Glycal intermediate

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). 

A full account has been given of the synthesis of cordycepin via a furanoid glycal intermediate (Vol. 29, p. 258), and a similar approach has been adopted for the synthesis of the puromycin aminonucleoside (25), as outlined in Scheme 1 ... 

Unsaturated sugars are useful synthetic intermediates (11). The most commonly used are the so-called glycals (1,5- or 1,4-anhydroalditol-l-enes). In the presence of a Lewis-acid catalyst, 3,4,6-tri-0-acetyl-l,5-anhydro-2-deoxy-D-arabinohex-l-enitol [2873-29-2] commonly called D-glucal triacetate, adds nucleophiles in both kineticaHy controlled and thermodynamically controlled (soft bases predominately at C-3 and hard bases primarily at C-1) reactions (11,13). 

Little used organic acceptors for these reactions, also [53,54], The use of a glycal as the radical source together with a functionalized enone as the radical acceptor is remarkable. Enones are swiftly reduced by Cp2TiCT [55] and thus epoxide activation must be considered as even more efficient. The product of the addition constitutes a valuable intermediate en route to derivatives of thyrsiferiol. 

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]. 

The term glycal is used to define sugar derivatives having a double bond between C-1 and C-2. Accordingly, C-1 glycals are A1,2 unsaturated carbohydrate derivatives with a carbon substituent at the anomeric position. These compounds are versatile synthetic intermediates, owing to the variety of transformations associated with their enol ether functionality, and have found ample use in the preparation of C-glycosyl compounds,1 carbohydrate mimics,2 and natural products.3... 

The determination of the lifetime of the ionic intermediates using the azide-clock method has been however useful in showing that electrophilic addition of Br2 can occur, even through a fully concerted mechanism, definable as SN2-like. Bromination of cyclic enol ethers (glycals) 8-10 in methanol in the presence of... 

The C-glycosylation of pentose glycals with silylacetylenes or allylsilanes through oxocarbenium ion intermediates proceeds with high regio- and stereo-selectivity, giving the 1,4-anti compounds as the main products. ... 

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