Substituted 3,4-pyridynes: Clean cycloadditions

Article abstract of DOI:10.1039/a909772d

The stabilisation of 3,4-pyridyne (1) by an alkoxy group adjacent to the ring nitrogen is reported. The regioselective lithiation of 2-ethoxy- (14), 2-methoxy- (18), 2-isopropoxy- (19) and 6-isopropoxy- (26) -3-chloropyridines with tertbutyllithium at low temperatures, followed by elimination of lithium chloride affords 2- and 6-alkoxy-3,4-pyridynes. These species are trapped m situ with furan in a Diels-Alder reaction to give 5-8 in 66-89% yield, and do not give products typical of polymerisation or nucleophilic addition to the 3,4-pyridyne intermediates. As a comparison treatment of 3-chloropyridine with furan and LDA gives only 19% of adduct (4). We also report the novel use of the isopropoxy (rather than methoxy) group in these systems, which can act as a heteroatomic electron donating group which inhibits a-lithiation by tert-butyllithium because of its increased steric bulk. The Royal Society of Chemistry 2000.