Synthesis and Aerobic Dehydrogenation of Indolizin-1-ol Derivatives.

The reaction between simple pyridines, Michael acceptors (cyclopentenone, N-methylmaleimide), and monoalkyl-3,3-difluorocyclopropenes affords 3-(1-hydroxyindolizin-3-yl)-succinimides or 3-(1-hydroxyindolizin-3-yl)-cyclopentanones in good yields. These air-sensitive products regenerate double bond in the incorporated Michael acceptors by selective and near-quantitative aerobic dehydrogenation, yielding intensively colored dyes. The purple 3-(1-hydroxyindolizin-3-yl)-maleimides are highly electrophilic and react smoothly with N-, S-, and P-nucleophiles at the maleimide double bond, which is again easily restored by aerobic dehydrogenation. In the particular case of hydrazine and hydroxylamine nucleophiles, their Michael adducts with the 3-(1-hydroxyindolizin-3-yl)-maleimides afford the novel pyrimido[6,1,2-cd]indolizin-5-one (5-aza[2.3.3]cyclazin-1-one) heterocyclic core by the proposed double-dehydrogenation-6π-electrocyclization-β-elimination reaction sequence. O-Protected 3-(1-hydroxyindolizin-3-yl)-succinimides are air-stable and not electrophilic. Deprotection returns the ability of the succinimides for aerobic dehydrogenation, yielding the appropriate electrophilic maleimides. This property may be employed in design of the switchable covalent-binding tool, activated by chemical or enzymatic cleavage of the O-protective group. Electron-withdrawing group at the C7 position of the indolizine core directly affects the dehydrogenation rate; hence, it can be used for kinetic tuning. Additionally, new stable indolizinium-based zwitterionic 3-oxo-3H-indolizin-4-ium-1-olate (1-oxo-1H-indolizin-4-ium-3-olate) was accessed by TEMPO oxidation of the C3-free indolizin-1-ol, generated by 3-hydroxypyrrole ring annulation with monoalkylcyclopropenone in situ.