An extended scope synthesis of an artificial safranine cofactor

Safranines hold great promise as artificial flavin-like electron transfer cofactors with tunable properties. We have previously reported the stepwise synthesis of a safranine analogue, p-methoxy safranine. We now report an improved synthetic pathway which enables the synthesis of safranine analogues containing electron donating phenyl substituents. Low potential safranine analogues were synthesized that extend the range of two electron midpoint reduction potentials to 249 mV, or 11.4 kcal/mol. NMR analysis of the safranine series demonstrates that the 15N chemical shift at the N(5) position correlates with the two-electron reduction midpoint potential.