Mesyl with potassium acetate

Our first synthesis of brassinolide started from stigmasterol which was converted to (20S)-6p-methoxy-3a,5-cyclo-5a-pregnane-20-carboxaldehyde (2). This pathway involved formation of the mesylate with methanesulfonyl chloride in pyridine and tetrahydrofuran, treatment of the mesylate with potassium acetate in methanol, and ozonolysis of die i-sterol with reductive work-up. 

Sn2 displacement of the mesylates by potassium acetate in hot acetic anhydride (the inversion step), and saponification of the resulting diacetate to diol 551 (40% overall yield). Cyclization of 551 with NBS-NaOH gives optically pure 552 [134]. 

The use of enzyme technology has also been exemplified in a Chirotech process for the synthesis of both enantiomers of the hydrogenation catalyst DuPHOS (80) [2,86]. A hexanediol mixture consisting of 50% meso and 50% racemic diol 77 was acylated with the aid of immobilized CAL-B (Scheme 23). Only the (i )-configured alcohol was converted by the enzyme and led to the monobutyrate (RyS)-7S and the bisbutyrate RyR)-79. The unchanged (S,S)-diol 77 could be removed by extraction with water and was purified by crystallization from ethyl acetate, to reach an optical purity of 98% ee and 88% de. The mesylate of alcohol (RyS)-7S could be inverted with potassium acetate, and saponification of this acetate and of compound RyR)-79 followed by a crystallization step provided the enantiopure diol (RyR)-77 in 48% yield. Although this procedure was used to produce approximately 30 kg of DuPHOS, it has not been used further for commercial catalyst production. 

Condensation of sodium phenoxide witli 2,2,2-trifluoroethyl iodide gives a product of direct substitution in a low yield, several other ethers are formed by eliminatton-addition reactions [7] Use of mesylate as a leaving group and hex amethyl phosphoramide (HMPA) as a solvent increases the yield of the substitution [S] Even chlorine can be replaced when the condensation is performed with potassium fluoride and acetic acid at a high temperature [9] (equations 6-8)... 

The iodo derivative is a useful intermediate for the preparation of a wide variety of different types of compounds. Primary mesyl esters also react with sodium iodide in acetone, but the selectivity of this cleavage is less because of the greater reactivity of secondary mesyl esters. oa( ) Methyl 2,3,4-tri-0-acetyl-6-0-mesyl-a-D-glucopyranoside is converted into methyl 2,3,4,6-tetra-O-acetyl-a-D-glucopyranoside with acetic anhydride and potassium acetate. Replacements of a primary mesyloxy group with fluorine by use of potassium fluoride in methanol,106 with chlorine by use of lithium chloride,102 and with pyridine to form a pyri-dinium deoxy derivative,106 have been reported. Primary tosyloxy groups have been replaced by hydrogen,106 by thiocyanate,107 and by... 

Two syntheses of racemic 2,3-dihydrotriquinacen-2-one (380) have been described. The first approach consisted in the selective monoketalization of diketone 364 with 2,2-dimethylpropane-l, 3-diol and Baeyer-Villiger oxidation of 375 (Scheme 60).357 Although two lactones were produced, 376 could be freed of its isomer by selective alkaline hydrolysis. Diisobutylalumium hydride reduction of 376 afforded 377, the acetate of which was converted by acid treatment to 378. Ketaliza-tion of this isomeric mixture, followed by hydrolysis, mesylation, and treatment with potassium f-butoxide in dimethyl sulfoxide afforded diene ketal 379. Deketa-lization then liberated the desired ketone. 

Terminally fluorinated carbohydrates may be obtained by treating the tosyl esters, or the more reactive mesyl esters, with potassium fluoride in methanol. Because of the alkaline reaction of potassium fluoride, the remaining hydroxyl groups must be protected, as for example, by acetal formation,... 

Compound 110, readily prepared from 111 and acrylic acid at 135°, underwent the same sequence of reactions used in the model series, yielding exclusively 112 in which it is inferred that the hydrogen at C-4 is trans to the bridging group. The ketone was converted into the alcohol 113 and thence to the mesylate 114 which in turn was transformed to the alkene 115 with potassium tertiary butoxide in DMSO. Only under these conditions was 115 obtained in high yield uncontaminated with rearranged products. Functionality at C-5 was introduced by oxidation with selenium dioxide in glacial acetic acid. Acetate 116 so obtained was hydrolysed to the alcohol 117 and oxidized to the racemic ketone 118. One of the enantiomers of 118 had already been prepared from annotinine (1) and comparison of the spectroscopic properties of 118 with the naturally derived sample established the identity of the two systems. 

A solution of the epoxide in acetic acid at room temperature is heated under reflux for 2-3 hr. The reaction mixture is then cooled to ca. 40° and concentrated in vacuo to ca. 1/4 volume. The concentrate is diluted with water and extracted with ether. The ether extract is washed with an equal volume of water, dried (MgS04) and concentrated in vacuo. The crude or recrystallized product is then treated with an equal weight of mesyl chloride in dry pyridine at 0° for about 24 hr. A refluxing mixture of the mesylate and 25 % methanolic potassium hydroxide (wt/wt) affords the -epoxide. 

The oxygen atom at 21 is similarly an expendable group. Reaction of 241 (obtained from 185 by the usual procedure for introduction of the 9a-fluoro group) with methanesulfonyl chloride affords the 21 mesylate (242a). Replacement of the leaving group at 21 with iodine by means of potassium iodide in acetone followed by reduction of the halogen with zinc in acetic acid leads to fluorometholone (243). ... 

Nitroalkene Synthesis. Henry reaction of (phenylthio)nitro-methane with aldehydes fohowed by dehydration has been used to prepare (. -l-nitro-l-phenylthioalkenes. potassium 7eri-butoxide-catalyzed addition and dehydration via mesylation (eq 1), or direct condensation catalyzed by piperidinium acetate (eq 2), are the methods of choice. Henry reaction can be also carried out by reaction of the nitroalkene dianion with aldehydes. ... 

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