Acetyl ester from diastereoselective synthesis

The treatment of /Vb-methoxycarbonyl-L-tryptophan methyl ester (9) with trifluoroacetic anhydride in trifluoroacetic acid gave directly the A -trifl uoroacetyl cyclic tautomer with excellent diastereoselectivity [90% yield, endo/exo 97 3 ( ll NMR)]60. Furthermore, pure A a-sulfonyl-entfo-derivatives (X-ray) were prepared from lo206,61 and were employed to prepare homochiral a-, / -, and a, -substituted tryptophan derivatives and pyrrolo[2,3-b]indole alkaloids206 59 62. Similarly, the /Va-acetyl-enc/o-pyrrolo-indole 10 was used for the synthesis of 5-substituted L-tryptophan derivatives63,64. 

Perhaps the most direct approach to 2,3-di-O-methyl tartrates is to treat the corresponding ester, such as lb, with a base like sodium hydride together with excess methyl iodide. Performed on a scale of up to one kilogram, this reaction provides in 66% yield diethyl (+ )-2,3-di-O-methyltartrate (569). Saponification of the diesters followed by anhydride formation with refluxing acetyl chloride furnishes 570, which is converted to 3,4-dimethoxy-thiopyrrolidone (571). This is the key intermediate for the synthesis of the antibiotic ani-somycin (572). The synthesis itself is not diastereoselective, and suffers from the tedious methodology required to introduce the correct acetoxy functionality present in 572 [183] (Scheme 126). 

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