Procedures for rapid assembly of multigram amounts of mannan components have been examined. Although these studies are reported in the context of the mannan moiety of the glycan anchors of membrane-bound glycoproteins, the procedures should be applicable to the wider family of mannose-containing glycoproteins. Readily prepared n-pentenyl ortho esters of mannose are shown to be versatile substrates that can serve as glycosyl donors in their own right or be used to furnish mannosyl bromides or n-pentenyl alpha-D-mannosides. Thus three glycosyl donors of different reactivities and stabilities are obtainable from the same precursor, all three being activated under mild conditions. Two approaches are described. in the first, a portion of the starting n-pentenyl ortho ester is converted into an n-pentenyl glycoside (NPG) by acid-catalyzed rearrangement, while another portion is titrated with bromine to give a glycosyl bromide. These are coupled under Koenigs-Knorr conditions to give an n-pentenyl disaccharide which is then processed to become a glycosyl acceptor. A third portion of the ortho ester, after suitable protecting group adjustments, is also titrated with bromine and coupled to the disaccharide acceptor to give the desired trimannan. The instability of glycosyl bromides detracts from this route, and so a second approach which avoids their use completely was pursued in which NPG obtained from the acid-catalyzed rearrangement was converted into a vicinal dibromide. The latter is then able to serve as a glycosyl acceptor for coupling to a donor obtainable by reaction of the n-pentenyl ortho ester with halonium ion. The dibromopentanyl disaccharide produced then becomes an acceptor for a donor derived from n-pentenyl mannoside. The second approach uses no unstable reactants, is therefore experimentally less demanding, and can be operated conveniently on a large scale.