Enols, from glycals

Conjugate additions of various organocopper reagents to the enones derived from glycals, e.g. 2-(diethoxyphosphoryl)hex-l-en-3-uloses, has been described. The reactions proved to be rapid, clean, and diastereoselective, giving rise to the formation of 3-oxo-2-phosphono-o -C-glycosides or the corresponding enol acetates.215... 

Anomeric triphenylphosphonium salts have been used as well as phenylsul-fides,but in the latter case extra stabilization is necessary (see below). Anomeric nitrosugars, which have been extensively studied in C-glycosylation reactions by Vasella, will be covered in Sect. 2.2.1 and ester enolates derived from 3-deoxy-2-ketoulosonic acids (sialic acid and KDO derivatives), which bear a structural similarity to 2-deoxy pyranosides, will be covered in Sect. 4.4. Deprotonation of anomeric phenylsulfones has been discussed in Sect. 2.1.1 and additional transformations on closely related compounds are presented in Scheme 14 [20]. Alkylation of phenylsulfone 54 with epoxide 55 provides adduct 56 which eliminates benzenesulfinic acid at room temperature to give the C(l)-alkylated glycal 57 a similar elimination is also observed with adducts derived from... 

Relevant pyranoid and furanoid compounds (with the exception of the furanoid 3-enes that, as enol ethers, are noted in Section IV. 1) have isolated double bonds in the sense that they are not components of vinyl ethers. In consequence their chemistry is distinct from that of the glycals and their derivatives in particular, their addition reactions are seldom regioselective as are those of the glycals. 

R. E. Ireland, S. Thaisrivongs, N. Vanier, and C. S. Wilcox, Enolate Claisen rearrangements from furanoid and pyranoid glycals, J. Org. Chem., 45 (1980) 48-61. 

Glycals serve as activated sugars because of the inherent reactivity of the endocyclic enol ether. Consequently, they have been extremely useful substrates in their complementarity to native activated sugars. Additionally, they have been used to demonstrate versatility not directly available from other sugar derivatives. In a good example of the utility of allylsilanes and silylacetylenes in C-glycoside chemistry, Ichikawa et al. [91] demonstrated a preference for the a anomer in all cases. The results, shown in Scheme 7.17, included a demonstration that a characteristic nOe can be used to confirm the stereochemical outcome of the reaction with hw-trimethylsilylacetylene. 

Dawe, R D, Fraser-Reid, B, a-C-glycopyranosides from Lewis acid catalysed condensations of acetylated glycals and enol silanes, J. Chem. Soc., Chem. Commun., 1180-1181, 1981. 

Another approach to the tricothecene skeleton proposed by Fraser-Reid takes advantage of carbohydrate enolate chemistry to constmct the A ring [366]. The C ring was formed by intramolecular sulfonate displacement by an appropriate amide enolate. A route to the same skeleton involving [2 + 2] cycloaddition on the double bond of an enone obtained from a glycal has been proposed by Fetizon [367]. Ring expansion of the cyclobutene obtained this way can be effected to constmct the C ring of these compounds. 

The potential of the ester enolate Claisen rearrangement for the stereocontrolled synthesis of highly functionalized, complex systems has been demonstrated in numerous applications in natural product synthesis. Utilizing the 1,3-chirality transfer Ireland has synthesized oxygen heterocycles with chiral side chains, such as are found as units in polyether antibiotics and macrolides, starting from enantiomerically pure furanoid or pyranoid glycal systems of type (41), which are easily accessible from carbohydrates (Scheme 65). ... 

Nitridomanganese Complexes. Stoichiometric amounts of chiral complexes of type 43 react with silyl enol ethers in the presence of trifluoroacetic or /Moluenesulfonic anhydride to give a-(A-trifluroacetyl)amino- and a-(N-p-tosylamino) ketones, respectively (see Eq. 160).350 - 353 With glycals, the 1-hydroxy-2-( A -trifluoroacetyljamino derivatives are formed (see Eq. 83).354 A mechanism involving approach of the enol ether from the least hindered side of the 43-TFA complex has been proposed.353... 

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