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The design and synthesis of lincomycin derivatives modified at the C-6 and C-7 positions are described. A substituent at the C-7 position is a 5-aryl-1,3,4-thiadiazol-2-yl-thio group that generates antibacterial activities against macrolide-resistant Streptococcus pneumoniae and Streptococcus pyogenes carrying an erm gene. An additional modification at the C-6 position was explored in application of information regarding pirlimycin and other related compounds. These dual modifications were accomplished by using methyl α-thiolincosaminide as a starting material. As a result of these dual modifications, the antibacterial activities were improved compared with those of compounds with a single modification at the C-7 position. The antibacterial activities of selected compounds in this report against macrolide-resistant S. pneumoniae and S. pyogenes with an erm gene were superior to those of telithromycin.

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'-NH 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.

The synthesis and antibacterial activity of (7S)-7-(5-aryl-1,3,4-thiadiazol-2-yl-thio)-7-deoxylincomycin derivatives are described. These derivatives were mainly prepared by the Mitsunobu reaction of 2,3,4-tris-O-(trimethylsilyl)lincomycin and the corresponding thiols. Exploring structure-activity relationships of the substituent at the 5 position of a thiadiazole ring revealed that compounds with the ortho substituted phenyl group showed improved antibacterial activities against Streptococcus pneumoniae and Streptococcus pyogenes with erm gene compared with the reported compound (1) that had an unsubstituted benzene ring.

Novel lincomycin derivatives possessing an aryl phenyl group or a heteroaryl phenyl group at the C-7 position via sulfur atom were synthesized by Pd-catalyzed cross-coupling reactions of 7(S)-7-deoxy-7-thiolincomycin (5) with various aryl halides. This reaction is the most useful method to synthesize a variety of 7(S)-7-deoxy-7-thiolincomycin derivatives. On the basis of analysis of structure-activity relationships of these novel lincomycin derivatives, we found that (a) the location of basicity in the C-7 side chain was an important factor to enhance antibacterial activities, and (b) compounds 22, 36, 42, 43 and 44 had potent antibacterial activities against a variety of Streptococcus pneumoniae with erm gene, which cause severe respiratory infections, even compared with our C-7-modified lincomycin analogs (1-4) reported previously. Furthermore, 7(S)-configuration was found to be necessary for enhancing antibacterial activities from comparison of configurations at the 7-position of 36 (S-configuration) and 41 (R-configuration).