Two approaches to the synthesis of 2-chloro-9-(2-deoxy-2-fluoro-β-D-arabinofuranosyl)adenine (1, clofarabine) were studied. The first approach consists in the chemical synthesis of 2-deoxy-2-fluoro-α-D-arabinofuranose-1-phosphate (12a, 2FAra-1P) via three step conversion of 1,3,5-tri-O-benzoyl-2-deoxy-2-fluoro-α-D-arabinofuranose (9) into the phosphate 12a without isolation of intermediary products. Condensation of 12a with 2-chloroadenine catalyzed by the recombinant E. coli purine nucleoside phosphorylase (PNP) resulted in the formation of clofarabine in 67% yield. The reaction was also studied with a number of purine bases (2-aminoadenine and hypoxanthine), their analogues (5-aza-7-deazaguanine and 8-aza-7-deazahypoxanthine) and thymine. The results were compared with those of a similar reaction with α-D-arabinofuranose-1-phosphate (13a, Ara-1P). Differences of the reactivity of various substrates were analyzed by ab initio calculations in terms of the electronic structure (natural purines vs analogues) and stereochemical features (2FAra-1P vs Ara-1P) of the studied compounds to determine the substrate recognition by E. coli nucleoside phosphorylases. The second approach starts with the cascade one-pot enzymatic transformation of 2-deoxy-2-fluoro-D-arabinose into the phosphate 12a, followed by its condensation with 2-chloroadenine thereby affording clofarabine in ca. 48% yield in 24 h. The following recombinant E. coli enzymes catalyze the sequential conversion of 2-deoxy-2-fluoro-D-arabinose into the phosphate 12a: ribokinase (2-deoxy-2-fluoro-D-arabinofuranose-5-phosphate), phosphopentomutase (PPN; no 1,6-diphosphates of D-hexoses as co-factors required) (12a), and finally PNP. The substrate activities of D-arabinose, D-ribose and D-xylose in the similar cascade syntheses of the relevant 2-chloroadenine nucleosides were studied and compared with the activities of 2-deoxy-2-fluoro-D-arabinose. As expected, D-ribose exhibited the best substrate activity [90% yield of 2-chloroadenosine (8) in 30 min], D-arabinose reached an equilibrium at a concentration of ca. 1:1 of a starting base and the formed 2-chloro-9-(β-D-arabinofuranosyl)adenine (6) in 45 min, the formation of 2-chloro-9-(β-D-xylofuranosyl)adenine (7) proceeded very slowly attaining ca. 8% yield in 48 h.
对2-氯-9-(2-脱氧-2-氟-β-D-阿拉伯呋喃核糖基)腺嘌呤(1,克洛法比林)的合成进行了两种方法的研究。第一种方法包括通过将1,3,5-三-O-苯甲酰基-2-脱氧-2-氟-α-D-阿拉伯呋喃糖(9)经过三步转化成磷酸2-脱氧-2-氟-α-D-阿拉伯呋喃糖(12a,2F Ara-1P)而无需中间产物的分离。使用重组大肠杆菌嘌呤核苷酸磷酸化酶(PNP)催化12a与2-氯腺嘌呤的缩合反应,形成克洛法比林,收率为67%。该反应还与多种嘌呤碱基(2-氨基腺嘌呤和次黄嘌呤)、它们的类似物(5-氮杂-7-脱氮鸟嘌呤和8-氮杂-7-脱氮次黄嘌呤)以及胸腺嘧啶进行了研究。结果与使用α-D-阿拉伯呋喃糖磷酸(13a,Ara-1P)的类似反应进行了比较。通过从头算计算分析了各种底物的反应性差异,考虑了电子结构(天然嘌呤与类似物)和立体化学特征(2F Ara-1P与Ara-1P)来确定大肠杆菌核苷酸磷酸酶对底物的识别。第二种方法从2-脱氧-2-氟-D-阿拉伯糖的级联一锅酶促转化开始,形成磷酸12a,然后与2-氯腺嘌呤缩合,从而在24小时内以约48%的产率制备克洛法比林。下列重组大肠杆菌酶催化了2-脱氧-2-氟-D-阿拉伯糖的顺序转化为磷酸12a:核糖激酶(2-脱氧-2-氟-D-阿拉伯呋喃糖-5-磷酸)、磷酸戊糖异构酶(PPN;不需要D-己糖的1,6-二磷酸作为辅因子)(12a),最后是PNP。研究了D-阿拉伯糖、D-核糖和D-木糖在相关2-氯腺嘌呤核苷的类似级联合成中的底物活性,并将其与2-脱氧-2-氟-D-阿拉伯糖的活性进行了比较。如预期,D-核糖表现出最佳的底物活性(30分钟内2-氯腺苷(8)产率达90%),D-阿拉伯糖在大约45分钟达到浓度平衡,形成2-氯-9-(β-D-阿拉伯呋喃核糖基)腺嘌呤(6),而形成2-氯-9-(β-D-木糖呋喃核糖基)腺嘌呤(7)的过程非常缓慢,在48小时内产率达到约8%。