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The synthesis and biological evaluation of sixteen-membered macrolides possessing a 4-O-alkyl-alpha-L-cladinosyl moiety as a neutral sugar are described. These potent novel derivatives have been efficiently synthesized avoiding glycosylations. Two hydroxyl groups in mycarose of the tri-(tert-butyldimethylsilyl) ether intermediate were successively alkylated. Sequential deprotections of silyl groups afforded 4-O-alkyl-L-cladinose analogues and 3,4-di-O-alkyl-L-mycarose analogues of leucomycin V. Some 4-O-alkyl-L-cladinose analogues exhibited potent antibacterial activities. The most active derivative, 3"-O-methyl-4"-O-(3-methylbutyl)leucomycin V, showed improved metabolic stability in rat plasma in vitro and extremely high concentrations in serum after oral administrations in mice and in hamsters.