Substituted Glycals

Glycal-substituted quinols and quinol ketals, derived from the 1,2-addition of lithiated glycals to quinones or quinone monoketals, are versatile intermediates for the synthesis of aryl C-glycosides, providing access to all four substitution patterns found in the natural products. For example, reduction of a quinol provides a p-hydroxyaryl glycal (8 9,11 -> 12). On... 

It has been recognized since 1931 that hydroxylation of glycals with peroxybenzoic acid results mainly in m-2,3-diols, whereas the compounds having trans groups at C-2 and C-3 preponderate amongst the oxidation products of glycals substituted at C-3. These findings are closely paralleled by the results of peroxidation of the model compounds cyclohex-2-en-l-ol (35, R = H) and its acetate (35, R = Ac) whereas... 

Glycals. Substituted glycos) rroup at the adjacent carbon on brief GU cals are obtained in good yields, Pinacol formation. Stereos ... 

There has been a full account of the synthesis of the 2 -stannylated alkene 135 (X = SnBua) by base-induced stannyl migration from C-6 (see Vol. 32, p. 275), and the application of this compound to the preparation of the alkenyl halides 135 (X = Cl, Br, I), and products with carbon substituents at C-2 through Stille couplings. Reaction of di-O-acetyl-L-rhamnal with silylated thjmiine gave the 2 -enopyranosyl nucleoside by allylic rearrangement, as a mixture of anomers. A paper discussing a glycal substituted at C-3 with a nucleobase is mentioned in Chapter 10, and a 3 -ene derived from thymidine is mentioned in Section 17. 

Perrier RJ (2001) Substitution-with-Allylic-Rearrangement Reactions of Glycal Derivatives. 215 153-175... 

Reaction with glycals.1 (E)-l reacts with glycals in the presence of BF, eth-erate to give pyrans substituted at C2 with a 3-methylpropenyl group that is trans to a substituent at C6. 

Palladium catalyzed cross-coupling reactions of 1-substituted glycals have not only been limited to tributylstannyl derivatives. In fact, the versatility of this approach is significantly enhanced by the fact that C-l zinc-, indium-, or iodine-substituted glycals (easily accesible from glycals, see Scheme 7)... 

Table 2 Transition metal-catalyzed coupling of different 1-substituted glycals... 

In related work, Rutjes and co-workers reported a four-step protocol from conveniently protected sugar derivatives to substituted C-l glycals (Scheme 16).77 The key step in their synthetic route included RCM of highly functionalized a-alkoxyacrylates. The potential of these glycal derivatives was illustrated by their conversion to natural products such as DAH (3-deoxy-D-arabino-2-heptulosonic acid) or KDO (3-deoxy-D-manno-2-ulosonic acid). 

Fig. 7. Oxepine isomers 134-136 that vary as to which ring C—C bond is unsaturated the substituted dihydropyran 137 possesses the enol ether functionality of a glycal, akin to oxepine 136.

---