Abstract
Incubation of 2-deoxy- D- lyxo-hexose with β-galactosidase, gave 2 in small (≈2%) yield a compound which was shown to be 3,6-anhydro-2-deoxy- D- lyxo-hexose (“ D-isogalactal” 2), identified by comparison of the reduced and acetylated product with an authentic sample of 2,3,6-tri- O-acetyl-1,4-anhydro-5-deoxy- D- arabino-hexitol. Enzymic synthesis of 2 is likely to start from the 2-deoxy- D- lyxo-hexopyranosyl cation ( 10) which undergoes β-elimination to give 2,3-dideoxy- D- threo-hex-2-enose ( 12, “ D-pseudogalactal”). Spontaneous intramolecular addition then yields 2. This pathway would support the hypothesis of a generally occurring formation of a free cyclic carboxonium ion in the action of β-galactosidase on sterically suitable aldohexopyranoses and aldohexopyranosides.
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