FREE-RADICAL THIOL-ENE AND THIOL-YNE COUPLINGS AS CLICK PROCESSES FOR GLYCOCONJUGATION - Click Chemistry in Glycoscience: New Developments and Strategies - page 64

FIGURE 3.6 Dually glycosylated peptides prepared via TYC.

of glutathione 57 (17 mg, 0.055 mmol) in MeOH (0.35 mL). The solution was stirred at
0 ◦ C for 1 hour, then warmed to room temperature, stirred for an additional 2 hours, and
then concentrated. A stirred solution of the crude product, lactosyl thiol 61 (79 mg,
0.22 mmol), and DPAP (5.5 mg, 0.022 mmol) in 2:1 MeOH-H 2 O(1.6mL)was
irradiated ( max 365 nm) at room temperature for 45 minutes and then concentrated.

The residue was eluted from a column of Sephadex LH-20 with MeOH to give 62
(mixture of diastereomers, 30 mg, 51%) as an amorphous solid.
As early experiments on bis-glycosylation of cysteine via TYC demonstrated that
the vinyl thioether intermediate that formed in the early stage of the reaction (see
mechanism in Scheme 3.1) showed some stability, the sequential hydrothiolation of
alkynyl peptides by two different thiyl radicals generated from different thiols was
examined. Thus, a solution of S -propargyl glutathione 60 in MeOH was treated with
glucosyl thiol 44 (1.1 eq.) and sensitizer DPAP and the mixture was irradiated at
max 365 nm for 15 minutes (Scheme 3.22) [39]. The 1 H NMR spectrum showed
the total consumption of the alkynyl derivative 60 (absence of the signal at 2.26
ppm corresponding to CH) and the formation of the vinyl thioether intermediate

as a mixture of E and Z isomers (presence of vinyl proton signals in the range
5.6-6.3 ppm). Then, a solution of biotin thiol 67 (2 eq.) in MeOH was added and
the mixture was again irradiated at

≡

max 365 nm for 45 minutes. The work-up of
the reaction mixture and chromatography allowed the glycosylated and biotinylated