Lithium mesylate displacement with

An acetonide is used as a protecting group in the synthesis of a corticoid that incorporates no fewer than four halogen atoms. The sequence starts with the conversion of the terminal hydroxyl on the side chain at C21 to a better leaving group, a mesylate, by means of methylsulfonyl chloride (30-2) (Scheme 7.30). Treatment of that intermediate with lithium chloride displaces the mesylate to afford the 21-chloro analogue (30-3). Addition of chlorine then leads to the 9a,l l)8-dichloro derivative (30-4). 

Treatment of the allylic mesylate (5) with various alkyl or alkynyl Grignard reagents gave mainly the products of direct displacement, whereas methylmagnesium bromide afforded the a,p-products of S 2 reaction (Scheme 5). Lithium dimethyIcuprate, on... 

Replacement of a primary or secondary hydroxyl function with deuterium is usually carried out by first converting the alcohol into a mesylate or tosylate ester, which can then be displaced by treatment with lithium aluminum deuteride. The... 

The mesylate-tosylate in the framework of the pyranose derivative reacted with lithium acetylide-ethylenediamine complex to give the seven-membered sultone (13) in 98% yield, as a result of displacement of a tosylate coupled with deprotonation at the mesylate producing a nucleophile and then ring formation (Scheme 6) <81CJC260>. 

Replacement of the C-3 hydroxyl group of D-ribonucleosides (or ribonucleotides) with azido or amino groups forms 3 -azidothymidine (AZT) or 3 -amino-3 -deoxythymidine, both of which are used in the treatment of AIDS and cancer. These compounds are synthesized by the mesylation of l-(2 -deoxy-5 -0-trityl-p-D-lyxofuranosyl)-thymidine [100]. The mesyl group is displaced by reaction with lithium azide in DMF at 100°C. The azide can then be catalytically hydrogenated to give the 3 -amino analog (reaction 4.94). 

A novel reaction of carbohydrate dithianes bearing a mesyloxy group at the S-posidon has been discovered on attempted substitution of the mesylate in compounds 3 by azide. As illustrated in Scheme 2, participation, first by 0-2 and then by sulfur, led to the formation of rearranged products 4 and 5 with little direct displacement. Pyrolysis of 3 in the absence of lithium azide gave the unsaturated product 6 in high yields. The reduction of dithioacetal nx>no-S-oxides to dithioacetals by use of lithium aluminium hydride has been reported. ... 

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