Two frame shifted pentasaccharides containing 4,6-dideoxy-4-formamido-D-mannose (4-deoxy-4-formamido-D-rhamnose) have been synthesized by application of a strategy utilizing S-ethyl disaccharide glycoside building blocks 16 and 25. The target molecules contain a single α1,3 linkage as either the first or last inter-residue linkage in an otherwise α1,2-linked homo-pentameric structure:[Formula: see text]and[Formula: see text]These structures represent possible biological repeating units of the Brucella M antigen and their synthesis is designed to facilitate the identification of the structural element that is polymerized to provide the intact polysaccharide antigen. The α1,2-linked tetrasaccharide component of each pentasaccharide was constructed by methyl triflate promoted activation of the α1,2-linked S-ethyl disaccharide glycoside 16. Glycosylation of methyl 4-azido-2-O-benzyl-4,6-dideoxy-α-D-mannopyranoside 3 by 16, followed by a second block extension, gave the pentasaccharide 22 with an α1,3 linkage at the reducing terminus. In analogous fashion the α1,3-linked S-ethyl derivative 25 was used in conjunction with methyl triflate to glycosylate the α1,2-linked trisaccharide 26, and provided the pentasaccharide 27 that possesses a 1,3 linkage at its non-reducing end. The synthesis of an α1,3-linked disaccharide and a trisaccharide are also reported. Keywords: Brucella M antigen, synthesis of frame shifted pentasaccharides, thioglycoside building units, 4,6-dideoxy-4-formamido-D-mannose, biological repeating unit.