Conformations of selected 3-substituted 4-hydroxycoumarins in solution by nuclear magnetic resonance. Warfarin and phenprocoumon

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Abstract
The chemical shift position of the benzylic proton, Hx, was diagnostic in indicating the preferred conformations of selected 3-substituted 4-hydroxycoumarins. In general, the nonrigid open-chain compounds, e.g., the open-chain tautomer of warfarin and phenprocoumon, existed in equal populations of the 2 conformations in which the benzylic proton was in the plane of the coumarin ring and was either cis or trans to the 3,4 double bond. The cyclic compounds, e.g., cyclocumarol, were constrained to 2 limiting conformations defined as axial2 or trans or intermediate conformations between these limits. The antivitamin K activity of warfarin may be due to its open side-chain tautomeric form.

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