Y. Jin, K. Akagawa and K. Kudo
Tetrahedron 84 (2021) 131998
Scheme 4. Sequential deprotection of side chains on a Cys-Cys dipeptide 7.
of a disulfide from the thiol product. TCEP has been used for
reductive cleavage of disulfide bonds [33]; however, in the present
case, the reagent served as a nucleophile to regenerate the thiol
from the sulfide. Other known disulfide-reducing agents such as
dithiothreitol (DTT) or NaBH4 did not show any deprotecting ability.
The stability of this protective group was confirmed under
weakly basic conditions (trimethylamine or piperidine). By
contrast, partial deprotection occurred under acidic conditions (4
eq. TFA in CH2Cl2, ca. 50% yield).
On the basis of the finding presented above, alcohol 1a was
applied as a protective group to the synthesis of a cysteine deriv-
ative (Scheme 3). The progress of the reaction could be easily
monitored by TLC as blue spots.
This protective group was further applied for sequential
deprotection of a differently protected CyseCys derivative (Scheme
4). First deprotection with DTT successfully underwent disulfide
bond cleavage, and the subsequent treatment with TCEP gave a
fully deprotected dipeptide. This methodology is considered to be
useful in the synthesis of Cys-containing peptides. As mentioned,
the guaiazulenylmethyl sulfide was stable under Fmoc-
deprotecting conditions (20% piperidine in DMF); the peptide 7
might be further elongated further.
Several azulene derived “blue” protective groups have been
reported to date; 1) azulen-5-ylethoxycarbonyl group was applied
as Fmoc-like protective group for amino group [34]; 2) 2-(azulen-1-
yl)-2-oxoacetyl chloride, prepared from azulene and oxalyl chloride
in situ, has been used to protect hydroxy groups [35]; and 3) azulen-
6-ylethoxy group was utilized to protect carboxylic acids [36]. Here
we have added another azulene-based protective group that is
unique to thiols. The present protection method is based on the
high benzylic cation stabilizing nature for the five membered ring
part of the azulene, which has not been utilized as a working
principle for the protection of nucleophilic function groups.
4. Experimental section
4-1) The typical procedure for the thiol substitution of azulene-
derived benzylic alcohol is as follows: To a 30 mL round-bottomed
flask containing alcohol 1a (42.0 mg, 0.173 mmol) in CHCl3 (3 mL)
was added 4-methoxybenzenethiol (20.2 mg, 0.144 mmol). The
reaction mixture was stirred at rt for 2 h, and then it was parti-
tioned between water and chloroform. The organic layer was dried
with MgSO4 and the solvent was removed by evaporation. The
crude mixture was purified by preparative thin layer chromatog-
raphy (hexane: ethyl acetate ¼ 95:5) to afford 43.5 mg of sulfide 2a
as a blue amorphous solid (83% yield).
4-2) The typical procedure for “deprotection”, or the formation
of the thiol from the sulfide, is as follows: To a 30 mL round-
bottomed flask containing a sulfide 2a (55 mg, 0.15 mmol) in
MeOH (3 mL) was added TCEP$HCl (64 mg, 0.22 mmol) and the
resulting mixture was stirred for 12 h at rt. The reaction was
quenched with saturated aqueous NaHCO3 and extracted with
CHCl3 for five times. The collected organic layer was dried over
MgSO4 and the solvent was removed to give 18.5 mg of 4-
methoxybenzenthiol as a colorless oil (88% yield).
The preparation of guaiazulene-3-ylmethyl alcohols, procedures
for the synthesis of cysteine derivatives and peptides, the charac-
terization data of isolated compounds, and the 1H and 13C NMR
spectra are presented in Supplementary data.
Declaration of competing interest
The authors declare that they have no known competing
financial interests or personal relationships that could have
appeared to influence the work reported in this paper.
Acknowledgments
This work was partially supported by JSPS KAKENHI Grant
Number JP20K05487. The authors thank Prof. Akimitsu Okamoto at
The University of Tokyo for the use of ESI-MS instrument.
3. Conclusion
A spontaneous substitution of guaiazulene-3-methanol de-
rivatives by thiols was developed. The reaction proceeded under
mild conditions in a range of nonpolar to aqueous solvents and
required no activators, albeit slight enhancement by an acid was
observed. The click-chemistry-like nature of this reaction could be
successfully applied to the blue labeling of immobilized biothiols as
detectable by the naked eye. Considering the unreactive nature of
alcohol 1 to disulfide, this detection of thiols should be potentially
applicable to the instant monitoring of thiol/disulfide-based redox-
responsive materials [37,38]. Another merit of this substitution is
that the original thiol can be regenerated by treating the 1-derived
sulfide with TCEP. When coupled with the stability toward DTT and
piperidine along with the inherent blue color, the guaiazulen-3-
ylmethyl group is expected to be applicable to the Fmoc solid-
phase synthesis of peptides with multiple Cys residues via an
orthogonal protection/deprotection pathway [39].
Appendix A. Supplementary data
Supplementary data to this article can be found online at
References
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