Hydroformylation, of glycals

Attempts to separate the heptoses (43) and (44) as their diethyl dithioacetals were unsuccessful. 

Under similar hydroformylation conditions, 3,4-di-O-acetyl-D-xylal a Forded, in 20 %yield, 4,5-di-0-acetyl-2,6-anhydro-3-deoxy-cidefi / io-D-/t/ o-(45) and -D-xy/o-hexose (46), and the anhydrodeoxyhexitols (47) and (48). The sugars were characterized by their conversion into l,5-anhydro-4-deoxy-D-flmhtno-(7) and -L-xyio-hexitol (6). 

Although the anhydrodeoxyhexitols (47) and (48) were of interest, the primary objective of the authors was to synthesize anhydrodeoxy-aldoses. As a consequence, various mild conditions of oxidation were sought, in order to convert the anhydrodeoxyalditols (having one free primary hydroxyl group) into aldoses. Of the various methods tried, only that involving oxidation with methyl sulfoxide in JV,N -dicyclo-hexylcarbodiimide, according to the procedure of Pfitzer and Moffett, resulted in a conversion of (47) and (48), in 35% yield, into the aldoses (45) and (46). 

Procedures that have been used by other workers to minimize the side reactions of the aldehydes during the oxo reaction were found to give exceedingly complicated mixtures of products when applied to the glycals. However, use of ethyl orthoformate to protect the unstable aldehyde (as the stable, diethyl acetal) gave, for the heptoses (43) and (44), a small yield of the acetal.  

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The hydroformylation of glycals has been described in detail,308,34 and the reaction has been extended to the hydroxyglycal series (see p. 220). 

The cycloaddition of various 13-dipoles to levoglucosenone has been described (Scheme 7) The rhodium catalyzed hydroformylation of glycals in the presence of xn ortho-tert-butylphenyl) phosphite to afford 2-formyl derivatives such as 33 has appeared. ... 

Because two epimers in about equal amounts are produced in the 0X0 reaction on the glycals, it was surprising that the epimer of (50) was not produced in the oxo reaction of 2-acetoxy-tri-O-acetyl-D-glucal. In order to determine whether a different catalyst might influence the composition of the mixture of products, compound (49) was allowed to react with carbon monoxide and hydrogen in the presence of a rhodium, instead of a cobalt, catalyst. Surprisingly, this led to deacetylation, but hydroformylation of the substrate did not occur. 

Hydroformylation, see also Oxo reaction, of epoxides, 63 of glycals, 86 of olefins, 61... 

Several acetylated glycals have been subjected to this reaction, and the main products obtained after deacetylation are (despite an initial report to the contrary ) the 2,6-anhydro-3-deoxyalditols. Thus, for example, 1,5-anhydro-4-deoxy-D-lj/xo-hexitol (42) (a 2,6-anhydro-3-deoxyhexitol), and the it-ribo isomer, were isolated in high combined yield, in the ratio 1 0.7, from di-O-acetyl-D-arabinal, after hydroformylation followed by deacetylation and reduction of the formyl compounds (which are produced together with the anhydrodeoxyalditols). Structural analyses of the products were carried out with the aid of periodic acid degradations, nuclear magnetic resonance spectroscopy, and x-ray crystallographic analysis. ... 

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