FREE-RADICAL THIOL-ENE AND THIOL-YNE COUPLINGS AS CLICK PROCESSES FOR GLYCOCONJUGATION - Click Chemistry in Glycoscience: New Developments and Strategies - page 62

S -Homoallyl Route

SH S

O O

N N

Br

HO 2 C CO 2 H HO 2 C CO 2 H

H H

NH 3, MeOH
r.t., 3h

NH 2 NH 2

O O

57 58

OH

O

OH

HO

SH

HO O

HO

HO S

HO

44 OH

S

O

(1.2 eq.)

N

HO 2 C CO 2 H

l

max 365 nm, 45 min

H

NH 2

DPAP (0.1 equiv) O

MeOH-H 2 O

59 (97%)

SCHEME 3.20 Post-translational approach to glutathione-based S -glycopeptide.

to give an S -linked glycoconjugate (route A). The other approach, instead, consisted
of a single step involving TEC between the unmodified peptide or protein and an
allyl C -glycoside to give in this case a C -linked glycoconjugate (route B).
In the first instance, we explored the viability of route A using the tripeptide
glutathione Glu-Cys-Gly 57 as a cysteine-containing substrate (Scheme 3.20) [37].
To this end, we introduced a homoallyl chain in 57 by reaction with 4-bromo-1-butene
in the presence of NH 3 . Notably, this reaction proceeded readily and specifically to
give exclusively S -butenyl glutathione 58 in almost quantitative yield due to the
superior nucleophilicity of the cysteine sulfhydryl group with respect to the terminal
amino group of glutamic acid. Then, the crude alkene-tagged 58 was submitted to
the photoinduced reaction (

max 365 nm) with glycosyl thiol 44 in the presence of
DPAP to give the S -glycoconjugate 59 in excellent isolated yield (97%). It has to be
noted that S -butenylation of glutathione 57 was a key factor for the success of this
approach. This functionalization was selected after some experimentation on cysteine
as a model substrate. In fact, the S -allylation of cysteine by allyl bromide occurred
readily and quantitatively but the photoinduced reaction of the S -allyl cysteine thus
formed with the thiol 44 gave the expected thioether in very low yield together with
several byproducts.
Synthesis of S -glucosyl glutathione 59 .NH 4 OH (1 mL of a 28% solution in H 2 O)
and butenyl bromide (16 L, 0.14 mmol) were added to a cooled (0 ◦ C), stirred

solution of glutathione 57 (40 mg, 0.13 mmol) in MeOH (1 mL). The solution
was stirred at 0 ◦ C for 1 hour, then warmed to room temperature, stirred for an
additional 1 hour, and concentrated. A stirred solution of the crude 58 , glucosyl
thiol 44 (30 mg, 0.15 mmol), and DPAP (4 mg, 0.015 mmol) in 1:2 MeOH-H 2 O
(1.6 mL) 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