Synthetic Strategies for the Synthesis and Transformation of Substituted Pyrrolinones as Advanced Intermediates for Rhazinilam Analogues
The biaryl core structure of rhazinilam with its fixed dihedral angle is a pivotal element for its unique in vitro cytotoxic activity. Most of the related natural products are oxidized versions of rhazinilam. Replacing the sensitive pyrrole ring by a pyrrolinone ring is the basis of our initial strategy towards rhazinilam analogues. With this goal, variants of the sequence crossed Mukaiyama aldol reaction followed by the Staudinger reaction were studied. Reacting a suitably substituted acetophenone with O-methyl O-trimethylsilyl ketene acetal gave pyrrolinones 8a and 8b in good to excellent yields. These intermediates could be transformed in four high-yielding steps into the pyrrolic precursors 7a–c containing all the atoms necessary for the construction of rings A, B, and C of rhazinilam. Our studies illustrate a lack of stability of these intermediates. Alternative synthetic approaches towards this central biaryl core structure are described.