FULL PAPER Efficient Generation of β-L-Rhamnosidic Linkages by the 2-Ulosyl Donor Approach: Synthesis of a Trisaccharide with a Central β-L-Rhamnose Unit Frieder W. Lichtenthaler* [a] and Thomas Metz [a] Keywords: Carbohydrates / Glycosylation / Oligosaccharides / Reductions Practical procedures for the production of variously blocked 6-deoxy-α-L-arabino-2-ketohexosyl bromides from L-rham- nose have been developed. These compounds are highly useful as indirect β-L-rhamnosyl donors: they undergo β-spe- cific glycosidations under Koenigs-Knorr conditions, and the 2-keto group of the resulting 6-deoxy-β-L-hexosiduloses is Introduction The 2-ulosyl donor approach, introduced in 1985, [1] has proven to be highly expedient for the generation of β-- mannosidic linkages, as evidenced by the straightforward synthesis of various β--mannose-containing oligosacchar- ides up to the hexasaccharide level. [1-5] Central to this pro- cedure is the ready preparation of variously blocked α-- arabino-2-ketohexosyl bromides of type II from - glucose, [1,4-7] their essentially β-specific glycosidation ( III) - the nonparticipating electron-withdrawing 2-keto Scheme 1. The ulosyl donor approach to β--mannosides [a] Clemens-Schöpf-Institut für Organische Chemie und Biochemie, Technische Universität Darmstadt, Petersenstr. 22, 64287 Darmstadt, Germany Fax: (internat.) + 49-(0)6151/166674 Eur. J. Org. Chem. 2003, 3081-3093 DOI: 10.1002/ejoc.200300149  2003 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3081 reduced with high β-L-rhamno selectivity. The straightfor- ward application of this 2-ulosyl donor approach for the syn- thesis of β-L-rhamnose-containing di- and trisaccharides is demonstrated. ( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003) group suppresses oxycarbenium ion formation at the anom- eric center thereby resulting in direct S N 2 displacement of the bromine - and manno-selective reduction of the re- sulting β--2-keto-glycosides III  IV (Scheme 1). When selectrides are used rather than borohydride only, the car- bonyl reduction is also essentially manno-specific. [8] Besides its preparative simplicity, the procedure has the further ad- vantage of providing the β--mannosides with free 2-OH groups (i.e., mannosyl acceptors ready for further glycosid- ation towards the oligosaccharides with β--Man-(12)-- Man linkages). Numerous bacterial antigens contain β--rhamnopyr- anose units, [9] and the 6-deoxy- enantiomers of II, ulosyl bromides of types 15, should, if reasonably readily access- ible, provide an expedient two-step procedure for their