Claisen‐Schmidt Condensation and Domino Claisen‐Schmidt Condensation ‐ Michael Addition of 16‐Formyl Steroids in the Presence of Switchable Polarity Solvents

Switchable polarity solvents (reversible ionic liquids) have been used to replace conventional catalysts and solvents in Claisen‐Schmidt condensation reactions of 16‐formyl steroids and aromatic ketones. The primarily formed α,β‐unsaturated ketones may undergo a Michael addition with the aromatic ketone reaction partner. By the proper choice of the reaction conditions, either the α,β‐unsaturated ketone or the products of the domino Claisen‐Schmidt condensation ‐ Michael addition could be obtained with good selectivity. The catalyst could successfully be recovered and reused. The steroidal products with androstane and estrane skeleton were characterized by different spectroscopic methods (NMR, IR, MS). 16‐Formyl‐androstane and ‐estrane derivatives were converted into two types of products formed either via Claisen‐Schmidt condensation or a sequential Claisen‐Schmidt condensation ‐ Michael addition with good selectivity depending on the reaction conditions. The guanidine catalyst and ethylene glycol solvent were recycled efficiently by a reversible switch between their molecular and ionic forms in the absence and presence of CO2, respectively.