J. orakt. Chem. 337 (19951 68-70

Journal fur praktische Chemie Chemiker-Zeitung 0 Johann Ambrosius Barth 1995

Chromium Trioxide-3,5-Dimethylpyrazole Complex in the Oxidation of A5-Sterols to Steroidal A4-3,6-Diones

Hardy Schabdach and Karlheinz Seifert Bayreuth, Lehrstuhl fur Organische Chemie I12 der Universitat

Received 21st June, respectively 17th August 1994

Steroidal A4-3,6-diones are naturally occurring substances found in several plants [l-71 and marine sponges [8-101. Moreover, A4-3,6-diones are of special interest because of their biological properties [3,11-151. In steroid chemistry they are key intermediates in the further chemical transformation of the steroid nucleus [16-181.

A number of methods are currently available for the con- version of A5-sterols into A4-3,6-diones, and several of these are based on the use of high oxidation state chromium species. The classical oxidation of A5-sterols with sodium dichromate is a reaction with a yield of only 3 9 4 0 % [19]. Oxidation of cholesterol with a Cr03-pyridine complex under vari- ous conditions produced cholest-5-en-3-one accompanied by varying amounts of cholest-4-ene-3,6-dione [20]. Pyridiniurn chlorochromate (PCC), in refluxing benzene, has recently been found to be an effective reagent for the oxidation of steroidal A5-3P-tetrahydropyranyl ethers to the correspond- ing A4-3,6-diones [21]. Other methods include the oxida- tion of cholesterol with tetra-n-propylammonium perruthen- ate with 4-methylmorpholine N-oxide as co-oxidant [22], ox- idation with tert. butylhydroperoxide and catalytic amounts of chromium trioxide [23] and the sodium peroxide oxidation of A4-3-ones [24].

Synthetically useful changes in the properties and reactiv- ity of chromium(V1) reagents have brought about the forma- tion of amine complexes. Complexation of chromium trioxide with benzotriazole or 3,5-dimethylpyrazole (DMP) has been successfully used for allylic oxidation of A5-sterol esters in position 7 [25, 261.

We now report, that chromium trioxide, if it is used in con- junction with 3,5-dimethylpyrazole, is a convenient and useful reagent for the oxidation of A5-sterols to the corresponding A4-3,6-diones in high yield. It is important to prepare the complex at a low temperature (-2O"C), as it rapidly loses its activity when allowed to stand at room temperature. This may be explained by the possibility of polymerization of the oxi- dant caused by bridging via pyrazol nuclei [26,27]. Treatment of cholesterol (l), p-sitosterol (2) and 3P-hydroxy-5-pregnen- 20-one (3) with ten molar equivalents of the Cr03-DMP com- plex in dichloromethane at -10°C to -20°C for 8 h resulted in the corresponding A4-3,6-diones with approximately 70 % yield (Figure 1). We have found that the oxidation with the Cr03-DMP complex is remarkably fast. In reactions where the molar ratio of the complex to steroid is 30:1, the reac-

R

HO JJp '

A5-sterol 1,2,3

CIOJDMP

CH,CI,, -2OOC

A4-3.6dione 4,5,6

tion is complete in less than 45 min. No significant amounts of A4-3-ones were detected in the reaction mixtures. The re- action proceeds probably via the initial formation of the A5- P,y-unsaturated ketone, which is immediately isomerized to the A4-a,P-unsaturated ketone with concomitant oxidation at C-6. A similar course of reaction has been described for the aqueous chromic acid oxidation of A5-3p-alcohols to A4-3,6- diones [28].

The structures of 4-6 have been determined primarily on the basis of spectroscopic data ('H, 'H,'H-COSY-45", I3C, 13C-APT and 1H,13C-COSY experiments; MS; IR). Accord- ing to APT and lH,13C COSY experiments of 5 the assign- ments of the signals C-8, C-10 (ref. [7]) C-20, C-22 (ref. [8]) have to be interchanged.

The results indicate that the Cr03-DMP complex is an effective agent for the conversion of A5-sterols to the corre- sponding A4-3,6-diones and extends the scope of the oxidant properties of this reagent.

Experimental

Melting points (m.p.) were measured on a Reichert hot stage microscope and are reported without correction. Infrared spectra (wavenumbers in cm-*) were recorded in CHC13 on a Bio-Rad FTS-40 spectrometer. Optical rotations were mea- sured on a Perkin Elmer 241 polarimeter. 'H- and 13C-nuclear magnetic resonance (NMR) spectra were taken in CDC13 on a Bruker AC-300 spectrometer. Chemical shifts are given in parts per million (&scale), coupling constants ( J ) in Hertz. 'H-chemical shifts were referenced to the residual CHCl3 signal (7.24). 13C-chernical shifts were referenced to CDCl3

H. Schabdach, K. Seifert, Oxidation of A5-Sterols 69

(77.0). MS were recorded at 70 eV on a Varian MAT-313 spectrometer.

Thin layer chromatography was carried out on precoated plates of PolygramR SILG/UV25, (layer thickness 0.25 mm, Machry/Nagel). Spots were visualized by UV (254 nm) and spraying with Rosenheim reagent (20 g of antimony trichlo- ride in 100 ml of chloroform/acetic acid, 3:l) followed by heating. Column chromatography was performed on Merck silica gel 60 (70-230 mesh ASTM).

Cholesterol, p-sitosterol, chromium trioxide and 3,s- dimethylpyrazole were purchased from Merck (Darmstadt, Germany); 3p-hydroxy-S-pregnen-20-one was supplied by Aldrich Chemical Co. (Milwaukee, WI, USA.).

Dichloromethane was distilled under nitrogen from Pz05. Chromium trioxide was dried over Pz05 before use.

General procedure for the preparation of 4-6 Chromium trioxide (3.0 g, 30 mmol) was suspended in dry dichloromethane (40 ml) at -2O"C, and DMP (2.9 g, 30 mmol) was added in one portion under nitrogen. The resulting mix- ture was stirred at -20°C for 20 min. The A5-sterols (1, 2 or 3, 3 mmol), were added under nitrogen, and the mixture was stirred for 8 h maintaining a temperature between -10 and -20°C. The solution was diluted with dichloromethane (100 ml) and washed with dilute hydrochloric acid (70 ml, 5 %) to remove the DMP. The dichloromethane phase was dried over anhydrous sodium sulfate, filtered, and concentrated to dryness under reduced pressure to give a residue, which was subjected to column chromatography (silica gel, 130 g) using cyclohexanelethyl acetate (1:l) as eluant.

Table 1 Conversion of A5-sterols to A4-3,6-diones with CrO3- DMP complex

substrate Cr03/DMP solvent volume time product (3 mmol) equiv. (mu (h) (YO yield)

1 10 40 8 73 20 50 3 71 30 60 0.75 69

2 10 40 8 71 3 10 40 8 70

Cholest-4-ene-3,6-dione (4)

(73 YO), m.p. 123-125 "C(ref. [19] m.p. 124125 "C). [a]? = -29.9" (c = 0.9 in CHC13). Rf = 0.76 (cyclohexaneiethyl acetate, 1:l) ' H NMR: 0.69 (s, 3H, 18-H3), 0.82 (d, 6H, 26,27-H3, J = 6.6), 0.88 (d, 3H, 21-H3, J = 6 3 , 1.14 (s, 3H, 19-H3), 6.14 (s, lH , 4-H). MS, WZLZ (Yo): 398 (100) [M'], 370 (70) [M+-CO], 285 (35) [M+-side chain]. IR: 1685.

Stignzast-4-ene-3,6-dione (5 )

(71 Y), m.p. 161-163°C (ref. [8] m.p. 160-163°C). [a]? = -37.2" (c = 0.9 in CHC13), (ref. [29] [a]: = -36.7") Rf = 0.76 (cyclohexane/ethyl acetate, 1:l). lH NMR: 0.69 (s, 3H, l8-H3), 0.79 (d, 3H, 26-H3, J = 6.8),

0.81 (d, 3H, 27-H3, J = 6.5), 0.82 (m, 3H, 29-H3), 0.91 (d, 3H,

13C NMR: 11.9 (C-29, C-18), 17.5 (C-19), 18.7 (C-21), 19.0 21-H3, J = 6.5), 1.14 (s, 3H, 19-H3), 6.14 (s, lH, 4-H).

(C-26), 19.8 (C-27), 20.9 (C-ll), 23.4 (C-28), 23.9 (C-15), 26.0 (C-23), 28.0 (C-16), 29.1 (C-25), 33.8 (C-22), 34.2 (C-2, C-S), 36.0 (C-l), 36.1 (C-20), 39.1 (C-12), 39.8 (C-lo), 42.5 (C-13), 45.8 (C-24), 46.8 (C-7), 51.0 (C-9), 55.8 (C-14), 56.5 (C-17), 125.4 (C-4), 161.0 (C-5), 199.4 (C-3), 202.3 (C-6). MS, m/z (YO): 426 (100) [M'], 398 (SO) [M+-CO], 285 (30) [M+-side chain]. I R 1685.

Pregn-4-ene-3,6,20-trione (6)

(70 %), m.p. 195-197°C (ref. [30] m.p. 197-203°C). [a]: = -7.4" (c = 0.9 in CHCl3). Rf = 0.36 (cyclohexane/ethyl acetate, 1:l). 'H NMR: 0.63 (s, 3H, l8-H3), 1.12 (s, 3H, 19-H3), 2.09 (s,

I3C NMR: 13.2 (C-18), 17.5 (C-19), 20.8 (C-ll), 22.8 (C-16), 3H, 21-H3), 6.13 (s, lH , 4-H).

24.7 (C-15), 31.3 (C-21), 33.9 (C-2), 34.0 (C-8), 35.5 (C-l), 38.1 (C-12), 39.6 (lo), 43.8 (C-13), 46.4 (C-7), 50.7 (C-9), 56.5 (C-14), 63.1 (C-17), 125.6 (C-4), 160.4 (C-S), 199.1 (C-3), 201.4 (C-6), 208.6 (C-20). MS, m/z (YO): 328 (100) [M'], 300 (50) [M+-CO]. IR: 1696, 168.5.

References

I. M. Fernandez, R. J. Pedro, E. Seoane, Phytochemistry 22 (1983) 2087 E. G. Gross, P. Cattaneo, S. N. Nolasco, Rev. Latinoam. Quim. 14 (1983) 72 H. Xue, J. Zhang, L. He, Nanjing Yoaxueyuan Xuebao 16 (1985) 7 H. Achenbach, M.Stocker, M. M. Constenla, Phytochem- istry 27 (1988) 1835 H. Achenbach, H. Hemrich, Phytochemistry 30 (1991) 1265 H. Ripperger, Pharmazie 33 (1978) 82 M. D. Greca, P. Monaco, L. Previtera, J. Nat. Prod. 53 (1990) 1430 A. Aiello, E. Fattorusso, S. Magno, M. Menna, J. Nat. Prod. 54 (1991) 281 M. Tischler, S. W. Ayer, R. J. Andersen, J. E Mitchell, J. Clardy, Can. J. Chem. 66 (1987) 1173 A. Migliuolo, V. Piccialli, D. Sica, J. Nat. Prod. 53 (1990) 1262 G. Aliotta, M. D. Greca, P. Monaco, G. Pinto, A. Pollio, L. Previtera, J. Chem. Ecol. 16 (1990) 2637 M. Yoshihama, K. Tamura, M. Nakakoshi, J. Nakamura, N. Fujise, G. Kawanishi, Chem. Pharm. Bull. 38 (1990) 2834 M. Numazawa, M. Tsuji, A. Mutsumi, J. Steroid Biochem. 28 (1987) 337 R. Rembiesa, W. Ptak, M. Bubak, Experientia 30 (1974) 82 R. Rembiesa, W. Ptak, M. Bubak, Mater. Nauk. Kra- iowego Ziazdu Endokrynol. Pol., 8th (1974) 105

[16] H. SFngh,-K.K. Bhutani, Indian J. Chem. 16 B (1978) 95

70 J. prakt. Chem. 337 (1995)

[17] M. Ishige, M. Shiota, F. Suzuki, Can. J. Chem. 54 (1976)

[18] Z. You, M. Koreeda, Tetrahedron Lett. 34 (1993) 2745 [19] L. F. Fieser, Organic Synthesis Coll. Vol. 4 (1963) 189 [20] E. Piers, P. M. Worster, Can. J. Chem. 55 (1977) 733 [21] E. J. Parish, S. A. Kizito, R. W. Heidepriem, Synth. Com-

[22] M. J. S. M. Moreno, M. L. S B e Melo, A. S. C. Neves,

[23] J. Muzart, Tetrahedron Lett. 28 (1987) 4665 [24] H. L. Holland, U. Daum, E. Riemland, Tetrahedron Lett.

[25] E. J. Parish, S. Chitrakorn, Synth. Commun. 15 (1985)

[26] W. G. Salmond, M. A. Barta, J. L. Havens, J. Org. Chem.

2581

mun. 23 (1993) 223

Terahedron Lett. 22 (1981) 5127

22 (1981) 5127

393

43 (1978) 2057

[27] S. Trofimenko, Chem. Rev. 72 (1972) 497 [28] L. F. Fieser, M. Fieser, Steroide, Verlag Chemie, Wein-

[29] G. Rucker, B. Langmann, N. S. de Siqueira, Planta Med.

[30] G. Gondos, J. C. Orr, Liebigs Ann. Chem. 1990, 213

heim 1961, 220

41 (1981) 144

Address for correspondence:

Prof. Dr. K. Seifert Universitat Bayreuth Lehrstuhl fur Organische Chemie 112 Universitatsstr. 30 D-95447 Bayreuth, Germany