Direct radical functionalization of native sugars

Abstract

Naturally occurring (native) sugars and carbohydrates contain numerous hydroxyl groups of similar reactivity^1,2. Chemists, therefore, rely typically on laborious, multi-step protecting-group strategies³ to convert these renewable feedstocks into reagents (glycosyl donors) to make glycans. The direct transformation of native sugars to complex saccharides remains a notable challenge. Here we describe a photoinduced approach to achieve site- and stereoselective chemical glycosylation from widely available native sugar building blocks, which through homolytic (one-electron) chemistry bypasses unnecessary hydroxyl group masking and manipulation. This process is reminiscent of nature in its regiocontrolled generation of a transient glycosyl donor, followed by radical-based cross-coupling with electrophiles on activation with light. Through selective anomeric functionalization of mono- and oligosaccharides, this protecting-group-free 'cap and glycosylate' approach offers straightforward access to a wide array of metabolically robust glycosyl compounds. Owing to its biocompatibility, the method was extended to the direct post-translational glycosylation of proteins.

Fig. 1. Design of a protecting-group-free ‘cap and glycosylate’ blueprint for direct functionalization of native sugars to robust glycosides.

a, Enzymatic synthesis of C-glycosyl compounds. b, Challenges in the non-enzymatic chemical synthesis of unprotected C-glycosyl compounds. c, Our biomimetic approach to achieve site- and stereoselective anomeric functionalization of native sugars. R, functional group; LG, leaving group; B, base; NTP, nucleoside triphosphate; NDP, nucleoside diphosphate; Dha, dehydroalanine; C₅F₄N–SH, 2,3,5,6-tetrafluoropyridine-4-thiol; and C₅F₄N, 2,3,5,6-tetrafluoro-4-pyridyl.

Fig. 2. Reaction development.

a, Selection of an appropriate activator for site-selective nucleophilic substitution. b, Identification of the most effective thioglycosyl donor for photoinduced cross-coupling. Yields were determined by ¹H NMR analysis of the crude reaction mixture; yields in parentheses denote isolated yields. α:β Anomeric ratios were determined by ¹H NMR and liquid chromatography–mass spectrometry (LC-MS) analysis. DMC, 2-chloro-1,3-dimethylimidazolinium chloride; CDMT, 2-chloro-4,6-dimethoxy-1,3,5-triazine; NMM, N-methylmorpholine; HE, Hantzsch ester (diethyl 1,4-dihydro-2,6-dimethyl-3,5-pyridinedicarboxylate); LED, light-emitting diode; RT, room temperature; and C₆F₄, 2,3,5,6-tetrafluorophenyl.

Fig. 3. Mechanistic studies.

a, Different anomers of the thioglycoside intermediate eventually converge to a stereoisomerically pure C-glycosyl product. b, Radical trap experiment supports the intermediacy of a glycosyl radical species. c, UV–vis absorption spectra of reaction components in DMSO. d, Plausible mechanisms for native sugar activation and photoinduced cross-coupling. Yields were determined by ¹H NMR analysis of the crude reaction mixture; yields in parentheses denote isolated yields. α:β Anomeric ratios were determined by ¹H NMR and LC-MS analysis. ESI, electrospray ionization; E, electrophile.

Fig. 4. Scope of the reaction with various native sugars.

Cross-coupling of mono- and oligosaccharides through unprotected glycosyl donors to directly afford unprotected C-alkyl glycosyl compounds. Yields were determined by ¹H NMR analysis of the crude reaction mixture; yields in parentheses denote isolated yields. α:β Anomeric ratios were determined by ¹H NMR and LC-MS analysis. Bn, benzyl.

Fig. 5. Synthesis of diverse classes of robust glycosides and glycoconjugates.

a, C-Alkyl glycosyl compounds by reaction with I. b, C-Alkenyl and C-heteroaryl glycosyl compounds by reaction with II (for 44) and III (for 45–47). c, Se-Glycosides by reaction with IV. d, S-Glycosides by reaction with V. Yields were determined by ¹H NMR analysis of the crude reaction mixture; yields in parentheses denote isolated yields. α:β Anomeric ratios, diastereomeric ratios (dr) and E:Z ratios were determined by ¹H NMR and LC-MS analysis. The asterisk indicates the value obtained as a 77:23 E:Z mixture. The dagger indicates d-galactose was used. Ar, aryl; X, halide; Ac, acetyl; Boc, tert-butyloxycarbonyl.

Fig. 6. Application to direct post-translational chemical glycosylation of proteins.

Glycosylation of proteins by cross-coupling of representative native sugars (through capping as thioglycosyl donors) to afford unprotected C-alkyl glycosylproteins. Yields were determined by LC-MS analysis based on conversion of the protein substrate; yields in parentheses denote reactions with in situ-generated and unpurified thioglycosyl donors. Tris, 2-amino-2-(hydroxylmethyl)-propane-1,3-diol; B₂Cat₂, bis(catecholato)diboron; Man, d-mannosyl; Gal, d-galactosyl; GlcNAc, N-acetyl-d-glucosaminyl.

Extended Data Fig. 1. Preliminary results in photoinduced O-glycosylation.

Reaction scope using different sugars and phenols. Yields denote isolated yields. β:α Anomeric ratios were determined by ¹H NMR and LC-MS analysis. TMEDA, tetramethylethylenediamine.