A new tricyclic ring and a nitrogen–sulfur analogue of the tri-pentagon bowl: cycloaddition reactions of the unstablised 1,3,4-thiadiazolium-3-methanide 1,3-dipole: steric influences on the endo-effect: substituted pyrrolo[2,1-b]-1,3,4-thiadiazole systems: azolium 1,3-dipoles
作者:Richard N. Butler、Georgina M. Smyth、Patrick McArdle、Desmond Cunningham
DOI:10.1039/b208544p
日期:——
1,3,4-Thiadiazolium-3-methanide 1,3-dipole 6 and the 2,5-diphenyl and 2,5-dimethyl derivatives, 4 and 5, were generated at â60 °C in dichloromethane. Cycloaddition reactions with substituted alkenes gave many new derivatives of the pyrrolo[2,1-b][1,3,4]thiadiazole ring system. The first examples of a bowl-shaped tricyclic nitrogen-sulfur analogue of the tripentagon bowl, a 3,4,10-triaza-6-thiatricyclo[6,3,0,03,7]undecane ring system were obtained from N-substituted maleimide dipolarophiles. The reactions displayed predominantly endo-stereochemistry but with decreasing size of the substituent at the incipient 7a-fusion bridgehead in the cycloaddition transition state, the extent of exo-cycloaddition increased. The cycloadduct endo
â¶
exo ratio was reduced from exclusively endo to ca. 2 â¶ 1 on changing the 1,3-dipole from the 2,5-diphenyl derivative 4 to the unsubstituted case 6. X-Ray crystal structures are reported for 2,7a-diphenyl-5,6,7,7a-tetrahydropyrrolo[2,1-b][1,3,4]thiadiazole-endo-6,7-N-methyldicarboxyimide 7a, 2,7a-diphenyl-5,6,7,7a-tetrahydropyrrolo[2,1-b][1,3,4]thiadiazole-endo-6,7-N-phenyldicarboxyimide 9e and 2,7a-diphenyl-5,6,7,7a-tetrahydropyrrolo[2,1-b][1,3,4]thiadiazole-7-endo-carbonitrile 13.
1,3,4-Thiadiazolium-3-methanide 1,3-二极 6 以及 2,5-二苯基和 2,5-二甲基衍生物 4 和 5 是在±60°C 的二氯甲烷中生成的。与取代烯烃的环加成反应产生了吡咯并[2,1-b][1,3,4]噻二唑环系统的许多新衍生物。从 N-取代马来酰亚胺二极性化合物中首次获得了碗状三环氮硫类似物--3,4,10-三氮杂-6-硫杂三环[6,3,0,03,7]十一烷环系统。反应主要表现为内-立体化学,但随着环化转变态中初生 7a 融合桥头取代基大小的减小,外-环化程度也随之增加。将 1,3-偶极从 2,5-二苯基衍生物 4 转变为未取代的情况 6 时,环加成产物的内和和(endo)和外和(exo)比率从完全内和(endo)降低到约 2 和 1。报告了 2,7a-二苯基-5,6,7,7a-四氢吡咯并[2,1-b][1,3,4]噻二唑内向-6,7-N-甲基二羧酰亚胺 7a、2,7a-二苯基-5,6,7、7a-四氢吡咯并[2,1-b][1,3,4]噻二唑-内向-6,7-N-苯基二羧酰亚胺 9e 和 2,7a-二苯基-5,6,7,7a-四氢吡咯并[2,1-b][1,3,4]噻二唑-7-内向甲腈 13。