The genesis and development of the 4,6-O-benzylidene acetal method for the preparation of β-mannopyranosides are reviewed. Particular emphasis is placed on the influence of the various protecting groups on stereoselectivity and these effects are interpreted in the framework of a general mechanistic scheme invoking a series of solvent-separated and contact ion pairs in dynamic equilibrium with a covalent α-glycosyl trifluoromethanesulfonate.
A wide variety of carboxylic acids in the form of their salts condense with aryl isocyanates at room temperature with loss of carbon dioxide to give the corresponding amides in high yield. Application of the reaction to acyl isocyanates gives unsymmetric imides. The reaction is compatible with hydroxyl groups and both Fmoc and Boc protecting groups for amines and is applicable to aliphatic, aromatic, and heteroaromatic acids.
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.Given the widespread availability of amines and of carboxylic acids, amide bond forming reactions are necessarily some of the most widely applied transformations in parallel synthesis and combinatorial chemistry. The needs for atom ecomony, ever milder conditions, and greater selectivity combine to drive the search for new and improved methods for the formation of amide bonds. 1 The condensation of isocyanides with carboxylic acids has recently received much interest in this regard, 2 and this despite the relatively limited range of commercially available isocyanides and the somewhat forcing reaction conditions. Our attention was caught by a little known reaction 3 involving condensations of the much more widely available isocyanates and isothiocyanates with monothiocarboxylates which result in the formation of amides with loss of a simple, volatile byproduct, carbon oxysulfide or carbon disulfide, respectively. Although this reaction was described in 1973 for use with very simple substrates it has seen very limited application subsequently, 4 perhaps due to the relative paucity of commercially available thioacids. 5 We reasoned that the advent of convenient mild methods for thioacid synthesis 6 would combine with this reaction to afford a convenient and mild method for amide formation, and report here that this is indeed the case. In addition, we report Dcrich@icsn.cnrs-gif.fr.
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