Glycals properties

This delightfully simple methodology is generally applicable to hydroxy-glycal esters and has provided acylated ketoximes of type 25 in large variety acylated pyranoid ketoximes, featuring such useful properties as high tendency for crystallization, ease of isolation and stability (29). 

General Physical Properties.—For some glycals and their derivatives the absorption spectrum in the ultraviolet,83 the heat of combustion83 and the rotatory dispersion84 have been measured and compared with one another and with like properties of the corresponding sugars and derivatives. 

As already mentioned in this chapter, nucleophilic carbohydrates have, until recently, been poorly investigated [218]. However, several groups have reported, at almost the same time, on the properties and availability of anomeric carbanions. Glycals were the first substrates to be investigated in this manner because no elimination can occur. 

Similarly, reaction of 1,4-diacetoxy-l,3-butadiene with methyl glyoxalate afforded glu-curonate glycal [228] and a 2,3-unsaturated isomer. Cycloaddition reactions performed on substrates with inverse electronic properties (e. g., enol ether as a dienofile and unsaturated carbonyl compound as a heterodiene) afford not only the expected products but also ones having high endo selectivity [229,230] as exemplified below by olivose 126 synthesis (O Scheme 43). 

For example, cassette coupling and protecting group manipulations provided trifluoromethansulfonyl (TF) disaccharide 28, which was evolved in solution to the trimeric TF cluster 29 (Scheme 5). Here, epoxide 25 derived from glycal 25 proved to be a powerful donor in reaction with cassette 26 to afford the /3-linked disaccharide 27 (Scheme 5). Similarly, 2,6-STF and STn clusters and the clustered LewisY were also prepared, conjugated to keyhole limpet hemocyanin (KLH) or bovine serum albumin (BSA), and their immunological properties were evaluated as anticancer vaccine. ... 

The properties of various glycals and their derivatives are given in Table I. 

Ness, R. K., and H. G. Fletcher, Jr. 2-Deoxy-D-er> f/iro-pentose. X. Synthesis of l,4-Anhydro-3,5-di-0-benzoyl-2-deoxy-D-er> //iro-pentose-l-enol. Derivatives of a Furanose-related Glycal. J. Organ. Chem. (U.S.A.) 28, 435 (1963) Haga, M., and R. K. Ness Preparation and Properties of 3,5-Di-0-p-anisoyl-l,2-dideoxy-D-er> t/iro-pentofuranose-l-ene. Various p-Anisoylated Derivatives of D-Ribofuranose. J. Organ. Chem. (U. S. A.) 30, 158 (1965). 

To inhibit the formation of the 3-substituted glycals, starting materials with allylic groups with enhanced leaving properties are recommended [81] or, otherwise, AT-trimethylsilylated bases can be used with catalysts such as trityl on lithium perchlorate [82]. 

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