tetrasubstituted ureas with high yields without side product. Our findings offer promising guidelines for direct preparation of useful ureas from N-Alloc-, N-Cbz-, and N-Boc-carbamates.     

Synthesis and conformational analysis of 9,10-bis-aminomethyl-11,12-dicarboxy-dibenzobarrelene derivatives

The synthesis of bis-γ-amino acid dibenzobarrelene derivatives (9,10-bis-aminomethyl-11,12-bis-carboxy-dibenzobarrelene) is presented. Bromomethylation of anthracene followed by azide substitution gave 9,10-bis-azidomethylanthracene. Azide reduction, N-Boc protection, and Diels-Alder cycloaddition in DMAD furnished the protected 9,10-bi-aminomethyl-11,12-dicarboxy-dibenzobarrelene derivative, which was further converted into the bis-γ-amino acid methyl ester, the N-Boc-protected bis-γ-amino methyl amide, and a bis-γ-lactam. Monte Carlo simulations and X-ray analysis of the 9,10-substituted dibenzobarrelenes revealed an exposed hydrophobic surface surrounded by amino and carboxy groups.     

An efficient green synthesis of proline-based cyclic dipeptides under water-mediated catalyst-free conditions

l-Proline-based cyclic dipeptides were synthesized from N-Boc-protected dipeptide methyl esters under catalyst-free condition using water as a solvent. One-pot deprotection and cyclization have been used as the key steps, providing an efficient and environmentally friendly approach. Clean reaction conditions, easy isolation, and good yields of cyclic dipeptides are the salient features of the proposed methodology.