Synthesis development of an aminomethylcycline antibiotic via an electronically tuned acyliminium Friedel–Crafts reaction

With the goal of improving the synthetic efficiency, the development of a convergent synthesis of a minocycline derivative PTK0796 via an intermolecular acyliminium Friedel–Crafts reaction (Tscherniac–Einhorn reaction) is described. The entire C9 neopentylaminomethyl side chain was installed in one step using an electronically optimized chloromethylacyliminium precursor in 83% yield. Deprotection and re-equilibration to the C4 α-epimer in the presence of CaCl2 and ethanolamine or NaOH afforded the target aminomethylcycline antibiotic. The corresponding crystalline tosylate salt was found to exhibit improved solid state stability.