Aldonolactones 3-elimination

As acylation of aldonolactones is related to -elimination reactions, the fully acylated aldonolactone derivatives are mentioned in Section IX (see also Sections VI and VIII). Only a few studies on selective acylation of aldonolactones have been described. The partially acylated derivatives have been employed as glycosylating agents or for the preparation of monometh-ylated sugars. 

The application of lithiumtrimethylsilyl acetate for the C-l elongation of aldonolactones has been examined (73). Although the reagent had been successfully used for the alkenation of lactone carbonyl groups (74), in the case of aldonolactones 10b or 25b only insignificant yields of the alkenes, but 30 - 40% of the lactols 49a or 50a, were obtained (73). However, these lactols, alternatively prepared in good yields by a Reformatsky-type reaction (53,54), were readily eliminated to the desired alkenes by simple treatment with methanesulfonyl chloride-triethylamine at 0°. Thus, from 49a or 50a separable E,Z mixtures (76a and 76b, or 77a and 77b, respectively) were obtained in good yields (73). 

Because of the activating effect of the carbonyl group, aldonolactone derivatives readily undergo -elimination reactions to yield unsaturated lactones. 

The elimination of benzoic acid from benzoylated aldonolactones takes place in successive steps. The relative extent of breaking the C-H bond and the C - OBz bond depends on the ease of removal of the hydrogen as a proton and the fact that the benzoate is a poor leaving-group. The H-2 proton in aldonolactones is acidic and it is likely that an E, cb mechanism, through an... 

The elimination of benzoylated 1,4-aldonolactones in pyridine is difficult to control, and further elimination reactions take place, decreasing the yield of monounsaturated derivative and consequently of the 3-deoxy sugar.83... 

R. M. de Lederkremer, M. I. Litter, and L. F. Sala, P-Elimination in aldonolactones A convenient synthesis of 2,4,6-tri-0-benzoyl-3-deoxy-D-ara6/no-hexono-1,5-lactone, Carbohydr. Res, 36 (1974) 185-187. 

O. Varela, A. Fernandez Cirrelli, and R. M. de Lederkremer, P-Elimination in aldonolactones. Synthesis of 3,6-dideoxy-L-araWno-hexose (Ascarylose), Carbohydr. Res., 70 (1979) 27-35. 

The acylation of aldonolactones over extended periods leads to the formation of unsaturated lactones, with -elimination being promoted by pyridine. Thus, treatment of D-glucono-1,5-lactone with an excess of benzoyl chloride and pyridine for 16 h at room temperature gave crystalline 2,4,6-tri-0-benzoyl-3-deoxy-D-eryt/n-u-hex-2-enono-1,5-lactone (31) in 97% yield.98 Other examples may be found in Ref. 37. 

Deoxy- and dideoxy-aldonolactones have also been prepared via the bromo derivatives, which on hydrogenolysis yield the corresponding deoxyaldonolactones.293 3-Deoxyaldonolactones have been obtained via /1-elimination reactions on aldonolactones and catalytic hydrogenation of the intermediate 2-enonolactone derivatives.37 The synthetic potential of this reaction was used for the synthesis of 3-deoxyaldonolactones by combining the base-catalyzed elimination of peracylated lactones with the in situ reduction.294... 

When the bromodeoxy lactones were boiled in water tetrahydrofurans were formed, whereas catalytic hydrogenation caused deoxygenation at C-2 and/or at the primary position, while the acetylated aldonolactones could also be reduced to 3-deoxylactones after elimination and stereospecific reduction. Substitution by azide introduced nitrogen functions at C-2 or at both bromo-substituted carbons, and these compounds could be converted into the corresponding amino acids. The new molecules could furthermore be reduced to the corresponding aldoses or alditols. 

Deoxy-sugars can be synthesized from aldonolactones via -elimination or bromlnatlon procedures. 3-Deoxy-D-arablno-hexono-1,5-lactone, readily available from D-glucono-1,5-lactone by sequential acetylation, B-eliminatlon, hydrogenation, and deprotection, has been converted to 2-deoxy-D-erythro-pentose In 92)J yield by Indirect electrochemical oxidation. 3-Deoxy-D-arablno-hexose, which exists as a 53 30j 17 mixture o-f the o- and B-pyranose, and a-furanose forms In aqueous solution, has been synthesized by reduction of the 2,U,6-trl-0-benzoyl derivative of... 

Per-benzoylated aldonolactones undergo a reductive elimination reaction with samarium diiodide leading to unsaturated derivatives (Scheme 3). Deoxygenation at the 2-position can also take place. The difference in the reductive elimination patterns between the D-g/uco and D-galacto isomers may be due to the axial disposition of the OBz in the galacto isomer. 

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