Total Synthesis of a Cis-Syn 2-Carbomethoxypsoralen Furan-Side Thymidine Monoadduct

The first total chemical synthesis of a cis -syn furan-side photoproduct between a psoralen derivative and thymidine is described. The key step in the synthesis was an intramolecular [2 + 2] photocycloaddition, which directed the stereochemical course of the reaction to afford a product equivalent to that formed when a psoralen molecule is allowed to react at a 5‘-TpA-3‘ site in DNA. A model system consisting of a simple benzofuranyl acid tethered to the 5‘ hydroxyl of thymidine showed that it was possible to bias the stereochemical outcome of the photochemical reaction in favor of the desired cis -syn product. Further refinement of the model system allowed for the elaboration of a benzofuran−thymidine photoproduct into a cis -syn 2-carbomethoxypsoralen furan-side thymidine monoadduct. The stereochemistry of all photoproducts as established by NMR and CD spectroscopy indicated that the cycloadditions occurred on the 3‘ face of thymine, the equivalent of a psoralen monoadduct in a 5‘-TpA-3‘ site in DNA. Previously, the inability to program the sequence context of psoralen−DNA adducts has constrained certain biological studies, such as experiments aimed at deciphering the transcriptional effects of adducts at TATA sites. This method is a key step toward overcoming that problem.