Synthesis and structure–activity relationships of novel lincomycin derivatives. Part 4: synthesis of novel lincomycin analogs modified at the 6- and 7-positions and their potent antibacterial activities

To modify lincomycin (LCM) at the C-6 and the C-7 positions, we firstly prepared various substituted proline intermediates ( 7 , 11 – 15 and 17 ). These proline intermediates were coupled with methyl 1-thio-α-lincosamide and tetrakis- O -trimethylsilylation followed by selective deprotection of the TMS group at the 7-position gave a wide variety of key intermediates ( 23 – 27 , 47 and 50 ). Then, we synthesized a variety of novel LCM analogs modified at the 7-position in application of the Mitsunobu reaction, an S N 2 reaction, and a Pd-catalyzed cross-coupling reaction. Compounds 34 and 35 (1′- N H derivatives) exhibited enhanced antibacterial activities against resistant pathogens with erm gene compared with the corresponding 1′- N -methyl derivatives ( 3 and 37 ). On the basis of reported SAR, we modified the 4′-position of LCM derivatives possessing a 5-(2-nitrophenyl)-1,3,4-thiadiazol-2-yl group at the C-7 position. Compound 56 showed significantly potent antibacterial activities against S. pneumoniae and S. pyogenes with erm gene, and its activities against S. pneumoniae with erm gene were improved compared with those of 34 and 57 . Although we synthesized novel analogs by transformation of a C-7 substituent focusing on the 1′-demethyl framework to prepare very potent analogs 73 and 75 , it was impossible to generate novel derivatives exhibiting stronger antibacterial activities against S. pneumoniae with erm gene compared with 56 .