β-Oxygen effect in the Barton-McCombie deoxygenation reaction: further experimental and theoretical findings

The chemistry of (S)-methyl xanthates derived from xylo- and ribo-furanose derivatives in the presence of the stannyl radical is investigated. Xanthate derived from β-xylo-furanose affords exclusively a deoxygenated product; whereas, under the same reaction conditions, the α-ribo-furanose xanthate derivative produces quantitatively a hemithioacetal compound. We reasoned that in the case of the β-xylo-furanose derivative, a favorable β-oxygen effect in the Barton-McCombie deoxygenation reaction is operating where, according to theoretical calculations, unusual molecular orbital interactions (and not strain, as previously proposed) are present. These orbital interactions involve the SOMO (intermediary generated from the stannyl radical addition) with the σ* orbital of the bond undergoing cleavage and this with the two C-O antibonding orbitals anti oriented. Such molecular orbital interactions are not present in the α-ribo-furanose; therefore, the β-scission is highly delayed, and due to the reversibly nature of the stannyl radical addition, the ribo-furanose xanthate is forced to take an alternative route: the homolytic substitution (S(H)2) of the sulfide sulfur by stannyl radical. This radical addition gives the alkoxythiocarbonyl radical, which is trapped by Bu3SnH before the elimination of carbonyl sulfide; subsequently, radical stannyl addition followed by radical reduction produces the hemithioacetal.