Pyranosiduloses. II. The synthesis and properties of some alkyl 2,3-dideoxy-2-enopyranosid-4-uloses

Ethyl 2,3-dideoxy-α-D-erythro-hex-2-enopyranoside (9a) an accessible and highly crystalline substance is selectively oxidized at the allylic position to ethyl 2,3-dideoxy-α-D-glycero-hex-2-enopyranosid-4-ulose (10a) in 80% yield. This hydroxyketone can be benzoylated, acetylated, and tosylated, and the resulting esters are all highly crystalline, as is the parent ketone. 3,4-Di-O-acetyl-D-xylal reacts with ethanol under boron trifluoride catalysis to give the anomeric mixture of ethyl 4-O-acetyl 2,3-dideoxy-D-glycero-pent-2-enopyranosides which may be separated after deacetylation. The deacetylated α-D anomer (14α) is readily oxidized by manganese dioxide to give an ethyl 2,3-dideoxy pent-2-enopyranosid-4-ulose (6-ethoxy-2,6-dihydropyran-3-one) (12); however, the corresponding β-D (14β) anomer resists oxidation. All of these ketones display a diagnostic nuclear magnetic resonance pattern consisting of clean doublets for H-1 and -3, and a doublet of doublets for H-2. Reduction of 10a with lithium aluminum hydride gives back the parent D-erythro alcohol exclusively, and catalytic hydrogenation saturates the olefinic double bond. These α,β-unsaturated ketones (10) in the hexose series are all levorotatory although their parent D-erythro alcohols (9) are strongly dextrorotatory; the saturated ketones derived from them are also strongly dextrorotatory. The epimeric D-threo alcohols which should also be oxidizable to 10 are however strongly levorotatory.