Nosyl azide

DOrner and Westermann [149] reported the preparation of the pseudodisaccharide 143 with a triazole tether (Scheme 37) as a prototype for the synthesis of carbohydrate-containing macrocycles, through a Huisgen-catalysed reaction and ring-closing methathesis as the key steps. Standard conditions were used, Cu(OAc)2 and sodium ascorbate as the Cu(I) source and a mixture of water and f-BuOH as solvent, for the conversion of protected glucopyra-nosyl azide 141 into pseudo-disaccharide 143 by using dialkyne 142 in a 77% yield. 

Nitrogen-containing reagents electrophilic at the N atom such as azodicarboxylate esters, tosyl and nosyl azides, and nitrosobenzene have been successfiilly applied to a variety of organocatalyzed enantioselective amination reactions of carbonyl compounds and other related pronucleophiles. Early work in this area has been covered in previous reviews [1 ], and here the most representative examples are presented. 

A series of other azidation reagents and diazotransfer reagents are described in academic research papers, including sulfonyl azides such as benzenesulfonyl azide, trifluorometh-anesulfonyl azide, pyridine-3-suIfonyl azide or also iodine azide, nosyl azide, tributyl-hexadecylphosphonium azide, tetramethylguanidinium azide, t-butoxycarbonyl azide. 

In order to solve this problem, we examined stractural variation of the sulfonyl azide and silane (Table 4.1). Changing the nitrogen source from ethane- (7) to methanesulfonyl azide (16) showed no improvements (entry 2). The use of tosyl azide (17) (entry 3) gave the first increase in selectivity affording a 89 11 ratio in favor of the desired azide. No conversion was observed with the electron-poor nosyl azide (18) (entry 4). 

Nosyloxyesters undergo substitution reactions with inversion of configuration (Scheme 15). The corresponding nosyl lactate 98 is prepared in quantitative yield from 97 upon treatment with 4-nitrobenzenesulfonyl chloride in the presence of triethylamine and DMAP [34]. Reaction of 98 with sodium azide in DMSO (55 °C) gives the (R)-azidoester 99. Hydrolysis of the ester with lithium hydroxide in aqueous THF furnishes the azido acid 101 (92% ee). Reduction of the azide group is easily accomplished either under catalytic conditions or with triphenylphosphine and water. 

The conversion of methyl (3,4,5-tri-C-acetyl-p-D-araZ)/ o-hex-2-ulopyra-nosyl)onate bromide (51) by sodium azide in Me2SO gave rise to the a-azide 52 in reasonable yield. An alternative approach suggested omission of phase-transfer methods and avoidance of dipolar aprotic solvents, and allowing gly-cosyl halides to react with NaNs in aqueous acetone or acetonitrile. " ... 

X-sulfonyloxy-(X,P-unsatuTated ester, and the formation in both cases are kept to a minimum by carrying out the reaction under relatively dilute conditions. Under mild alkaline conditions, the monosulfonates (85) are converted to glycidic esters (86) in high yield. The nosylates, but not the tosylates, undergo displacement by azide ion to give ot-azido-P-hydroxy esters, which are envisioned as precursors to P-hydroxyamino acids [69]. 

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