Guanine derivatives substituted at N7 with 4-R-benzyl groups (R = H, MeO, NO2) have been evaluated in the regioselective N9-alkylation of guanine. Given the capricious removal of (substituted) benzyl groups from guanine derivatives and pent-4-enoylation of guaninium hydrochloride, an improved alternative approach has been elaborated consisting in the pent-4-enoylation and per-O-acetylation of guanosine (8), 4-nitrobenzylation at N7 followed by N-glycoside hydrolysis (10), N9-alkylation (13) and deprotection with sodium dithionite to afford the peptide nucleic acid building block tert-butyl [N2-(pent-4-enoyl)guanin-9-yl]- acetate (15) in 36% overall yield. This avoids N7 regioisomer formation, solubility problems and any chromatographic purification. A remarkable influence of the O- and/or N2-acyl groups on the stability of N-glycosidic bond and reactivity of 2-amino group was observed. The structure of a pyrimidine by-product 12 arising from the imidazole ring-opening of guaninium salt 4d in alkaline medium has been elucidated by 2D NMR.
鸟嘌呤衍生物在N7位置被4-R-苄基基团(R = H,MeO,NO2)取代,已在鸟嘌呤的N9-烷基化中进行评估。鉴于从鸟嘌呤衍生物中(取代的)苄基基团的善变去除和鸟嘌呤盐酸盐的pent-4-烯酰化,已制定了一种改进的替代方法,包括pent-4-烯酰化和guanosine的per-O-乙酰化(8),N7位置的4-硝基苄基化,后接N-糖苷水解(10),N9-烷基化(13)和用亚硫酸钠去保护以得到肽核酸构建块tert-丁基[N2-(pent-4-烯酰)鸟嘌呤-9-基]-乙酸酯(15),总收率为36%。这避免了N7位置异构体的形成,溶解度问题和任何色谱纯化。观察到O-和/或N2-酰基对N-糖苷键的稳定性和2-氨基基团的反应性具有显著影响。通过2D NMR阐明了在碱性介质中由鸟嘌呤盐4d的咪唑环开环产生的嘧啶副产物12的结构。