Synthesis of 3-Deoxy-d-threopentofuranose 5-Phosphate,a Substrate of Arabinose 5-Phosphate Isomerase

3-Deoxy-d-threopentofuranose 5-phosphate, a substrate of arabinose 5-phosphate isomerase, has been synthesised starting from d-arabinose. Selective protection of the hydroxyl groups at C-1, C-2, and C-5 allowed deoxygenation of position 3 by conversion into a thiocarbamate and radical reduction. Deprotection and phosphorylation of the primary hydroxyl group and final deprotection of the other hydroxyl groups afforded the desired compound.     

A Simple Synthesis Route for Selectively Methylated β-Cyclodextrin Using a Copper Complex Sandwich Protecting Strategy

This communication reports a novel synthesis route for the preparation of monofunctionalized β-cyclodextrin in a single stage. The approach involves only the in-situ protection of secondary hydroxyl groups as an excellent alternative to the classical procedure involving a series of five steps of protection and deprotection of hydroxyl groups (both primary and secondary ones) belonging to β-cyclodextrin.     

Biodegradable brushlike graft polymers. I. Polymerization of ϵ‐caprolactone onto water‐soluble hydroxypropyl cellulose as the backbone by the protection of the trimethylsilyl group

After the protection of partial hydroxyl groups with trimethylsilyl (TMS) groups, hydroxypropyl cellulose becomes soluble in organic solvents, and the number of hydroxyl groups as initiating groups can be controlled. As a result, a new brushlike graft poly(?‐caprolactone) can be prepared with hydroxypropyl cellulose as the backbone polymer by homogeneous ring‐opening graft polymerization and deprotection. The protection and deprotection of the TMS group during the entire procedure were carefully monitored with Fourier transform infrared (FTIR) and NMR, and the final graft copolymers were characterized with FTIR, ¹H NMR, ¹³C NMR, gel permeation chromatography, and differential scanning calorimetry. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 273–280, 2003     

Self‐Catalysis of Phthaloylchitosan for Graft Copolymerization of ε‐Caprolactone with Chitosan

Summary: A protection‐graft‐deprotection method was developed to prepare chitosan‐g‐polycaprolactone graft copolymers, during which the ring‐opening copolymerization of ε‐caprolactone onto phthaloylchitosan (PHCS) happened without any additional catalysis. The intermediate PHCS was introduced primarily to protect the active amino group of chitosan. After controlled experiments, the phthalimido compound was proposed to be a novel kind of organic catalyst for the ring‐opening polymerization of caprolactone monomers, while the hydroxyl group acted as an initiator. Hence, in this graft system, PHCS was endowed with both self‐catalysis and self‐initiation at the same time, and the PCL side chains grew from the hydroxyl groups of the chitosan backbone.

     

Reactions at Only One of Two Equivalent Functional Groups

A common problem encountered by synthetic chemists is to carry out a reaction at just one of two similar functional groups in the substrate molecule. Examples include protection/deprotection of just one hydroxyl group in a diol, addition to only one carbonyl of a diketone, etc. The problem, as depicted below, is to find conditions which maximize formation of the monoproduct (XP) while minimizing the amounts of diproduct (P₂) and starting material (X₂).     

HPLC Detection of Sulphated Carbohydrates with a New Detection Method,that is Based on Measurement of Optical Activity by a Polarized Laser (ChiramonitorR)

ABSTRACT

The synthesis of sulphated pentasaccharides with heparin-like activity^1,2,3 proceeds through many chemical steps of protection and deprotection. As long as the pentasaccharide is fully protected with hydroxyl protecting groups such as benzylic and ester groups, the purity of intermediates can be determined by TLC. During deprotection and sulphation procedures, this determination of the purity with TLC becomes more difficult and reversed phase HPLC turns out to be a better chromatographic technique. The purification and purity control of the deprotected and sulphated pentasaccharide I (ORG31540/CY234) is performed on ion-exchange columns with a NaCl gradient as a mobile phase and UV-detection at 215 nm.     

Deprotection of durable benzenesulfonyl protection for phenols — efficient synthesis of polyphenols

A robust protection method for phenol was demonstrated by the use of durable benzenesulfonyl group, which survives various harsh reaction conditions using Grignard reagent, organolithium reagent, metal alkoxide, phosgene, mineral, and Lewis acids. A facile deprotection condition utilizing pulverized KOH (5 equiv) and t-BuOH (10 equiv) in hot toluene makes this protocol as a practical method, which can be applied to the multistep synthesis of biologically and medicinally important polyphenol compounds.     

Preparation of a set of selectively protected disaccharides for modular synthesis of heparan sulfate fragments: toward the synthesis of several <Emphasis Type="Italic">O</Emphasis>-sulfonated [β-D-GlcUA-(1→4)-β-D-GlcNAc]OPr types

A concise method for a stereocontrolled synthesis of a set of selectively protected disaccharides is reported. Coupling of the donor 11 onto acceptors 23 and 24, promoted by trimethylsilyl triflate-N-iodosuccinimide (TMSOTf-NIS), generated the disaccharides 25 and 26. Under typical conditions, condensation of the fully protected donor 12 onto acceptors 23 and 24 produced the disaccharides 27 and 28. The building blocks 25–28 were prepared in moderate yields having exclusive β-stereoselectivity. A unique pattern of protecting groups distinguished clearly between positions to be sulfated and functional groups remaining as free hydroxyl groups. Acetyl and/or levulinoyl esters temporarily protected the positions to be sulfated, while benzyl ethers were used for permanent protection. The anomeric positions were protected as allyl ethers, whereas the 4′-positions were masked as p-methoxybenzyl (PMB) ethers. The orthogonality of the PMB and allyl groups can then be used for further elongation of the chain by recurrent deprotection and activation steps. The hydroxyl group, OH-6, of glucosamine moieties was protected as a TBDPS ether to avoid oxidation. A five-step deprotection/sulfonation sequence was applied to the disaccharide 27 to generate the corresponding sulfated [β-D-GlcUA-2-OSO₃Na-(1→4)-β-D-Glc pNAc]-(1→O-Pro) 34.     

Cyclodextrin chemistry. Selective modification of all primary hydroxyl groups of α- and β-cyclodextrins

Two efficient methods are described for the selective modification of all six primary hydroxyl groups of α-cyclodextrin (α-CD, 1 1 ). One, using an indirect strategy, involves protection of all 18 hydroxyl functions as benzoate esters, followed by selective deprotection of the six primary alcohol groups. The other, using a direct strategy, involves selective activation of the primary hydroxyl groups via a bulky triphenylphosphonium salt, which is then substituted by azide anion as the reaction proceeds. A number of modified α-cyclodextrin derivatives have been prepared and fully characterized, among which are: the useful intermediate α-cyclodextrin-dodeca (2, 3) benzoate ( 3 ); hexakis (6-amino-6-deoxy)-α-cyclodextrin hexahydrochloride ( 7 ); hexakis (6-amino-6-deoxy)-dodeca (2, 3)-O-methyl-α-cyclodextrin hexahydrochloride ( 9 ), hexa (6)-O-methyl-α-cyclodextrin ( 13 ). The direct substitution is shown to be even more efficient for β-cyclodextrin ( 16 ), giving the heptakis (6-azido-6-deoxy)-β-CD-tetradeca (2, 3)acetate ( 17 ), while the indirect strategy fails. The compounds are characterized by extensive use of ¹³C- and ¹H-NMR. spectroscopy. The steric and statistical problems of selective polysubstitution reactions for the cyclodextrins are discussed, and possible reasons for the observed differences in reactivity between α- and β-cyclodextrins are examined. The dodecabenzoate 3 presents a very marked solvent effect on physical properties (IR. and NMR. spectra, optical rotation); the effects observed may be ascribed to an unusually strong intramolecular network of hydrogen bonds which severely distorts the α-cyclodextrin ring and lowers the symmetry from six-fold to three-fold.     

(S)-Mandelic acid enolate as a chiral benzoyl anion equivalent for the enantioselective synthesis of non-symmetrically substituted benzoins

A strategy for the enantioselective synthesis of non-symmetrically substituted benzoins from (S)-mandelic acid and aromatic aldehydes has been developed. This strategy is based on a diastereoselective aldol reaction of the lithium enolate of the 1,3-dioxolan-4-one derived from (S)-mandelic acid and pivalaldehyde with aromatic aldehydes, which gives the corresponding aldols in good yields. Subsequent hydroxyl group protection as MEM ethers, basic hydrolysis of the dioxolanone ring, oxidative decarboxylation of the α-hydroxy acid moiety, and hydroxyl group deprotection provides chiral non-symmetrically substituted benzoins with high enantiomeric excesses.     

Microwave-Assisted Protocols Applied to the Synthesis of 1′,2,3,3′,4,4′-Hexa-O-benzylsucrose

The sucrose derivative 1′,2,3,3′,4,4′-hexa-O-benzylsucrose is a key intermediate for the chemoselective synthesis of various useful materials, such as macrocycles, crown ether analogs, and polymers. Several strategies for the synthesis of this compound were explored by applying microwave-assisted protocols, thus permitting significant reductions of time and energy compared to other routes. The outcomes of the different approaches were compared and the optimal one, in terms of yield and reproducibility, was found to be the initial protection at the positions 6 and 6′, with tert-butyldiphenylsilylchloride (TBDPSCl) in the presence of 4-(dimethylamino)pyridine (4-DMAP) and pyridine as a solvent, then perbenzylation of the remaining hydroxyl groups, followed by selective deprotection of the TBDPS groups to obtain the title compound.     

Synthesis and characterization ofα-butyl-ω-{3-[(2,2-dihydroxymethyl)-propionyloxy]}propylpolydimethylsiloxanes

A series of polydimethylsiloxanes containing two primary hydroxyl groups at one single chain end were synthesized by five-step reactions which included esterification, hydroxyl protection, anionic ring-opening polymerization, hydrosilylation and deprotection. The prepared compounds in each step were characterized. The results showed that each step synthesis was successfully carried out and objective products could be achieved.     

Synthesis of non-hydrolyzable substrate analogs for Asp-tRNA/Glu-tRNA amidotransferase

Non-hydrolyzable substrate analogs for tRNA-dependent amidotransferase, 2′- or 3′-aspartyl or -glutamyl adenosine, were synthesized from adenosine without protection of the adenine base. The hydroxyl groups of adenosine were selectively protected, followed by a series of oxidation/reductions to alter the stereochemistry. DFT calculations revealed the driving forces for the ketone hydrate formation at C-2′, but not the C-3′ carbon during the oxidation step. Subsequently, triflation and azide replacement yielded azidoadenosines, which were coupled to protected amino acids after deprotection and reduction. After global deprotection, the target substrate analogs were obtained in 2–14% overall yields from adenosine.     

A New Synthetic Approach to Elvucitabine

A new synthetic approach to elvucitabine started from L-xylose via the reactions of 10 steps in an overall yield of 20% was developed. The key steps included trimethylsilyl trifluoromethanesulfonate(TMSOTf)-mediated stereocontrolled β-glycosidation and exquisite choice of chloroacetyl group for the protection of hydroxyl groups as well as the corresponding deprotection under notably mild conditions. The structure of elvucitabine, in particular, the stereochemistry thereof, was unambiguously determined by comparison of the physical properties, such as ¹H NMR data and the specific rotation, of the synthesized sample with those reported.     

DDQ mediated regiospecific protection of primary alcohol and deprotection under neutral conditions: Application of new p-methoxy benzyl-pixyl ether as reagent of choice for nucleoside protection

A simple and efficient protocol is described for regiosepecific protection of primary hydroxyl group both in nucleosides and other molecules with p-methoxy-benzyl 2,7-dimethyl pixylether (MBDPE) in presence of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). Furthermore, swift deprotection of 2, 7-dimethylpixyl (DMPx) is accomplished with DDQ in MeOH. Both procedures are successfully implemented on gram-scale synthesis of modified nucleosides. This protocol offers mild and neutral conditions for selective protection and deprotection of DMPx group while compatible in presence of other conventional protecting groups such as benzoyl, benzyl, THP, TBDPS and acetonide.     

Sequential SNAr Reaction/Suzuki–Miyaura Coupling/C−H Direct Arylations Approach for the Rapid Synthesis of Tetraaryl‐Substituted Pyrazoles

A rapid synthesis of 1,3,4,5‐tetraaryl‐substituted pyrazoles has been achieved through a sequence of S_NAr reaction/Suzuki–Miyaura coupling/Pd‐catalyzed direct arylations that used 3‐iodo‐1H‐pyrazole as a scaffold. Pyrazoles with four different aryl groups were synthesized in a straightforward manner with no extra synthetic steps, such as protection/deprotection or the introduction of activating/directing groups, using readily available substrates and reagents. The developed synthetic approach enabled the structurally diverse synthesis of multiaryl‐substituted pyrazoles without using a glovebox technique.     

Thermolytic carbonates for potential 5'-hydroxyl protection of deoxyribonucleosides

Thermolytic groups structurally related to well-studied heat-sensitive phosphate/thiophosphate protecting groups have been evaluated for 5'-hydroxyl protection of deoxyribonucleosides as carbonates and for potential use in solid-phase oligonucleotide synthesis. The spatial arrangement of selected functional groups forming an asymmetric nucleosidic 5'-O-carbonic acid ester has been designed to enable heat-induced cyclodecarbonation reactions, which would result in the release of carbon dioxide and the generation of a nucleosidic 5'-hydroxyl group. The nucleosidic 5'-O-carbonates 3-8, 10-15, and 19-21 were prepared and were isolated in yields ranging from 45 to 83%. Thermolytic deprotection of these carbonates is preferably performed in aqueous organic solvent at 90 degrees C under near neutral conditions. The rates of carbonate deprotection are dependent on the nucleophilicity of the functional group involved in the postulated cyclodecarbonation reaction and on solvent polarity. Deprotection kinetics increase according to the following order: 4 < 5 < 10 < 6 < 12 < 7 < 13 < 8 < 14 congruent with 19-21 and CCl4 < dioxane < MeCN < t-BuOH < MeCN:phosphate buffer (3:1 v/v, pH 7.0) < EtOH:phosphate buffer (1:1 v/v, pH 7.0). Complete thermolytic deprotection of carbonates 7, 8, 13, and 14 is achieved within 20 min to 2 h under optimal conditions in phosphate buffer-MeCN. The 2-(2-pyridyl)amino-1-phenylethyl and 2-[N-methyl-N-(2-pyridyl)]aminoethyl groups are particularly promising for 5'-hydroxyl protection of deoxyribonucleosides as thermolytic carbonates.     

Selective protecting group manipulations on the 1-deoxynojirimycin scaffold

Iminosugars are inhibitors of glycoprocessing and are of interest as scaffolds for medicinal chemistry, as their successful application as peptide mimetics has shown. The synthesis of novel peptidomimetics based on 1-deoxynojirimycin (DNJ) requires practical strategies that allow introduction of amino acid side chains or pharmacophore groups at each of its hydroxyl groups or to the nitrogen atom. This paper describes one approach towards achieving selective protection and deprotection at the hydroxyl and amino groups of DNJ and a novel synthesis of DNJ from l-sorbose is included.     

Combinatorial Synthesis of Deoxyhexasaccharides Related to the Landomycin A Sugar Moiety,Based on an Orthogonal Deprotection Strategy

In this report, we describe the stereoselective synthesis of a combinatorial library comprised of 16 deoxyhexasaccharides that are related to a landomycin A sugar moiety, based on an orthogonal deprotection strategy. The use of an olivosyl donor containing a benzyl ether at the C3 position and benzoyl ester at the C4 position, and the olivosyl donor, a naphthylmethyl ether, and a p‐nitrobenzylethyl or benzyl sulfonyl ester enabled the synthesis of a set of four diolivosyl units containing a hydroxyl group at the C3 or C4 position by a simple glycosylation and deprotection procedure. Using a phenylthio 2,3,6‐trideoxyglycoside, α‐selective glycosidation proceeded without anomerization of the 2,6‐dideoxy‐β‐glycosides. In addition, alkylhydroquinone and levulinoyl groups were found to be an effective set of orthogonal protecting groups for the anomeric position and a hydroxyl group. The coupling of all combinations of trisaccharide units in a β‐selective manner was accomplished by activation of the glycosyl imidate with I₂ and Et₃SiH. No cleavage of the acid‐labile 2,3,6‐trideoxyglycoside was observed under the conditions used for the reactions. Finally, all of the protected hexasaccharides were deprotected by hydrolysis of the esters, microwave (MW) assisted cleavage of the 2‐trimethylsilylethoxymethoxy (SEM) ether, and a Birch reduction.     

Tailor-made crosslinkers for high performance PUR coatings – hyperbranched polyisocyanates

The introduction of S elf- C rosslinking I socyanates ( SCI ) as AB₂-building blocks makes it possible to synthesize hydroxyl- or isocyanate-group terminated dendrimers. Furthermore, inherent reactivity differences of non equivalent NCO-groups of specific diisocyanates (such as isophorone diisocyanate [IPDI] or toluylene diisocyanate [TDI]) can be used to build up hyperbranched polyurethane structures in a one pot synthesis without the need of arduous protection/deprotection steps. This synthetic approach allows the construction of tailor-made hyperbranched molecular architectures which are end-functionalized with either hydroxyl or isocyanate groups. These products were then tested as crosslinkers in 2-component coating formulations where they displayed better hardness than any other aliphatic isocyanate raw material.