CHIRAL NMR SOLVATING AGENT
709
protonated primary amine needed for strong association with
the 18-crown-6 moiety.
differentiation with no overlap of the particular substrate res-
onance with other substrate or crown ether resonances). For
2–5, one or more resonances of each show substantial
While
1 has been examined with a number of
1
monofunctional primary amines, we wanted to explore its util-
ity with diamino compounds. We will describe the use of 1 as
a chiral solvating agent for a series of diamines. Some of the
compounds have stereocenters, whereas others are
atropisomers. When solubility in methanol is permitted, sub-
strates were mixed with 1 in their neutral and protonated
forms. Also, we describe the utility of 1 as a chiral NMR sol-
vating agent for a series of bicyclic β-amino acids.
enantiomeric differentiation in the H NMR spectrum with
1. Comparative data are also provided for mixtures of 1 with
the neutral (2–5), monoprotic (2), and diprotic (3–5) amine.
When the neutral or monoprotic amine is used, the acid–
base neutralization reaction will produce one or more anionic
forms of 1, depending on the stoichiometric equivalents of
the two compounds in the solution. In all cases, a complex,
concentration-dependent stoichiometry of the association of
1 and substrate likely occurs. There is the possibility of two
substrates associating simultaneously with 1 where one
substrate binds at each of the faces of the crown ether. Alter-
natively, two different crown ethers could simultaneously
bind at each of the amine groups. Depending on the relative
concentrations of the amine and 1, neutral, mono-anionic,
and di-anionic forms of 1 may occur in the solution.
Compound 2 was available in its neutral form. Addition of
two equivalents of DCl to a solution of 2 in methanol-d4
resulted in a precipitate. Addition of one equivalent of DCL
to a solution of 2 in methanol-d4 did not. Hence, 2 was
studied with 1 in its neutral and monoprotic form. The behav-
ior of the methine resonance of 2 with (+)-1 shows markedly
different behavior depending on whether neutral or
monoprotic 2 is examined (Fig. 2). Significant deshielding
of the methine hydrogen occurs upon protonation of the
amine groups. When neutral 2 (10 mM) is mixed with (+)-1,
large enantiomeric differentiation of the methine resonance
occurs and the resonance of the (R)-enantiomer is consis-
tently deshielded relative to that of the (S)-enantiomer (Fig. 2,
bottom series of spectra). When monoprotic 2 (10mM) is
mixed with (+)-1, the methine resonance of the (S)-enantiomer
is more deshielded when the concentration of (+)-1 is in the
10–15 mM range (Fig. 2, top series of spectra). At higher
concentrations of (+)-1 (20–30 mM), the two resonances
coalesce and then reverse their order in the spectrum
such that the (R)-enantiomer is deshielded relative to the
(S)-enantiomer. The same series of spectra recorded for
monoprotic 2 with (–)-1 shows the opposite behavior, as
the (R)-enantiomer of 2 is more deshielded at lower con-
centrations of (–)-1 and then the two resonances coalesce
and reverse order as the concentration of (–)-1 is raised.
The concentration behavior of the nature seen in Figure 2b
indicates that either differences in the association constants of
the two enantiomers with 1 or a concentration-dependence
difference in the stoichiometry likely dominates the origin of
the enantiomeric differentiation.37,38
EXPERIMENTAL
Reagents
1–12, 23 and methanol-d4 were obtained from commercial suppliers
(Sigma-Aldrich, Milwaukee, WI; Cambridge Isotope, Andover, MA) and
used as received. 2–9 were obtained in their neutral forms. 10–13 were
obtained as their dihydrochloride salts. The synthesis of the enantiomers
of monoterpene-based β-amino acids 14–17 were carried out by formerly
reported methods.26–30 Chlorosulfonyl isocyanate addition to α-pinane,
δ-pinane, and apopinene furnished β-lactams in highly regio- and
stereospecific reactions. Acid-catalyzed hydrolysis of lactams resulting
in cis-β-amino acids 15 and 17.28,29 Acid-catalyzed ethanolysis of β-
lactams derived from α-pinene and apopinene led to cis-β-amino esters.
Hydrolysis of α-pinane-based ester under mild conditions resulted in
14,27 while the trans amino acid 16 was obtained by base-catalyzed isom-
erization of the corresponding cis-β-amino ester, followed by hydrolysis.29
The 3-phenylisoserine-derived side-chain is essential for the antitumor
activity of Taxol, currently considered to be among the most important
drugs in cancer chemotherapy31. Racemic ethyl 3-amino-3-phenyl-2-
hydroxypropionate [( )-19] and (3S*,4R*)-3-hydroxy-4-phenylazetidin-2-
one [( )-22], used for the enzymatic resolutions, were synthesized ac-
cording to literature methods.32–34 The lipase PS-IM-catalyzed hydrolysis of
( )-1935 (E > 200) resulted in (2R,3S)-3-amino-3-phenyl-2-hydroxypropionic
acid [(2R,3S)-20] and unreacted β-amino ester (2R,3S)-19, while the
CAL-B-catalyzed enantioselective ring cleavage of racemic [ )-2236
(E > 200) furnished β-amino acid (2R,3S)-20 and unreacted β-lactam
(3S,4R)-22 with high enantiomeric excess (ee) values (>98%) and in
good yields (>45%).
Instrumentation
All 1H NMR spectra (eight scans) were collected on a Bruker (Billerica,
MA) Advance 400 MHz NMR Spectrometer. Spectra were obtained in
methanol-d4 at room temperature and calibrated using TMS (0.05%) as
an internal reference. If necessary, 2D COSY spectra were also collected
to facilitate assignment of the resonances.
Procedures for Chiral Differentiation Studies
The 10-mM solutions of the neutral forms of 2–9 and the
dihydrochloride salts of 10–13 were prepared in methanol-d4. Substrate
solutions were enriched in one of the enantiomers when it was available.
The dihydrochloride salts of 2–9 were prepared by addition of an appro-
priate amount of concentrated aqueous deuterium chloride to the
methanol-d4 solution. A stock solution of 1 (0.30 M) was prepared in
methanol-d4. A series of spectra were obtained by adding appropriate vol-
umes of the stock solution of 1 to 600 μl of the 10 mM substrate solution
in an NMR tube.
The H2’ phenyl resonance of 2 shows substantial enantio-
meric differentiation in the presence of 1, as seen in Figure 3.
Confirmation of that conclusion is reached by recording the
spectrum of 2 in the presence of (+)-(1) (Fig. 3b) and (–)-
(1) (Fig. 3c). Reversal of the position of the H2’ resonances
of the (R)- and (S)-enantiomers of 2 is expected with the re-
versal of configuration of 1, which is what is observed in
the NMR spectra.
RESULTS AND DISCUSSION
1
Analysis of Diamines
The H NMR spectra obtained for the diprotic and neutral
The structures of the diamino-containing substrates are
shown in Figure 1. 3 and 5 are aryl-containing atropisomers
in which the two amine groups are symmetrical. The enantio-
meric differentiation observed in the 1H NMR spectra of 2–5
with 1 is provided in Table 1 (values are reported for the con-
forms of 4 mixed with (+)-1 are shown in Figure 4. Substan-
tial enantiomeric differentiation of the methine, H3’ and H4’
resonances of 4 is observed in mixtures of the diprotic form
with (+)-1 (Fig. 4b,c). Also, the methine and H4’ resonances
of 4 are deshielded in the presence of (+)-1, whereas the H3’,
H5’, and H6’ resonances are shielded. Finally, whereas the
Chirality DOI 10.1002/chir
centration of
1 that caused the largest enantiomeric