Aldehydes trifluoromethanesulfonic anhydride

The Frasca method for obtaining 1-arylindazoles also involves a C(3)—C(3a) ring closure (67CJC697). It consists in the cyclization of p-nitrophenylhydrazones of ketones and aldehydes with polyphosphoric acid. The Barone computer-assisted synthetic design program has found several new methods for preparing indazoles (79MI40409). The selected method involves the transformation of jV, jV -diphenylhydrazides (596) into 1-phenylindazoles (597) by means of trifluoromethanesulfonic anhydride. The yields vary from 2% (R = H) to 50% (R = Ph). 

Aldehyde 26 was treated with hydroxylamine hydrochloride in refluxing methanol to give a mixture of (E)- and (Z)-pyrrolotriazine 40 in 59% and 21% yield, respectively. Dehydration of aldoxime 40 with trifluoromethanesulfonic anhydride and triethylamine in dichloromethane afforded triazine 41. Conversion of the nitrile 41 to the deprotected amide 42 was accomplished in 96% yield on treatment of 41 with basic hydrogen peroxide in ethanol <2001CAR77>. 

Electrophilic attack by aldehydes can also take place at positions 3 and 4 in suitably substituted thiophenes. Thus, 2,5-bis(trimethylsilyloxy)thiophene reacts with aromatic aldehydes in the presence of the Lewis acid trimethylsilyl trifluoromethanesulfonate (0.1-1.0%) to form 3,4-disubstituted thiosuccinic anhydrides (88) in quantitative yield. With aliphatic aldehydes, the yields are lower, the catalyst being ZnBry, the products so obtained have been hydrolyzed in the presence of lead acetate to form the lactones (89) in 30-40% yields. 2,5-Bis(trimethylsilyloxy)thiophene also reacts with orthoesters in the presence of trimethylsilyl triflate to form (90) <87C152,92LA615>. 

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