David Crich

The invention of new radical chain reactions. Part VIII. Radical chemistry of thiohydroxamic esters; A new method for the generation of carbon radicals from carboxylic acids

Abstract The aliphatic and alicyclic esters of N-hydroxypyridine-2-thione are readily reduced by tributylstannane in a radical chain reaction to furnish nor-alkanes.1 In the absence of the stannane a smooth decarboxylatlive rearrangement occurs to give 2-substituted thiopyridines.1 The radicals present in this reaction provoke with t -butylthiol an efficient radical reaction with formation of nor-alkane and 2-pyridyl- t -butyl disulphide.1Similarly these carbon radicals can be captured by halogen atom transfer to give noralkyl chlorides, bromides and iodides. 2 With oxygen in the presence of t-butylthiol the corresponding noralkyl hydroperoxides are formed in another radical chain reaction.3

Mechanism of a chemical glycosylation reaction.

Glycosylation is arguably the most important reaction in the field of glycochemistry, yet it involves one of the most empirically interpreted mechanisms in the science of organic chemistry. The beta-mannopyranosides, long considered one of the more difficult classes of glycosidic bond to prepare, were no exception to this rule. A number of logical but circuitous routes for their preparation were described in the literature, but they were accompanied by an even greater number of mostly ineffective recipes with which to access them directly. This situation changed in 1996 with the discovery of the 4,6-O-benzylidene acetal as a control element permitting direct entry into the beta-mannopyranosides, typically with high yield and selectivity. The unexpected nature of this phenomenon demanded study of the mechanism, leading first to the demonstration of the alpha-mannopyranosyl triflates as reaction intermediates and then to the development of alpha-deuterium kinetic isotope effect methods to probe their transformation into the product glycosides. In this Account, we assemble our observations into a comprehensive assessment consistent with a single mechanistic scheme. The realization that in the glucopyranose series the 4,6-O-benzylidene acetal is alpha- rather than beta-directing led to further investigations of substituent effects on the stereoselectivity of these glycosylation reactions, culminating in their explanation in terms of the covalent alpha-glycosyl triflates acting as a reservoir for a series of transient contact and solvent-separated ion pairs. The function of the benzylidene acetal, as explained by Bols and co-workers, is to lock the C6-O6 bond antiperiplanar to the C5-O5 bond, thereby maximizing its electron-withdrawing effect, destabilizing the glycosyl oxocarbenium ion, and shifting the equilibria as far as possible toward the covalent triflate. beta-Selective reactions result from attack of the nucleophile on the transient contact ion pair in which the alpha-face of the oxocarbenium ion is shielded by the triflate counterion. The alpha-products arise from attack either on the solvent-separated ion pair or on a free oxocarbenium ion, according to the dictates of the anomeric effect. Changes in selectivity from varying stereochemistry (glucose versus mannose) or from using different protecting groups can be explained by the shifting position of the key equilibria and, in particular, by the energy differences between the covalent triflate and the ion pairs. Of particular note is the importance of substitutents at the 3-position of the donor; an explanation is proposed that invokes their evolving torsional interaction with the substituent at C2 as the chair form of the covalent triflate moves toward the half-chair of the oxocarbenium ion.

Direct chemical synthesis of β-mannopyranosides and other glycosides via glycosyl triflates

Abstract High yield, highly stereoselective methods for the synthesis of β-mannopyranosides of primary, secondary, and tertiary alcohols are presented. Activation of mannosyl sulfoxides or mannosyl thioglycosides with trifluoromethanesulfonic anhydride or benzenesulfenyl triflate, respectively, leads to the efficient formation of α-mannosyl triflates at −78 °C in dichloromethane, in the presence of 2,6-di- tert -butyl-4-methylpyridine. These triflates then react S N 2-like with alcohols to give the β-mannosides. The use of a 4,6- O -benzylidene protected mannose is required for high selectivity, as is the use of non-participating protecting groups on O -2 and O -3 of the donors. It is further demonstrated that the thioglycoside/benzenesulfenyl triflate activation is applicable in the glucoside series, when both armed and disarmed protecting groups are tolerated.

A PRACTICAL ALTERNATIVE TO THE HUNSDIECKER REACTION

Abstract Carboxylic acid esters derived from N-hydroxy-pyridine-2-thione react with carbon tetrachloride, bromotrichloromethane or iodoform in a radical chain reaction to give the corresponding noralkyl chlorides, bromides or iodides in high yield.

Dissecting the mechanisms of a class of chemical glycosylation using primary 13C kinetic isotope effects

Although arguably the most important reaction in glycoscience, chemical glycosylations are among the least well understood of organic chemical reactions, resulting in an unnecessarily high degree of empiricism and a brake on rational development in this critical area. To address this problem, primary 13C kinetic isotope effects have now been determined for the formation of β- and α-manno- and glucopyranosides using a natural abundance NMR method. In contrast to the common current assumption, for three of the four cases studied the experimental and computed values are indicative of associative displacement of the intermediate covalent glycosyl trifluoromethanesulfonates. For the formation of the α-mannopyranosides, the experimentally determined KIE differs significantly from that computed for an associative displacement, which is strongly suggestive of a dissociative mechanism that approaches the intermediacy of a glycosyl oxocarbenium ion. The application of analogous experiments to other glycosylation systems should shed further light on their mechanisms and thus assist in the design of better reactions conditions with improved stereoselectivity. Chemical glycosylations are perhaps the most important reactions in glycoscience, but the mechanisms are not well understood. Here, quantum chemical calculations combined with natural-abundance NMR measurements of 13C kinetic isotope effects reveal both associative and dissociative mechanisms at the extremes of a continuum that depends on the relative stereochemistry of the substrate and the anomeric configuration of the product.

On the mechanism of the deoxygenation of secondary alcohols by the reduction of their methyl xanthates by tin hydrides

Abstract Two alternate proposals for the mechanism of reduction of xanthates by tributylstannane have been examined. Evidence has been secured that under normal reduction conditions the thiocarbonyl group is attacked reversibly. At a high enough temperature the carbon radical fragments to give eventually the reduction product. Under modified conditions, where no reducing agent (Sn-H) is present, the radical formed eliminates methylthiotributylstanne and affords the thiocarbonyloxy radical observed in the e.s.r. spectrum.

The invention of new radical chain reactions. Part 9. Further radical chemistry of thiohydroxamic esters; formation of carbon–carbon bonds

Carbon radicals derived from the esters of several types of thiohydroxamic acids have been trapped in a number of different ways. Particular attention has been paid to carbon–carbon bond formation by addition to suitably activated ethylenic linkages. Carboxylic acids can be conveniently converted into homo- and bishomo-acids by free radical chemistry based on thiohydroxamic esters. In a coda the tautomerism of the thiohydroxamic acids have been examined by physical methods. It is confirmed that the esters used are derivatives of the thione tautomer.

The free radical chemistry of carboxylic esters of 2-selenopyridine-N-oxide: a convenient synthesis of (L)-vinylglycine

Abstract Optically pure (L)-vinylglycine has been synthesised by two different methods. The first of these involves protected (L)-glutamate esters of N -hydroxy-2-seleno-pyridine. Such esters are shown to undergo the same decarboxylotive rearrangement as their thio-analogues. Oxidative elimination of the selenopyridine residue with ozone, and with the aid of hex-1-ene as sacrificial olefin for the work-up, gave the desired (L)-vinylglycine derivatives. Similarly, the modified Hunsdiecker reaction on the terminal carboxyl of suitably protected (L)-glutamic derivatives gave the nor-bromide which readily afforded the corresponding phenylselenides on treatment with phenylselenide anion. The sequence was then as above. Using the methyl ester with corbobenzyloxy protection for the amino-function an overall yield of crystalline optically pure (L)-vinylglycine of about 45% was obtained by either route.

Methodology Development and Physical Organic Chemistry: A Powerful Combination for the Advancement of Glycochemistry

This Perspective outlines work in the Crich group on the diastereoselective synthesis of the so-called difficult classes of glycosidic bond: the 2-deoxy-β-glycopyranosides, the β-mannopyranosides, the α-sialosides, the α-glucopyranosides, and the β-arabinofuranosides with an emphasis on the critical interplay between mechanism and methodology development.

Reaction of Thioacids with Isocyanates and Isothiocyanates: A Convenient Amide Ligation Process

Thiocarboxylates, prepared conveniently by cleavage of 9-fluorenylmethyl or trimethoxybenzyl thioesters, react at room temperature with isocyanates and isothiocyanates to give amide bonds in good to excellent yield. A carboxylate salt is also shown to react with an electron-deficient isocyanate to give the corresponding amide in excellent yield at room temperature.