The development of N-aryl trifluoroacetimidate-based benzyl and allyl protecting group reagents

Comparison of disaccharide donors for heparan sulfate synthesis: uronic acids vs. their pyranose equivalents

Late oxidation of hexose based building blocks or the use of uronic acid containing building blocks are two complementary strategies in the synthesis of glycosaminoglycans, the latter simplifiying the later stages of the process. Here we report the synthesis and evaluation of various disaccharide donors-uronic acids and their pyranose equivalents-for the synthesis of heparan sulfate, using an established protective group strategy. Hexose based "imidate" type donors perform well in the studied glycosylations, while their corresponding uronate esters fall short; a uronate ester thioglycoside performs equal to, if not better than, a hexose thioglycoside equivalent.

α-Selective glycosylations using glycosyl N-(ortho-methoxyphenyl)trifluoroacetimidates

Six N-(o-methoxyphenyl)trifluoroacetimidate glycosyl donors have been synthesized and their role as glycosyl donors has been investigated. The donors were synthesized with complete β-selectivity, except in one case, and were found to be stable. When Bi(OTf)3, Fe(OTf)2, and Zn(OTf)2 were employed as catalysts, the glycosylations were found to be highly α-selective in Et2O. The selectivity and reaction rate changed with a change in the acceptor reactivity.

Trifluoroacetimidoyl halides: a potent synthetic origin

In the review, recent advances in the synthetic applications of trifluoroacetimidoyl halides have been summarized.

Glycosyl nitrates in synthesis: streamlined access to glucopyranose building blocks differentiated at C-2

In an attempt to refine a CAN-mediated synthesis of 1,3,4,6-tetra-O-acetyl-α-d-glucopyranose (2-OH glucose) we unexpectedly discovered that this reaction proceeds via the intermediacy of glycosyl nitrates. Improved mechanistic understanding of this reaction led to the development of a more versatile synthesis of 2-OH glucose from a variety of precursors. Also demonstrated is the conversion of 2-OH glucose into a variety of building blocks differentially protected at C-2, a position that is generally hard to protect regioselectively in the glucopyranose series.

Study of O-Allylation Using Triazine-Based Reagents

Acid-catalyzed allylating reagent 2,4,6-tris(allyloxy)-1,3,5-triazine (TriAT-allyl) and its substituted derivatives have been developed. The reaction of acid-, and alkali-labile alcohols with these reagents in the presence of a catalytic amount of trifluoromethanesulfonic acid (TfOH) afforded the corresponding allyl ethers in good yields. Reactions using these reagents with an unsymmetrically-substituted regioisometric allyl group suggested that a single isometric allylic cation species would be involved.

Removal of some common glycosylation by-products during reaction work-up

With the aim of improving the general glycosylation protocol to facilitate easy product isolation it was shown that amide by-products from glycosylation with trichloroacetimidate and N-phenyl trifluoroacetimidate donors could be removed during reaction work-up by washing with a basic aqueous solution. Excess glycosyl acceptor or lactol originating from glycosyl donor hydrolysis could equally be removed from the reaction mixture by derivatization with a basic tag and washing with an acidic solution during reaction work-up.

Formation of DPM ethers using O-diphenylmethyl trichloroacetimidate under thermal conditions

Alcohols are effectively converted to their corresponding diphenylmethyl (DPM) ethers by reaction with O-diphenylmethyl trichloroacetimidate in refluxing toluene without the requirement of a catalyst or other additives. A number of acid and base sensitive substrates were protected in excellent yield using this new method without disturbing the pre-existing functionality present in these molecules. This reaction is the first example of the formation of an ether from stoichiometric amounts of a trichloroacetimidate and an alcohol without the addition of a Brønsted or Lewis acid catalyst.

Regioselective Benzylation of 2-Deoxy-2-Aminosugars Using Crown Ethers: Application to a Shortened Synthesis of Hyaluronic Acid Oligomers

The combination of benzyl bromide, sodium hydroxide and 15-crown-5 in tetrahydrofuran is shown to be an efficient method for installing benzyl groups at both the 4- and 6-positions regioselectively directly from peracetylated N-trichloroacetyl-protected glucosamine and galactosamine. Application of this benzylation strategy proved to significantly shorten the synthetic route to hyaluronic acid tetra- and hexamers.