Regioselective de-O-benzylation of monosaccharides

Practical Synthesis from Streptomycin and Regioselective Partial Deprotections of (-)-(1R,2S,3R,4R,5S,6S)-1,3-Di(deamino)-1,3-diazido-2,5,6-tri-O-benzylstreptamine

We describe the gram-scale synthesis of (-)-(1R,2S,3R,4R,5S,6S)-1,3-di(diamino)-1,3-diazido-2,5,6-tri-O-benzylstreptamine from streptomycin by (i) hydrolysis of the two streptomycin guanidine residues, (ii) reprotection of the amines as azides, (iii) protection of all alcohols as benzyl ethers, and (iv) glycosidic bond cleavage with HCl in methanol. Protocols for regioselective monodebenzylation and regioselective reduction of a single azide in the product are also described, providing four optically pure building blocks for exploitation in novel aminoglycoside synthesis.

Tf2O- and Cu(OTf)2-Assisted Acylamination Reaction of Unactivated Alcohols with Nitriles: A One-Pot P(IV) Activation, Stereoretention in Cycloalkanols and Deprotection Approach

Described herein is a simple, novel, one-pot acylamination reaction of unactivated alcohols. This reaction employs the combination of PCl₃ and triflic anhydride (Tf₂O) or copper triflate Cu(OTf)₂, which serves as a source of P(IV)-activated complex for nitriles to react under the Ritter-type mechanism. The synthetic utility of Tf₂O-promoted reactions was demonstrated by its effectiveness to generate different acylaminated products. By employing Cu(OTf)₂, this method represents a rare example of α-selective acylamination reaction. With chiral cycloalkanols, using the Cu(OTf)₂-promoted procedure, acylaminated products are formed with complete retention of configuration. The synthetic utility of the copper-assisted reaction in acetonitrile was readily demonstrated as a mild deprotection strategy.

Streamlined access to carbohydrate building blocks: Methyl 2,4,6-tri-O-benzyl-α-d-glucopyranoside

Presented herein is an improved synthesis of a common 3-OH glycosyl acceptor. This compound is a building block that is routinely synthesized by many research groups to be used in glycosylation refinement studies. The only known direct synthesis by Koto lacks regioselectivity and relies on chromatography separation using hazardous solvents. Our improved synthetic approach relies on Koto's selective benzylation protocol, but it is followed by acylation-purification-deacylation sequence. Although this approach involves additional manipulations, it provides consistent results and is superior to other indirect strategies. Also obtained, albeit in minor quantities, is 4-OH acceptor, another common building block.

Systematic Study of Regioselective Reductive Ring-Opening Reactions of 4,6-O-Halobenzylidene Acetals of Glucopyranosides

Reductive openings of cyclic acetals are widely used in modern synthetic organic chemistry for the regioselective introduction of protecting groups. A systematic study was performed on the applicability and efficacy of various hydride donor and protic or Lewis acid reagent combinations in the reductive ring opening of glucosidic 4,6-halobenzylidene acetals bearing an ortho-, meta-, and para-chloro- or -bromo substituent on the benzene ring. Most of the reagent combinations tested cleaved the 4,6-O-halobenzylidene acetal rings at O4 or O6 efficiently and with the expected regioselectivity. The LiAlH₄-AlCl₃ and the BH₃·THF-TMSOTf combinations produced the 4-O-halobenzyl ether/6-OH products with complete regioselectivity and high yields. The use of Me₃N·BH₃-AlCl₃ reagent system in toluene was also effective in cleaving the acetal ring at O6 but was accompanied by Al-chelation-assisted debenzylation side reactions. The NaCNBH₃-HCl and the Et₃SiH-BF₃·Et₂O combinations were highly effective in yielding the 6-halobenzyl ether/4-OH derivatives. Et₃SiH, in combination with TfOH, produced the 6-O-ether/4-OH products in rapid reactions but also triggered silylation and reductive halobenzylation as secondary transformations. Reductive opening of the 1,3-dioxane ring of pyranosidic 4,6-O-halobenzylidene acetals by the proper reagent combination was found to be an efficient method for the regioselective introduction of versatile halobenzyl protecting groups onto the pyranose ring.

Synthesis of carbohydrate building blocks via regioselective uniform protection/deprotection strategies

Discussed herein is the synthesis of partially protected carbohydrates by manipulating only one type of a protecting group for a given substrate. The first focus of this review is the uniform protection of an unprotected starting material in a way that only one (or two) hydroxyl group remains unprotected. The second focus involves regioselective partial deprotection of uniformly protected compounds in a way that only one (or two) hydroxyl group becomes liberated.

MCM-41-supported phosphotungstic acid-catalyzed cleavage of C–O bond in allyl aryl ethers

A solid acid (PWA/MCM-41) efficiently catalyzed the deprotection of allyl group from the allyl aryl ethers of various types of substrates, in the presence of other oxygen protecting groups.

o-Xylylene protecting group in carbohydrate chemistry: application to the regioselective protection of a single vic-diol segment in cyclodextrins

A systematic study of the suitability of α,α'-dibromo-o-xylene as a reagent for cyclic o-xylylene protection of vic-diols in different monosaccharide substrates is reported. The installation of this protecting group, formally equivalent to a di-O-benzylation reaction, proceeds with good regioselectivity toward 1,2-trans-diequatorial diol systems in pyranose and furanose rings. Initially, the benzyl ether-type derivative of the more acidic hydroxyl is preferentially formed. Subsequent intramolecular etherification toward the equatorial-oriented vicinal OH is kinetically favored. The methodology has been implemented for the simultaneous protection of the secondary O-2 and O-3 positions of a single d-glucopyranosyl unit in cyclic oligosaccharides of the cyclodextrin (CD) family (cyclomaltohexa-, -hepta-, and -octaose; α, β, and γCD).

Highly efficient and mild method for regioselective de-O-benzylation of saccharides by Co2(CO)8-Et3SiH-CO reagent system

A highly efficient and mild method for the de-O-benzylation of protected saccharides was developed by transforming terminal benzyl ethers into silyl ethers using Co(2)(CO)(8)-Et(3)SiH under 1 atm of CO. The method was successfully used for the de-O-benzylation of perbenzylated monosaccharides with various anomeric protecting groups, as well as natural disaccharides and trisaccharides such as sucrose, raffinose, and melezitose in good yields (>80%).

Fe(0)-mediated synthesis of tri- and tetra-substituted olefins from carbonyls: an environmentally friendly alternative to Cr(II)

Fe(0) was investigated as a cost-effective, environmentally friendly alternative to Cr(II) for the olefination of carbonyls by activated polyhalides. In many instances, Fe(0) was equivalent or superior to Cr(II). Notably, Fe(0), but not Cr(II), proved compatible with a wide range of functionality, inter alia, unprotected phenol, aryl nitro, carboxylic acid, and alkyl nitrile. A surprising reversal of stereoselectivity for aldehydes versus ketones was observed using both metals. The resultant alpha-halo-alpha,beta-unsaturated or alpha,beta-unsaturated carboxylic acids, esters, and nitriles are common structural elements in numerous compounds of interest as well as key intermediates in the preparation of other functionality.

Alkylalanes and methyl furanosides: regioselective O-debenzylation or acetal cleavage

Perbenzylated methyl pentofuranosides were submitted to the action of three alkylalanes and regioselective debenzylation at O-2 of the four pentoses was observed when choosing the right match between anomeric configuration and aluminium reagent. Diisobutylalane (DIBAL-H) allowed an easy access to reduced open-chain compounds, whereas trimethylalane (TMAL) stereoselectively produced methylated open chain derivatives.

Regioselective debenzylation of C-glycosyl compounds by boron trichloride

Boron trichloride has been found to promote selective deprotection of 1,2- or 1,3-cis oriented secondary benzyl ethers of per-benzylated C-glycosyl derivatives. The reactivity towards BCl(3) follows the order: C-4>or=C-2>C-6>C-3 for C-glucopyranosyl derivatives and C-3>or=C-4>C-6>C-2 for C-galactopyranosyl derivatives. Preparatively useful selective debenzylation at secondary positions was possible after careful control of reaction conditions.