Lemieux's extensive work on replacement reactions at the anomeric center helped to establish the fact that the O-2-protecting group of a donor exerts powerful control over stereoselectivity in glycoside coupling reactions. This manuscript shows that the O-2-protecting group of a donor also exerts powerful, indeed sometimes total, control over regioselectivity in glycosidation of diols. The latter acceptors also exhibit preferences over the donor, thereby providing evidence for the concept of reciprocal donor acceptor selectivity (RDAS). The latter concept is put to the test by simultaneously presenting an acceptor diol with equivalent amounts of two donors, in the hope of achieving double differential glycosidation leading to one-pot assembly of a trisaccharide. When the pair of donors did not conform to RDAS principles the reaction did not proceed beyond a dissacharide. However, when the pair was RDAS sanctioned, a single trisaccharide (out of four possibilities) was obtained.Key words: regiocontrolled glycosidation, armed and disarmed donors, di- and trioxolenium ions, oxocarbenium ion.

Three sucrose monoalcohols with free hydroxyl groups at C-1', C-6, and C-6' (1, 4, and 6) were prepared selectively and in good yield from 2,3,3',4,4'-penta-O-benzylsucrose. These compounds were oxidized to aldehydes and reacted with stabilized ylide, Ph₃P=CHCO₂Me to afford appropriate α,β-unsaturated esters 10, 11, and 12. Each olefin was cis-hydroxylated with OsO₄/NMO to stereoisomeric diols 13/14, 15/16, and 17/18, configurations of which were assigned by chemical correlation and CD evaluation. Stereoselectivity of the osmylation reaction was surprisingly low (ca 3:2), especially as compared to a similar process performed on simple derivatives of 6,7-unsaturated methyl glycosides for which the ratio of isomeric diols was assigned as 10:1. The osmylation of 11 (derivative homologated by a C₂-unit at the glucose part) did not obey Kishi's rule. Horner-Emmons reaction of sucrose aldehyde 7 with a sugar-derived phosphonate 22 afforded α,β-unsaturated derivative 24, homologated by a C₇-unit at the glucose end.