Highly facile and regio-selective synthesis of pyrazolo[1,5-a]pyrimidines via reactions of 1,2-allenic ketones with aminopyrazoles

An efficient procedure for the syntheses of pyrazolo[1,5-a]pyrimidines through reactions of 1,2-allenic ketones with aminopyrazoles under extremely mild conditions without using any catalyst or promoter has been developed. The reactions showed excellent regio-selectivity with all the allenic ketone substrates except for those bearing an alkyl group at the internal position of the allene moiety. The reason behind this regio-selectivity dissimilarity has been explored with the aid of the B3LYP/6-31G* level of density functional theory. Moreover, this novel procedure has been successfully applied in the preparation of nucleoside-pyrazolo[1,5-a]pyrimidine chimeras with potent antiviral activities.