Characteristic Conformation of Mosher’s Amide Elucidated Using the Cambridge Structural Database

Conformations of the crystalline 3,3,3-trifluoro-2-methoxy-2-phenylpropanamide derivatives (MTPA amides) deposited in the Cambridge Structural Database (CSD) were examined statistically as Racid-enantiomers. The majority of dihedral angles (48/58, ca. 83%) of the amide carbonyl groups and the trifluoromethyl groups ranged from –30° to 0° with an average angle θ1 of −13°. The other conformational properties were also clarified: (1) one of the fluorine atoms was antiperiplanar (ap) to the amide carbonyl group, forming a staggered conformation; (2) the MTPA amides prepared from primary amines showed a Z form in amide moieties; (3) in the case of the MTPA amide prepared from a primary amine possessing secondary alkyl groups (i.e., Mosher-type MTPA amide), the dihedral angles between the methine groups and the carbonyl groups were syn and indicative of a moderate conformational flexibility; (4) the phenyl plane was inclined from the O–Cchiral bond of the methoxy moiety with an average dihedral angle θ2 of +21°; (5) the methyl group of the methoxy moiety was ap to the ipso-carbon atom of the phenyl group.


Introduction
NMR using chiral resolving agents is a powerful technique, along with X-ray crystallography and circular dichroism, for assignment of the absolute configuration of organic compounds [1]. Mosher et al. developed 3,3,3-trifluoro-2-methoxy-2-phenylpropanoic acid (MTPA, 1, Figure 1) and constructed the conformational model of the MTPA amide and the MTPA ester derived from a primary amine and a secondary alcohol, respectively (Figure 2a) [2][3][4]. Considering the shielding effect of the phenyl ring, the relative stereochemistry of the MTPA amide and the MTPA ester could be elucidated based on a mutual comparison of the 1 H-NMR chemical shifts of their diastereomers; namely, upfield shifts are observed in substituent L 2 . Therefore, the absolute configuration of the amine moieties and the alcohol moieties could be clarified using stereochemistry of the MTPA moiety as an internal standard. Kusumi et al. modified this method using two-dimensional NMR spectroscopy [5][6][7][8].   [3] was modified in (b). Three covalent bonds separate the two chiral centers C1′ and C2; therefore, L 3 -C1′-L 2 moiety is more flat in this projection. The MTPA ester exhibited the equivalent conformation with substitution of NH for O [9].
In 2013, we elucidated the crystal structure of Mosher's salt prepared from (R)-1 and (R)-1-phenylethylamine using X-ray crystallography [9]; Mosher et al. prepared this compound via the enantioresolution of rac-1 in ethanol as a less soluble salt [2]. In the course of this study, we found that a number of crystal structures of MTPA amides and MTPA esters were deposited in the CSD.
Each molecular structure is influenced by packing force in crystal [25]. However, statistical analyses of the crystal structures elucidated the relative stability of each conformer [25]. Therefore, we reported statistical analyses of the crystal structures of MTPA esters [9].
The properties of the major conformation of the crystalline MTPA ester are as follows [9]: (1) the ester carbonyl group is synperiplanar (sp, dihedral angle 0° to ±30°) [26] to the trifluoromethyl group; (2) the trifluoromethyl group is in the staggered conformation; (3) the methine group of the alcohol moiety is syn to the carbonyl group; (4) the phenyl plane is inclined from the O-Cchiral bond of the methoxy moiety; (5) the methyl group of the methoxy moiety is antiperiplanar (ap, dihedral angle ±150° to 180°) [26] to the ipso-carbon atom of the phenyl group. Thus, our database study proposed a modified conformational model of the MTPA ester.
Conformational features of the MTPA amide moiety were similar to those of the MTPA esters, with less diversity (Figure 2b). The features of the major conformation of crystalline MTPA amides are as follows: (1) the amide carbonyl group is sp to the trifluoromethyl group with an average dihedral angle θ 1 of −13°; (2) the trifluoromethyl group is in the staggered conformation; (3) the amide moiety of the secondary MTPA amide is in the Z form (i.e., R ap = H, R sp = alkyl group); (4) H1′ of the amine moiety is syn to the carbonyl carbon atom C1; (5) the phenyl plane is inclined from the O2-C2 bond with an average dihedral angle θ 2 of +21°; (6) the methyl group of the methoxy moiety is ap to C5 of the phenyl group.
Structural elucidation of chiral amines is important, because a considerable number of biologically active natural products and pharmaceuticals contain key chiral amine moieties [67][68][69]. The number of entries in the CSD is increasing rapidly; therefore, the crystal database is important for structural chemistry. The statistical analyses of crystal data have increased our understanding on the properties of acid 1 and have established valuable insights for Mosher's method and crystal engineering [70,71] of MTPA derivatives.

Crystal Structures of MTPA Amides Deposited in the CSD
The crystal structures of the MTPA amide moieties were searched in the CSD using ConQuest software. Table 1 shows the original dihedral angles of (Racid)-and (Sacid)-MTPA amides .       All obtained MTPA amide moieties (a total of 58) were processed as Racid-enantiomers in the following sections; that is, the dihedral angles of (Sacid)-MTPA amides cited in Table 1 were substituted by those of the mirror images (i.e., Racid-enantiomers) with plus/minus sign reversal. Despite the various structures of the amine moiety, the MTPA amide moieties showed little conformational diversity. Therefore, the conformations of all MTPA amide moieties were processed together, excluding the dihedral angle H1′-C1′-N-C1, which was specific to the MTPA amides prepared from amines possessing a secondary alkyl group (see Section 2.6.).

Dihedral Angles of Amide Carbonyl Group and Trifluoromethyl Group: O1-C1-C2-C3
The distribution of the dihedral angles O1-C1-C2-C3 (θ 1 ) exhibited a concentration of entries between −30° and 0° (Figure 3). All dihedral angles ranged from −70° to +40°; the median angle was −14.5°. These data confirmed Mosher's hypothesis that the carbonyl and trifluoromethyl groups of the MTPA amide are syn [3]. In addition, the majority of dihedral angles θ 1 (48/58, ca. 83%) ranged from −30° to 0° with the average angle θ 1 of −13°. That is, the carbonyl group was close to the phenyl group. This phenomenon was also observed for the MTPA esters and MTPA salt; we observed the short contacts between the two oxygen atoms and the ortho-hydrogen atoms of phenyl group in the MTPA anion [9]. The tetrakis-MTPA amide derivative CCDC 1310848 also exhibited another irregular θ 1 of −41(2)°. CCDC 739753 [34] showed an irregular θ 1 of −36.0(2)°; the methoxy group of the MTPA moiety formed an intramolecular aromatic C-H···π interaction with the phenyl group of the amine moiety. CCDC 122980 [36] also exhibited an irregular θ 1 of −31(1)°; the methoxy group of the MTPA moiety formed an intramolecular C-H···O hydrogen bond with the oxygen atom of the amine moiety. In addition, CCDC 603055 [29] showed an irregular θ 1 of −35.0(4)° as Racid-enantiomer; the amine moiety possessed a sulfonamide moiety and a phenyl group.
Previously, the dihedral angle θ 1 was discussed in relation to the steric repulsion between the phenyl group and L 2 or L 3 substituents of the amine moiety; it was estimated that the larger repulsion resulted in a lager dihedral angle θ 1 , and a smaller deshielding of the fluorine atoms by the amide or ester carbonyl groups [4]. However, Kusumi et al. observed the inaccuracies of the MTPA method using 19 F-NMR spectroscopy [5,8]. In 2007, Brand et al. reported that the origin of the sp conformation is the hyperconjugative interactions between the carbonyl group and the electronegative trifluoromethyl group [74].

Staggered Conformation of Trifluoromethyl Group: C1-C2-C3-F3
The distribution of the dihedral angle C1-C2-C3-F3 showed a concentration of entries around −180° (Figure 4). The median angle was −174°. This was suggestive of the staggered conformation of the trifluoromethyl group [9]; that is, the two oxygen atoms O1 and O2 were as far as possible from the fluorine atoms. Khan et al. reported the nonequivalence of three fluorine atoms of the MTPA amide prepared from secondary amines on the 19 F-NMR spectra at low temperatures [75]. They also reported that barriers to the hindered rotation on the C2-C3 bonds were in the range of 36-46 kJ/mol. This suggested the severe steric crowding of the MTPA moiety.

Resonance Effects of Amide Bond: C1′-N-C1-O1
The distribution of the dihedral angle C1′-N-C1-O1 exhibited a concentration of entries around 0° ( Figure 5). The average angle and the median angle were +2° and +0.8°, respectively. This represents the resonance effects of the amide bond ( Figure 6). Similar planarity was observed in the ester moieties of the crystalline MTPA esters [9].  . Z/E forms and their resonance hybrids of MTPA amides. R 1 indicates the substituents with higher Cahn-Ingold-Prelog (CIP) priority (e.g., secondary alkyl group); R 2 indicates the substituents with lower CIP priority (e.g., hydrogen atom and methyl group).
The trivalent nitrogen atom afforded the Z and E forms in the amide moiety ( Figure 6). All crystalline secondary MTPA amides prepared from primary amines exhibited the Z form, in which the N-substituent was sp to the amide carbonyl group (Figure 2 and Table 1).
The four conformers of an N-methyl MTPA amide exhibited the E forms (i.e., CCDC 707825 [49]). It is noteworthy that the two aromatic rings of the larger secondary alkyl group R 1 were bound to the MTPA's phenyl groups via the aromatic C-H···π and π···π interactions [73,76], respectively. By contrast, Nakagawa and Somei reported the Z form of a crystalline N-methyl MTPA amide in the course of the total synthesis of ergot alkaloids [65]. It is possible that the application of Mosher's method using 1 H-NMR could be expanded to the cyclic secondary amines [77,78]. In 1996, Hoye and Renner applied the MTPA method for assignment of the absolute configuration in chiral cyclic amines; they observed equilibrium mixtures of the Z and E forms of amide moieties in the 1 H-NMR spectra [77]. Similar peptidyl-prolyl isomerization is a key issue in protein chemistry [79]. In 2001, Azumaya reported the E-preference of aromatic N-methylamides (e.g., N-methylbenzanilide) [80].

Resonance Effects of Amide Bond: X1′′-N-C1-O1
The distribution of the dihedral angle X1′′-N-C1-O1 also exhibited a concentration of entries around −180° (Figure 7). The median angle was −174°. This also represents for the resonance effects of the amide bond (see above).

Conformation of the Amine Moiety: H1′-C1′-N-C1
The distribution of the dihedral angle H1′-C1′-N-C1 was examined in the case of the ten crystalline MTPA amides prepared from the primary amines possessing secondary alkyl groups (i.e., Mosher-type MTPA amides). All dihedral angles were distributed between −60° and +50° with an average angle of −5° (Figure 8). Besides, the median angle was −11°. These data agree with Mosher's hypothesis of the MTPA plane [3,5]. In addition, the broad distribution of dihedral angles was indicative of a moderate conformational flexibility of the C1′-N bond. The same is true for the C1′-O bond of the crystalline MTPA esters prepared from secondary alcohols [9]. Rzepa analyzed the relationship between the dihedral angle H-N-C=O and the distance from H1' of the amine moiety to the carbonyl oxygen atom in the crystalline secondary amides (a total of 619); that is, the major conformer exhibited a syn-co-planar alignment of the C-H bond with the plane of the C=O bond in the Z form [81].
These data confirmed Mosher's hypothesis of MTPA amide, in which the (Racid)-MTPA's phenyl group shields the amine's substituent L 2 (Figure 2a) [3]. Figure 9 also suggested that the substituent L 2 is not just above the phenyl ring. We reported that the phenyl group was inclined by +19° in the MTPA ester [9].

Caution
Acylation of an amine with (S)-MTPA chloride yields (Racid)-MTPA amide. In the same way, acylation of an amine with (R)-MTPA chloride yields (Sacid)-MTPA amide. This nominal change in the absolute configuration has often caused confusion. The same is true for MTPA esters.

Conclusions
We conducted a database study of the crystal structures of MTPA amides deposited in the CSD. The properties of the major conformation of the MTPA amide elucidated from our database study confirmed Mosher's empirical model on the conformation of MTPA amide; that is, the methine group of the amine moiety, the amide carbonyl group, and the trifluoromethyl group are on the MTPA plane. All secondary MTPA amides prepared from the primary amines exhibited the Z form in the amide moiety. The ratio of Z/E forms of amide moieties was 13:4 in the case of the tertiary MTPA amides prepared from the secondary amines; the cyclic secondary amines yielded the Z forms, whereas the N-methyl amines yielded the both Z and E forms. The amide carbonyl group was sp to the trifluoromethyl group with the average dihedral angle θ 1 of −13°. The trifluoromethyl group was in the staggered conformation. In addition, the C1′-N bond of the amine moiety exhibited moderate conformational flexibility. The phenyl plane was inclined by θ 2 = +21° from the O-Cchiral bond of the methoxy moiety. This dihedral angle θ 2 was suggestive of the inefficient shielding of the phenyl ring. Finally, the methyl group of the methoxy moiety was ap to the ipso-carbon atom of the phenyl group. These conformational properties were similar to those of the crystalline MTPA ester. Besides, the minor conformer of the crystalline MTPA amides was observed in which the amide carbonyl group and the methoxy group were anti. Mosher's method using NMR spectroscopy is crucial for the structural elucidation of chiral amines in combination with X-ray crystallography. This report increases our understanding of Mosher's method and acid 1 and can be used for crystal engineering of MTPA derivatives.