Further reactions of furans with trithiazyl trichloride; mechanistic considerations
作者:Charles W. Rees、Tai-Yuen Yue
DOI:10.1039/a700488e
日期:——
The reaction of 2,5-diarylfurans with trithiazyl trichloride 1 to
give 5-aroyl-3-arylisothiazoles in a useful one-step synthesis of
isothiazoles has been extended to both weakly and strongly polarised
unsymmetrical 2,5-diarylfurans. These react in an entirely analogous
manner; the more electron releasing aryl group becomes incorporated into
the 5-aroyl group of the isothiazole as the exclusive (strong
polarisation) or the major (weak polarisation) product. However, with
3-bromo-2-(4-methoxyphenyl)-5-(4-nitrophenyl)furan 7, where the more
reactive furan β-position is now substituted, this
regiospecificity is reversed (to give isothiazole 8). When one of the
α-aryl groups in the furan is replaced by methyl the same
regiospecific isothiazole formation is now accompanied by some ring and
side chain chlorination
(15 → 16 + 17 + 18). All
of these results can be explained by mechanisms (Schemes 2 and 5) which
involve initial electrophilic attack of the furan to give a
β-thiazyl derivative. This highly reactive (nitrenoid)
substituent then induces a novel opening of the furan ring 21 to give a
highly delocalised intermediate 22 which recyclises to the
isothiazole.
2,5-二芳基呋喃与三氯化三噻唑的反应1
在有用的一步合成中得到 5-芳酰基-3-芳基异噻唑
异噻唑已扩展到弱极化和强极化
不对称2,5-二芳基呋喃。这些以完全类似的方式发生反应
方式;掺入的电子释放芳基越多
异噻唑的 5-芳酰基是唯一的(强
极化)或主要(弱极化)产物。然而,随着
3-溴-2-(4-甲氧基苯基)-5-(4-硝基苯基)呋喃7,其中更多
反应性呋喃β位现已被取代,这
区域特异性被逆转(得到异噻唑8)。当其中之一
呋喃中的α-芳基被甲基取代,同样
区域特异性异噻唑的形成现在伴随着一些环和
侧链氯化
(15 → 16 + 17 + 18)。全部
这些结果可以通过机制(方案 2 和 5)来解释
涉及呋喃的初始亲电攻击以给出
β-噻唑基衍生物。这种高反应性(氮素)
然后取代基诱导呋喃环 21 发生新的开环,得到
高度离域的中间体 22 循环至
异噻唑。