RADICAL-CHAIN REDOX REARRANGEMENTS

H-.-S. Dang, B. P. Roberts, J. S. Sekhon, and T. M. Smits, Deoxygenation of carbohydrates by thiol-catalysed radical-chain redox rearrangement of the derived benzylidene acetals, Org. Biomol. Chem., 1 (2003) 1330-1341. 

B. P. Roberts and T. M. Smits, Radical-chain redox rearrangement of cyclic benzylidene acetals to benzoate esters in the presence of thiols, Tetrahedron Lett., 42 (2001) 137-140. 

Radical-chain Redox Rearrangement of Cyclic Acetals. 

There may be common themes in the role of protein-coenzyme contacts in these B -dependent enzymatic processes. In particular, these contacts could alter the relative stability of the Co(III)—R, Co(II), and Co(I) states to enhance reactivity. For coenzyme B 12-dependent enzymes, the deoxyadenosyl radical generates a substrate-derived radical, either directly or via a radical chain mechanism through the intermediacy of a protein-side-chain-based radical, such as S of cysteine or O of tyrosine. This protein-bound substrate-derived radical then undergoes rearrangement, possibly assisted by protein contacts. Thus, cofactor-protein contacts are probably very important in the activation of the Co—C bond, in altering the Co redox potentials, and in assisting in the rearrangements. 

This whole domino sequence becomes a chain reaction, which is not absolutely required for a domino reaction. For efficiently functionahzing domino products, individual rearrangements must be faster than termination of combination, disproportionation, and redox reactions of R , and faster than their reactions with the solvent, precursor, and initiator molecules. Also, the final intermediate R"" should react selectively with S while the rest of the radical species do not Hence, if the final rearrangement results in changes in polarity or reactivity of the propagating radical species, the generation of versatile O-centered radicals or vinyl-type radicals for efficient domino synthesis can be made. 

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