Exo-glycals

Synthesis from exo-glycals. Several routes to C-l glycals from 1-exo-methylene pyranoses and furanoses have been described recently. 

E. Caneva, Synthesis of C-disaccharides through dimerization of exo-glycal, J. Org. Chem. 57 1304 (1992) A. Boschetti, F. Nicotra, L. Panza, G. Russo, and L. Zucchelli, Easy synthesis of a C-disaccharide, J. Chem. Soc. Chem. Commun. p. 1085 (1989). 

An important conversion of exo-glycals for our work on altromycin was hydroboration (Scheme 4). We observed that the exo glucal 27 of entry 10 led to a 3/1 ratio of a(28) and J C-glycosides. By contrast, similar treatment of the altrose-derived glycal 29 of entry 11 afforded a 1/10 ratio. Since our program required an a-C-glycoside, the product 28 was carried further to finally produce an altromycin model compound.(2/)... 

In conclusion, the Ramberg- Backlund reaction of glycosyl sulfones is a useful and general route to a variety exo glycals. Simple conversions of the exo-glycals make available a variety of C-glycosides. 

The cycloaddition of o-glycals like 168 with nitrones and nitrile oxides provided spirohete-rocylic compounds 156 (O Scheme 16) with good stereoselectivity and yields [93,94]. The exo-glycal intermediates proved also useful for the s)mthesis of spirocompounds based on a diazepine 155 (O Fig. 12). 

Toth, M., Somsak, L. exo-Glycals from glycosyl cyanides. Eirst generation of C-glycosylmethylene carbenes from 2,5- and 2,6-anhydroaldose tosyihydrazones. J. Chem. Soc., Perkin Trans. 1 2001, 942-943. 

An interesting strategy towards synthesis of KDO proceeding through exo-glycals as a key intermediate has been reported by Sinay [123] (Scheme 44). 

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