On the [2+2] Cycloaddition of 2-Aminothiazoles and Dimethyl Acetylenedicarboxylate. Experimental and Computational Evidence of a Thermal Disrotatory Ring Opening of Fused Cyclobutenes
作者:Mateo Alajarín、José Cabrera、Aurelia Pastor、Pilar Sánchez-Andrada、Delia Bautista
DOI:10.1021/jo060664c
日期:2006.7.1
The reaction of 2-(phenylamino)- and 2-(dimethylamino)thiazoles with dimethyl acetylenedicarboxylate led unexpectedly to dimethyl 6-(phenylamino)- and 6-(dimethylamino)-3,4-pyridinedicarboxylates. Those compounds reasonably result from a sequence of reactions initiated by a [2 + 2] cycloaddition of the alkyne to the formal CC of the thiazole ring. These pyridines were obtained in nearly all the cases
2-(苯基氨基)-和2-(二甲基氨基)噻唑与乙酰二羧酸二甲酯的反应出乎意料地导致了6-(苯基氨基)-和6-(二甲基氨基)-3,4-吡啶二甲酸二甲酯。这些化合物合理地由炔烃[2 + 2]环加成至噻唑环的形式C C引发的一系列反应产生。在几乎所有情况下,这些吡啶都是唯一的反应产物在相当温和的条件下并以中等至良好的收率。相反,仅在一种特定情况下获得了由[4 + 2]环加成然后硫磺挤出而产生的区域异构体2-氨基-3,4-吡啶二羧酸酯。通过计算研究了导致两种区域异构体交替出现的两个反应路径。发现各自的[2 + 2]和[4 + 2]环加合物是由共同的偶极中间体逐步形成的。值得注意的是,[2 + 2]环加成之后的步骤(即,稠合的环丁烯中间体的开环得到全部-顺式1,3-噻氮平)以旋转方式发生。尽管几何约束和电子因素可能会降低旋转的能量,但是导致电子重组的1 - 5噻嗪类化合物的稠密五元环的含义是决定性的。