Amino acids C-glycosyl

The tetra-O-benzylated A-Fmoc derivative of 165 was then incorporated by solid-phase synthesis into a helix-forming glycoheptadecapeptide, Ac-Tyr-Lys-Ala-Ala-Ala-Ala-Lys-Ala-Ala-Cgaa((3-D-Gal-C-)-Ala-Lys-Ala-Ala-Ala-Ala-Lys-NH2 (Cgaa= C-glycosylated amino acid)J128l... 

S. Czernecki, S. Horns, and J. M. Valery, Chain-extension of carbohydrates. 5. Synthesis of C-glycosyl amino acid moiety of miharamycins involving stereocontrolled ethynylation of methyl 2,3,4-tri-0-benzyl-a-D-g/nco-hexadialdo-l,4-pyranoside, J. Org. Chem., 60 (1995) 650-655. 

Reaction of 1,2-anhydro-mannose with lithiated indole in the presence of BF3-OEt2 as Lewis acid yielded a-C-mannosylindole, which was further transformed into C-mannosyltryptophan, a naturally occurring C-glycosyl amino acid found in some proteins (O Scheme 38) [59]. 

Carbohydrates with a hydrazono ester at the C-2 position underwent intramolecular radical C-glycosylation upon reaction with tin hydride and AIBN, to afford the corresponding lactone in good yield (O Scheme 47). Although the stereoselectivities of the amino acid moiety were generally low, the methods have been used for the synthesis of a variety of C-glycosyl amino acids [73], The following reaction was a key step in the total synthesis of furanomycin, an antibiotic. 

C-Glycopyranosides may be obtained from glycopyranosyl halides via intermolecular addition of glycopyranosyl radicals [129]. In a more useful example, the a-aminoacrylate 192 was used as the radical acceptor for preparation of C-glycosyl amino acids 193 and 194 [130] (Scheme 66). In a concise synthesis of showdomycin (197), Barton utilized the trigger reaction of the 7V-hydroxy-2-thiopyridone derivative and the exceptional radicophilicity of tellurides in concocting the conditions for the conversion from the anisyl telluride 195 to the intermediate 196 after oxidative elimination [131] (Scheme 67). In Keck s synthesis of (-t-)-pseudomonic acid C (201), the intermediate 200 was prepared via stereocontrolled intermolecular addition of the radical generated from the iodide 198 to the allylic sulfone 199 [132] (Scheme 68). 

Several C-glycosylated amino acids were obtained by 13-DC of (-)-menthonc-derived chiral nitrone (-)-90 and allyl- or vinyl glycosides. For instance the cycloaddition of (-)-90 and 91 occurred with complete diastereo- and regioselectivity to afford the isoxazolidine 92 in 92% yield. The a-amino acid moiety was then obtained by Sm(ll) mediated cleavage of the N-O bond followed by fV -acetal and amide hydrolysis <01OL1375>. 

Nishikawa T, Ishikawa M, Isobe M (1999) Synthesis of a a-C-mannosyltryptophan derivative, naturally occurring C-glycosyl amino acid found in human ribonuclease. Synlett 123-125... 

C-Glycosyl amino acids of type 68, for example, are robust to degradation by glycosidases with respect to C>-glycosides. They can be obtained directly by CM of C-allyl glycosides with unsaturated amino esters [58]. They were recently produced by CM of vinylcarbohydrate 65 with the amino ester 66. The mercury(II)-mediated cyclization of 67 affords the C-glycosyl amino esters 68 (Scheme 28) [59]. 

The g]ycal ester (137) was converted into (138) (Scheme 35) and hence into the illustrated C-glycosyl amino-acid derivatives. ... 

---