Epoxides, preparation from tosyl alcohols

As shown in Scheme 8.11.3, C-sucrose was prepared from the illustrated olefin. Osmylation of the olefin followed by tosylation of the primary alcohol and treatment with base gave the epoxide. The acetonide was then hydrolyzed and the benzyl groups were removed. Spontaneous cyclization of the furanose ring was effected on removal of the acetonide. In all, an efficient synthesis of C-sucrose was accomplished in five steps from the starting olefin. 

Compound (E) was prepared from levoglucosan and, after transformation into its tosylate and the corresponding epoxide, was opened with DMSO anion to give a dimethylated alcohol after desulfurization. The alcohol was then transformed into the corresponding lactol. Reduction of the lactol to the expected triol followed by a tandem protection-oxidation process afforded an aldehyde, which was then coupled with the vinyllithium reagent generated from the corresponding vinyl iodide [190]. The major diastereoisomer was transformed, after a... 

Successful application of the Mitsonobu epimerization procedure to an eudesmanic alcohol 44 to bring about inversion of configuration at C(l) is the crucial step in the Harapanhalli synthesis of erivanin (50) from santonin (Scheme 7) [16]. Reduction of enone 43, prepared from santonin in 10 steps, with sodium borohydride furnished the )8-alcohol 44 as the sole product. This product results from the approach of the hydride anion from the less hindered Of-face of the molecule. The chemical modification of the C(3)-C(4) double bond to give a 3a-hydroxy-A4-i4 rnoiety was accomplished via the epoxide 46 and its rearrangement in a basic medium. Epoxidation of 44 with MCPA yielded only one product without any directing effect exerted by the homoallylic alcohol. Treatment of 46 with lithium diisopropylamide (EDA) afforded l-e/>/-erivanin (47). For the synthesis of erivanin (50), epimerization at C(l) prior to the A -modification sequence was required. Attempts to epimerize this carbon atom in 44 by acetolysis of the tosyl derivative 45 were unsuccessful as they led to eliminated product 13 (Scheme 3). 

Alcohol 143 (Scheme 6.26), prepared from (R)-glyceraldehyde derivative, was subjected to deoxygenation and epoxidation to give the racemic epoxide 144. Kinetic resolution with (S,S)-Jacobsen catalyst gave diol 145, which on further transformations was converted into the alcohol 146. Swern oxidation of 146 followed by Wittig olefination, acetonide deprotection under acidic conditions furnished the diol 147. Primary alcohol on deoxygenation through LAH reduction of tosylate afforded the alcohol 148. 

Methanesulfonates. The most common use of methanesulfonyl chloride is for the synthesis of sulfonate esters from alcohols. This can be readily accomplished by treatment of an alcohol with mesyl chloride in the presence of a base (usually Triethy-lamine or Pyridine). The methanesulfonates formed are functional equivalents of halides. As such they are frequently employed as intermediates for reactions such as displacements, eliminations, reductions, and rearrangements. Selective mesylation of a vicinal diol is a common method of preparation of epoxides." Alkynyl mesylates can be used for the synthesis of trimethylsilyl allenes. Oxime mesylates undergo a Beckmann rearrangement upon treatment with a Lewis acid. Aromatic mesylates have been used as substrates for nucleophilic aromatic substitution. Mesylates are more reactive than tosylates toward nucleophilic substitution, but less reactive toward solvolysis. 

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