Rhamnoside derivatives

Z. Zhang, P. G. Wang, N. Ding, G. Song, and Y. Li, Total synthesis of cleistetroside-2, partially acetylated dodecanyl tetra-rhamnoside derivative isolated from Cleistopholis patens and Cleistopholis glauca, Carbohydr. Res., 342 (2007) 1159-1168. 

Chapter 6, Scheme 1)/ Fluoren-9-ylidenc acetals, such as the methyl L-rhamnoside derivative (11), have been synthesized by use of 9,9 dichlorofluorene in pyridine at 1(X) C. Their hydrogenolysis to fluorenyl ethers is covered in Chapter 5 ... 

Thus, the O-benzylated glucosamine derivative 60 was condensed with the spacer-linked L-rhamnoside 61, followed by the removal of the base-labile protecting group at the site of the chain extension to afford the disaccharide acceptor 62. The trisaccharide block 67 was prepared in a similar approach. Thus, condensation of the glucosamine derivative 63 with rhamnoside 64 afforded the disaccharide... 

For a five-membered dioxolane ring, the regioselectivity depends on the stereochemistry of the benzylidene acetalic carbon atom. For example, treatment of the methyl endo 2,3-0-benzylidene protected rhamnoside with LiAlILi-AlClj gave a 2-0-benzyl derivative, but the same reaction with the 2-exo isomer yielded mainly the 3-0-benzyl derivative.32... 

The exo and endo isomers of benzylidene acetals of the carbohydrate series can be obtained separately. Their behavior on reduction with HzAlCl accords with the analysis outlined above frequently only one product is obtained. Thus, the 1,2,4,6-di-O-benzylidene derivatives of glucose (42) and (43 R = H or Bn) cleave rapidly with 1 equiv. of H2AICI at 0 °C (30 min for 42 and 10 min for 43) at the marked bonds." Here, at least, it is clear that steric effects outweigh polar ones. The 2,3-endo and -exo isomers of the a-D-mannoside series (44) and (45) likewise cleave mainly equatorial (78 22) and axial (96 4), respectively, when treated with 1 equiv. of H2AICI at room temperature. The same behavior is seen with the methyl 2,3-benzylidene-a-L-rhamnosides," the methyl 3,4-0-benzylidene-P-L-arabinosides" and... 

The naturally occurring aryl L-rhamnosides include quercitrin, namely, quercetin substituted at C-3. This is a fairly common derivative in plant tissues. - Mono-L-rhamnosides of kaempferol, in which substitution occurs at the hydroxyl groups at C-3 and C-7, are known, and kaemp-feritrin (lespedin, 3,7-di-L-rhamnosylkaempferol) has also been reported. ... 

In rare instances, 3-L-rhamnosyl derivatives of anthocyanidins occur in plant tissues.Frangulin (franguloside), an L-rhamnoside of 4,5,7-tri-hydroxy-2-methylanthraquinone, is found in Rhamrms spp. it probably has the glycosyloxy group at C-7. 

The selective tritylation of pentoses has been studied, and providing that the reaction is conducted at below 25 C and with only one equivalent of trityl chloride, the 5-0-tritylate is the major product. However, even under these conditions lyxose afforded substantial amounts of diethers, and the 5-monoether was isolated in only 32% yield. The monotritylation of D-xylose has been studied in detail at 50 °C in the presence of either pyridine or AgOAc-HMPT, and four of the monotrityl ethers were isolated as tetra-O-acetyl derivatives. The ratio of 1-, 3-, 4-, and 5-substitution was 36 0 25 100 (in pyridine) and 0 49 57 100 (AgOAc-HMPT). The products were also prepared unequivocally by the tritylation of the appropriate D-xylose tetra-acetate. The selective tritylation of methyl a- and j3-L-rhamnosides has also been studied the a-anomer giving the 3-, 4-, and 2-trityl ethers in 57, 3, and 1% yields respectively. The j3-anomer afforded the 3- and 4-trityl ethers in 34 and 17% yields respectively. Pyridinium perchlorate has been used for the selective de-O-tritylation of otherwise peracetylated methyl gluco- and manno-pyranosides. ... 

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