Model Substrate/Inactivation Reactions for MoaA and Ribonucleotide Reductases: Loss of Bromo, Chloro, or Tosylate Groups from C2 of 1,5-Dideoxyhomoribofuranoses upon Generation of an α-Oxy Radical at C3.

We report studies on radical-initiated fragmentations of model 1,5-dideoxyhomoribofuranose derivatives with bromo, chloro, and tosyloxy substituents on C2. The effects of stereochemical inversion at C2 were probed with the corresponding arabino epimers. In all cases, the elimination of bromide, chloride, and tosylate anions occurred when the 3-hydroxyl group was unprotected. The isolation of deuterium-labeled furanone products established heterolytic cleavage followed by the transfer of deuterium from labeled tributylstannane. In contrast, 3--methyl derivatives underwent the elimination of bromine or chlorine radicals to give the 2,3-alkene with no incorporation of label in the methyl vinyl ether. More drastic fragmentation occurred with both of the 3--methyl-2-tosyloxy epimers to give an aromatized furan derivative with no deuterium label. Contrasting results observed with the present anhydroalditol models relative to our prior studies with analogously substituted nucleoside models have demonstrated that insights from biomimetic chemical reactions can provide illumination of mechanistic pathways employed by ribonucleotide reductases (RNRs) and the MoaA enzyme involved in the biosynthesis of molybdopterin.