Synthesis of Racemic and Chiral Albicanol, Albicanyl Acetate and Cyclozonarone: Cytotoxic Activity of ent-Cyclozonarone

A síntese completa da mistura racêmica do cyclozonarone ((±)-3), foi obtida a partir do E,Efarnesol (4) em uma seqüência de oito passos com um rendimento geral de 6,6%. O albicanol ((±)-1) e seu acetato ((±)-2) são intermediários. Uma seqüência inicial, similar a partir do produto natural (-)-drimenol (5), produziu o(+)-albicanol (1) e o(+)-cyclozonarone (3) com 42% e 11%, respectivamente. A atividade citotóxica do composto(+)-cyclozonarone foi avaliada e mostrou alguma seletividade para MS-1 (células endoteliais de camundongos).


Introduction
Synthetic efforts towards naturally occurring drimane sesquiterpenes have been constant due to their interesting biological activities.
The more concluding data to establish (±)-8 as the 7--chlorinated epimer, was the double doublet centred at 1.95 ppm for H-5, the constants (J 3.1 and 9.1 Hz) are corresponding to axial-equatorial and axial-axial couplings with each H-6.The signal centred at 2.29 ppm for H-6 eq (ddd, J 2.6, 5.0 and 12.6 Hz) evidenced an axial-equatorial coupling with H-5 and H-7ax, besides the geminal coupling.Moreover, the coupling constants for H-7 (signal centred at 4.35 ppm) suggest an axial disposition for this proton.Thus, confirming the equatorial disposition for the chloro atom in (±)-8.On the basis of a similar analysis for H-5, H-6 and H-7 on both chlorinated albicanyl acetate (7a and 7b) the major epimer was assumed to have an axial chloro in C-7, while the minor epimer should correspond to the equatorial chloro in C-7.
The lack of reactivity for the equatorial chloro epimer towards Zn could be attributed to esteric hindrance for effective overlapping.Nevertheless, the ability of 7b to react with Zn was confirmed submitting a sample of pure 7b to the same experimental conditions as the epimeric mixture, although the reaction seems to be slower.
The possibility of readily preparing diene 9 from (±)-1 prompted us to assay a different approach to (±)-cyclozonarone.(±)-Albicanol (1) was dissolved in pyridine and added to a mixture of Tf 2 O in pyridine, the reaction yielded 62% of diene 9. Racemic cyclozonarone was obtained from (±)-9 and benzoquinone as previously described 14 (Scheme 2).Since racemic drimenol ((±)-5) has been previously resolved in its enantiomers 19 the sequence described represents a formal synthesis of both enantiomers of cyclozonarone.

General procedures
Melting points were determined on a Stuart Scientific SMP3 apparatus and are uncorrected.IR spectra were recorded on a Bruker Vector 22-FT in KBr disc or film.Optical rotations were obtained for CHCl 3 solutions in an Optical Activity, Ltd instrument in a 1 dm cell and their concentrations are expressed in g per mL.NMR spectra were recorded on a Bruker AC 200P (200.13MHz for 1 H, 50.13 MHz for 13 C) in CDCl 3 solutions with TMS as internal standard.Carbon multiplicity was established by a DEPT pulse sequence and signals were assigned based on 2D experiments.All two-dimension spectra were acquired with a Bruker AVANCE 400 Spectrometer with a Bruker inverse 5 mm Z gradient probe.The HMBC spectra were obtained using the inv4gplrndqf pulse sequence in the Bruker software.HRMS were determined on a MAT 95XP, Thermo Finnigan spectrometer.Chromatographic separations were carried out on Merck silica gel 60 (230-400 Mesh) using hexane-ethylacetate gradients of increasing polarity.All organic extracts were dried over magnesium sulfate and evaporated under reduced pressure, below 65ºC.
An ice-water mixture was added and the solution was neutralized with NaHCO 3 and extracted with ethyl ether.The organic layer was dried (MgSO 4 ) and the solvent was removed in vacuum.The crude was purified by column chromatography to afford an oily product (38 mg, 62%) as a mixture of endo and exo dienes (1:4).Spectral data of the major isomer is in good agreement with those reported for exo-diene (9). 14)-Cyclozonarone (3)  Benzoquinone (27.5 mg, 0.25 mmol) was added to a solution of diene (±)-9 (1:4 mixture of endo and exo dienes) (40 mg, 0.19 mmol) in benzene (10 mL) and the mixture was refluxed under N 2 atmosphere for 28 h.The solvent was removed under reduced pressure and the residue was purified by column chromatography to afford 21 mg of quinone 10, which was eluted after the unchanged endodiene.Quinone 10 was dissolved in benzene (10 mL) and DDQ (15 mg, 0.062 mmol) was added.The mixture was refluxed for 4 h and after removing the solvent under reduced pressure and column chromatography, (±)-cyclozanorone (3) 18.2 mg, (46% overall yield from (±)-9) was obtained as a yellow oil, with identical spectroscopic characteristics as previously described for natural (-)-cyclozonarone.13 The same protocol was used for the synthesis of (+)-cyclozonarone (3) starting from natural (-)-drimenol.20