Effects of neurodegeneration have been linked to inefficient detoxification of free radicals due to lowered concentrations of antioxidants, especially glutathione, in the brain. In the biosynthesis of glutathione, cysteine concentration is generally the limiting factor. Glutathione and cysteine administrations are not effective treatments for neurodegeneration because glutathione inefficiently crosses cell membranes and cysteine is neurotoxic at high concentrations. Prodrugs of glutathione and cysteine may have more favorable uptake and/or toxicity profiles. Three such prodrugs were synthesized with a 13C-label such that in vivo uptake of each and conversion to glutathione in the brain could be monitored by magnetic resonance imaging. L-[3-13C]-Cysteine was treated with sodium acetate trihydrate and acetic anhydride to give 2(R)-N-acetyl-[3-13C]-cysteine ([13C]-NAC; 96%). Addition of triphosgene to L-[3-13C]-cysteine provided 4(R)-[5-13C]-2-oxothiazolidine-4-carboxylic acid ([13C]-OTZ; 65%). A four-step pathway was used to synthesize ethyl γ-L-glutamyl-[3-13C]-L-cysteinate ([13C]-GCEE). L-[3-13C]-Cysteine was esterified (100% yield) and then cyclized with acetaldehyde to give ethyl 2(R,S)-methyl-[5-13C]-thiazolidine-4(R)-carboxylate (73%) as a mixture of two diastereomers (65:35). The thiazolidine was silylated (bis(trimethylsilyl)trifluoroacetamide) and reacted with N-phthaloyl-L-glutamic anhydride. Treatment with hydrazine afforded ethyl N-[γ-4′(S)-glutamyl]-2(R,S)-methyl-[5-13C]-thiazolidine-4(R)-carboxylate (48%; 73:27 mixture of diastereomers). This was converted to the desired product, [13C]-GCEE (49%), using mercury (II) acetate and hydrogen sulfide.
神经变性的影响与大脑中
抗氧化剂(尤其是
谷胱甘肽)浓度降低导致自由基解毒效率低下有关。在
谷胱甘肽的
生物合成过程中,半胱
氨酸浓度通常是限制因素。
谷胱甘肽和半胱
氨酸不能有效治疗神经变性,因为
谷胱甘肽不能有效穿过细胞膜,而半胱
氨酸在高浓度时具有神经毒性。
谷胱甘肽和半胱
氨酸的原药可能具有更有利的吸收和/或毒性特征。我们合成了三种带有 13C 标记的原药,以便通过磁共振成像监测每种原药在体内的吸收情况以及在大脑中转化为
谷胱甘肽的情况。用三
水醋酸钠和
醋酸酐处理 L-[3-13C]-半胱
氨酸可得到 2(R)-N-乙酰基-[3-13C]-半胱
氨酸([13C]-
NAC;96%)。将
三光气加入 L-[3-13C]-半胱
氨酸,可得到 4(R)-[5-13C]-2-氧代
噻唑烷-4-
羧酸([13C]-OTZ;65%)。γ-L-谷
氨酰-[3-13C]-
L-半胱氨酸乙酯([13C]-GCEE)的合成采用了四步途径。将 L-[3-13C]-半胱
氨酸酯化(产率 100%),然后与
乙醛环化,得到 2(R,S)-甲基-[5-13C]-
噻唑烷-4(R)-
羧酸乙酯(73%),它是两种非对映异构体的混合物(65:35)。
噻唑烷被
硅烷化(双(三甲基
硅基)三
氟乙酰胺),并与
N-邻苯二甲酰-L-谷氨酸酐反应。用
肼处理后得到 N-[γ-4′(S)-谷
氨酰]-2(R,S)-甲基-[5-13C]-
噻唑烷-4(R)-
羧酸乙酯(48%;73:27 非对映异构体混合物)。使用
乙酸汞 (II) 和
硫化氢将其转化为所需产物 [13C]-GCEE (49%)。