Resolution of a Configurationally Stable Hetero[4]helicene

We have developed an efficient chemical resolution of racemic hydroxy substituted dithia-aza[4]helicenes (DTA[4]H) 1(OH) using enantiopure acids as resolving agents. The better diastereomeric separation was achieved on esters prepared with (1S)-(−)-camphanic acid. Subsequent simple manipulations produced highly optically pure (≥ 99% enantiomeric excess) (P) and (M)-1(OH) in good yields. The role of the position where the chiral auxiliary is inserted (cape- vs. bay-zone) and the structure of the enantiopure acid used on successful resolution are discussed.


Introduction
Chirality is one of the most crucial assets of nature and is of paramount importance in several areas of science, technology and medicine. Molecular chirality has been recognized for a long time and has provided guidance in the design of drugs and functional materials. Furthermore, a smart combination of chiral phenomena and supramolecular chemistry resulted in an emerging interdisciplinary field called supramolecular chirality [1].
Helicenes are compounds with a screw-shaped skeleton formed by ortho-condensed (hetero)aromatic rings with a non-planar structure due to the steric superimposition of terminal rings or/and the substituents on these rings [2], which force the molecule to adopt a helical conformation ( Figure 1). This important class of axially chiral compounds has a barrier of interconversion between M and P enantiomers increasing with the increase of the ring number forming the helicene backbone. Dithia-aza [4]helicenes (DTA [4]H) 1 (Scheme 1) can be described as bisphenothiazines with an aryl ring and a nitrogen atom in common forced into a helical shaped structure by the four long carbon-sulfur bonds. These [1,4]benzothiazino [2,3,4kl]phenothiazines represent one of the attractive rare examples of geometrically stable [4]helicenes with racemization barriers higher than those measured for all carbon [5]helicenes.
Along with their peculiar helical-shaped structure, derivatives 1 show a very good one-electron donor ability, being easily, and reversibly, oxidized to the corresponding exceptionally stable crystalline chiral radical cations 1 •+ [18,21] (Scheme 2). We have also demonstrated that the oxidative process is extremely sensitive to the medium, and under acidic conditions, molecular oxygen becomes an efficient single electron transfer (SET) oxidant, giving rise to the formation of 1 •+ . Furthermore, radical cations 1 •+ can be generated also via irradiation at 240-400 nm of helicenes in the presence of PhCl [21] (Scheme 2). Indeed, we have also prepared, via ring-opening metathesis polymerization, dithiaaza [4]helicene functionalized polynorbornenes showing a pH depending reversible redox behavior as a new class of tunable material reversibly switchable by pH-triggered redox processes [22].
The availability of differently functionalized enantiomerically pure helicenes, avoiding the limitation associated with chiral HPLC resolution [17,23] is mandatory for the development of the appealing applications of these peculiar systems [21,22,24]. Therefore, in recent years we tried to set off regio-, stereo-and enantioselective synthetic approaches for the preparations of 1.
Actually, the synthetic procedure depicted in Scheme 1, while failing to control the absolute stereochemistry of the process, allowed for the control of the regiochemistry during ring closure as well as the possibility of inserting different substituents in specific positions. Thus, we have studied the chemical resolutions of 1 using the classical temporary insertion of chiral auxiliaries.
The DTA [4]H 1 derivatives requested for the above described applications require the insertion, as an anchoring unit, of a hydroxyl group in different positions of the helical backbone. Thus, we decided to take advantage of these phenolic groups for the introduction of chiral auxiliaries through esterification with enantiopure acids 7 and in order to verify whether the diastereoisomeric mixture of esters 8 obtained can be separated. Herein we report how the helicene topology and chiral auxiliary structure could be matched to allow the resolution of phenolic DTA [4]H 1(OH) (Scheme 3). Indeed, we have also prepared, via ring-opening metathesis polymerization, dithiaaza [4]helicene functionalized polynorbornenes showing a pH depending reversible redox behavior as a new class of tunable material reversibly switchable by pH-triggered redox processes [22].
The availability of differently functionalized enantiomerically pure helicenes, avoiding the limitation associated with chiral HPLC resolution [17,23] is mandatory for the development of the appealing applications of these peculiar systems [21,22,24]. Therefore, in recent years we tried to set off regio-, stereo-and enantioselective synthetic approaches for the preparations of 1.
Actually, the synthetic procedure depicted in Scheme 1, while failing to control the absolute stereochemistry of the process, allowed for the control of the regiochemistry during ring closure as well as the possibility of inserting different substituents in specific positions. Thus, we have studied the chemical resolutions of 1 using the classical temporary insertion of chiral auxiliaries.
The DTA [4]H 1 derivatives requested for the above described applications require the insertion, as an anchoring unit, of a hydroxyl group in different positions of the helical backbone. Thus, we decided to take advantage of these phenolic groups for the introduction of chiral auxiliaries through esterification with enantiopure acids 7 and in order to verify whether the diastereoisomeric mixture of esters 8 obtained can be separated. Herein we report how the helicene topology and chiral auxiliary structure could be matched to allow the resolution of phenolic DTA [4]H 1(OH) (Scheme 3). Indeed, we have also prepared, via ring-opening metathesis polymerization, dithiaaza [4]helicene functionalized polynorbornenes showing a pH depending reversible redox behavior as a new class of tunable material reversibly switchable by pH-triggered redox processes [22].
The availability of differently functionalized enantiomerically pure helicenes, avoiding the limitation associated with chiral HPLC resolution [17,23] is mandatory for the development of the appealing applications of these peculiar systems [21,22,24]. Therefore, in recent years we tried to set off regio-, stereo-and enantioselective synthetic approaches for the preparations of 1.
Actually, the synthetic procedure depicted in Scheme 1, while failing to control the absolute stereochemistry of the process, allowed for the control of the regiochemistry during ring closure as well as the possibility of inserting different substituents in specific positions. Thus, we have studied the chemical resolutions of 1 using the classical temporary insertion of chiral auxiliaries.
The DTA [4]H 1 derivatives requested for the above described applications require the insertion, as an anchoring unit, of a hydroxyl group in different positions of the helical backbone. Thus, we decided to take advantage of these phenolic groups for the introduction of chiral auxiliaries through esterification with enantiopure acids 7 and in order to verify whether the diastereoisomeric mixture of esters 8 obtained can be separated. Herein we report how the helicene topology and chiral auxiliary structure could be matched to allow the resolution of phenolic DTA [4]H 1(OH) (Scheme 3).

Results and Discussion
Selected DTA [4]H 1 can be resolved through HPLC in the chiral stationary phase as we previously reported [17,23]; however, this method is unsuitable to obtain enantiopure DTA [4]H in multigram scale. Instead, the diastereomeric process-based resolution has advantages in the the viewpoint of cost, generality and the amount of enantiopure products achieved. Thus, we decided to study the insertion of chiral auxiliaries, for example through esterification reactions, to verify whether the mixture of diastereomers obtained can be separated by flash chromatography allowing isolation of pure M and P DTA [4]H in relevant quantity. We have demonstrated that N-arylphenothiazines PTZ 3 are suitable substrates for the synthesis of unsymmetrically hydroxy substituted helicenes 1(OH) [20]. We selected helicene 1a(OH) and 1b(OH) (Scheme 4) to prepare diastereoisomeric esters using enantiopure acid 7 (Scheme 5).
Selected DTA [4]H 1 can be resolved through HPLC in the chiral stationary phase as we previously reported [17,23]; however, this method is unsuitable to obtain enantiopure DTA [4]H in multigram scale. Instead, the diastereomeric process-based resolution has advantages in the the viewpoint of cost, generality and the amount of enantiopure products achieved. Thus, we decided to study the insertion of chiral auxiliaries, for example through esterification reactions, to verify whether the mixture of diastereomers obtained can be separated by flash chromatography allowing isolation of pure M and P DTA [4]H in relevant quantity.

Scheme 4. Synthesis of hydroxy substituted helicenes 1a(OH) and 1b(OH).
Firstly, we planned the introduction of chiral auxiliaries in 2-hydroxy-substituted ADT [4]H 1a(OH) presenting a hydroxyl group in the 2-position (that we indicate as capezone) of the helicene, which was relatively easy to prepare [20].
Racemic 1a(OH) was esterified with different enantiopure acids 7a-h yielding a diastereomeric mixture (D1+D2) of esters 8a(a-h). Esterification reactions were carried out in presence of diisopropylcarbodiimide (DIC) and 4-dimethylaminopyridine (DMAP) as catalysts, in dry CH2Cl2 at room temperature (Scheme 5A). Regardless, all chiral acids 7a-h (Scheme 5 panel A and Table 1) allowed the formation of diastereomeric esters 8a(a-h) (Scheme 5 panel A and Table 1) in good yields, in none of these cases it was possible to separate the diastereoisomeric mixtures by flash chromatography or crystallization.
Using chiral acids 7d-h (Scheme 5 panel B and Table 1), the corresponding diastereomeric esters 8b(d-h) (Scheme 5 panel B and Table 1)were obtained in moderate yields Firstly, we planned the introduction of chiral auxiliaries in 2-hydroxy-substituted ADT [4]H 1a(OH) presenting a hydroxyl group in the 2-position (that we indicate as capezone) of the helicene, which was relatively easy to prepare [20].

Regardless, all chiral acids 7a-h (Scheme 5 panel A and
Using chiral acids 7d-h (Scheme 5 panel B and Table 1), the corresponding diastereomeric esters 8b(d-h) (Scheme 5 panel B and Table 1)were obtained in moderate yields generally lower than those of the corresponding esters prepared using phenol 1a(OH), indicating, as expected, a more difficult access to the OH group of 1b(OH) laying the bay-zone (Table 1). For several esters 8b it was possible to identify the presence of the Despite the introduction of the chiral auxiliary in the bay-zone diastereoisomeric esters D1 and D2 of 8b(d-f) were not separable by flash chromatography in spite of an accurate selection of eluent mixtures. However, esterification of 1b(OH) with N-boc pipecolic acid 7g allowed for the partial separation by flash chromatography on silica gel of diastereomers 8bgD1 and 8bgD2, which were characterized by 1 H and 13 C NMR. Optical rotation of 8bgD1 was: [α] −157 (c 0.1, CH2Cl2), while for 8bgD2 was: [α] +49 (c 0.1, CH2Cl2). 1 H NMR spectra show that the product 8bgD1 was isolated as single diastereomer, while the product 8bgD2 was isolated as a roughly 3:1 mixture of the two diastereomers.
Esters 8bgD1 and 8bgD2 were hydrolysed with 3 eq. of NaOH in CH2Cl2/MeOH to give enantiomeric phenols (M)-1b(OH) and (P)-1b(OH). Phenols were analysed by HPLC in the chiral stationary phase in order to calculate the enantiomeric ratio.  . 1 H NMR spectra show that the product 8bgD1 was isolated as single diastereomer, while the product 8bgD2 was isolated as a roughly 3:1 mixture of the two diastereomers.
Esters 8bgD1 and 8bgD2 were hydrolysed with 3 eq. of NaOH in CH 2 Cl 2 /MeOH to give enantiomeric phenols (M)-1b(OH) and (P)-1b(OH). Phenols were analysed by HPLC in the chiral stationary phase in order to calculate the enantiomeric ratio.   The assignment of the absolute configuration of DTA [4]H 1 derivatives has been established as P-(+) and M-(−), which is typical for helicene systems; opposite assignment is quite unusual, as we have established in ref [17,19]. In this work the absolute configuration of 1b(OH) was validated by comparison of the electronic circular dichroism (ECD) spectra of the two optical enantiomers-(+)-1b(OH) and (−)-1b(OH), assigned to (P)-1b(OH) and (M)-1b(OH), with the calculated spectrum of the M structure.
In order to assign the configuration, DFT and TD-DFT calculations have been conducted with the Gaussian16 package [39]. Two orientations are possible for the hydroxylgroup; the two optimized structures in the M configuration are reported in Figure 4 with their Boltzmann populations. Two functionals have been considered; the two differing in the amount of the exact exchange included M06 with 27% HF exchange and M06-2X with 54% HF exchange [40]. The solvent has been treated at the iefpcm level. Structural results are similar for the two functionals. CD and absorption spectra have been calculated at the same level of theory, a constant Gaussian 0.2 eV bandwidth was applied to each transition. The experimental CD and absorption spectra have been recorded for the two enantiomers in 4.2 mM dichloromethane solution in a 0.1 mm quartz cuvette using a JASCO-815SE instrument. The assignment of the absolute configuration of DTA [4]H 1 derivatives has been established as P-(+) and M-(−), which is typical for helicene systems; opposite assignment is quite unusual, as we have established in ref [17,19]. In this work the absolute configuration of 1b(OH) was validated by comparison of the electronic circular dichroism (ECD) spectra of the two optical enantiomers-(+)-1b(OH) and (−)-1b(OH), assigned to (P)-1b(OH) and (M)-1b(OH), with the calculated spectrum of the M structure.
In order to assign the configuration, DFT and TD-DFT calculations have been conducted with the Gaussian16 package [39]. Two orientations are possible for the hydroxylgroup; the two optimized structures in the M configuration are reported in Figure 4 with their Boltzmann populations. Two functionals have been considered; the two differing in the amount of the exact exchange included M06 with 27% HF exchange and M06-2X with 54% HF exchange [40]. The solvent has been treated at the iefpcm level. Structural results are similar for the two functionals. The assignment of the absolute configuration of DTA [4]H 1 derivatives has been established as P-(+) and M-(−), which is typical for helicene systems; opposite assignment is quite unusual, as we have established in ref [17,19]. In this work the absolute configuration of 1b(OH) was validated by comparison of the electronic circular dichroism (ECD) spectra of the two optical enantiomers-(+)-1b(OH) and (−)-1b(OH), assigned to (P)-1b(OH) and (M)-1b(OH), with the calculated spectrum of the M structure.
In order to assign the configuration, DFT and TD-DFT calculations have been conducted with the Gaussian16 package [39]. Two orientations are possible for the hydroxylgroup; the two optimized structures in the M configuration are reported in Figure 4 with their Boltzmann populations. Two functionals have been considered; the two differing in the amount of the exact exchange included M06 with 27% HF exchange and M06-2X with 54% HF exchange [40]. The solvent has been treated at the iefpcm level. Structural results are similar for the two functionals. CD and absorption spectra have been calculated at the same level of theory, a constant Gaussian 0.2 eV bandwidth was applied to each transition. The experimental CD and absorption spectra have been recorded for the two enantiomers in 4.2 mM dichloromethane solution in a 0.1 mm quartz cuvette using a JASCO-815SE instrument. CD and absorption spectra have been calculated at the same level of theory, a constant Gaussian 0.2 eV bandwidth was applied to each transition. The experimental CD and absorption spectra have been recorded for the two enantiomers in 4.2 mM dichloromethane solution in a 0.1 mm quartz cuvette using a JASCO-815SE instrument.
The comparison of experimental data with calculations are presented in Figure 5. In order to compare with data, +4 nm shift has been applied to the results obtained with M06, +26 nm with M06-2X; calculation of similarity index between experimental and calculated spectra suggested the best shift for the best correspondence, as reported in the Supplementary Materials paragraph. It is also shown that the two conformers give very similar spectra, while in Figure 5 the Boltzmann weighed average is presented. Both functionals permit confirmation of the configuration as M-(-) (correspondingly P-(+)). This conclusion agrees with what was obtained for the parent molecule triarylamine hetero [4]helicene examined in reference [19]. The comparison of experimental data with calculations are presented in Figure 5. In order to compare with data, +4 nm shift has been applied to the results obtained with M06, +26 nm with M06-2X; calculation of similarity index between experimental and calculated spectra suggested the best shift for the best correspondence, as reported in the Supplementary Materials paragraph. It is also shown that the two conformers give very similar spectra, while in Figure 5 the Boltzmann weighed average is presented. Both functionals permit confirmation of the configuration as M-(-) (correspondingly P-(+)). This conclusion agrees with what was obtained for the parent molecule triarylamine hetero [4]helicene examined in reference [19]. Overall, our results confirm that, on chemical resolution of helicenes, the position where the chiral auxiliaries are inserted is extremely important, being the bay-zone that allows for the higher effect on enantiomeric discrimination. At the same time we have confirmed previous studies reporting chromatographic resolutions of [7]carbo-and [7]hetero-helicenes by means of tetra-and monocamphanate esters [25][26][27][28][29][30][31]. In each of Overall, our results confirm that, on chemical resolution of helicenes, the position where the chiral auxiliaries are inserted is extremely important, being the bay-zone that allows for the higher effect on enantiomeric discrimination. At the same time we have confirmed previous studies reporting chromatographic resolutions of [7]carbo-and [7]heterohelicenes by means of tetra-and monocamphanate esters [25][26][27][28][29][30][31]. In each of these cases, the (1S)-camphanate of the (P)-helicenol moves more slowly upon chromatography on silica gel than the (1S)-camphanate of the (M)-helicenol [27].

Materials and Methods
1 H and 13 C NMR spectra were recorded with Varian Mercury Plus 400, Varian Inova 400, using CDCl 3 as solvent. Residual CHCl 3 at δ = 7.26 ppm and central line of CDCl 3 at δ = 77.16 ppm were used as the reference of 1 H-NMR spectra and 13 C NMR spectra, respectively. FT-IR spectra were recorded with a Spectrum Two FT-IR Spectrometer. ESI-MS spectra were recorded with a JEOL MStation JMS700. Melting points were measured with a Stuart SMP50 Automatic Melting Point Apparatus. Optical rotation measurements were performed on a JASCO DIP-370 polarimeter (JASCO, Easton, MD, USA) and the specific rotation of compounds was reported [41].
All the reactions were monitored by TLC on commercially available precoated plates (silica gel 60 F 254) and the products were visualized with acidic vanillin solution. Silica gel 60 (230-400 mesh) was used for column chromatography. Dry solvents were obtained by The PureSolv Micro Solvent Purification System. Chloroform was washed with water several times and stored over calcium chloride. Pyridine and TEA were freshly distilled from KOH. Phthalimide sulfenyl chloride was prepared from the corresponding disulfide (purchased from Chemper snc) as reported elsewhere. Helicenes 1a and 1b were described elsewhere [17].
General Procedure for the synthesis of diastereomeric esters from 1 by Steglich esterification: To a solution of 1 in dry CH 2 Cl 2 (roughly 0.03-0.04 M), the enantiopure acid 7 (1.2 eq), DMAP (0.1 eq) and DIC (1-1.2 eq) were added at 0 • C. The solution was stirred at room temperature under a nitrogen atmosphere for 2-29 h, then was diluted with CH 2 Cl 2 (60 mL), washed with a saturated solution of NH 4 Cl (2 × 40 mL), with a saturated solution of NaHCO 3 (3 × 40) then with NH 4 Cl (3 × 40 mL). The organic layer was dried over Na 2 SO 4 , filtered and evaporated under reduced pressure. (51 mg, 0.20 mmol) is added at 0 • C. After 10 min the solution was allowed to warm at room temperature and was stirred for 18 h under a nitrogen atmosphere. The mixture was diluted with AcOEt (25 mL) and washed with water (3 × 20 mL). The organic layer was dried over Na 2 SO 4 , filtered, and then evaporated under reduced pressure. The crude was purified by flash chromatography on silica gel (petroleum ether/AcOEt: 5/1, Rf 0.38) to afford the mixture of the two diastereomeric compounds 8abD1 and 8abD2 (55 mg, 56% yield) as a white solid (mp 190-195 • C). 1