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228
B. Luan et al.
Letter
Synlett
next reaction step after flash column chromatography,
which could decrease the purification difficulty and in-
crease the reaction yield. Compound 4 was produced in 66%
yield by treating 4-1 with bromoacetyl bromide.
Oligoamides 1 bear side chains with a chiral moiety de-
rived from L-alanine, which could serve to bias the handed-
ness of the well-known H-bonded helical columns, which,
in this case are also covalently tethered, formed by BTA
units. The potential chiral conformations could be revealed
with circular dichroism (CD) spectroscopy. Chiral com-
pound 2, corresponding to a single BTA unit of 1, was also
examined with CD as a control.
between sulfhydryl and bromoacetyl groups.11 The unopti-
mized yields of oligoamides 1a–d, after being subjected to
purification with column chromatography, were quite sat-
isfactory ranging from 63–85%. The coupling–deprotection
strategy described here is straightforward and should be of
value to the preparation of more oligomers or polymers
consisting of a variety of basic units. Preliminary CD experi-
mental study showed that the backbone of the oligoamides
indeed adopted folded conformations that are most likely
helical, as revealed by the introduced chiral side chains that
bias the handedness of the folded molecules. Future studies
will focus on the folded (secondary) structures of these
oligomers.
Thus, the potential handedness of oligoamides 1 was in-
vestigated with CD spectroscopy in CHCl3.10 Notable CD sig-
nals in the range of 260–400nm were detected for 1b–d,
while no CD signal could be detected for 2 in the range of
260–400nm (Figure 2 and Supporting Information S1), sug-
gesting the transfer of chirality from the UV-silent side
chains to the oligoamide backbone. As shown in Figure 2
(a), CD maxima at λ = 277 nm and 319 nm were observed in
the spectrum of 1b. The CD spectra of 1c and 1d shared
similar shapes, with signals at around λ = 280 nm (λ = 279
nm for 1c and λ = 275 nm for 1d) and 319 nm (Figure 2, b
and c). In the range from 20–100 μM, the CD spectra of 1b–
d showed growth in the intensity of CD signals with in-
creasing concentration. These observations indicated that
1b–d adopted similar chiral conformations with a biased
handedness, which may be owing to chirality transfer from
the chiral side chains to the stacked aromatic rings derived
by the well-known triple H-bonding interaction between
BTA motifs. However, only weak CD signals could be record-
ed for 1a from 20–100 μM in CHCl3, which is likely due to
the insufficient stability of the biased handedness structure
of this short oligomer.
Acknowledgment
This work was financially supported by the Natural Science Founda-
tion of China (NSFC Grants 21072187, 20772123, 20428202).
Supporting Information
Supporting information for this article is available online at
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References and Notes
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Figure 2 Concentration-dependent CD spectra of oligomers 1 and
compound 2: (a) 1b, (b) 1c, and (c) 1d recorded in CHCl3 at r.t.
In summary, a series of oligoamides consisting of differ-
ent numbers of BTA units connected via thioether linkers
have been prepared based on the highly efficient reaction
© Georg Thieme Verlag Stuttgart · New York — Synlett 2016, 27, 225–230