N–N Bond-Forming Cyclization for the One-Pot Synthesis of N-Aryl[3,4-d]pyrazolopyrimidines

[Image: see text] An efficient one-pot synthesis of N-aryl[3,4-d]pyrazolopyrimidines in good yield and under mild reaction conditions is described. By exploiting electron-deficient hydroxylamines, the substituted oxime products were formed with very high E-diastereoselectivity. The key step utilizes a cyclization reaction upon an oxime derived from hydroxylamine-O-sulfonic acid to form the N–N bond of the product.