Chiral HPLC Separation and CD Spectra of the Enantiomers of the Alkaloid Tacamonine and Related Compounds SALVATORE CACCAMESE, 1 * GRAZIA PRINCIPATO, 1 REIJA JOKELA, 2 ARTO TOLVANEN, 2 AND DAVID DIN BELLE 2 1 Dipartimento di Scienze Chimiche, Universita ` di Catania, Catania, Italy 2 Laboratory of Organic Chemistry, Helsinki University of Technology, Espoo, Finland ABSTRACT The HPLC enantiomeric separation of racemic indole alkaloids tacamo- nine, 17-hydroxytacamonine, deethyleburnamonine, and vindeburnol was accom- plished using Chiralpak AD and Chiralcel OD as chiral stationary phases. Small struc- tural differences affect the enantioselectivity ability of these phases. Single enantiomers of tacamonine and vindeburnol were isolated by semipreparative HPLC and their CD spectra and optical rotations were measured. Chirality 13:691–693, 2001. © 2001 Wiley-Liss, Inc. KEY WORDS: enantioselective chromatography; circular dichroism spectra; optical ro- tations; polysaccharide chiral stationary phases; alkaloids Tacamonine (1) is an indole alkaloid of the Iboga type isolated from the African plant Tabernaemontana eglandu- losa. 1 Because of its structural similarity to the pharmaco- logically valuable eburnamonine-vincamine alkaloids, 2 tacamonine was synthesized as a racemic mixture by some groups, 3–6 and in one case as the naturally occurring en- antiomer. 7 17-Hydroxytacamonine (2) was synthesized as a racemic mixture. 8,9 NOE difference spectroscopy proved that the trace compound occurring in the same plant 1 was the 17-isomer. 9 Unnatural 14-deethylderivatives also dis- play interesting pharmacological activities and racemic deethyleburnamonine (3) and vindeburnol (4), RU24722, have been recently synthesized. 10 The syntheses per- formed by the Finnish group proceed via epimerization of indolo[2,3-a]quinolizidine ester 6 or nitrile 8 derivatives. In any case, no attempt of enantiomeric separation was done. Thus, considering that individual enantiomers can have different pharmacological activities 11 and that great atten- tion has to be given to enantiomeric purity of chiral drugs 12 and due to our experience in the separation of the four enantiomers of the vincamine alkaloids, 13 we tried to re- solve the enantiomers of the racemic compounds 1–4. In this article, we wish to report the direct enantiosepa- ration of 1–4 by chiral HPLC on two polysaccharide de- rivatives chiral stationary phases (Chiralpak AD and Chi- ralcel OD). In addition, the good enantioselectivity and resolution factor allowed us to separate the single enantio- mers of tacamonine (1) and vindeburnol (4) and to mea- sure the circular dichroism (CD) spectra and specific op- tical rotation of the isolated enantiomers. MATERIALS AND METHODS Apparatus The HPLC system consisted of a Varian 5060 liquid chro- matograph with Valco sample loops, a Jasco Uvidec III UV spectrophotometer operating at 280 nm, and an HP 3396 A integrator or Houston Omniscribe recorder for fraction col- lecting. The columns (250 × 4.6 mm) were Chiralpak AD (amylose tris-3,5-dimethylphenylcarbamate) and Chiralcel OD (cellulose tris-3,5-dimethylphenylcarbamate), both coated on 10 μm silica gel from Daicel (Tokyo). Column void volume (t 0 ) was measured by injection of tri-tert- butylbenzene as a nonretained sample. 14 Resolution (R s ) was evaluated according to R s = 2 (t 2 -t 1 )/(w 1 +w 2 ), i.e., the peak separation divided by the mean value of the base- line widths. Retention times (t) were mean values of two replicate determinations. Other HPLC chromatographic parameters were those typically employed. 15 Experiments were performed at ambient temperature. CD spectra were recorded on a Jasco 600 spectropolarimeter and optical rotations were measured with a Jasco DIP-730 digital po- larimeter using a 10-cm microcell. Chemicals The synthesis and full characterization of compounds 1–4 have been described elsewhere. 6,8,9,10 RESULTS AND DISCUSSION Table 1 shows the chromatographic results for the enan- tioseparation of compounds 1–4 (formulas shown in Fig. 1) using as Chiral stationary phases (CSP) the Chiralcel OD and Chiralpak AD columns. Chiralpak AD (amylose tris-3,5-dimethylphenylcarbamate) was superior in the en- Contract grant sponsor: MURST. *Correspondence to: Prof. Salvatore Caccamese, Dipartimento di Scienze Chimiche, Universita ` di Catania, viale A. Doria 6, 95125 Catania, Italy. E-mail: scaccamese@dipchi.unict.it. Received 16 January 2001; Accepted 4 May 2001 CHIRALITY 13:691–693 (2001) © 2001 Wiley-Liss, Inc.