Alcohols glycals

The glycals are easily converted into the 1,2-dihalo-derivatives, which in principle can act as glycosyl donors. However, these derivatives have not found wide application in glycoside synthesis, mainly because of the low facial selectivity in the initial addition of the electrophilic species [143-145]. In an example of a successful application, 2-deoxy-2-bromo-a-D-glucopyranosyl bromide [146] has been shown to give predominantly the 2-deoxy-P-D-glucopyranosides in silver-triflate-promoted reactions with alcohols. 

Finally, S-(2,6-dideoxyglycosyl)phosphorodithioates 90 were prepared from 6-deoxy-L-glycals 89 [158] and transformed to either a- or [1-2-dcoxyglycosides 91 and 92 depending on the conditions used in adding the alcohol (Scheme 5.55). 

Finally (pathway e, Scheme 5.56), triazoline 103 formed by cyclo addition of azide to glycal 1 can be photolytically converted into a 1,2-aziridine intermediate 104, from which 2-benzylamino-2-deoxy-P-glucosides can be formed on addition of an alcohol and catalytic scandium triflate [176]. 

Glycals, for example D-glucal (23), being vinyl ethers are susceptible to acid catalysed additions of alcohols which result in the formation of 2-deoxyglycosides, but elimination reactions accompany additions of this type 86) go such products are better prepared by way of halogen adducts or by the alkoxymercuration reaction (see below). On the other hand, the addition procedure can be more suitable than the alcoholysis... 

The analogous product from the pentose glycals and 2-deoxy-pentoses would be furfuryl alcohol, but, as it is unstable and is readily converted into levulinic acid under the conditions of formation, it is difficult to isolate. The spectral data52 and the fact that levulinic acid is the common product from 2-deoxy-D-erythro-pentose, D-ara-binal, and furfuryl alcohol19 substantiate the supposition that the mechanism is analogous to that just described. Other (unknown) products are formed in significant yield from the dehydration of 2-deoxy-D-ert/fhro-pentose.52,84 The mechanism of formation of levulinic acid is discussed in Section V (see p. 212). 

When heated in aqueous acidic medium, D-glucose, 2-deoxy-D-erythro-pentose, furfuryl alcohol, 5-(hydroxymethyl)-2-furaldehyde, and five-carbon glycals yield levulinic acid. Because of the importance of developing tests for 2-deoxy-D-en/ihro-pentose, the reactions have been extensively studied, and several mechanisms have been suggested.3,198-208 For converting furfuryl alcohol (101) into levulinic... 

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