concentrated below 30 °C to give the title compound, 0.79 g (70%).Cbz-Aep(OLi)-Gly(OLi) [18, (aa)i = Gly]. An ethanol solution (30 mL) of 11 [(aa)x = Gly, R = Me, 2.0 g, 5.6 mmol) was mixed with 2 N lithium hydroxide (17 mL). After 24 h at room temperature, the reaction mixture was neutralized with 1 N hydrochloric acid and concentrated below 30 °C. Upon mixing the residue with anhydrous ethanol, the title compound was precipitated as a powder, which was purified by reprecipitation using water and ethanol as dissolving and precipitating solvents, respectively, 1.96 g (52%).H-Gly-Aep(OLi)-Gly(OLi) (23). The tripeptide diester 16a (0.24 g, 0.58 mmol) was hydrogenated using Pd/C (10%, 0.12 g) in ethanol (10 mL) at atmospheric pressure and 25 °C for 1 h. After removal of the catalyst with suction filtration, the filtrate was mixed with 2 N lithium hydroxide (1.7 mL) and allowed to stand at room temperature for 20 h. The hydrolysate was then neutralized with 1 N hydrochloric acid and concentrated to afford the residue, which upon triturating with anhydrous ethanol gave amorphous powder, 0.11 g. The 1H NMR spectrum agreed with the assigned structure of the title compound, but also indicated
Direct fluorination of enol-type 3-substituted pyruvates, e.g., those having aromatic and acyl substituents, by using molecular fluorine proceeds smoothly to produce a mixture of ketoand enol-type 3-substituted 3-fluoropyruvates in moderate to good yields, while the reaction with keto-type 3-substituted pyruvates, e.g., those having alkyl substituents, gives complex mixtures of products. The resultant 3-substituted 3-fluoropyruvates have been subsequently converted into silyl enol ether derivatives, which were hydrolyzed under mild conditions to afford the crystalline enol-type 3-substituted 3-fluoropyruvates. The results obtained in this study suggest the feasibility of direct a fluorination of ketones via free enol compounds.
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