Triflic anhydride/sodium acetate

Additive Pummerer reactions of the type described above using racemic a,P-unsaturated sulfoxides can be accomplished using the following electrophiles acyl chlorides [205,206], dithioacetic acid [207], acetic anhydride [208], mineral acids/alcohols [209], phosphorus pentachloride [210], silyl ketene acetals/zinc iodide [211], thionyl chloride [212], oxalyl chloride [213], trifluoroacetic acid and its anhydride [214-218], triflic anhydride/sodium acetate [219], and dichloroketene (see below). Selected recent examples of work in this area are presented here. 

Treatment of the alcohol ( ) with trifluoromethylsulfonic anhydride (triflic anhydride) at -78 C afforded the ester (1 ) which could be isolated and characterized. We knew from previous experience (2J that sulfonyl esters vicinal to an isopropylidene acetal are relatively stable. The triflate T,) reacted cleanly with potassium azide and 18-crown-6 in dichloromethane at room temperature. The crystalline product [68% overall from (1 )] was not the azide ( ) but the isomeric A -triazoline ( )- Clearly the initially formed azide (18) had undergone intramolecular 1,3-cyclo-addition to the double bond of the unsaturated ester (21- ). The stereochemistry of the triazoline (1 ), determined by proton nmr spectroscopy, showed that the reaction was stereospecific. There are several known examples of this reaction ( ), including one in the carbohydrate series ( ). When the triazoline was treated with sodium ethoxide ( ) the diazoester ( ) was rapidly formed by ring-opening and was isolated in 85% yield, Hydrogenolysis of the diazo group of (M) gave the required pyrrolidine ester ( ) (90%). 

Vinylic sulfoxides such as 1 react readily with electrophiles to give highly reactive species, and the overall reactions have been likened to "generation" of the synthon 2. Treatment of 1 with TFAA, for example, results in what is referred to as an "additive Pummerer reaction", and gives the diester 3. Reaction of 1 with triflic anhydride and sodium acetate in acetic anhydride, by contrast, gives an 85% yield of the protected aldehyde 4. 

A solution of triflic anhydride (1.57 ml, 10 mmol) [84] in dichloroethanc (30 ml) at 0"C is added to a solution of triphenylphosphine oxide (5.55 g, 20 mmol) or equivalent phosphinamide in dichloroethanc. After appearance of a precipitate (usually in less than 15 min), a solution containing n-phcnylenediamine (0.44 g, 4 mmol) and benzoic acid (0.61 g, 5 mmol) in dichloroethanc (10 ml) is added drop wise. After stirring (0.5 h), the solution is washed with 5% sodium bicarbonate solution, dried (MgS04) and evaporated. The residue is passed through a short column packed with silica and eluted with bexanc-cthyl acetate (3 1) to remove excess phosphine oxide. Evaporation gives 2-phcnylbenzimida7.ole (0.66 g, 85%), m.p. 287°C. 

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