1,3-Benzoxathiolium salts, 2-substituted

Benzoxathiolium salts have proven to be effective masked acylating agents (79S223). 2-Substituted 1,3-benzoxathiolium tetrafluoroborates have now been utilized in the preparation of esters. Reaction of the salt (334) with two equivalents of an alcohol gave the 2-alkoxy-2-alkyl-benzoxathiole (335). Hydrolysis of (335) with red mercury(II) oxide and boron trifluoride etherate in aqueous THF delivered ethyl benzoate in excellent yield (Scheme 72). 

Esterification.—Another way to improve the sometimes unsatisfactory N,N-dicyclohexylcarbodi-imide esterification method is to add catalytic quantities of toluene-p-sulphonic acid to the reaction mixture (c/. 3, 149). Esters can be obtained very rapidly and generally in high yields by reactions between 2-substituted 1,3-benzoxathiolium salts and alcohols (Scheme 30). One drawback could be the rather acidic conditions employed in the second step. A neat modification of the mixed-anhydride esterification method requires no added acid or base in the final step but uses an intramolecular cyclization as the driving... 

Substituted-1,3-benzoxathiolium tetrafluoroborates (53 Scheme 25) can be prepared readily from acids, anhydrides, esters and trihalomethyl compounds. The thiolium salts can be reduced easily with NaBH4 in acetonitrile at 0-20 C to give aldehydes in 71-91% overall yields. 

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