5-Aminofuro[3,2-c]pyridinium Tosylates and Substituted Furo[3,2-c]pyridine N-Oxides: Synthesis and Reactions

5-Aminofuro[3,2-c]pyridinium tosylates 2a-2c were synthesized by direct N-amination of furo[3,2-c]pyridines 1a-1c with O-(4-methylbenzenesulfonyl)hydroxylamine in dichloromethane. Zwitterionic furo[3,2-c]pyridinium N-imides 3a-3c generated from 2a-2c and anhydrous potassium carbonate in N,N-dimethylformamide afforded by 1,3-dipolar cycloaddition reactions with dimethyl butynedioate or ethyl propiolate the corresponding furo[3,2-c]pyrazolo[1,5-a]pyridinecarboxylic esters 4a-4c and 5a-5c. Furo[3,2-c]pyridine N-oxides 6a-6c and their benzo derivative 6d were synthesized by reaction of 1 with 3-chloroperbenzoic acid in dichloromethane. Treatment of N-oxides 6 with benzoyl chloride and cyanide anion (Reissert-Henze reaction) was shown to produce the corresponding furo[3,2-c]pyridine-4-carbonitriles 7. In further transformations (acid and alkaline hydrolysis), the cyano group was converted successively to amide and carboxylic acid.