6 熱グルコシル化反応によるシクロ-L-ラムノヘキサオースの全合成(口頭発表の部)  [in Japanese] 6 TOTAL SYNTHESIS OF CYCLO-L-RHAMNOHEXAOSE BASED ON A NOVEL THERMAL GLYCOSIDATION  [in Japanese]

A very simple procedure of O-glycosidation has been developed without using any metal salt. After dried up the reaction system containing cholesterol (5) and benzylated glucosyl chloride 1 (1.2 equiv) along with tetramethylurea (1.2 equiv) in dichloromethane by using apparatus I, the concentrated mixture was heated at 120℃ for 2h affording α- and β-glycosides (60:40) in 92% yield after silica gel column chromatography. Glycosyl chlorides 2, 3, and 4, are also employed for glycosyl donor toward a variety of primary and secondary alcohols. Although this procedure provides glycosides non-stereoselectively, glycosidation with rhamnosyl chloride 4 takes place stereospecific manner to give rise to α-rhamnoside as seen in Table I. Although wide attention have been focussing to the inclusion chemistry of cyclodextrins, synthesis of cyclooligosaccharides have been reported only in few cases. Herein we disclose an novel synthesis of the first L-series cyclooligosaccharide by means of stereoselective thermal glycosidation of rhamnosyl residue. The thermal glycosidation of 15 with 17 in the presence of TMU gave a dimer 18 in 64% yield. The dimer 19 derived from 18 was condensed with 17 under thermal condition to give trimer 20 in 62% yield. Condensation of trimer chloride 21 and trimer alcohol 22 at 120℃ for 2h afforded a hexamer 23 in 24% yield. Cycloglycosidation of hexamer 24 under thermal condition took place smoothly to give the first L-cyclooligosaccharide 25 in 77% yield.