作者:Stephen J. Connon、Anthony F. Hegarty
DOI:10.1039/a909772d
日期:——
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 tert-butyllithium at low temperatures, followed by elimination of lithium chloride affords 2- and 6-alkoxy-3,4-pyridynes. These species are trapped in 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 α-lithiation by tert-butyllithium because of its increased steric bulk.
邻环氮的烷氧基对3,4-吡啶炔(1)的稳定作用已有报道。2-甲氧基(18)、2-乙氧基(14)、2-异丙氧基(19)和6-异丙氧基(26)-3-氯吡啶在低温下与叔丁基锂进行区域选择性锂化反应,随后脱去氯化锂生成2-和6-烷氧基-3,4-吡啶炔。这些中间体与呋喃在Diels-Alder反应中就地捕获,生成5-8,产率为66-89%,并且不会生成3,4-吡啶炔中间体的聚合或亲核加成反应的典型产物。作为对比,3-氯吡啶与呋喃和LDA反应仅得到19%的加成物(4)。我们还报道了在这些系统中异丙氧基(而非甲氧基)的新颖应用,由于其增加了的空间位阻,可以作为抑制叔丁基锂α-锂化的杂原子电子供体。