Hybrids of Salicylalkylamides and Mannich Bases: Control of the Amide Conformation by Hydrogen Bonding in Solution and in the Solid State

3-Aminomethylation of salicylalkylamides afforded hybrids with a Mannich base. In addition, it triggered the rotation of the amide bond. The observed conformational switch is driven by strong intramolecular hydrogen bonding between the Mannich base and phenolic group. Crystal structure analysis reveals the stabilization of the hybrid molecules by double hydrogen bonding of the phenolic OH, which acts as an acceptor and donor simultaneously. The molecules contain an amide site and a Mannich base site in an orthogonal spatial arrangement. The intramolecular hydrogen bonds are persistent in a nonpolar solvent (e.g., chloroform). The conformational change can be reversed upon protection or protonation of the Mannich base nitrogen.

N-monosubstituted 2-hydroxybenzamides prefer the α-form in solution, which can be switched to the β-form by chemical manipulation. Deprotonation of the phenolic group [45][46][47][48] or substitution through alkylation [49] removes the hydrogen atom necessary for the stabilization of the α-form. Concomitantly, the hydrogen acceptor strength of the oxygen atom (oxyanion or alkoxy group) is enhanced. Upon rotation of the amide function, the alternative hydrogen bonding between the phenolate oxygen and amide hydrogen can be established. The result is a switch from the α-form (I) to the β-form ((II) and (III) in Figure 2). Another possibility to control the conformation is 3-aminoalkylation of salicylamides, which introduces a basic functionality with an available nitrogen lone pair able to compete with the amide oxygen for the acidic hydrogen of the phenolic group. Such a reaction affords a hybrid composed of salicylamide and a Mannich base. The presence of a basic nitrogen of the 3-aminomethyl residue is in ideal premise for the formation of an intramolecular hydrogen bond to the phenolic hydrogen, which should trigger the conformational change from the α-to the β-form (Figure 3, IV). Intramolecular hydrogen bonding in Mannich bases has been extensively investigated by different techniques [50][51][52][53][54] and, in particular, by NMR spectroscopy [55,56]. Furthermore, introduction of a Mannich base into position 3 of salicylamides creates an aminoalcohol motif, which is a recognized pharmacophore in drugs (e.g., antimalarials).

Hybrids of Salicylalkylamides and Mannich Bases
Synthesis of hybrids composed of salicylamide and a Mannich base started with aminomethylation of 2-hydroxy-N-(3-methyl-butyl)-benzamide (1) (Scheme 1; the isopentyl residue was chosen to mimic an isoprene moiety). The applied Mannich reaction is a widely-used standard C-C bond-forming procedure in organic chemistry [57]. The electrophilic substitution of salicylamides with iminium ions affords N-, as well as C-Mannich bases, depending on the substrate used and the reaction conditions applied. Parent salicylamide yields N-Mannich bases, which have been investigated as models for prodrug systems to deliver in water sparingly soluble drugs into systemic circulation [58,59]. Salicylamides bearing more than one hydroxy group, such as β-resorcylic acid amide or gentisic acid amide, afford C-Mannich bases upon aminomethylation [60], as well as niclosamide [61]. From the reaction of 1 with diethyl amine and formaldehyde, no N-Mannich base was obtained, although the reaction starts on the amide nitrogen [62]. This is in line with our observation that the corresponding ester congener of 1 (isopentyl ester) did not give any product under the conditions applied for the syntheses of 2-4.
Formation of mixtures of C-Mannich bases is programmed due to unsubstituted 3-and 5-positions in 1. Both positions are amenable to electrophilic attack by iminium ions. We attempted to isolate all possible isomers by offering one equivalent of amine for the reaction. Only isomers 3 and 4 contain N-diethylaminomethyl residues in position 3 of the salicylamide scaffold, which would be mandatory for intramolecular hydrogen bonding of the acidic phenol-hydrogen to the basic nitrogen of the Mannich base. The nitrogen of the 5-aminomethyl residue in isomer 2 is expected to form intermolecular hydrogen bridges. The synthesis is depicted in Scheme 1. Aminomethylation of 1 with diethyl amine (free base) and formaldehyde worked without the addition of acid in a protic solvent (ethanol). For the separation of the basic products 2-4, sequential preparative chromatographic separations had to be applied. Finally, all three isomers were obtained from one experiment in sufficient purity for analytical characterization. The structures of 2-4 were verified by 1D and 2D NMR spectroscopy (atom numbering for resonance assignment is shown in Scheme 1). Isomer 2 is characterized by only one coupling of H-6 over four bonds to H-4 (δ 7.29 ppm, constants of 3 J = 7.9 Hz and 4 J = 1.5 Hz, which is indicative of unsubstituted 4-and 5-positions. In addition, the exocyclic methylene protons of the aminomethyl substituent showed a strong cross-peak to C-2 of the phenyl ring in the HMBC spectrum, which is in agreement with the substitution in position 3. The structure of the 3,5-bis-diethyl aminomethyl derivative 4 is proven by a doublet at δ 7.07 ppm ( 4 J = 2.8 Hz) for H-4 and a second doublet at δ 7.90 ppm ( 4 J = 2.8 Hz) for H-6 (the full assignment of NMR resonances is provided in the Supporting Information).
2.1.1. Assessment of the Conformation of Salicylalkylamides in Chloroform-d1 by NMR Spectroscopy The conformation of salicylalkylamides in solution can be determined by NMR spectroscopy. The distinctive chemical shift changes (δ) revealed by NMR for the transition from the α-into the β-form will be briefly discussed in the following. Our starting material 1 is comparable to 2-hydroxy-N-methyl-benzamide (A, Figure 4), the parent N-alkyl salicylalkylamide. Compound A, as well as 5-chloro congener B have been shown to adopt the α-conformation in 1,2-dichloroethane, as well as in DMSO by emission spectroscopy. Large Stokes shifts were indicative of excited-state intramolecular proton transfer, which is in good agreement with strong intramolecular hydrogen bonding [63]. We inspected the 13 C/ 1 H-NMR (CDCl3) spectra reported for A, B [63,64], as well as for an additional 25 N-benzyl salicylamides D [34]. We found out that the α-form of salicylalkylamides reveal δ < 7 ppm for amide-NH, δ 6.8-7.5 ppm for aromatic H-6 (outliers from [34] removed, e.g., 5-NO2) and δ 11.5-12.6 ppm for phenolic 2-OH, which appears mainly as a sharp singlet. It has to be noted that investigation of hydrogen bonding by NMR is very sensitive to the polarity of the solvent used, the sample concentration, the water content and temperature, respectively. Further spectroscopic and structural data with regard to 2-hydroxybenzamides are available [65]. 2-Methoxy-N-methylbenzamide (C) exemplifies the removal of the α-form stabilizing phenolic hydrogen from parent N-alkyl salicylamide A upon alkylation [63,66]. Compound C has been shown to contain an intramolecular H-bond by fluorescence spectroscopy [49], which can only be ascribed to the presence of the β-form in solution. Derivatives of C have received much attention for design of arylamide oligomers [67,68] and supramolecular recognition units [69]. Ab initio methods revealed a persistent intramolecular hydrogen bond between the 2-methoxy group and NH of the amide in a nonpolar environment, such as chloroform. In protic solvents (methanol, water), the intramolecular H-bond is disturbed by interaction with the solvent [70]. Significant changes of chemical shifts in the NMR spectra (CDCl3) reflect the conformational switch. In the β-form of C, the amide-NH, as well as H-6 reveal downfield shifts to δ 7.8 and δ 8.2 ppm, respectively ( Table 1). The observed downfield shift for amide-NH in the β-form is in agreement with intramolecular hydrogen bonding. Deshielding of H-6 is probably due to the closer proximity of H-6 to the amide carbonyl in the β-form. Both conformers α and β are fairly planar with respect to amide and the aromatic ring and are quite rigid due to intramolecular hydrogen bonding [70]. The amide carbonyl is exo to H-6 in the α-form, whereas it is endo in the β-form. Therefore, H-6 is spatially closer to the amide carbonyl in the β-conformation. The magnetic anisotropy of the amide carbonyl contributes to downfield shift of H-6 in the β-form, similar to the effects found for carbonyl compounds [71][72][73][74][75][76][77]. Alternatively, electric field effects, as well as orbital interactions have to be taken into account for the explanation of the observed shift change [78]. Furthermore, also 13 C chemical shifts are indicative of the conformational switch from the α-to the β-form. The 2-hydroxy congeners A and B adopt the α-conformation in a nonpolar environment. The phenolic hydrogen of A and B is in the ideal position for the formation of intramolecular resonance-assisted hydrogen bonding (RAHB) to the oxygen of the amide group [79]. 13 C chemical shifts are sensitive to mesomeric effects. The RAHB established in the α-form can be described by quasi-aromatic resonance structures resembling C-1 and C-2 quinoid forms (see Supporting Information, Figure S1) with less delocalization of the nitrogen lone pair into the amide carbonyl [80]. Consequentially, in the α-form, the 13 C resonance for C-1 of A and B should experience an upfield shift and the 13 C resonance for C-2 and the carbonyl a downfield shift compared to the β-form. Analysis of the shift changes for C-1, C-2, and C=O of A and B and those found for 2-methoxy derivative C shows the expected shift changes as discussed above (Table 1). Compound C reveals for the carbon resonance of the carbonyl group and for C-2 an upfield shift compared to A and B, whereas C-1 experiences a downfield shift, respectively. In addition, both carbons C-6 and C-4 are in meta-position to the 2-substituents, OH for A, B and OCH3 for C. Meta-positions are the least affected by the mesomeric effects of the substituents. Nevertheless, C-6 reveals a downfield shift by ~6.8 ppm for the β-form of C, whereas C-4, in contrast, an upfield shift by ~2 ppm. This opposite shift change displayed for C-4 and C-6 is also connected to the conformational switch.

Assessment of the Conformation of Hybrids 2-4 in Chloroform-d1 by NMR Spectroscopy
For the discussion of the conformation adopted by isomers 2-4, selected 13 C/ 1 H chemical shifts (δ) of 1-4 in chloroform-d1 are summarized in Table 2.
Comparing the 1 H/ 13 C δ-values of isomers 2-4 with the one recorded for the α-form of the starting material 1 (Table 2) showed significant differences. Compound 2, the 5-aminomethyl-substituted isomer revealed very similar δ for amide-NH, as well as for H-6 of the aromatic ring compared to 1, suggesting also an "α-form" for 2. The phenolic hydrogen of 2 was not detectable in the spectrum, due to a rapid exchange (most probably an intermolecular exchange driven by the basic aminomethyl moiety in position 5). However, isomers 3 and 4, both bearing a diethylaminomethyl residue in position 3, have changed the conformation in chloroform-d1 with respect to salicylamide 1. Most remarkable is a large downfield shift of the amide proton resonance by approximately 2 ppm, indicating a strong intramolecular NH···OH hydrogen bonding interaction. In addition, the H-6 resonance is downfield shifted due to a deshielding effect of the amide group. The 1 H-NMR spectra reveal that both compounds 3 and 4 adopt a β-form conformation in chloroform-d1. This is consistent also with the shift changes of the 13 C resonances of 3 and 4 with respect to those of 1: Our findings suggest that the intramolecular hydrogen bonding within the Mannich base motif in hybrids 3 and 4 is indeed established and favored over the alternative OH···O=CNH bonding present in starting material 1 and 5-aminomethylated congener 2. In addition, the formation of a phenolate anion via deprotonation of the phenolic group by the Mannich base nitrogen can be excluded taking into account an upfield shift of C-2 (Δ(δ) ~2-3 ppm found for hybrids 3 and 4. Phenolate carbon 2 would reveal a tremendous downfield shift of at least 10 ppm. The conformation in the solid state will be discussed in Section 2.2.

Synthesis of a Small Library of Hybrids of Mannich Bases and Salicylalkylamide 5
We extended the investigations by the synthesis of a small library of hybrids using different secondary amines for the Mannich reaction with 5. The synthesis is depicted in Scheme 2. The 5-chloro substitution was chosen for several reasons. Blocking position 5 might facilitate the work-up by the suppression of isomers and concomitantly increasing the yield of the desired hybrids, 6-12. Additionally, more drug-like molecules are obtained by metabolic stabilization of the scaffold. Full characterization of compounds 6-12 by NMR spectroscopy and analytical data is provided in the Supporting Information.
2.1.4. Assessment of the Conformation of Hybrids 6-12 in Chloroform-d1 by NMR Spectroscopy Selected chemical shifts useful for the assessment of the conformation adopted by 6-12 in comparison to 5 are summarized in Table 3 (for a discussion of 12, see Section 2.1.5). The molecular structures are depicted in Figure 5.  Starting material 5 adopts the α-form, as revealed by the very similar 13 C/ 1 H δ-values with respect to 1 and reference compounds A and B. From the chemical shifts summarized in Table 2, it can be deduced that all novel hybrids 6-11 prefer the β-conformation in chloroform-d1. In particular, this is indicated by downfield shifts of H-6 δ-values >8 ppm, a shift-range of δ 8.3-8.6 ppm for CONH and broadening of 2-OH resonance, respectively. Additional support for the assessment of the β-conformation to hybrids 6-11 is obtained from the 13 C spectra. The recorded changes of δ-values follow the same trend as already discussed for isomers 3 and 4: (C=O) Δ(δ) ~−4, (C-6) Δ(δ) ~+5 and (C-1) Δ(δ) ~+5 ppm. The conformation adopted in the solid state will be discussed in Section 2.2.

Reversal of the β-Conformation of Salicylalkylamide-Mannich Base Hybrids
The conformational switch of salicylamides (1, 5) from the α-to the β-conformation in hybrids with a Mannich base is triggered by the exocyclic basic nitrogen of the Mannich base. Consequently, blocking the nitrogen in a hybrid structure by a protecting group, which removes the basicity of the nitrogen, should switch back the β-conformation of the hybrid to the α-form of the salicylamide. We demonstrated this by synthesis of N-protected derivative 12 ( Figure 5). Salicylamide 5 was subjected to a Tscherniac-Einhorn reaction, which installs a non-basic phthalimidomethyl residue in position 3 of the aromatic ring in 5 [81,82]. Such reactions are known with salicylic acids [83]. The NMR spectrum of 12 in chloroform-d1 revealed δ 7.20 ppm for H-6 and δ 6.57 ppm for CONH, respectively. These values correspond indeed to the shifts recorded for 5 in the α-conformation.
Competition for the basic nitrogen with a strong acid, such as hydrogen chloride, should also destroy the hydrogen bonding of the less acidic phenol and consequently switch back the β-conformation of the hybrids into the α-form of the parent amides. For this purpose, we prepared hydrogen chloride salts of salicylamide-Mannich base hybrids 3, 6 and 9, respectively ( Figure 6). Unfortunately, NMR inspection in chloroform-d1 was not feasible, due to the insolubility of the hydrochlorides. We were able to grow X-ray diffraction quality crystals of hydrochlorides of 9 (×HCl: 13) and 3 (×HCl: 14). The hydrochloride of 5-chloro substituted derivative 6 (×HCl: 15) was isolated as a powder of tiny crystals. As expected, at least in the solid state, the conformation switched back from the β-to the α-form for both hydrochlorides 13 and 14 (

Crystallographic Structure Determination
Crystal structures of parent primary salicylamide (2-hydroxybenzamide) have been reported by Sasada et al. [84] and Pertlik [85]. Solid state structures for secondary and tertiary salicylamides are documented in several publications [80,[86][87][88][89][90][91]. To the best of our knowledge, there are no crystal structures for hybrids of salicylamide and the Mannich base reported (a substructure search in the database of Cambridge Crystallographic Data Centre (CCDC) did not reveal comparable structures). (9) The crystals grown for 9 are shown in Figure 7a. The molecular structure is depicted in Figure 7b, while selected geometrical parameters are given in Table 4. The residues of amine and the amide site of 9 are close to orthogonal. Such a scaffold would be able to bind to a target with one site and concomitantly provide a molecular architecture for filling a steep pocket. The nature of the biological information offered in the two diversity sites is a matter of design. Polar, as well as hydrophobic interactions can be built into the orthogonal arms of molecule 9.

5-Chloro-2-hydroxy-N-(3-methyl-butyl)-3-(4-phenyl-piperidin-1-ylmethyl)-benzamide
Molecules of 9 are not involved in intermolecular hydrogen bonding in the crystal, as revealed by the crystal packing pattern (Figure 8). Two intramolecular hydrogen bonds are evident in 9, namely O1-H1···N1 and N2-H2···O1, with the geometric parameters quoted in Table 5, which force the molecule to adopt the β-conformation. The interactions of the molecules within the crystal are confined to hydrophobic contacts. There are no π-π* interactions between the aromatic fragments of the molecule in the crystal.     (13) The crystals grown for 13 are shown in Figure 9a. The molecular structure of 13 is depicted in Figure 9b, while selected geometrical parameters are quoted in Table 4. The crystal structure of 13 reveals the change of molecule conformation upon protonation of free base 9. The lone pair of the nitrogen in the Mannich base is no longer available for the phenolic hydrogen to establish a hydrogen bond. This triggers the β-conformation of hybrid 9 back to the α-form. The two molecules, 9 and 13, address very different spaces in the pharmacophoric room.

[5-Chloro-2-hydroxy-3-(3-methyl-butylcarbamoyl)-benzyl]-4-phenylpiperidinium Chloride
The crystals of 13 are composed of centrosymmetric dimers in which the monomers are held together by hydrogen bonding and electrostatic interactions. Each protonated molecule in 13 forms two hydrogen bonds, namely N1-H···Cl2 i and N2-H···Cl2 ii , and acts as a proton donor to two symmetry-related chloride counteranions as proton acceptors ( Figure 10). Furthermore, a marked positive partial charge on the amide hydrogen is presumably responsible for the geometric nonsymmetry of the two H-bonds (see Table 6 and Figure 10). We can describe the first interaction as an ionic hydrogen bond [92]. Being predominantly Coulombic in nature, it remains directional, with N1-H pointing at chloride counteranion Cl2 i . A similar interaction is well documented as salt-bridges between primary ammonium and carboxylate groups in biological systems [93].   (14) The protonation of the Mannich base nitrogen in hybrid 3 triggers the same conformational switch from β-to α-form as shown for hybrid 9 (Section 2.2.2). The molecular structure of 14 (3 ×HCl) is shown in Figure 11 and the selected geometrical parameters in Table 4. Furthermore, the crystals of 14 are composed of centrosymmetric dimers held by H-bonding interactions, which are nonsymmetric due to an additional Coulombic component in the interaction between the chloride anion and the ammonium cation. Like in 13, each protonated molecule acts as a proton donor in two H-bonds, N1-H···Cl1 i and N2-H···Cl1 ii (Figure 12 and Table 7).

Materials and Methods
Starting materials (amines, salicylic acid derivatives) were purchased from various commercial sources and were used without further purification. Solvents used in the synthesis and chromatographic purification steps were distilled prior use (ethyl acetate, petrol ether, n-hexane).

Aminomethylation of 1: Separation of Isomers 2-4
The synthesis of 2-4 by the Mannich reaction is depicted in Scheme 3. To a solution of 2-hydroxy-N-(3-methyl-butyl)-benzamide (1) (4.03 g, 19.4 mmol) in ethanol (25 mL) was added diethyl amine (1.70 g, 1.2 equiv) and formaldehyde (1.89 g, ~37% in water). The mixture was transferred to a screw cap tube, sealed and kept in an oil bath at 90 °C for 20 h. The reaction mixture was cooled to room temperature. Volatile materials were removed under reduced pressure on a rotary evaporator. The oil obtained was stirred with hydrochloric acid (2 M, 20 mL) for 10 min. The acidic phase was extensively extracted with methyl t-butyl ether (MTBE) (4 × 25 mL). To the aqueous phase was added sodium hydroxide (2 M, 25 mL) and brine (25 mL). The basic products were extracted with ethyl acetate (2 × 30 mL). The organic phase was washed with water to pH 7 and dried over Na2SO4. After filtration, the solvent was removed on a rotary evaporator to give 2.9 g of residual material. The latter was subjected to MPLC-chromatography. Elution was started with MTBE (100%), and the fractions containing pure isomer 2 and 3 were collected. 5-Diethylaminomethyl-2-hydroxy-N-(3-methyl-butyl)-benzamide (2): 160 mg (3%, yellowish oil); 3-diethylaminomethyl-2-hydroxy-N-(3methyl-butyl)-benzamide (3): 834 mg (15%, yellowish oil). The eluent was changed to a polar mixture made of ethyl acetate, methanol and aqueous ammonia (6/3.5/0.5). All residual material was eluted from the column: 966 mg, orange colored oil. The mixture was dissolved in MTBE (10 mL), and hydrogen chloride was added (1 mL, 4 M in dioxane). To the solution was added dry diethyl ether (20 mL). After cooling to 4 °C for 2 h, the solvents were decanted from the formed oil. The oil was washed with dry ether (3 × 10 mL). The basic products were liberated by the addition of NaHCO3 and extracted with ethyl acetate (80 mL). The organic phase was washed with brine, dried with sodium sulfate and evaporated. The residue was subjected to a second column chromatography applying ethyl acetate, methanol and aqueous ammonia at a ratio of 4/0.8/0.2 as the eluent. Fractions containing pure bis-aminomethyl substituted product were collected: 3,5-bis-diethylaminomethyl-2-hydroxy-N-(3methyl-butyl)-benzamide (4): 190 mg (13%, slightly yellowish oil). Structural assignment of 2-4 was performed by NMR spectroscopy (Section 2.1.; the full assignment of NMR spectra is provided in the Supporting Information).
3.4.3. Aminomethylation of 5-Chloro-2-hydroxy-(3-methyl-butyl)-benzamide (5) The general procedure for the syntheses of compounds 6-11 (Scheme 2): A mixture of salicylamide 5 (1 mmol), formaldehyde (~37% in water) (1.2 mmol) and the corresponding secondary amine, 6-11 (1.2 mmol), in ethanol (4 mL) was prepared in a screw cap tube. The tube was sealed and kept in an oil bath at 90 °C for 20 h. The mixture obtained was diluted with water and extracted with ethyl acetate. The organic phase was washed once with sodium bicarbonate, two times with water and dried over magnesium sulfate. From the dried solution, the solvent was removed under reduced pressure on a rotary evaporator. The resulting residue was purified by column chromatography (silica gel, eluent: ethyl acetate/n-hexane). (12) The Tscherniac-Einhorn reaction is described for salicylic acids, but not for salicylamides [82]. A literature search for imidomethylation of salicylamides did not deliver any result. Only three simple salicylamides with the 3-phthalimidomethyl substituent were found (CAS RN: 1243458-02-7, 1243454-79-6, 1243288-23-4), but no references, experiments or characterization were available (commercial sources). Synthesis of compound 12 from 5 was performed by following Scheme 2.

Crystallographic Structure Determination
X-ray diffraction measurements were performed on Bruker D8 VENTURE diffractometers. Single crystals were positioned at 50, 35, and 40 mm from the detector, and 2281, 2534, and 3016 frames were measured, each for 16, 5.6, and 24 s over a 0.4° scan width for 9, 13, and 14, correspondingly. The data were processed using SAINT software [97]. Crystal data, data collection parameters and structure refinement details are given in Table 8. The structures were solved by direct methods and refined by full-matrix least-squares techniques. Non-H atoms were refined with anisotropic displacement parameters. H atoms were inserted in the calculated positions and refined with a riding model. The following computer programs were used: structure solution, SHELXS-97, and refinement, SHELXL-97 [98]; molecular diagrams, ORTEP [99].

Conclusions
Salicylalkylamides undergo a conformational switch from the α-to the β-form upon aminomethylation in the 3-position. The reaction creates a hybrid composed of salicylamide and a Mannich base. The conformational change can be reversed by N-protection or protonation of the Mannich base nitrogen. The preferred β-conformation of the hybrid molecules adopted in solution has been determined by NMR spectroscopy. Single crystal X-ray diffraction revealed the β-form for hybrids of salicylalkylamide and the Mannich base, also in the solid state. The reversal of the conformation upon protonation has been demonstrated for the solid state by the X-ray diffraction data of the hydrochlorides of hybrid molecules. The structural changes observed upon transformation of salicylalkylamide-Mannich base hybrids from the α-to the β-conformation provide powerful tools for the design of more complex molecular architectures, as well as of novel drugs. Decorating salicylamides with a Mannich base motif in the 3-position creates an extended pharmacophore whose conformation can be controlled. Hybrid molecules in the α-form can cover more extended interactions with targets, whereas the β-form provides an L-shape conformation able to fill steep pockets, e.g., in a protein. The two amine diversity sites, one in the Mannich base, the other in the amide part of the hybrid, can be addressed independently by the design. In addition, double intramolecular hydrogen bonding in the β-form of hybrid molecules will influence the pharmacological properties by masking the polarity of the functional groups involved.

Author Contributions
C.D. and H.G. designed the study, performed the Tscherniac-Einhorn experiments and wrote the manuscript. M.M. synthesized and separated the hybrid isomers. B.K. prepared the library of hybrid molecules. V.A. and A.R. performed the X-ray structural analysis. H.K. and S.F. provided NMR support and spectroscopic analysis.