Thermal inverse-electron-demand Diels-Alder reactions of 5-aminoimidazoles and 2,4,6-tris(ethoxycarbonyl)-1,3,5-triazine (2) with spontaneous retro-Diels-Alder loss of ethyl cyanoformate and elimination of ammonia give 2,6-bis(ethoxycarbonyl)purines. A report that selective alkaline hydrolysis followed by acid-catalyzed decarboxylation gave 6-(ethoxycarbonyl)purine products was not in harmony with known reactions in purine chemistry. Our reinvestigation has shown that the 6-(ethoxycarbonyl) group undergoes preferential base-promoted hydrolysis, as expected, but regioselectivity for attack of hydroxide at the carbonyl group at C6 is not high (relative to hydrolysis of both C2 and C6 esters). The structure of 9-benzyl-2-(ethoxycarbonyl)purine was determined by X-ray crystallography and confirmed by Curtius rearrangement of the azidocarbonyl analogue to give 2-amino-6-benzylpurine. Acid-catalyzed decarboxylation of the 2,6-dicarboxylate formed during hydrolysis gave 9-benzylpurine, and Curtius rearrangement of 2,6-bis(azidocarbonyl)-9-benzylpurine gave 2,6-diamino-9-benzylpurine. Attempted applications of inverse-electron-demand Diels-Alder reactions of 2 with nucleoside derivatives were problematic.
5-氨基咪唑和2,4,6-三(乙氧羰基)-1,3,5-三嗪的热反向电子需求Diels-Alder反应与乙基氰甲酸乙酯自发性逆Diels-Alder失去和氨的消除反应,形成2,6-双(乙氧羰基)嘌呤。一份报告指出,选择性碱水解后接酸催化脱羧可得到6-(乙氧羰基)嘌呤产物,但与嘌呤化学中已知的反应不一致。我们的再研究表明,6-(乙氧羰基)基团更倾向于碱促水解,但羟基攻击C6羰基的区域选择性并不高(相对于C2和C6酯的水解)。9-苄基-2-(乙氧羰基)嘌呤的结构经X射线晶体学确定,并通过Curtius重排验证了其氮氧羰基类似物的2-氨基-6-苄基嘌呤。在水解过程中形成的2,6-二羧酸酯的酸催化脱羧产生了9-苄基嘌呤,而2,6-双(氮氧羰基)-9-苄基嘌呤的Curtius重排产生了2,6-二氨基-9-苄基嘌呤。尝试使用反向电子需求Diels-Alder反应与核苷衍生物的2进行应用时出现问题。