Phase-transfer-catalyzed synthesis of aryl alpha-ketosides of N-acetylneuraminic acid. A 2-methylfluoran-6-yl glycoside of N-acetylneuraminic acid, 2-methyl-6-(5-acetamido-3,5-dideoxy-alpha-D-glycero-D-galacto- nonulopyranosylonic acid)xanthene-9-spiro-1'-isobenzofuran-3'-one, a new substrate for neuraminidase assay.

Glycosidation of N-acetylneuraminic acid by phase-transfer catalysis in chloroform-aqueous alkali gave several known and some new aryl alpha-ketosides in a short reaction time and in good yields. The 4-methylumbelliferyl alpha-ketoside, the standard substrate for neuraminidase, was prepared in a yield of up to 70%. New Neu5Ac ketosides were prepared with fluorescein and the fluorescein analog, 2-methyl-6-hydroxyfluoran (2-methyl-6-hydroxyxanthene-9-spiro-1'-isobenzofuran-3'-one) as aglycons, the latter being synthesized from 2-(2-hydroxy-5-methyl-benzoyl) benzoic acid and 3-fluorophenol. The alpha configuration was ascertained by 400-MHz 1H-n.m.r. spectroscopy and by cleavage of the ketosides with neuraminidases from Vibrio cholerae and Clostridium perfringens. The enzymic hydrolysis of the 2-methylfluoran-6-yl ketoside gave Km values of 82 microM (V. cholerae) and 96 microM (C. perfringens).