FREE-RADICAL THIOL-ENE AND THIOL-YNE COUPLINGS AS CLICK PROCESSES FOR GLYCOCONJUGATION - Click Chemistry in Glycoscience: New Developments and Strategies - page 63

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SCHEME 3.21 Glutathione dual glycosylation via thiol-yne coupling.

1:2 MeOH-H 2 Otogive 59 (70 mg, 97%) as an amorphous solid; [ ] D =+ 13.4

( c 0.5, H 2 O).
The same strategy was followed in the intent of introducing two sugar fragments
on the same peptide via free-radical thiol-yne chemistry (TYC) according to the
mechanism presented in Scheme 3.1. We thought that peptide double glycosylation
could affect much more substantially than single glycosylation the peptide structure
and biological activity. Also, in this case the whole process consisted of two oper-
ations; that is, peptide modification by introduction of a chemical handle bearing
an alkynyl tag and then the execution of the photoinduced TYC with a sugar thiol.
Suitable conditions for performing these two operations in one-pot sequence were
established by using glutathione 57 as a substrate (Scheme 3.21) [39]. Succinctly,
S -alkynylation of 57 was effectively carried out using propargyl bromide and NH 3 .
Then a mixture of the crude S -propargyl glutathione 60 and an excess of lactosyl thiol
61 (4 eq.) was irradiated (

max 365 nm) in the presence of catalytic DPAP. Also in this
case the photoreaction was conducted in a glass vial at room temperature without cau-
tion to exclude air and moisture. Indeed, the double hydrothiolation took place with
great efficiency with conversion 95% into the corresponding S -lactosyl peptide 62 .

NMR analysis revealed that this product was actually a mixture of two diastereomers
in ca. 1:1 ratio but no efforts were made for their separation. The same procedure
was successfully followed for S -glycosylation of other cysteine-containing peptides
including the synthetic tetrapeptide Arg-Gly-Asp-Cys and octapeptides Asp-Leu-
Tyr-Cys-Tyr-Glu-Gln-Leu and Phe-Gly-Trp-Cys-Tyr-Lys-Leu-Val. The structures of
the S -glycopeptides 63-66 thus obtained are shown in Figure 3.6. It is worth noting
in this respect that the latter higher peptides contained basic lysine residues and,
therefore, there was the possibility of nonselective propargylation. This event did not
occur due to the superior nucleophilicity of the cysteine sulfhydryl group with respect
to the basic nitrogen atoms of other amino acid residues.
Synthesis of glycopeptide 62 .NH 4 OH (0.35 mL of a 28% solution in H 2 O) and
propargyl bromide (4.3

>

L, 0.06 mmol) were added to a cooled (0 ◦ C), stirred solution