作者:Jérôme Guillard、Christelle Lamazzi、Otto Meth-Cohn、Charles W. Rees、Andrew J. P. White、David J. Williams
DOI:10.1039/b101157j
日期:——
Premixed ethyl carbamate, thionyl chloride and pyridine (which generate thiazyl chloride, NSCl) in boiling benzene or toluene convert 2,5- and 2,3,5-substituted furans into 5-acylisothiazoles regiospecifically. The reactions are much faster and generally higher yielding in boiling chlorobenzene with more thionyl chloride and with pyridine or isoquinoline as base. Under the more vigorous conditions, even fully substituted 3-bromofurans give isothiazoles, with the displacement of bromine. Deactivated furans, with electron-withdrawing groups such as ester, cyano, benzoyl and phenylsulfonyl in the α-position, react under the more vigorous conditions to give 5-acylisothiazoles with the electronegative group in the 3-position. The ‘activated’ 2-methyl-5-phenyl- and 5-phenyl-2-phenylthio-furans react analogously, with the more electron-releasing group becoming part of the 5-acyl substituent, exclusively or predominantly. These results are explained by initial
electrophilic attack of the furan ring to give a β-thiazyl derivative which spontaneously ring-opens and closes to the isothiazole. The X-ray structures of five of the differently substituted isothiazole compounds are reported. All have very similar patterns of bonding within their isothiazole rings that appear to be independent of the electron-withdrawing or -donating nature of the substituents. Three of the compounds (8a, 8g and 13) have loosely linked chain structures in the solid state, adjacent molecules being connected by combinations of hydrogen bonding and π–π stacking interactions.
预混的乙基氨基甲酸、亚硫酰氯和吡啶(生成噻唑氯,NSCl)在沸腾的苯或甲苯中能够进行区域特异性地将2,5-和2,3,5-取代的呋喃转化为5-酰基异噻唑。在沸腾的氯苯中,反应更快,产率普遍更高,且使用了更多的亚硫酰氯及吡啶或异喹啉作为碱。在更激烈的反应条件下,甚至完全取代的3-溴呋喃也能与溴发生取代反应生成异噻唑。对于那些带有电子吸引基团(如酯、氰、苯甲酰和苯磺酰)且位于α位的非活性呋喃,在较为激烈的条件下也能反应,生成5-酰基异噻唑,其中电负性基团位于3位。这些“活化”型的2-甲基-5-苯基和5-苯基-2-苯硫基呋喃也表现出类似的反应,其中电子供给基团会独占或主导成为5-酰基取代基。其结果可以通过呋喃环的初始亲电攻击解释,生成β-噻唑衍生物,随后自发环打开并闭合成异噻唑。报告了五种不同取代异噻唑化合物的X射线结构,所有化合物在其异噻唑环内的键合模式非常相似,似乎与取代基的电子吸引或供给特性无关。三种化合物(8a、8g和13)在固态下具有松散联接的链结构,邻近的分子通过氢键和π–π堆叠相互作用相连。