First synthesis of cellooctaose by a convergent synthetic method

The first chemical synthesis of cellooctaose by a convergent synthetic method is described. A challenging glycosylation between cellotetraosyl donor 5 and acceptor 7 proceeded in a one-step reaction using a high-vacuum system for anhydrous glycosylation and minimizing imidate side reactions such as hydrolysis and glycosyl fluoride formation. Pivaloyl, allyl, and benzyl protecting groups of cellooctaose derivative 8 were completely removed with SeO2-AcOH, NaOMe-MeOH, and H2/Pd(OH)2-C, respectively. The acetylation after each deprotection step finally led to cellooctaose hexacosaacetate (20), which is useful for purification and structural identification. Finally, the acetyl derivative 20 was deacetylated with 1,8-diazabicyclo[5.4.0.]undec-7-ene (DBU) in 20% MeOH-CH2Cl2 to give pure cellooctaose (21). The analogous synthetic route to the present convergent synthetic design of cellooctaose may be a most promising one that enables us to synthesize cellulose with a defined degree of polymerization (dp).