Synthesis of functional 1,2-dithiolanes from 1,3-bis-: Tert -butyl thioethers

Article abstract of DOI:10.1039/d0ob01577f

We report the one-step synthesis of diversely substituted functional 1,2-dithiolanes by reacting readily accessible 1,3-bis-tert-butyl thioethers with bromine. The reaction proceeds to completion within minutes under mild conditions, presumably via a sulf

Full text of DOI:10.1039/d0ob01577f

Organic &
Biomolecular Chemistry
View Article Online
View Journal
COMMUNICATION
Synthesis of functional 1,2-dithiolanes from 1,3-
bis-tert-butyl thioethers†
Cite this: DOI: 10.1039/d0ob01577f
a
a
b
a
Georg M. Scheutz, Jonathan L. Rowell, Fu-Sheng Wang, Khalil A. Abboud,
Received 30th July 2020,
Accepted 10th August 2020
b
a
Chi-How Peng
and Brent S. Sumerlin
*
DOI: 10.1039/d0ob01577f
rsc.li/obc
We report the one-step synthesis of diversely substituted func- for the direct polydisulfide-mediated cytosolic delivery of
2
7
5
28
tional 1,2-dithiolanes by reacting readily accessible 1,3-bis-tert- various cargo, such as proteins, quantum dots, or silica
2
9
butyl thioethers with bromine. The reaction proceeds to com- particles.
pletion within minutes under mild conditions, presumably via a In addition to the CSSC dihedral angle, a determining
sulfonium-mediated ring closure. Using X-ray crystallography and factor for the reactivity of 1,2-dithiolanes is the ring substi-
1
2,30,31
UV-vis spectroscopy, we demonstrate how substituent size and tution pattern.
For example, Matile’s group reported pro-
2
4
ring substitution pattern can affect the geometry and photo- found differences in the polymerization behavior and the
2
physical properties of 1,2-dithiolanes.
cell-uptake efficiency of lipoic acid and asparagusic acid
Fig. 1A). Whitesides and coworkers showed that higher-substi-
tuted 1,2-dithiolanes are more resistant towards reduction and
(
1
,2-Dithiolanes are five-membered heterocyclic molecules con-
taining a disulfide bond. The intricate reactivity of this class of
disulfides, arising from the geometric constraints imposed
upon the sulfur–sulfur (S–S) bond, has been exploited for cell-
22
ring-opening (Fig. S1B†).
Considering these results, we
believe there is great potential in controlling 1,2-dithiolane
reactivity via tailored substituent selection. However, most
application-focused reports revolve around commercially avail-
1
–6
uptake applications,
reversible protein–polymer conju-
7
8
9–14
gation, biosensors, dynamic networks,
and functional
1
5,16
polymer synthesis.
Distinct from linear disulfides, which usually exhibit CSSC
dihedral angles around 90°, the five-membered cyclic geome-
try of 1,2-dithiolanes forces the disulfide scaffold into confor-
mations with CSSC dihedral angles often lower than 35°
1
7,18
(
Fig. 1A and S1A†).
boring fully occupied non-bonding sulfur orbitals overlap,
causing a destabilizing four-electron interaction, also known
At such low dihedral angles, the neigh-
1
9,20
2
1
as closed-shell repulsion (Fig. S1A†). This stereoelectronic
effect weakens the S–S bond, rendering 1,2-dithiolanes prone
2
2,23
to rapid thiol-disulfide exchange
and ring-opening
Such polydisulfides generated from 1,2-
dithiolanes are typically dynamic and can be reversibly
7
,16,24
polymerization.
1
2,25,26
depolymerized,
a phenomenon that has been exploited
a
George & Josephine Butler Polymer Research Laboratory, Center for Macromolecular
Science & Engineering, Department of Chemistry, University of Florida, Gainesville,
FL 32611, USA. E-mail: sumerlin@chem.ufl.edu
b
Department of Chemistry and Frontier Research Center on Fundamental and
Fig. 1 Electronic properties and synthesis of 1,2-dithiolanes. (A) The
low CSSC dihedral angle (φ) imparts 1,2-dithiolanes, such as lipoic or
asparagusic acid, with unique reactivity. (B) Previous syntheses of 1,2-
dithiolanes involved a two-step reaction sequence. (C) Our strategy pro-
vides hydroxy-functional 1,2-dithiolanes in a single step from readily
available 1,3-bis-tert-butyl thioether substrates.
Applied Sciences of Matters, National Tsing Hua University, 101, Sec 2, Kuang-Fu
Rd., Hsinchu 30013, Taiwan
†
Electronic supplementary information (ESI) available: Procedures, calculations,
and characterization data. CCDC 2011482 and 2011483. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/d0ob01577f
This journal is © The Royal Society of Chemistry 2020
Org. Biomol. Chem.

View Article Online
Communication
Organic & Biomolecular Chemistry
able lipoic acid (Fig. 1A) and its amide or ester derivatives. addition of 1.3 equivalents of Br
This lack of substrate variety is arguably due to the limited syn- sion of 1a, lower reaction temperatures required increased
thetic accessibility of substituted 1,2-dithiolane derivatives amounts of Br for the reaction to complete (entry 3). Other
2
typically led to full conver-
2
with functional handles for downstream modification. 1,2- electrophilic halogen reagents, such as N-bromosuccinimide
Dithiolanes are commonly synthesized in a two-step sequence, (NBS) or 1,2-dibromotetrachloroethane (C Cl Br ), were less
2
4
2
which includes generation of the 1,3-dithiol via hydrolysis or effective or showed no reaction (entries 4–6).
1
reduction of suitable precursors, followed by oxidation to the
Analysis of the crude reaction mixture by H NMR spec-
corresponding 1,2-dithiolane (Fig. 1B). However, formation of troscopy and GC-MS showed the formation of 1,2-dibromo-2-
the 1,3-dithiol often requires harsh reaction conditions that methylpropane and tert-butyl bromide as the major byproducts
limit functional group tolerance and often lead to undesired (Fig. S2–5†). Based on these results, we propose that ring
polydisulfide formation. To overcome these synthetic limit- closure proceeds via the initial formation of sulfonium
3
7
ations and expand the substrate toolbox for applications of bromide A, followed by elimination of isobutylene (Fig. 2).
,2-dithiolanes, we developed a modular one-step synthesis of The activated sulfenyl bromide B could then undergo intra-
diversely substituted 1,2-dithiolanes from readily accessible molecular cyclization to compound C, which yields the target
,3-bis-tert-butyl thioethers (Fig. 1C). Furthermore, the 1,3-bis- 1,2-dithiolane after another elimination of isobutylene.
tert-butyl thioethers were designed to feature a hydroxy group Isobutylene (T = −6.9 °C) either evaporates from the reaction
that can be used as a handle for downstream functionaliza- mixture or reacts with Br2 and HBr to form 1,2-dibromo-2-
tions of the 1,2-dithiolane product. methylpropane and tert-butyl bromide, respectively (Fig. 2).
tert-Butyl protection of thiols is typically known for robust- The higher solubility of isobutylene in the mixture at lower
1
1
b
3
2,33
34
35
ness and stability.
However, Mayor and later Feringa
temperatures would also explain the increased consumption of
at 0 °C (Table 1, entry 3).
Having established this synthetic strategy, we aimed to
leveraged the tert-butyl group for the direct transformation of Br
S-tert-butyl thioethers into thioacetates using acetyl chloride
2
and a catalytic amount of Br or TiCl , respectively. Specifically generate multiple 1,3-bis-tert-butyl thioether compounds
2
4
intriguing to us was Mayor’s observation of disulfide side pro- (Fig. S6†) for the subsequent transformation into 1,2-dithio-
3
4
ducts during the reaction. Based on these reports, we tested lanes (Fig. 1C). Specifically, we synthesized 1,3-bis-tert-butyl
if S-tert-butyl cleavage in combination with intramolecular di- thioethers 1a–1l from various 1,3-dichloropropan-2-ol deriva-
sulfide formation promoted by electrophilic halogen reagents tives, α,α′-halogenated ketones, and 2-(chloromethyl)oxiranes
could provide access to 1,2-dithiolanes from 1,3-bis-tert-butyl with tert-butylthiol and K
thioethers in a single step. Optimization of the reaction con- erature (Fig. S6†). This broad range of suitable starting
ditions with thioether 1a as a substrate showed that Br , in materials suggests that a variety of 1,3-bis-tert-butyl thioether
2 3
CO as a base in DMF at room temp-
2
combination with hydrated silica gel, was most effective for substrates can be readily generated by this protocol. Notably,
the targeted transformation, yielding 4-hydroxy-4-phenyl-1,2- the reaction could be conducted on multigram scales (1a and
dithiolane PhDL in 77% isolated yield (Table 1). The addition 1f) and all products were obtained in good yield and purity,
of silica gel to the reaction mixture improved the yield (entries often without the need of chromatographic purification.
3
6
1
and 2), presumably by scavenging reactive byproducts,
Following the 1,3-bis-tert-butyl thioether preparation, we
-induced ring-closure conditions
which could be visually confirmed by the discoloration of the used the optimized Br
2
silica gel over the course of the reaction. While the slow (Table 1) to convert 1,3-bis-tert-butyl thioethers 1a–1f and 1l
into 4-hydroxy-1,2-dithiolanes with moderate to good yields
(
Fig. 3). This approach provided access to seven new 1,2-dithio-
lane derivatives with unprecedented ring substitution and
a
Table 1 Optimization of reaction conditions
b
Entry
Deviation from standard conditions
Yield (%)
1
2
None
No silica gel
0 °C
77
52
24
28
n. r.
n. r.
c
3
4
d
NBS instead of Br
Cl Br instead of Br
instead of Br
2
5
6
C
2
4
2
2
I
2
2
a
All reactions were run to full conversion of 1a unless no reaction
n. r.) was observed. Standard conditions: 1a (1.0 mmol), silica gel ([g
Fig. 2 Proposed reaction mechanism for the deprotection-disulfide
formation sequence affording 4-hydroxy-4-phenyl-1,2-dithiolane
(PhDL). After elimination, isobutylene either evaporates or is converted
into brominated byproducts.
(
silica gel]/[mmol 1a] = 2), DCM (20 mL), room temperature (r. t.).
b
c
d
Isolated yield after column chromatography. 2.2 equiv. Br2.
3
equiv. NBS.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Organic & Biomolecular Chemistry
Communication
could potentially contribute to the reactivity associated with
such compounds, warranting future investigations.
Next, we turned to UV-vis spectroscopy to analyze the
maximum absorbance wavelength (λmax) of the first electronic
transition (S → S ), which provides information about the S–S
0
1
bond geometry in 1,2-dithiolanes. Specifically, the energy of
→ S in disulfides is dependent on the overlap of the fully occu-
pied non-bonding sulfur orbitals, which in turn is determined by
S
0
1
19,20
the CSSC dihedral angle (Fig. S1A†).
For example, a stronger
orbital overlap at lower CSSC dihedral angles raises the HOMO
20
energy while the LUMO remains largely unaffected, thus redu-
cing the photon energy required for the excitation of S → S
0
1
.
For the 1,2-dithiolane products tested in this report, we
recorded a slight increase of λmax upon geminal substitution
on C4 (Fig. 4, S8, and Table S3†). HDL exhibited a λmax of
327 nm, whereas derivatives with alkyl and aromatic substitu-
ents on C4 showed λmax values around 335 and 339 nm,
respectively. The absorbance bands of derivatives with aro-
matic substituents were generally broader than those of deriva-
tives with alkyl substituents, suggesting differences in the anti-
3
8
bonding character of S
1
.
Substitution on C3 in DiMeDL
resulted in a large λmax red shift to 354 nm, which corroborates
with the lower CSSC dihedral angle revealed in the crystal
Fig. 3 Preparation of hydroxy-functional 1,2-dithiolanes with isolated structure. These results suggest that ring substitution affects
yields. The lower yield of HDL is likely due to auto-polymerization the geometry and the photophysical properties of 1,2-dithio-
during purification. The X-ray crystal structures of DiMeDL and C12DL
show a shortened S–S bond length (d) and a compressed CSSC dihedral
angle (φ).
functionality (Fig. 3). Geminal to the hydroxy group, the substi-
tuents were varied from hydrogen (HDL) to propyl (nPrDL), iso-
propyl (iPrDL), and dodecyl (C12DL); phenyl (PhDL), thiophe-
nyl (TphDL), and bromothiophenyl (BrTphDL) groups could
be installed as aromatic analogues. Additionally, we created an
intriguing alternative 1,2-dithiolane scaffold with gem-
dimethyl substitution vicinal to the disulfide bond (DiMeDL).
Importantly, this reaction proved efficient on multigram scales
(
iPrDL and PhDL), which will be beneficial in 1,2-dithiolane
applications.
With the compounds in hand, we sought to evaluate the
effect of substituent size and substitution pattern on 1,2-
dithiolane ring conformation, since the geometry and dihedral
angle of the disulfide moiety profoundly affects the properties
of 1,2-dithiolanes (Fig. S1A†). The crystal structures of C12DL
and DiMeDL (Fig. 3) revealed similarly elongated S–S bond
lengths around 2.06 Å (linear disulfide bond lengths are typi-
17
cally around 2.03 Å) but differing CSSC dihedral angles of
5.2° and 23.4°, respectively. Interestingly, this CSSC dihedral
3
angle reduction coincided with a sharp decrease of the CCSS
dihedral angle from 21° to 1° (Table S1†). Comparison with all
available crystal structures of unbridged and monocyclic 1,2-
dithiolanes showed a similar, almost linear relationship
between the CSSC and the CCSS dihedral angles (Tables S1
and S2, Fig. S7†). We believe that such eclipsed CCSS confor-
Fig. 4 Substituent effects on the photophysical properties of 1,2-
dithiolanes. (A) Overlay of representative UV-vis absorbance spectra
taken at 10 mM in DMSO. (B) Bar diagram showing the variation of the
maximum absorbance wavelength (λmax) with respect to 4-hydroxy-1,2-
mations in 1,2-dithiolanes with low CSSC dihedral angles dithiolane substituent.
This journal is © The Royal Society of Chemistry 2020
Org. Biomol. Chem.

View Article Online
Communication
Organic & Biomolecular Chemistry
lanes. Furthermore, based on the linear increase of λmax with
C4 substituent A-values (Fig. S9†), we propose that the substi-
tuent effects in this position are mostly of steric nature.
Typically, low CSSC dihedral angles in 1,2-dithiolane com-
pounds are associated with ring strain and S–S bond instabil-
ity. For example, auto-polymerization has been commonly
1
2,18,25,39,40
observed for 1,2-dithiolanes,
an issue we also
encountered during purification of the compounds. We noted
dramatic stability differences between HDL and higher substi-
tuted iPrDL, and C12DL. For example, HDL could be used only
in solution due to rapid polymerization upon concentration,
whereas iPrDL and C12DL were bench-stable over weeks.
While the crystalline nature of C12DL could have a stabilizing Fig. 5 Downstream functionalization of 1,2-dithiolanes, exploiting the
hydroxy functionality. Triethylamine (TEA) was used as a base with 4-di-
methylaminopyridine (DMAP) as a catalyst in the esterification of PhDL.
effect, HDL and iPrDL are liquids and still showed substantial
differences in stability.
1,8-Diazabicylcoundec-7-ene (DBU) was employed in the base-cata-
To further investigate this observation, we estimated the ring
lyzed thia-Michael addition of 1,2-dithiolane acrylate 3 with benzyl
strain for HDL and iPrDL via quantum chemical calculations of mercaptan.
the enthalpy of reaction (ΔrxnH) for the isodesmic reaction
between 1,4-butanedithiol (BuSH) and HDL or iPrDL (Table 2).
The value of ΔrxnH reflects the additional ring strain of the 1,2-
dithiolane compound with respect to the relatively unstrained
In conclusion, we disclosed a scalable and straightforward
synthetic protocol for diversely substituted new 1,2-dithiolane
compounds featuring a hydroxy functionality as a valuable
handle for downstream conjugation. X-ray crystallography, UV-
vis spectroscopy, and quantum chemical calculations revealed
profound substitution effects on the stereoelectronic properties
and the stability of the 1,2-dithiolane derivatives, suggesting that
1
,2-dithiane (BuSS). The calculations revealed a ΔrxnH of −27.9 kJ
−1
mol for HDL, which is slightly higher than the ring strain for
41
1,2-dithiolane determined by Sunner via iodine oxidation.
Upon geminal substitution on C3 with an isopropyl group, Δrxn
H
−1
was reduced to −2.9 kJ mol , corroborating with the higher
stability of iPrDL observed experimentally, emphasizing the stabi-
1,2-dithiolane reactivity can be tuned by careful substituent
12,22,42,43
lizing effect from substitution in cyclic structures.
selection. We believe this report represents an attractive avenue
for the future design of 1,2-dithiolanes in advanced applications,
such as cargo delivery and stimuli-responsive polymer materials.
Finally, to test if the hydroxy group incorporated in the 1,2-
dithiolane structure was suitable for downstream modifi-
cations, we reacted PhDL with isobutyryl chloride, affording
1
,2-ditholane ester 2 in good yield (Fig. 5). Using the same
strategy, we synthesized 1,2-dithiolane acrylate 3 from iPrDL
and acryloyl chloride. The subsequent base-catalyzed thia-
Michael addition between benzyl mercaptan and 3 provided
Michael adduct 4 in high yield. We believe that such trans-
formations could be particularly useful in applications that
require covalent conjugation of 1,2-dithiolanes to substrates
such as proteins or polymers.
Conflicts of interest
The authors have no conflicts to declare.
Acknowledgements
This research was supported by the DoD (W911NF-17-1-0326)
and the NSF (DMR-1904631, CHE-1808234). The University of
Florida Department of Chemistry Mass Spectrometry Center
acknowledges NIH (S10 OD021758-01A1), and K.A.A wishes to
acknowledge NSF for funding the X-ray diffractometer through
CHE-1828064. The authors thank Prof. Yuan-Chung Cheng
Table 2 Ring strain calculations via the isodesmic reaction of 1,4-buta-
nedithiol (BuSH) and 1,2-dithiane (BuSS) with a 1,2-dithiolane derivative
in the ring-closed (DL) and 1,3-dithiol (C3SH) form
(
National Taiwan University) for providing the calculation
resource and Dr. Ion Ghiviriga (University of Florida) for assist-
ance with NOE experiments.
a
(kJ mol−1
b
R′
R″
Compound
Δ
rxn
H
)
λmax (nm)
H
iPr
H
H
HDL
iPrDL
−27.9
−2.9
327
336
Notes and references
a
Calculated via ΔrxnH = [Δ
f f
f
H (BuSS) + Δ
f
H (C3SH)] − [Δ
f
H (BuSH) +
Δ H (DL)] All enthalpies of formation (Δ H) were calculated for the gas
1
G. Gasparini, E. K. Bang, G. Molinard, D. V. Tulumello,
S. Ward, S. O. Kelley, A. Roux, N. Sakai and S. Matile, J. Am.
Chem. Soc., 2014, 136, 6069–6074.
b
phase at 298 K using a B3LYP/6-311G** basis set. Determined from
UV-vis spectroscopy (Table S4†). Higher λmax values indicate lower
CSSC dihedral angles.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2020

View Article Online
Organic & Biomolecular Chemistry
Communication
2
3
G. Gasparini, G. Sargsyan, E. K. Bang, N. Sakai and 21 E. V. Anslyn and D. A. Dougherty, Modern Physical Organic
S. Matile, Angew. Chem., Int. Ed., 2015, 54, 7328–
331.
Chemistry, University Science Books, Sausalito, California,
2006.
7
D. Abegg, G. Gasparini, D. G. Hoch, A. Shuster, 22 J. Houk and G. M. Whitesides, J. Am. Chem. Soc., 1987, 109,
E. Bartolami, S. Matile and A. Adibekian, J. Am. Chem. Soc.,
017, 139, 231–238.
J. Fu, C. Yu, L. Li and S. Q. Yao, J. Am. Chem. Soc., 2015,
37, 12153–12160.
L. Qian, J. Fu, P. Yuan, S. Du, W. Huang, L. Li and
6825–6835.
2
23 R. W. Singh and G. M. Whitesides, J. Am. Chem. Soc., 1990,
112, 6304–6309.
24 E. K. Bang, G. Gasparini, G. Molinard, A. Roux, N. Sakai
and S. Matile, J. Am. Chem. Soc., 2013, 135, 2088–2091.
4
5
1
S. Q. Yao, Angew. Chem., Int. Ed., 2018, 57, 1532– 25 B. Nelander, Acta Chem. Scand., 1971, 25, 1510–1511.
536. 26 J. P. Danehy and V. J. Elia, J. Org. Chem., 1972, 37, 369–373.
N. Chuard, G. Gasparini, D. Moreau, S. Lorcher, C. Palivan, 27 S. Du, S. S. Liew, L. Li and S. Q. Yao, J. Am. Chem. Soc.,
1
6
W. Meier, N. Sakai and S. Matile, Angew. Chem., Int. Ed.,
017, 56, 2947–2950.
J. Lu, H. Wang, Z. Tian, Y. Hou and H. Lu, J. Am. Chem.
Soc., 2020, 142, 1217–1221.
L. Zhang, D. Duan, Y. Liu, C. Ge, X. Cui, J. Sun and J. Fang,
J. Am. Chem. Soc., 2014, 136, 226–233.
Q. Zhang, C.-Y. Shi, D.-H. Qu, Y.-T. Long, B. L. Feringa and
H. Tian, Sci. Adv., 2018, 4, eaat8192.
2018, 140, 15986–15996.
2
28 E. Derivery, E. Bartolami, S. Matile and M. Gonzalez-
Gaitan, J. Am. Chem. Soc., 2017, 139, 10172–10175.
29 C. Yu, L. Qian, J. Ge, J. Fu, P. Yuan, S. C. Yao and S. Q. Yao,
Angew. Chem., Int. Ed., 2016, 55, 9272–9276.
30 A. Carmine, Y. Domoto, N. Sakai and S. Matile, Chem. Eur.
J., 2013, 19, 11558–11563.
7
8
9
31 J. A. Burns and G. M. Whitesides, J. Am. Chem. Soc., 1990,
112, 6296–6303.
1
0 Q. Zhang, Y.-X. Deng, H.-X. Luo, C.-Y. Shi, G. M. Geise,
B. L. Feringa, H. Tian and D.-H. Qu, J. Am. Chem. Soc., 32 P. G. M. Wuts, Greene’s Protective Groups in Organic
019, 141, 12804–12814. Synthesis, John Wiley & Sons, New Jersey, 2014.
1 G. A. Barcan, X. Zhang and R. M. Waymouth, J. Am. Chem. 33 O. Nishimura, C. Kitada and M. Fujino, Chem. Pharm. Bull.,
Soc., 2015, 137, 5650–5653. 1978, 26, 1576–1585.
2 X. Zhang and R. M. Waymouth, J. Am. Chem. Soc., 2017, 34 A. Błaszczyk, M. Elbing and M. Mayor, Org. Biomol. Chem.,
39, 3822–3833. 2004, 2, 2722–2724.
3 Y. Deng, Q. Zhang, B. L. Feringa, H. Tian and D.-H. Qu, 35 T. C. Pijper, J. Robertus, W. R. Browne and B. L. Feringa,
Angew. Chem., Int. Ed., 2020, 59, 5278–5283. Org. Biomol. Chem., 2015, 13, 265–268.
4 G. M. Scheutz, J. L. Rowell, S. T. Ellison, J. B. Garrison, 36 M. H. Ali and M. McDermott, Tetrahedron Lett., 2002, 43,
2
1
1
1
1
1
T. E. Angelini and B. S. Sumerlin, Macromolecules, 2020, 53,
038–4046.
6271–6273.
37 E. Anklam, Synthesis, 1987, 841–843.
4
1
5 H. Tang and N. V. Tsarevsky, Polym. Chem., 2015, 6, 6936– 38 L. Antonov and D. Nedeltcheva, Chem. Soc. Rev., 2000, 29,
945. 217–227.
6 Y. Liu, Y. Jia, Q. Wu and J. S. Moore, J. Am. Chem. Soc., 39 G. Claeson, Acta Chem. Scand., 1955, 9, 178–180.
6
1
2
019, 141, 17075–17080.
40 L. Field and R. B. Barbee, J. Org. Chem., 1969, 34, 36–41.
41 S. Sunner, Nature, 1955, 176, 217.
1
1
1
2
7 R. Steudel, Angew. Chem., Int. Ed. Engl., 1975, 14, 655–664.
8 L. Teuber, Sulfur Rep., 1990, 9, 257–333.
9 G. Bergson, Ark. Kemi, 1958, 12, 233–237.
0 D. B. Boyd, J. Am. Chem. Soc., 1972, 94, 8799–8804.
42 M. E. Jung and G. Piizzi, Chem. Rev., 2005, 105, 1735–1766.
43 H. Liu, A. Z. Nelson, Y. Ren, K. Yang, R. H. Ewoldt and
J. S. Moore, ACS Macro Lett., 2018, 7, 933–937.
This journal is © The Royal Society of Chemistry 2020
Org. Biomol. Chem.