Oligosaccharide Analogues of Polysaccharides. Part 8. Orthogonally Protected Cellobiose-Derived Dialkynes. A Convenient Method for the Regioselective Bromo- and Protodegermylation of Trimethylgermyl- and Trimethylsilyl-protected Dialkynes

The cellobiose-derived dialkynes 14 and 15 were prepared by glycosidation of the acceptor 9 with the thioglycosides 12 (82%) and 13 (85%), respectively. The acceptor 9 was prepared from the known alcohol 2 via the lactone 7 in five steps (48% overall), and the donors 12 and 13 were prepared from the alkynylated anhydroglucose derivative 10 (60% overall). Acetolytic debenzylation of 14 and 15 (→ 16 and 17 , resp.) followed by deacylation of 16 yielded 60% of the cellobiose-derived dialkyne 18 . Deacylation of 14 (→ 19 ), methoxymethylation (→ 20 ) and trimethylgermylation led to the orthogonally protected dialkyne 21 (69% overall). Protodesilylation of 21 with K₂CO₃/MeOH gave 22 (90%), while the Me₃Ge group was selectively removed with CuBr (19 mol-%) in THF/MeOH to give 20 (95%). Treatment of 21 with aqueous HCl solution led to 19 (80%). Bromodegermylation of 21 (NBS/AgOOCCF₃) led to a mixture of 23 (85%) and 24 (11%). Similar conditions using CuBr instead of AgOOCCF₃ gave exclusively the bromoalkyne 23 (93%). The temperature dependence of the δ values of the OH resonances of 18 in (D₆)DMSO evidence a strong intramolecular H-bond between C(5′)? O…?HO? C(5).