Oxidative desulfurization–fluorination of thioethers. Application for the synthesis of fluorinated nitrogen containing building blocks
作者:Verena Hugenberg、Roland Fröhlich、Günter Haufe
DOI:10.1039/c0ob00560f
日期:——
An oxidative desulfurizationâfluorination protocol has been used to synthesize (2S)-2-(difluoromethyl)-N-tosylpyrrolidine (6a) and (2S)-2-(trifluoromethyl)-N-tosylpyrrolidine (7a) from the (2S)-prolinol-derived (2S)-2-(4-chlorophenylthiomethyl)-N-tosylpyrrolidine (9) or (2S)-2-(dithian-2-yl)-N-tosylpyrrolidine (5). Efforts to prepare 3,3-difluoroalanine similarly from an N-protected S-aryl-cysteine ester 17 gave only traces of the target compound 18. Instead, an unique N-(α,α-difluorobenzyl)-N-αâ²,αâ²-dibromoglycine ester 19 was formed by an unprecedented sequence of reaction steps. A plausible mechanism is suggested involving a sulfur-assisted deoxygenation-difluorination of an imino oxygen and a haloform reaction like carbonâcarbon bond fission as key-steps. Efforts to prepare (2S)-2-(fluoromethyl)-N-tosylpyrrolidine (12) from (2S)-N-tosylprolinol (3) by treatment with Fluolead⢠(1-tert-butyl-4-trifluorosulfanyl-3,5-dimethylbenzene) gave only 5% of the target compound, but 95% of (3R)-3-fluoro-N-tosylpiperidine (11a) by ring enlargement.
氧化脱硫-氟化方案已用于从 (2S)-2-(二氟甲基)-N-甲苯磺酰吡咯烷 (6a) 和 (2S)-2-(三氟甲基)-N-甲苯磺酰吡咯烷 (7a) 合成-脯氨醇衍生的(2S)-2-(4-氯苯硫基甲基)-N-甲苯磺酰吡咯烷(9)或(2S)-2-(二噻烷-2-基)-N-甲苯磺酰吡咯烷(5)。类似地从 N-保护的 S-芳基-半胱氨酸酯 17 制备 3,3-二氟丙氨酸的努力仅得到了目标化合物 18 的痕迹。相反,独特的 N-(α,α-二氟苄基)-N-α α,β-二溴甘氨酸酯19是通过前所未有的一系列反应步骤形成的。提出了一种可能的机制,涉及亚氨基氧的硫辅助脱氧-二氟化和碳碳键裂变等卤仿反应作为关键步骤。通过 Fluolead™ (1-叔丁基-4-三氟硫基-3,5) 处理,从 (2S)-N-甲苯磺酰脯氨醇 (3) 制备 (2S)-2-(氟甲基)-N-甲苯磺酰吡咯烷 (12) -二甲基苯)仅得到 5% 的目标化合物,但通过扩环得到 95% 的 (3R)-3-氟-N-甲苯磺酰哌啶 (11a)。