Preparation of new nitrogen-bridged heterocycles. XXIV. Syntheses and reactions of pyrazolo(1,5-a)pyridine-2-thiols. (2).

Abstract

Various 2-(acylmethylthio)-, 2-(benzylthio)-, and 2-(propargylthio)pyrazolo[1, 5-a]pyridine derivatives (4a-n' and 7a-n') were prepared in moderate to good yields by the reactions of potassium pyrazolo[1, 5-a]pyridine-2-thiolates (2 and 6), readily available by the treatment of 2-[(2-ethoxycarbonylethyl)thio]- and 2-[(2-cyanoethyl)thio]pyrazolo[1, 5-a]pyridines (1a-h and 5a-h) with potassium tert-butoxide in N, N-dimethylformamide (DMF), with alkylating agents such as chloroacetone (3a), phenacyl bromides (3b-d), ethyl bromoacetate (3e), benzyl bromides (3f-k), and propargyl bromide (31). Of these S-functionalied pyrazolo[1, 5-a]pyridines only 2-(acetonylthio)- and 2-(phenacylthio)pyrazolo[1, 5-a]pyridine-3-carbonitriles (4a-t) were converted to new heterocyles, thieno[2', 3' : 3, 4]pyrazolo[1, 5-a]pyridines (8a-t) by heating them with 1, 8-diazabicyclo[5.4.0]undec-7-ene (DBU); the other compounds did not provide any tricyclic heterocycles under various alkaline conditions. The structures of thieno[2', 3' : 3, 4]pyrazolo[1, 5-a]pyridines (8a-t) were determined mainly by elemental analyses and spectral inspections, and the structural assignment was confirmed by the X-ray analysis of compound 8d.