Assignment of Absolute Configuration to Enantiomers of Anti-Alzheimer Drug Candidate Blarcamesine

Blarcamesine is a promising investigational drug for the treatment of Alzheimer's disease. The international nonproprietary name blarcamesine refers to a racemic compound, although it seems likely that it will be marketed in an enantiopure form. A resolution process has been described in the literature, but the absolute configurations of the enantiomers have not yet been disclosed. In the present study, crystals of ( R )-(-)- and ( S )-(+)-mandelate salts of (+)- and (-)-blarcamesine and also that of ( R )-(+)-blarcamesine itself, suitable for single-crystal X-ray diffraction measurement were prepared and the absolute configurations of (+)- and (-)-blarcamesine have been determined.

The enantiomers of 1 were liberated from the diastereomeric salts 3 and obtained as solids after evaporation of ethereal solutions to dryness. A method for the synthesis of blarcamesine enantiomers via separation of covalent diastereomeric derivatives of a racemic intermediate was also described [16]. Racemic alcohol 4 (Scheme 2) was acylated with (S)-(-)camphanic chloride [(S)-5] and diastereomeric ester (-)-6 was isolated. Hydrolysis of the ester afforded enantiomerically pure (-)-4 which was transformed to (-)-1 by conventional methods. A similar procedure was carried out for the synthesis of (+)-1 using (R)-5 as the acylating agent. The enantiomeric purities were characterized by optical rotation measurements, ee values were not determined.
For the sake of completeness, it has to be mentioned that an enantioselective chemoenzymatic desymmetrization of one of the early intermediates of blarcamesine (1) using Amano Lipase PS30 enzyme has also been described [16]. However, the yields and the information disclosed suggest that the procedure is inappropriate for an economical scaled up implementation.

Results and discussion
In the course of our work on the process development of blarcamesine drug substance (1) it appeared to us that the absolute configuration of its enantiomers had not been disclosed in the literature. Therefore, we aimed to improve the resolution process and to obtain crystals suitable for evaluation of the absolute configurations by single-crystal X-ray diffraction (SCXRD) measurements.
This result was further confirmed by the preparation and SCXRD measurement of salt (+)-3. The mother liquor of salt (-)-3 was treated with (R)-(-)-mandelic acid [(R)-(-)-2] to give salt (+)-3 (99.92% ee). SCXRD measurement of the latter supported the expected R absolute configuration of the chiral center in the furan ring ( Fig. 2 (b)). The absolute configuration of blarcamesine in salts (-)-3 and (+)-3 was determined relative to the known configuration of mandelic acid and supported also by anomalous dispersion, which latter was weak due to the lack of heavy atoms.
The resolution of 1 was also carried out by changing the sequence of addition of the resolving agents with a smaller batch size (starting from 5.30 g, 18.7 mmol blarcamesine). In this case we first prepared mandelate (+)-3 (38%, 99.72% ee) and then, from the mother liquor the diastereomeric salt (-)-3 (38%, 99.74% ee).

Scheme 2 Resolution of blarcamesine intermediate (4) via covalent diastereomeric derivatives
Scheme 3 Improved process for the resolution of blarcamesine (1)

Supporting information
The "Supporting Information" contains the ORTEP diagrams, cif, and checkcif files, and structure report files of (-)-3, (+)-3 and (R)-(+)-1 as a zip file and is available on the journal website.

Experimental section
All melting points were determined on a Büchi Melting Point B-540 melting point apparatus and are uncorrected. IR spectra were obtained on a Bruker ALPHA FT-IR spectrometer operating in transmission mode, in KBr pellets. NMR spectra were recorded at 295 K on a Bruker Avance III HD 600 (600 and 150 MHz for 1 H and 13 C NMR spectra, respectively) spectrometer equipped with a Prodigy cryo-probehead. 1 H assignments were accomplished using general knowledge of chemical shift dispersion with the aid of the 1 H-1 H coupling pattern. CDCl 3 was used as the solvent and tetramethylsilane (TMS) as the internal standard. Chemical shifts (δ) and coupling constants (J) are given in ppm and in Hz, respectively. All reagents were purchased from commercial sources and were used without further purification. Reactions were followed by HPLC-MS on a Shimadzu LC-30 HPLC equipment (Kyoto, Japan) equipped with a quaternary pump, degasser, autosampler, column oven, diode array detector and an LCMS-2020 quadrupole mass spectrometer. Elemental analyses were performed on an Elementar Vario MICRO cube CHNS elemental analyzer. Single-crystal X-ray diffraction (SCXRD) measurements were carried out on a Rigaku R-Axis Spider diffractometer with imaging plate area detector using graphite monochromatic Cu-K α radiation. Single-crystal X-ray structures were deposited at the