Radical-mediated brominations

This article deals with the state of knowledge of ring photobromination of carbohydrate derivatives about 12 years after the phenomenon was first observed. The reaction is very unusual in providing means by which hydrogen atoms bonded to carbon atoms of cyclic carbohydrate derivatives may be substituted directly and, in many cases, affording selectivity at ether carbon atoms in preference to the dioxygenated anomeric centers. Some aspects of it have been reviewed from the standpoint of the operation of the captodative effect.24 

Radical-mediated Brominations 1. General Reaction Conditions and Suitable Compounds 

Radical-mediated brominations of carbohydrate derivatives have usually been conducted in refluxing carbon tetrachloride, under a tungsten (150— 250 W) or heat (250-450 W) lamp, with either JV-bromosuccinimide (1.2— 5 molar equivalents) or bromine (2-5 equivalents) as the source of halogen. Addition of bromotrichloromethane to the carbon tetrachloride can be advantageous.25 Substrate concentrations have ranged from 0.02 to 0.2 mol.l-1 and have depended to some degree on solubility factors, and ordinary laboratory glassware has most often been used. 

Hydroxyl groups are always protected prior to reaction with bromine radicals, and derived esters have proved suitable. Benzoates are particularly useful and are preferable to acetates, which are susceptible to methyl-group bromination, particularly when the acetoxyl groups are bonded to carbon atoms in the -relationship to carbon radicals. Conceivably, this susceptibility can be accounted for as follows. 

Methyl ethers have been employed, but alkyl glycosides may be unstable following hydrogen abstraction from the anomeric center (see Section 11,9). Benzyl ethers and benzylidene (and other aldehyde-based) acetals, which themselves undergo ready radical bromination,27 can be expected to be entirely unsuitable protecting groups. Ketone-derived acetals, on the other hand, should be stable, at least in the absence of acid, and a few examples of successful brominations in their presence are reported in Section 11,4. 

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Radical-mediated brominations, at ring positions of carbohydrates, 49, 37-92... 

RADICAL-MEDIATED BROMINATIONS AT RING POSITIONS OF CARBOHYDRATES... 

R. J. Ferrier and R. H. Fumeaux, Photobromination of carbohydrate derivatives. 6. Functionalization at C-6 of 2,3,4-tri-0-acetyl-1.6-anhydro-/i-i)-glucopyranosc, Aust. J. Chem., 33 (1980) 1025-1036 for other references, see L. Somsak and R. J. Ferrier, Radical-mediated bromination at ring positions of carbohydrates, Adv. Carhohydr. Chem. Biochem., 49 (1991) 37-92. 

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