Diels–Alder reactions of trichloro-1,2,4-triazine: intramolecular additions with 1,5 and 1,6 dienes

Abstract

Trichloro-1,2,4-triazine reacts with cycloheptene and cyclododecene to give the 2,6-dichloropyridine derivatives 6 and 7 via an intermediate dihydropyridine formed by Diels–Alder addition and loss of N₂, and with cyclopenta-1,3-diene and indene by addition of a second molecule of alkene to the intermediate dihydropyridine. Bicyclo [2.2.1]hepta-2,5-diene and quadricyclane give mainly 2,3,6-trichloropyridine by loss of cyclopentadiene from the intermediate dihydropyridine. With hexa-1,5-diene, the dihydropyridine is trapped by an intramolecular Diels–Alder reaction, forming tricyclic compounds 17 and 18. With diallyl ether, the dihydropyridine partly undergoes intramolecular cycloaddition to give 23 and partly [1,5]-sigmatropic shift of hydrogen before intramolecular cycloaddition to give 25. With cyclododeca-1,5,9-triene formation of pyridine derivatives is incomplete and the immediate precursor can be trapped with water.