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The Yu glycosylation reaction is a gold(I)-catalyzed glycosylation method involving ortho-alkynylbenzoate as the glycosyl donor. Developed by Biao Yu in 2008, it involves using Au(I) complexes to activate glycosyl ortho-alkynylbenzoate to generate an oxocarbenium ion, which can be trapped by various glycosyl acceptors.[1] The method has enabled synthesis of complex oligosaccharides, C- and N-glycosides, and glycopolymers.[2]

The earliest example of O-glycosidic bond formation is the phenol O-glycosylation reported by Michael in 1897, in which peracetyl glucopyranosyl chloride is substituted with potassium phenolate.[3] Since then, tremendous efforts have been given to the glycosylation reaction to make glycosidic bonds. Even though various methods have been reported, synthesis of glycans still remains to be challenging to operate. For example, in the Koenigs-Knorr reaction, glycosyl bromides/chlorides are unstable and cumbersome to handle, and their activation requires more than 1 equivalent of Hg(II)/Ag(I) salt, which renders the condition to be harsh and toxic. At the same time, glycosylation reaction is complicated by factors such as poorly nucleophilic acceptors, limited functional group compatibility, promotor interference, and acid sensitivity. For example, when a dammarane acceptor was treated with a glucosyl sulfoxide donor activated by Tf2O in the presence of the hindered base DTBMP, in situ-generated TolSOTf triggered olefin activation and delivered the furan product in 78% yield.[4] Next, Schmidt-type glycosylation caused dehydration, whereas the gold(I)-catalyzed glycosylation furnished the desired product with 80% yield.

Reaction mechanism

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Most of homogenous gold(I)-catalyzed processes reactions are initiated by the addition of heteroatom nucleophiles onto LAu+ activated C-C π-bonds.[5] In a similar fashion, for the gold(I)-catalyzed glycosylation, the gold(I) complex first coordinates with the ortho-alkynylbenzoate donor followed by intramolecular nucleophilic addition of activated alkyne, leading to an isochromen-4-yl-gold(I) complex and a sugar oxocarbenium ion.[6] [7] The latter is trapped by the nucleophilic acceptor to furnish the glycoside, while the proton released from nucleophilic addition facilitates protodeuration, regenerating the active Au(I) species. Introduction of externally added strong protic acid (10 mol% TfOH) can be applied to reduce the loading of the gold(I) catalyst significantly.[7]

Synthetic applications

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The method can be applied to the glycosylation of nucleobases.[2] [8] In the total synthesis of tunicamycins, a late-stage glycosylation of uracil with an ortho-alkynylbenzoate successfully yielded desired nucleoside in 73% yield.[9]

The method was used in the total synthesis of trioxacarcins. The robustness and divergency of the method allowed them to decipher the structural assignment of trioxacacin C.[10]

While Koenigs-Knorr-type N-mannosylation only provided trace amount of the desired product, the gold(I)-catalyzed glycosylation method using o-alkynylbenzoate donors successfully yielded the desired product with high yields. The acetyl group is necessary to control the stereoselectivity through neighboring group participation.[11]

Mild gold(I)-catalyzed glycosylation method can be employed for assembly of oligosaccharide, as shown in the following example.[12] During the process, they found the addition of DMF could change the diastereochemical outcome. In the second step, α/β ratio is 3:1 without the addition of DMF. On the other hand, 12 equivalents of DMF reversed the α/β ratio into 1:4.[13]

References

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  1. ^ Li, Y.; Yu, B. (2008). "An efficient glycosylation protocol with glycosyl ortho-alkynylbenzoates as donors under the catalysis of Ph3PAuOTf". Tetrahedron Letter. 49 (22): 3604–3608. doi:10.1016/j.tetlet.2008.04.017.
  2. ^ a b Zhang, Q.; Sun, J.; Zhu, Y.; Zhang, F.; Yu, B. (2011). "An Efficient Approach to the Synthesis of Nucleosides: Gold(I)-Catalyzed N-Glycosylation of Pyrimidines and Purines with Glycosyl ortho-Alkynyl Benzoates". Angewandte Chemie International Edition. 50 (21): 4933–4936. doi:10.1002/anie.201100514.
  3. ^ Michale, A. (1879). "NA". Journal of the American Chemical Society (1): 305–312.
  4. ^ Yu, B. (2013). "Synthetic access toward the diverse ginsenosides". Chemical Science (4): 3899–3905.
  5. ^ Tang, Y.; Zhu, Y.; Li, Y.; Yu, B. (2013). "Mechanistic Insights into the Gold(I)-Catalyzed Activation of Glycosyl ortho-Alkynylbenzoates for Glycosidation". Journal of the American Chemical Society. 135 (49): 18396–18405. doi:10.1021/ja4064316.
  6. ^ Yu, B. (2018). "Gold(I)-Catalyzed Glycosylation with Glycosyl o-Alkynylbenzoates as Donors". Accounts of Chemical Research. 51 (2): 507–516. doi:10.1021/acs.accounts.7b00573.
  7. ^ a b Zhu, Y.; Yu, B. (2011). "Characterization of the isochromen-4-yl-gold(I) intermediate in the gold(I)-catalyzed glycosidation of glycosyl ortho-alkynylbenzoates and enhancement of the catalytic efficiency thereof". Angewandte Chemie, International Edition in English. 50 (36): 8329–8332. doi:10.1002/anie.201103409.
  8. ^ Yang, F.; Zhu, Y.; Yu, B. (2012). "A dramatic concentration effect on the stereoselectivity of N-glycosylation for the synthesis of 2'-deoxy-β-ribonucleosides". Chemical Communication. 48: 7097–7099. doi:10.1039/c2cc33155a.
  9. ^ Li, J.; Yu, B. (2015). "A Modular Approach to the Total Synthesis of Tunicamycins". Angewandte Chemie International Edition. 54 (22): 6618–6621. doi:10.1002/anie.201501890.
  10. ^ Nicolaou, K. C.; Cai, Q.; Sun, H.; Qin, B.; Zhu, S. (2016). "Total Synthesis of Trioxacarcins DC-45-A1, A, D, C, and C7″-epi-C and Full Structural Assignment of Trioxacarcin C". Journal of the American Chemical Society. 138 (9): 3118–3124. doi:10.1021/jacs.5b12687.
  11. ^ Wang, B.; Liu, Y.; Jiao, R.; Feng, Y.; Li, Q.; Chen, C.; Liu, L.; He, G.; Chen, G. (2016). "Total Synthesis of Mannopeptimycins α and β.". Journal of the American Chemical Society. 138 (11): 3926–3932. doi:10.1021/jacs.6b01384.
  12. ^ Zeng, J.; Sun, G.; Yao, W.; Zhu, Y.; Wang, R.; Cai, L.; Liu, K.; Zhang, Q.; Wan, Q. (2017). "3-Aminodeoxypyranoses in Glycosylation: Diversity-Oriented Synthesis and Assembly in Oligosaccharides". Angewandte Chemie International Edition. 56 (19): 5227–5231. doi:10.1002/anie.201700178.
  13. ^ Chen, J.; Ruei, J.; Mong, K. (2014). "Iterative α-Glycosylation Strategy for 2-Deoxy- and 2,6-Dideoxysugars: Application to the One-Pot Synthesis of Deoxysugar-Containing Oligosaccharides". European Journal of Organic Chemistry. 2014 (9): 1827–1831. doi:10.1002/ejoc.201400006.
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