Seven (5-carba)isoalloxazines with restricted rotation about the C(1′)-N(10) single bond were synthesized for the first time. They were optically resolved by a liquid chromatographic method. Six of these atropisomers (92.1-100% enantiomeric excess) (e.e.) did not racemise thermally (at 70°C) or by visible light irradiation (at 30°C). On the other hand, the (5-carba)isoalloxazines with a 2′-substituted phenyl group or a naphthyl group at N(10) racemised invariably when they were reduced to 1,5-dihydro forms [by four different reducing agents including photoreduction with ethylene diamine tetra-acetic acid (EDTA)]. The kinetic studies confirm that the racemisation process consists of slow, rate-determining reduction of chiral (5-carba)isoalloxazines followed by rapid rotation of the C(1′)-N(10) single bond. The easy rotation in the reduced state is due to conversion of the 'planar' oxidised forms into the 'bent' reduced forms. These (5-carba)isoalloxazines are novel examples for atropisomers with a C-N axis, the rotation of which is induced only by the redox reaction.
|Number of pages||7|
|Journal||Journal of the Chemical Society, Perkin Transactions 1|
|Publication status||Published - 1988|
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