Synthesis of 3,5-disubstituted pyridazines by regioselective [4+2] cycloadditions with ethynyltributyltin and subsequent replacement of the organotin substituent

Cycloadditions of 3-aryl-1,2,4,5-tetrazines 1a-o with ethynyltributyltin 5 occur with high regioselectivity to yield 3-aryl-5-tributylstannyl-pyridazines 9-23 in 71-95% yield. Moreover, the stannanes obtained could be utilized as starting materials for the preparation of various 3,5-disubstituted pyridazines as shown for 3-phenyl-5-tributylstannyl-pyridazine 9b. Stille cross-couplings with aryl halides yield 3,5-diaryl-pyridazines 26-29, whereas exposure to elemental halogens furnished 3-phenyl-5-halogeno-pyridazines 24 and 25, which can be used as starting compounds for further functionalizations like Heck reaction. (C) 1998 Elsevier Science Ltd. All rights reserved.