Chemical transformation of tylosin derivatives into neutral macrolides having a 3′-methoxyl group

This paper describes the chemical transformation of the basic 16-membered macrolides, tylosin derivatives, into neutral macrolides having a 3′-methoxyl group. 2′,4′-Di-O-acetyl-3,23-bis(O-tert-butyldimethylsilyl)mycaminosyltylonolide 9,20-bis(ethylene acetal) N-oxide (1b) was treated with Ac2O–pyridine in CH2Cl2 to afford the 3′-ketone 1c and the 3′-N-acetyl-3′-N-demethyl derivative 1d in 67 and 5% yield, respectively. Reduction of 1c with Zn(BH4)2 gave the 3′-alcohol 1e in 84% yield stereoselectively. O-Methylation of 1e with MeOTf and 2,6-di-tert-butylpyridine gave the 3′-methyl ether 1f in 49% yield in spite of the presence of the adjacent acetoxyl groups. Deprotection of 1f provided the desired neutral macrolide 1g. Similar synthetic routes were also used for transformation of the suitably protected 4′-deoxymycaminosyltylonolide 2b and desmycosin 3c into neutral macrolides having a 3′-methoxyl group. It was found that the mycinose moiety of a neutral macrolide plays an important role in its antimicrobial activity.