Elongation through the Aldonolactone Carbonyl Group

Chain Elongation Through the Aldonolactone Carbonyl Group 

The addition of organometallic reagents to the carbonyl group of conveniently substituted aldonolactones constitutes a viable chain-extension method. The reaction leads to the formation of hemiacetals of glyculoses, 1-methylene sugars, and C-glycosyl compounds, which are precursors of, or occur as subunits of, a variety of natural products. 

Carbohydrate lactones have been used as the carbonyl reagent in the Reformatsky reaction. Thus, 2,3 5,6-di-O-cyclohexylidene-D-mannono-1,4-lactone [44, obtained by oxidation of the mannofuranose derivative (49)] reacted with ethyl bromoacetate and zinc to give the protected 2-deoxy-3-octulosonic acid ethyl ester (45a) in 69% yield (50). Ketonic hydrolysis with potassium hydroxide in aqueous methanol, followed by acidification and heating, afforded the 1-deoxyheptulose derivative 45b. Similarly, starting from compound 44, the 1-C-substituted allyl and propar-gyl lactols were prepared on reaction with allyl or propaigyl bromides in the presence of zinc (51). 

Condensation of 2,3 5,6-di-O-isopropylidene-D-gulono-1,4-lactone (7a) with ethyl bromoacetate in the presence of zinc also gave the expected 

Reformatsky product (46). Treatment of 46 with methanol and an acid ion-exchange resin afforded the methyl glycoside 47a, the product of partial hydrolysis (47b), and the 3,8-anhydro sugar 48. Synthetic transformations were performed on these products (52). 

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V. Chain Elongation Through the Aldonolactone Carbonyl Group... 

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