Arenesulfonyl esters

When /V-arenesulfonyl-a-amino acid derived boranes 13 and 14 are used in substoichiometric amounts in order to mediate enantioselective aldol additions of a,a-dimethyl substituted ketcnc acetal 15, /J-hydroxycarboxylic esters 16 are obtained in enantiomeric excess of 84 to > 99 %3fi. 

The H and 13C CIDNP studies have shown that not only the sulfone 14, but also the sulfmic ester 15, is generated as cage products from the phenyl/p-toluenesulfonyl radical pair during the thermal decomposition of phenylazo aryl sulfone (13)54 (Scheme 2). The cross-termination of arenesulfonyl and triethylgermyl radicals was found to occur exclusively via the formation of germyl sulfinate, ArS(0)0GeEt333. 

Arenesulfinate esters are usually prepared from an arenesulfinyl chloride and an alcohol in ether and pyridine. The arenesulfinyl chloride is usually prepared from the sodium arenesulfinate which is made by reduction of the arenesulfonyl chloride, preferably by aqueous sodium sulfite. After the crystalline sulfinate epimer has been removed by filtration, the equilibrium between the epimers remaining in the mother liquor may be reestablished by the addition of hydrogen chloride as shown by Herbrandson and Cusano . In this way the yield of the least soluble diastereomer may be increased beyond that which exists in the original reaction mixture (Scheme 1). Solladie prepared sulfinate ester 19 in 90% yield using this technique and published the details of his procedure. Estep and Tavares also published a convenient recipe for this method, although their yields were somewhat lower than Solladie s. 

Esters of arenesulfonic acids are prepared by reaction of the corresponding alcohol with the arenesulfonyl chloride in the presence of a basic reagent, which has the function of activating the alcohol and binding the hydrogen chloride. The reaction is often carried out with pyridine, which is used as solvent. In the procedure for tosylates of primary aliphatic alcohols, described in A.l. Vogel, A Textbook of Practical Organic Chemistry, an aqueous solution of sodium hydroxide is used. Esters of primary alcohols are formed more easily than secondary-alkyl esters, while tertiary alcohols cannot be esterified under the usual conditions. 

If desired, glycidic esters can be derived from a,3-dihydroxy esters, such as (110), by either of two methods. In one method, reaction of the diol with an arenesulfonyl chloride is regioselective, producing the a-arenesulfonate (111) in preference to the 3-sulfbnate. Treatment of (111) with an equivalent of base produces the erythro-glycidic ester (112) in good yield. In the second method, the diol is converted to a bromohydrin (114) via the acetoxy bromide (113). The brotmhydrin (114) affords the threo-g ycidic ester (115) on exposure to potassium caitxHiate in methanol. 

Aromatic sulfonyl azides react with enol ethers of cyclic ketones to form arenesulfonyl imidate esters (96) with ring contraction, the addition-rearrangement process is very stereospecific (Scheme 63). 

P-Arylthwpropanoic esters. Reduction of arenesulfonyl chlorides in the presence of conjugated esters is followed by Michael reaction. 

Ketene dimethyl acetals react with phenyl azide to give a-anilido esters after acid hydrolysis of the intermediate triazolines, but yields are low.291 The reaction of ketene acetals with arenesulfonyl azides does not appear to have been investigated. 

---