, 2003, 13(3), 97–99
Pasteur-like resolution of quasi-racemates in solid and gas phases
Remir G. Kostyanovsky,*a Eugene N. Nikolaev,b Oleg N. Kharybin,b Gul’nara K. Kadorkinaa
and Vasilii R. Kostyanovskya
a N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 095 938 2156; e-mail: kost@chph.ras.ru
b Institute of Energy Problems of Chemical Physics, Russian Academy of Sciences, 119991 Moscow, Russian Federation.
Fax: +7 095 137 8258; e-mail: olmon@chph.ras.ru
10.1070/MC2003v013n03ABEH001801
Isotopomeric quasi-racemates (IQR), i.e., 1:1 mixtures of enantiomers one of which contains an isotopic label, can undergo
crystallisation as conglomerates or true quasi-racemates. In the former case, each single crystal contains, predominantly or
exclusively, either labelled or non-labelled enantiomers, whereas in the latter case, it contains both enantiomers (1:1). If so lution
sputtering is used to ionise quasi-racemates, the formation of homochiral protonated and metallated enantiomers and their homo-
chiral oligomers in the gas phase is detected in ion cyclotron resonance (ICR) mass spectra; these compounds differ in their m/z
values; hence, Pasteur-like ‘manual sorting’ can be employed to catch them successively in the ion trap of the spectrometer.
Louis Pasteur was the first to report1 quasi-racemates, i.e., the
mixtures (1:1) of chiral compounds with close structures but
opposite absolute configurations.2 The examples of these include
the mixtures of monoammonium (+)-tartrate and (–)-malate1,2
or the dimethyl esters of (+)-dicarboxylic acids and the diethyl
(or diisopropyl) esters of their (–)-enantiomers.3(a)–(d) Quasi-
racemic mixtures of enantiomers, one of which contains an
isotopic label,3(a),(b),(e) e.g., (+)-alaninamide and (–)-alanin-
amide-15N,3(a),(b) are most similar to racemates. Unlike true
racemates and conglomerates, the components of such isotopo-
meric quasi-racemates (IQRs) can be detected by not only
chiroptical methods or chiral chromatography but also many
‘achiral’ spectral methods (MS,3(a),(b),(e) NMR,3(c),(d) IR, etc.).
Therefore, IQRs can be used to study the self-association of
chiral molecules in solid and gas phases, as well as in solutions.
In many cases, the isotopic label should probably not change
the crystal structure, and the single crystals of IQRs can be used
in tests for conglomerate formation or racemic twinning and
also for studying other crystal properties by spectral methods.
IQRs have previously been used to determine the enantiomeric
composition by 1H NMR in a study of absolute asymmetric
synthesis by photodimerisation in a chiral crystal.4(a) Recently,
IQRs have been employed in an elegant study of the chiral self-
assembly of (±)-Lys and (±)-Glu derivatives into two-dimen-
sional crystallites at the air–water interface using MS (and other
techniques).4(b)–(d) This method is widely used in studies on the
chiroselective self-assembly of IQR molecules.5
obtained by spontaneous resolution of (±)-Asn.6,†
As expected, crystallisation of their equimolar mixture (IQR-1)
from water gives optically pure or highly enriched crystals
containing labelled (S)-Asn or non-labelled (R)-Asn. This was
1
confirmed by H NMR spectra and ICR mass spectra of solu-
tions of these crystals (Figure 1) and by the measurement of
optical rotation angles.† The ICR mass spectrum‡ of the equi-
of 15NH2 protons; the mass spectrum contains only the signal of an ion
with m/z 133 [M + H+] but no signal with m/z 134. It follows that the
crystal contains only (R)-Asn. For another crystal with a mass of 7 mg:
20
20
20
20
[a]
+27.6°, [a]
+35.6°, [a]
+50.5°, [a]
+63.5° (c 0.2,
578
546
436
406
1N HCl). The peak of labelled (S)-Asn with m/z 134 [M + H+] pre-
dominates in the mass spectrum.
IQR-2. Betaines 1 and 1-d5 were obtained by reactions of maleic acid
with pyridine and pyridine-d5 in water (three weeks at 20 °C) in 98 and
90% yields, respectively; mp of separate crystals are 214 (decomp.)
and 200–204 °C (decomp.), respectively.
1: 1H NMR (D2O) d: 3.4 (m, 2H, CH2, AB part of ABX spectrum, ∆n
68.0, 2JAB –18.0 Hz, 3JAX 9.9 Hz, 3JBX 4.4 Hz), 5.6 (dd, 1H, HC, X-part),
3
3
3
8.0 (dd, 2H-β, J 7.8 Hz, J 6.1 Hz), 8.5 (t, 1H, H-γ, J 7.8 Hz), 8.9 (d,
2H, 2H-α, 3J 6.1 Hz). 13C NMR (D2O) d: 37.0 (t, CH2, J 131.0 Hz),
1
71.3 (d, CH, 1J 144.2 Hz), 127.6 (d, CH-γ, 1J 175.7 Hz), 144.2 (d, CH-β,
1J 193.7 Hz), 146.0 (d, CH-α, J 171.5 Hz), 168.1 (s, CO2), 170.8 (s,
1
CO2H).
1-d5: 1H NMR (D2O) d: 3.4 (m, 2H, CH2, AB part of ABX spectrum,
2
3
3
∆n 68.0, JAB –18.0 Hz, JAX 9.9 Hz, JBX 4.4 Hz), 5.6 (dd, 1H, HC,
X-part). The spectrum of a 1/1-d5 (1:1) mixture is identical to the
spectrum of 1, but the integral intensity of signals in the region d 8–8.9
is two times lower. Crystallisation of 1 and 1-d5 from distilled water
and separation of crystals according to the sign of optical rotation gave
portions of crystals of (R)-(+)-1 with [a]D20 +71.5° (c 0.8, H2O) and
(S)-(–)-1-d5 with [a]D20 –69.9° (c 0.8, H2O). Subsequent co-crystallisation
of their equimolar amounts gave six crystals with total masses of 1.5 to
6.7 mg. Based on mass spectra, the ee was found to be 100% in one
crystal, only a peak of 1-d5 with m/z 201 [M + H+] was observed; in
another three crystals, the ee was 56, 50 or 35% with predominance
In this work, we studied three IQRs. The first one, IQR-1,
was obtained on the basis of asparagine (Asn) forming a con-
glomerate (space group P212121, Z = 4),6 whose ICR mass
spectra contained the following ions: [2AsnCuII + H+],5(b) as
well as [Asn + H+], [2Asn + H+], [4Asn + H+] and [Asn + M+],
[2Asn + M+] (M = Na, K)6 upon electrospray ionisation (ESI) of
solutions.7 Using known procedures,8 (S)-(–)-Asn-(15N-amide)
was obtained from (S)-(+)-Asp and 15NH3;† (R)-(+)-Asn was
1
of 1: the peak with m/z 196 was observed. Based on H NMR of solu-
†
NMR spectra were measured on a Bruker WM-400 spectrometer
tions of two more crystals, the ee was found to be 66.6 and 60% for 1-d5
and 1, respectively, from the signal ratio at d 5.6 (HC) and 8.5 (H-γ).
IQR-3. (R,R)-(+)-Dimethyl tartrate (+)-2 {mp 58–61 °C, [a]2D0 +21.0°
(c 1.0, H2O)} and (S,S)-(–)-dimethyl tartrate (–)-2 [mp 59–60 °C, [a]D20
–20.9° (c 1.0, H2O)] from Fluka were used.
(400.13 MHz for 1H and 100.61 MHz for 13C); optical rotation was
measured on a Polamat-A polarimeter; melting points were determined
on a Boetius heating stage and corrected.
IQR-1. (R)-Asn was obtained by spontaneous resolution of (±)-Asn
(see ref. 6). (S)-Asn (15N-amide) was synthesised from (S)-(+)-Asp and
15NH3 (isotopic enrichment 98.7%) (see ref. 8). 1H NMR spectrum
([2H6]DMSO, at 60 °C) d: 2.52 (m, 2H, CH2, AB part of ABX spectrum,
(S,S)-(–)-Dimethyl tartrate-d6 (–)-2-d6 was obtained by transesterifica-
tion: concentrated HCl (five drops) was added to a solution of (–)-2
(1.3 g) in 30 ml of methanol-d4, and the solution was kept for four days.
The mixture was treated with an ethereal suspension of NaHCO3, filtered,
dried with anhydrous Na2SO4, filtered again, concentrated and evacuated.
The product was recrystallised three times from CCl4 to give 1 g of
anhydrous crystals, mp 50 °C, [a]D20 –21.7° (c 1.0, H2O). 1H NMR
(CDCl3) d: 3.20 (d, 2H, 2HO, 3J 6.5 Hz), 4.56 (d, 2H, 2HC, 3J 6.5 Hz).
Equimolar amounts of (+)-2 and (–)-2-d6 were dissolved in hot CCl4;
crystals of IQR-3 were obtained, mp 90–92 °C. For a separate crystal,
2
3
3
∆n 380.0, JAB –16.1 Hz, JAX 8.7 Hz, JBX 4.0 Hz), 3.40 (dd, 1H, HC,
1
2
X-part), 7.02 (dd, 1H, HN, JHN 87.4 Hz, JHH 2.3 Hz), 7.69 (dd, 1H,
HN, 1JHN 88.7 Hz, 2JHH 2.3 Hz) (cf. with the spectrum of formamide-15N).14
The spectra of HC protons of the isotopomers coincide, whereas those of
HN do not overlap (cf. ref. 6); this makes it possible to determine their
ratio. (R)- and (S)-Asn (18 mg of each) were dissolved in 2 ml of dis-
tilled water and kept at 18–20 °C. After 11 days, six crystals with a total
mass of 30 mg were obtained. For a crystal with a mass of 14 mg:
3
1H NMR (CDCl3) d: 3.20 (d, HO, J 6.5 Hz), 3.87 (s, MeO), 4.55 (d,
20
20
546
20
436
20
406
3
[a]
–29.4°, [a]
–37.7°, [a]
–53.6°, [a] –67.1° (c 0.3,
HC, J 6.5 Hz); the integral intensity ratio of the signals at d 3.87 and
578
1N HCl); the 1H NMR spectrum ([2H6]DMSO) does not contain signals
4.55 is 3:2.
– 97 –