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.