Di(1-naphthyl) methanol ester of carboxylic acids for absolute stereochemical determination

Article abstract of DOI:10.1002/chir.22775

The absolute stereochemistry of chiral carboxylic acids is determined as a di(1-naphthyl)methanol ester derivative. Computational scoring of conformations favoring either P or M helicity of the naphthyl groups, capable of exciton-coupled circular dichroic coupling, leads to a predicted stereochemistry for the derivatized carboxylic acids.

Full text of DOI:10.1002/chir.22775

Received: 26 June 2017
DOI: 10.1002/chir.22775
Revised: 5 September 2017
Accepted: 8 September 2017
S P E C I A L I S S U E A R T I C L E
Di(1‐naphthyl) methanol ester of carboxylic acids for
absolute stereochemical determination
Jun Zhang1
| Wei Sheng1 | Hadi Gholami1
| Tatsuo Nehira2 | Babak Borhan1
1 Department of Chemistry, Michigan
State University, East Lansing, Michigan,
USA
2 Graduate School of Integrated Arts and
Sciences, Hiroshima University,
Hiroshima, Japan
Abstract
The absolute stereochemistry of chiral carboxylic acids is determined as a di(1‐
naphthyl)methanol ester derivative. Computational scoring of conformations
favoring either P or M helicity of the naphthyl groups, capable of exciton‐
coupled circular dichroic coupling, leads to a predicted stereochemistry for
the derivatized carboxylic acids.
Correspondence
Babak Borhan, Department of Chemistry,
Michigan State University, East Lansing,
Michigan, 48824, USA.
KEYWORDS
absolute stereochemistry, carboxylic acids, chiral sensing, chirality, computational analysis, ECCD
Email: babak@chemistry.msu.edu
Funding information
National Science Foundation; Division of
Chemistry, Grant/Award Number: CHE‐
1213759
1 | INTRODUCTION
are not without their own limitations.27,28 The
derivatizing agent must be a chiral agent; the reactions
Over the past 4 decades, chiroptical spectroscopy, and in
particular circular dichroism (CD), has played a pivotal
role in the absolute stereochemical determination of
organic molecules.1-11 Numerous strategies have been
devised to address this important issue; nonetheless, it
is clear that a unified solution to the problem is not
forthcoming. The best solution for a particular func-
tional group, or a subfamily of molecules, is not neces-
sarily the best choice for another, and thus, the
continual optimization of old methods and new
creative approaches are needed. Amongst the many
different chiroptical techniques, exciton‐coupled circular
dichroism (ECCD), a method based on the through
space electronic coupling between two or more noncon-
jugated chromophores,12-14 has enjoyed great success as
a result of its ability to directly correlate the sign of
the ECCD signal to the helicity of the coupling chromo-
phores.15-26 Nuclear magnetic resonance (NMR)
methods are also widely used in stereochemical determi-
nations, although chemical derivatizations are necessary
to convert enantiomers to diastereomers. The Mosher
ester analysis is an example of the latter, although they
are often not microscale and thus require substantial
amounts of analyte. The NMR readouts that lead to
absolute stereochemical assignments require assump-
tions regarding the highest populating conformation of
a complex system that can have a number of conformers
with small energetic differences.29 Recently, we disclosed
the marriage of the latter two approaches, combining
the stability in structure of the derivatized sample, with
the routine determination of helicity via the ECCD
method in determining the absolute stereochemistry of
asymmetric amines.30 Herein, we apply this approach
to the absolute stereochemical determination of chiral
carboxylic acids.
In our recent report, we demonstrate that 1,1′‐
(bromomethylene)dinaphthalene (BDN) bearing
2
naphthalene groups can easily alkylate an asymmetric
amine (Figure 1A). The naphthyl rings of the derivatized
molecule adopt a preponderance of either the P or M
helicity as a consequence of intramolecular interactions,
yielding either a positive or negative ECCD signal. Low‐
cost computational analysis provides
a theoretical
population distribution of conformers. Statistical analysis
Chirality. 2017;1–6.
wileyonlinelibrary.com/journal/chir
© 2017 Wiley Periodicals, Inc.
1

2
ZHANG ET AL.
(A)
(B)
(C)
FIGURE 1 A, Chiral amines derivatized
with BDN; B, Synthesis of DNM 1 and
chiral DNM esters; C, P and M helical
conformations of chiral DNM esters
of the helicity of the computed structures that predict an
excess population of either P or M helicity leads to the
prediction of an ECCD signal for the chirality of the
computed derivatized compound. Comparison with
experimental ECCD data leads to the absolute stereo-
chemical determination of the derivatized amine. In a
similar fashion, this methodology is ported for use with
chiral carboxylic acids. Instead of BDN, di(1‐naphthyl)
methanol 1 (DNM, Figure 1B) is used, enabling com-
plete derivatization of carboxylic acids within 2 hours
at room temperature. As described with BDN for
amines, the induced helical twist of the bisnaphthyl
groups in the carboxylic acid derivatized DNM complex
leads to a distinct ECCD spectrum that is the direct
result of the neighboring chiral center. Conformer distri-
bution analysis (MMFF molecular force field) yields pre-
diction for populations that lead to either positive or
negative helicity of the bisnaphthyl core. Comparison
with the experimentally obtained ECCD signals enables
the absolute stereochemical determination of the
derivatized carboxylic acid.
2.2 | Instrument
1H‐NMR and 13C‐NMR spectra were obtained on Varian
500 MHz and are reported in parts per million (ppm) rela-
tive to the solvent resonances (δ), with coupling constants
(J) in Hertz (Hz). The CD spectra were recorded on a
JASCO J‐810 spectropolarimeter, equipped with a temper-
ature controller (Neslab 111) and are reported as λ [nm]
(Δεmax [mol−1 cm−1]). High Resolution Mass Spectrometry
(HRMS) analysis was performed on a Q‐TOF Ultima sys-
tem using electrospray ionization in negative mode.
2.3 | CD measurement
Chiral carboxylic acids derivatized with DNM 1 were
dissolved in acetonitrile to make a 0.01 M stock solution.
From the stock solution, 1 μL was dissolved in acetonitrile
(1 mL), yielding a 10 μM final solution ready for CD
measurement. Background spectrum of acetonitrile was
recorded from 350 to 200 nm with a scan rate of
100 nm/min at room temperature. The CD spectra of ester
samples measured with 10 accumulations were subtracted
from the background spectra and normalized based on
the concentration to obtain molecular CD (Mol CD).
2 | MATERIALS AND METHODS
2.1 | Materials
3 | RESULTS AND DISCUSSION
Anhydrous spectral grade solvents used for CD measure-
ments were purchased from Sigma‐Aldrich. Acetonitrile
was dried over molecule sieves, and THF was dried over
sodium. Column chromatography was performed using
SiliCycle silica gel. Chiral carboxylic acids not synthesized
here were purchased from commercial sources and were
used without further purification.
The ECCD signals require coupling of the electric transi-
tion dipole moments of 2 or more independently conju-
gated chromophores. The structure of DNM 1, with its 2
naphthyl rings, has the prerequisites for generating an
ECCD signal, given that the 2 aryl groups adopt a
nonracemic helical population. The methine group that

ZHANG ET AL.
3
separates the two rings not only provides a handle for
derivatization31-34 but also aids in reducing the rotational
barrier for the two aryl groups, thus enabling them to
adopt a preferred conformation as dictated by the chirality
of the derivatized carboxylic acid. At the onset, we predict
that upon esterification of DNM 1 with a chiral carboxylic
acid, either the P or M helicity of the bisnaphthyl group
would be favored (Figure 1C), resulting in an ECCD
signal that can be directly correlated to the chirality of
the carboxylic acid.
DNM 1 with chiral carboxylic acids requires less than
two hours for completion. A short silica plug purification
is all that is needed for preparation of the sample for CD
analysis.
As expected, DNM 1 is CD silent in acetonitrile. On
the other hand, derivatization of carboxylic acid 2 with
DNM 1 yields ester (R)‐2a that generates a positive ECCD
spectrum. As expected, its enantiomer (S)‐2a exhibits the
opposite ECCD signal (see Figure S1). The observed
spectrum is the result of induced helicity of the coupling
Although DNM 1 is commercially available, it can be
easily synthesized and purified by recrystallization in
gram scale from the Grignard reaction of 1‐
bromonaphthalene with ethyl formate in 90% overall
yield (Figure 1B). The DCC mediated derivatization of
TABLE 2 Exciton‐coupled circular dichroism data for chiral aryl
DNM esters
Predicted
λ(nm),
Entry Chiral Derivativea
ECCD (P:M)b Δεc
Ad
1
Neg
(8.1/83.1)
226, −52 −63
227, +48 +58
227, −30 −48
227, +31 +43
225, +31 +42
TABLE 1 Exciton‐coupled circular dichroism data for chiral alkyl
DNM esters
Predicted
λ(nm),
2
3
4
5
Pos
(83.1/8.1)
Entry Chiral Derivativea ECCD (P:M)b Δεc
Ad
1
2
3
4
5
6
7
Pos
(54.7/38.8)
226, +14 +18
226, −19 −24
227, −11 −16
226, −10 −16
Neg
(32.8/57.5)
Neg
(38.8/54.7)
Pos
(57.5/32.8)
Neg
(36.5/56.3)
Pos
Neg
(44.1/46.4)
(56.9/33.8)
6
Pos
(51.9/38.3)
227, +70 +92
Neg
(40.7/49.8)
227, −8
227, +5
226, +8
−13
+6
7
8
Pos
(49.6/43.1)
228, +11 +25
228, +8 +15
Pos
(49.8/40.7)
Pos
Pos
+10
(48.1/41.1)
(51.1/39.3)
aAll CD measurements were recorded with 10 μM ester derivative in acetoni-
aAll CD measurement were recorded with 10 μM ester derivative in acetoni-
trile at rt.
trile at rt.
bCalculated populations of P and M helical conformers were obtained via tab-
ulating the population of conformations that yield positive and negative
helicity (for computational details, see the text and the Supporting
Information).
bCalculated populations of P and M helical conformers were obtained via tab-
ulating the population of conformations that yield positive and negative
helicity (for computational details, see the text and the Supporting
Information).
cThe reported Δε is from the long wavelength portion of the couplet.
cThe reported Δε is from the long wavelength portion of the couplet.
dA refers to the amplitude of the ECCD couplet.
dA refers to the amplitude of the ECCD couplet.

4
ZHANG ET AL.
naphthyl groups, adopting specific conformations as a
result of their interaction with the nearby groups attached
to the asymmetric center. The asymmetry observed in the
ECCD spectra resulting from the coupling of naphthyl
groups has been observed before.35 The asymmetrical
shape of the couplet is closely related to the relevant elec-
tric transition moment, one of the factors responsible for
the Cotton effects.12 Namely, the transition fall‐offs from
the vibrational bands reflect the flatness of the shorter
wavelength. The shape also seems to be deformed by the
strong absorption of solvents and/or minute impurities
close to 200 nm, which diminishes the signal because of
reduce signal/noise ratio. The spectra remained virtually
identical as the temperature was reduced to 0°C. It should
be noted that although naphthyl groups have two main
(A = +18). The same analysis for molecules listed in
Table 1 yields predictions that are in line with the ECCDs
obtained for each molecule.
Table 2 summarizes the extension of the same princi-
ples and analysis to chiral carboxylic systems that are aryl
substituted. As expected, enantiomeric esters (R)‐7a and
(S)‐7a show opposite ECCD signals (Figure 2A). Interest-
ingly, the aryl substituted esters yield the opposite ECCD
sign in comparison to their alkyl carboxylic acid deriva-
tives. For example, ester (S)‐5a, with its medium sized
chloride and large sized isopropyl substituent, yields a
negative ECCD signal. Ester (S)‐8a, having the same dis-
position of substituents based on size as determined by
A‐value steric parameters (medium sized methyl group
and large sized aryl group), produces a positive ECCD.
Importantly, the conformational analysis for both leads
to the correct prediction, thus providing an operationally
expedient system that considers all mechanical interac-
tions that lead to a preferred helicity. In general, the
derivatized aryl substituted carboxylic acids (Table 2)
exhibits a stronger ECCD signal as compared to the
aliphatic substituted carboxylic acid derivatives listed in
Table 1. This is presumably the result of stronger interac-
tions of the bisnaphthyl group with the aryl substituents.
Similar to that discussed with aliphatic substrates, compu-
tational studies also predict the correct P:M ratio,
matching the observed ECCD signals. Of note, the benzyl
substituted derivatives (S)‐11a and (S)‐12a (Table 2,
entries 7 and 8) do lead to observable ECCD spectra,
albeit weaker in strength as compared to other aryl‐
substituted examples. This is presumably the result of
having the phenyl group further away from the
dinaphthyl moiety, thus lessening its influence on dictat-
ing the P:M population difference. A greater difference
in helical population should translate to the higher ECCD
amplitudes. As depicted in Figure 2B, a trend was
observed between the P and M populations for esters 2a‐
12a (alkyl and aryl systems), plotted as a function of the
observed amplitude.
1
transitions (1La ~270 nm and Bb ~220 nm), we observe
1
the coupling from the more intense Bb band (long axis
1
transition) and see no evidence of the short axis La tran-
sition in the ECCD spectra.
Table 1 illustrates a set of diverse chiral alkyl
carboxylic acids derivatized with DMN 1 (2a‐6a) that pro-
duce ECCD active species. Notably, the system is sensitive
to the discrimination of substituents with similar size
such as a methyl vs ethyl group (Table 1, entry 4). Our
approach to the prediction of ECCD relies on evaluating
conformational distribution of populations that adopt
either the P (leading to positive ECCD) or the M (leading
to negative ECCD) helicity. The estimate for the percent-
age of P and M populations for conformers with over
90% abundance (spanning 2.3 Kcal/mol from the most
stable conformer) was calculated by routine and expedi-
tious conformational modeling (MMFF94S force field).
For example, molecular modeling of ester (R)‐2a
(Table 1, entry 1) leads to a set of conformers, of which
93.5% reside within 1.12 Kcal/mol. Conformational analy-
sis of this group of molecules shows a P:M calculated ratio
of 54.7:38.8. The dominance of the P helical population
would predict a positive ECCD spectrum, which in fact
is in agreement with the experimentally observed data
FIGURE 2 A, Circular dichroism
spectra of chiral DNM esters (R)‐2a and
(S)‐2a show the opposite exciton‐coupled
circular dichroism (ECCD) signals. B, A
correlation is observed for the strength of
the ECCD spectra (Δε of the high
wavelength cotton effect) and the
population difference calculated for P and
M helicities

ZHANG ET AL.
5
FIGURE 3 A, Crystal structure of (S)‐8a
(CCDC deposition number: 1441836)
exhibits the P helicity that would yield a
positive exciton‐coupled circular
dichroism (ECCD) signal. B, Theoretical
calculation of ECCD for (S)‐8a, overlapped
with the experimentally observed
spectrum
Although we find that a simple conformational analy-
sis, requiring less than a few hours per substrate, is suffi-
cient to yield predictable populations for absolute
stereochemical determinations, a higher‐level calculation
was also performed for substrate (S)‐8a. For this purpose,
all conformers above 1% population, obtained from
MMFF minimization, were further optimized by DFT
calculation (hybrid functional B3LYP, double‐zeta basis
set 6‐31G(d) in gas phase). The energetically most stable
conformers accounting for 91.0% of the population (4 con-
formers) were then subject to TDDFT calculation with a
hybrid functional B3LYP and a double‐zeta basis set cc‐
pVDZ. The calculated ECCD spectra were integrated by
considering their population, resulting a theoretical
ECCD spectrum for substrate (S)‐8a as shown in
Figure 3B. As anticipated, the calculated ECCD matches
the Cotton effects observed experimentally for (S)‐8a, thus
confirming that the calculated P:M ratios results in the
observed sign for the ECCD spectra. Fortuitously, ester
(S)‐8a succumbed to crystallization, exhibiting the P
helical conformation, matching the predicted dominant
population by modeling (66.8% P helicity, Figure 3A and
Supporting Information).
ACKNOWLEDGMENTS
We are grateful to the NSF (CHE‐1213759) for funding.
We also thank Dr Richard Staples (MSU) for the X‐Ray
crystallography.
ORCID
Jun Zhang http://orcid.org/0000-0002-9986-0312
Hadi Gholami http://orcid.org/0000-0001-6520-0845
Babak Borhan http://orcid.org/0000-0002-3193-0732
REFERENCES
1. You L, Zha D, Anslyn EV. Recent advances in supramolecular
analytical chemistry using optical sensing. Chem Rev.
2015;115:7840‐7892.
2. Yashima E, Nimura T, Matsushima T, Okamoto Y. Poly ((4‐
dihydroxyborophenyl) acetylene) as a novel probe for chirality
and structural assignments of various kinds of molecules includ-
ing carbohydrates and steroids by circular dichroism. J Am
Chem Soc. 1996;118:9800‐9801.
3. Zahn S, Canary JW. Electron‐induced inversion of helical chiral-
ity in copper complexes of N, N‐dialkylmethionines. Science.
2000;288:1404‐1407.
4 | CONCLUSION
4. Mazaleyrat J‐P, Wright K, Gaucher A, et al. Induced axial chiral-
ity in the biphenyl core of the Cα‐tetrasubstituted α‐amino acid
residue Bip and subsequent propagation of chirality in
(Bip)n/Val oligopeptides. J Am Chem Soc. 2004;126:12874‐12879.
A
simple and microscale derivatization of chiral
carboxylic acids with DNM 1 yields ECCD active species.
The observed signal is the result of interactions that lead
to a specific orientation of the naphthyl groups, adopting
either P or M helicity. Low‐cost conformational modeling
of the derivatized carboxylic acid yields energetically
preferred populations; a weighted average of conformers
that lead to P and M helicity yields a prediction for the
absolute stereochemistry of the parent carboxylic acid.
In all cases, computational modeling for alkyl and aryl
carboxylic acids yields predictions that are in agreement
with the observed ECCD spectra. Ongoing studies with
other functional groups are underway, along with
alterations to the chromophore to redshift absorption.
5. Mei X, Wolf C. Enantioselective sensing of chiral carboxylic
acids. J Am Chem Soc. 2004;126:14736‐14737.
6. Pira SL, Wallace TW, Graham JP. Enantioselective route to 5‐
methyl‐and 5, 7‐Dimethyl‐6, 7‐dihydro‐5 H‐dibenz [c, e] azepine:
secondary amines with switchable axial chirality. Org Lett.
2009;11:1663‐1666.
7. Yu S, Pu L. Pseudoenantiomeric fluorescent sensors in a chiral
assay. J Am Chem Soc. 2010;132:17698‐17700.
8. Kuwahara S, Nakamura M, Yamaguchi A, Ikeda M, Habata Y.
Combination of a new chiroptical probe and theoretical calcula-
tions for chirality detection of primary amines. Org Lett.
2013;15:5738‐5741.

6
ZHANG ET AL.
9. Seo M‐S, Lee A, Kim H. 2, 2′‐Dihydroxybenzil: a stereodynamic
probe for primary amines controlled by steric strain. Org Lett.
2014;16:2950‐2953.
25. Gholami H, Anyika M, Zhang J, Vasileiou C, Borhan B. Host‐
guest assembly of a molecular reporter with chiral cyanohydrins
for assignment of absolute stereochemistry. Chem Eur J.
2016;22:9235‐9239.
10. Akdeniz A, Mosca L, Minami T, Anzenbacher P. Sensing of
enantiomeric excess in chiral carboxylic acids. Chem Commun.
2015;51:5770‐5773.
26. Gholami H, Zhang J, Anyika M, Borhan B. Absolute stereo-
chemical determination of asymmetric sulfoxides via central to
axial induction of chirality. Org Lett. 2017;19:1722‐1725.
11. Zhang P, Wolf C. Sensing of the concentration and enantiomeric
excess of chiral compounds with tropos ligand derived metal
complexes. Chem Commun. 2013;49:7010‐7012.
27. Dale JA, Mosher HS. Nuclear magnetic resonance enantiomer
regents. Configurational correlations via nuclear magnetic
resonance chemical shifts of diastereomeric mandelate, O‐
12. Harada N, Nakanishi K. Circular Dichroic Spectroscopy: Exciton
Coupling in Organic Stereochemistry. Mill Valley, CA: University
Science Books; 1983.
methylmandelate,
and.
alpha.‐methoxy‐.
alpha.‐
trifluoromethylphenylacetate (MTPA) esters. J Am Chem Soc.
1973;95:512‐519.
13. Berova N, Polavarapu PL, Nakanishi K, Woody RW. Compre-
hensive Chiroptical Spectroscopy, Vol. 2: Applications in
Stereochemical Analysis of Synthetic Compounds, Natural
Products, and Biomolecules. Hoboken, NJ: Wiley; 2012.
28. Wenzel TJ, Wilcox JD. Chiral reagents for the determination of
enantiomeric excess and absolute configuration using NMR
spectroscopy. Chirality. 2003;15:256‐270.
29. Seco JM, Quinoá E, Riguera R. The assignment of absolute con-
14. Mason SF. Molecular Optical Activity & Chiral Discrimination.
figuration by NMR. Chem Rev. 2004;104:17‐118.
Cambridge, UK: Cambridge University Press; 1982.
30. Zhang J, Gholami H, Ding X, et al. Computationally aided abso-
lute Stereochemical determination of Enantioenriched amines.
Org Lett. 2017;19:1362‐1365.
15. Huang X, Rickman BH, Borhan B, Berova N, Nakanishi K.
Zinc porphyrin tweezer in host‐guest complexation: determi-
nation of absolute configurations of diamines, amino acids,
and amino alcohols by circular dichroism. J Am Chem Soc.
1998;120:6185‐6186.
31. Yang X, Birman VB. Homobenzotetramisole‐catalyzed kinetic
resolution of α–aryl‐, α–Aryloxy‐, and α–Arylthioalkanoic acids.
Adv Synth Catal. 2009;351:2301
16. Li X, Borhan B. Prompt determination of absolute configuration
for epoxy alcohols via exciton chirality protocol. J Am Chem Soc.
2008;130:16126‐16127.
32. Shiina I, Nakata K, Ono K, Onda Y‐S, Itagaki M. Kinetic resolu-
tion of racemic α‐arylalkanoic acids with achiral alcohols via the
asymmetric esterification using carboxylic anhydrides and acyl‐
transfer catalysts. J Am Chem Soc. 2010;132:11629‐11641.
17. Anyika M, Gholami H, Ashtekar KD, Acho R, Borhan B.
Point‐to‐axial chirality transfer: a new probe for “sensing”
the absolute configurations of monoamines. J Am Chem Soc.
2014;136:550‐553.
33. Yang X, Birman VB. Kinetic resolution of α‐substituted Alkanoic
acids promoted by homobenzotetramisole. Chem Eur J.
2011;17:11296‐11304.
18. You L, Berman JS, Anslyn EV. Dynamic multi‐component cova-
lent assembly for the reversible binding of secondary alcohols
and chirality sensing. Nat Chem. 2011;3:943‐948.
34. Nakata K, Gotoh K, Ono K, Futami K, Shiina I. Kinetic resolu-
tion of racemic 2‐hydroxy‐γ‐butyrolactones by asymmetric
esterification using diphenylacetic acid with pivalic anhydride
and a chiral acyl‐transfer catalyst. Org Lett. 2013;15:1170‐1173.
19. Li X, Burrell CE, Staples RJ, Borhan B. Absolute configuration
for 1, n‐glycols: a nonempirical approach to long‐range stereo-
chemical determination. J Am Chem Soc. 2012;134:9026‐9029.
35. Tartaglia S, Pace F, Scafato P, Rosini C. A new case of induced
helical chirality in a bichromophoric system: absolute configura-
tion of transparent and flexible diols from the analysis of the
electronic circular dichroism spectra of the corresponding di(1‐
naphthyl)ketals. Org Lett. 2008;10:3421‐3424.
20. Proni G, Pescitelli G, Huang X, Quraishi NQ, Nakanishi K,
Berova N. Configurational assignment of α‐chiral carboxylic
acids by complexation to dimeric Zn–porphyrin: host–guest
structure, chiral recognition and circular dichroism. Chem
Commun. 2002;1590‐1591.
21. Yang Q, Olmsted C, Borhan B. Absolute stereochemical determi-
SUPPORTING INFORMATION
nation of chiral carboxylic acids. Org Lett. 2002;4:3423‐3426.
Additional Supporting Information may be found online
in the supporting information tab for this article.
22. Joyce LA, Maynor MS, Dragna JM, et al. A simple method for
the determination of enantiomeric excess and identity of chiral
carboxylic acids. J Am Chem Soc. 2011;133:13746‐13752.
23. Tanasova M, Borhan B. Conformational preference in
Bis(porphyrin) tweezer complexes: a versatile chirality sensor
How to cite this article: Zhang J, Sheng W,
Gholami H, Nehira T, Borhan B. Di(1‐naphthyl)
methanol ester of carboxylic acids for absolute
stereochemical determination. Chirality. 2017;1–6.
https://doi.org/10.1002/chir.22775
for α‐chiral carboxylic acids. Eur
2012;2012(17):3261‐3269.
J
Org Chem.
24. Tanasova M, Anyika M, Borhan B. Sensing remote chirality: ste-
reochemical determination of beta‐, gamma‐, and delta‐chiral
carboxylic acids. Angew Chem Int Ed. 2015;54:4274‐4278.