Bromine migration, intramolecular

Among the halogens, fluorine does not undergo rearrangement, and what evidence there is suggests that the rearrangement of chlorine is an intramolecular process (1,2 shift) whereas that of bromine appears to take place by both inter-and intramolecular routes. Less is known about iodine migration. 

The basic process is, we see, the same as in rearrangements intramolecular (1,2) nucleophilic attack. Indeed, there is rearrangement here in half the molecules formed, the bromine has migrated from one carbon to the next. 

Synthesis from xylose The xylose derivative 47, obtained from 5,5-bis-benzyloxy-7-oxa-bicyclo[2.2.1]hept-2-ene (45), has been used in the synthesis of (-P)-castanospermine (1) (Scheme 6). Bromination of 45 occurred exclusively on the less hindered convex face of 45, followed by stereoselective migration of the endo OBn group of the acetal to give 46, which subsequently converted to 47. Mesylation of 47 followed by cyclization with ammoifia gave 48, whose protection, hydrolysis, acetylation and cyclization by an intramolecular Wittig-Homer condensation gave 49. Conversion of 49 into epoxide 50... 

Aromatic sulphonic acids (X = SO3H) for example, are isomerized in this way. The change of position in the aromatic ring of the tertiary alkyl groups and bromine likewise cannot be confined as intramolecular mechanism with successive 1,2-shifts of the migrating group. Thus, the interconversions of ortho- and para-disubstituted benzenes with substituents of the above type proceed faster than their conversion into a thermodynamically more stable meta isomer... 

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