An Ming, Maitra Uday, Neidlein Ulf, Bartlett Paul A
Center for New Directions in Organic Synthesis, Department of Chemistry, University of California, Berkeley, California 94720-1460, USA.
J Am Chem Soc. 2003 Oct 22;125(42):12759-67. doi: 10.1021/ja036627+.
A chemical synthesis of both diastereomers of the tetrahedral intermediate involved in 5-enolpyruvylshikimate 3-phosphate synthase (EPSPS) catalysis has been accomplished. Combination of methyl dibromopyruvate with a protected shikimic acid derivative, phosphorylation, and lactonization afforded the intermediates (S)-15 and (R)-15, whose configurations were assigned by NMR. After introduction of the 3-phosphate group and deprotection, photoinitiated radical debromination of the dibromo analogues (S)-5 and (R)-5 was accomplished with tributyltin hydride in mixed aqueous solvents in the presence of surfactant to give the pyruvate ketal phosphates (R)-TI and (S)-TI, respectively. These compounds are stable at high pH, but decompose at pH 7 with a half-life of ca. 10 min. (R)-TI proved to be inert to EPSPS, while (S)-TI was converted by the enzyme to a mixture of 5-enolpyruvylshikimate 3-phosphate, shikimate 3-phosphate, and phosphoenolpyruvate. The demonstration that the enzymatic intermediate possesses the S-configuration at the ketal center confirms the mechanism as an anti addition followed by a syn elimination. Furthermore, it appears that the syn stereochemistry of the second step requires the phosphate leaving group to serve as the base in catalyzing its own elimination.
已完成对参与5-烯醇丙酮酸莽草酸-3-磷酸合酶(EPSPS)催化的四面体中间体的两种非对映异构体的化学合成。将二溴丙酮酸甲酯与一种受保护的莽草酸衍生物相结合、进行磷酸化反应和内酯化反应,得到中间体(S)-15和(R)-15,其构型通过核磁共振(NMR)确定。在引入3-磷酸基团并脱保护后,在表面活性剂存在下,于混合水性溶剂中用三丁基氢化锡对二溴类似物(S)-5和(R)-5进行光引发自由基脱溴反应,分别得到丙酮酸缩酮磷酸酯(R)-TI和(S)-TI。这些化合物在高pH值下稳定,但在pH 7时分解,半衰期约为10分钟。(R)-TI对EPSPS呈惰性,而(S)-TI被该酶转化为5-烯醇丙酮酸莽草酸-3-磷酸、莽草酸-3-磷酸和磷酸烯醇丙酮酸的混合物。对酶促中间体在缩酮中心具有S构型的证明证实了该反应机理为反式加成后顺式消除。此外,第二步的顺式立体化学似乎要求磷酸离去基团在催化自身消除时充当碱。