Letter
to some extent. When the ester group at the para-position of
the aromatic ring was changed to an electron-donating
methoxy group, the dithioacetal product 3e was given in
only 55% yield. It is likely that electron-deficient substrates
were preferred in this reaction. In addition, benzocyclohex-
anone 1f with two chlorine atoms at the α-methylene position
of the carbonyl group provided no desired product 3f. Besides,
dichlorinated substrates with aliphatic carbonyl and ester
groups were also tested and furnished the corresponding
dithioacetal products 3g−h in 36 and 39% yields. However, in
the case of substrate with a sulfonyl group, no desired product
3
i was obtained. In view of the above results, a substrate with
the benzoyl group is more suitable for this reaction. The para-
substitution site was optimum. Therefore, we chose the
substrate with an ester group at the para-position as the
standard substrate 1d for further investigation.
With the optimum results in hand, different thiol substrates
were investigated. As shown in Table 3, all the substrates with
a single thiol group can furnish the corresponding
dithioacetals. For thiol 2b with an ester at the β-position, the
reaction proceeded with a slight decrease in the yield. It is also
found that the yield of dithioacetal 3k is only 29% within 2 h
by using thiol 2c bearing a longer aliphatic chain, although the
yield can reach 80% after 16 hours. Notably, dichloroaceto-
phenone 1d was also proved to be effective to trap N-Ac-
cysteine ester 2d in this transformation to give the
corresponding 3l as a single product in 83% yield. Meanwhile,
when ethane-1,2-dithiol 2e and propane-1,3-dithiol 2f were
employed in the reactions, cyclized products 3m and 3n could
be obtained in 74% and 52% yields, respectively. These results
indicate that an ester group at the α-position of thiol plays an
important role in disulfide bridging, and the effect of
intramolecular disulfide bridging is better than that of the
intermolecular one.
Figure 4. Disulfide bridging of lanreotide and N-Ac oxytocin. (a)
Starting from lanreotide with dichloroacetophenone 1d. (b) Starting
from N-Ac oxytocin with dichloroacetophenone 1d. (c) Starting from
N-Ac oxytocin with dichlorinated substrate 1j bearing fluorescein
isothiocyanate (FITC).
Subsequently, the chemoselectivity of dichloroacetophenone
d toward various functional groups was investigated. Glycine
1
methyl ester and methyl 2-hydroxyacetate were chosen to
mimic the amino group and hydroxyl group widely found in
peptides and proteins (Figure 3). To our delight, dichlor-
oacetophenone 1d displayed excellent chemoselectivity to thiol
2a. Only desired dithioacetal product 3d was assembled in
good yields, and no corresponding byproducts were generated.
Interestingly, when treating dichloroacetophenone 1d with the
mixture of N-Ac-cysteine ester 2d and 1-hexanethiol 2c under
the same reaction conditions, only cysteine-containing product
3l was obtained in excellent yield. This is possibly attributed to
the ester group at the α-position of cysteine 2d, which acts as
the electron-withdrawing group to make the nucleophilic
thiolate anion more stable in buffer solution. These results
demonstrate that dichloroacetophenone 1d is inclined to react
with functionalized thiol substrates such as cysteine, regardless
of other nucleophiles existing in the reaction. This reaction is
highly chemoselective.
the pH value was decreased from 7.6 to 6, the reaction
dropped dramatically (entry 1 vs 9). Similarly, the yield was
3
0% higher in PBS buffer (pH 7.6) than in ultrapure water
(
entry 8 vs 10). Since NaHCO buffer (pH 8.0) gave the
3
product in lower yield than NH HCO buffer (pH 8.0) (entry
4
3
1
1 vs 12), the latter proved to be far more superior probably
because of the better solubility of the reagents. As PBS buffer
pH 7.6) and NH HCO buffer (pH 8.0) gave the product in
(
4
3
comparable yields (entries 8, 13, and 14), these buffers were
used in subsequent studies. Overall, PBS buffer (pH 7.6) or
NH HCO buffer (pH 8.0) with 10% aprotic cosolvent at 30
4
3
°
C is found to be the best for the conjugation of the cysteine
residue by dichloroacetophenone.
The efficiency of disulfide bridging was then investigated by
varying types and substituents of dichlorinated substrates
under the reaction conditions as depicted in Table 2 (entry 8).
Based on the success of dichloroacetophenone 1a, we
continued to investigate the scope of substituents in the
phenyl ring. The reaction of three dichloroacetophenones with
the same ester group but with different substitution sites,
ortho-, meta-, and para-, with thiol 2a were conducted, and the
corresponding yields were 40%, 58%, and 72%, respectively. It
was revealed that the position of the electron-withdrawing
group in the phenyl ring also had an influence on the reactivity
Encouraged by these results, we further investigated the
application of this method in disulfide bridging of commer-
cially available peptides through dichloroacetophenone 1d.
First, lanreotide 2g, an analogue of somatostatin to treat
11
acromegaly, was investigated by the reaction with dichlor-
oacetophenone 1d. After treatment with tris[2-carboxyethyl]-
12
phosphine (TCEP), two free thiols were released, and
further disulfide bridging led to the desired product 3o
confirmed by LCMS (Figure 4a). Next, commercialized
oxytocin is widely used during and after childbirth and served
C
Org. Lett. XXXX, XXX, XXX−XXX
Wu, Liu-Hai
