HK1019878B - Total synthesis of antitumor acylfulvenes - Google Patents

Description

Background of the Invention

Natural products from plants and microorganisms have proven to be a major source of active anticancer agents and lead compounds for cancer chemotherapy. Mushrooms of the class Basidiomycetes are an exception. Although they occur widely and some are well known to contain a variety of highly poisonous substances, only Omphalotus illudens (jack o'lantern mushroom) is known to produce promising anticancer compounds. These are the sesquiterpenes illudin S and illudin M. The illudins are extremely cytotoxic compounds but have a low therapeutic index particularly in solid tumor systems. However, modification of their structures has yielded several analogs, which possess a greatly improved therapeutic index. Remarkable efficacy has been observed in tests on mouse xenografts of leukemias and various solid tumors.

First and second generation analogs, for example, dehydroilludin M and acylfulvene, have been described (WO 91/04754). A promising compound is a third generation analog hydroxymethylacylfulvene (HMAF). In tests with MV 522 metastatic lung carcinoma xenografts in nude mice, complete tumor regression was observed in all animals. HMAF also exhibited outstanding activity against breast (MX-1), colon (HT-29) and skin cancers.

The structures of illudin S and illudin M were first published in 1963 (McMorris et al., J. Am. Chem. Soc. 85:831 (1963)). Until recently only one total synthesis of these compounds had been reported (Matsumoto et al., Tetrahedron Lett. 1171 (1970)). This synthesis involved Michael addition of a cycylopropane intermediate to an appropriately substituted cyclopentenone. The resulting product was then transformed into an intermediate which could undergo aldol condensation to form illudin's six-membered ring. A number of further reactions were required to complete the synthesis.

Padwa et al., (J. Am. Chem. Soc. 116:2667 (1994)), have published a synthetic approach to the illudin skeleton using a dipolar cycloaddition reaction of a cyclic carbonyl ylide dipole with cyclopentenone to construct the six-membered ring. Kinder and Bair (J. Org. Chem. 59:6955 (1994)), have also employed the Padwa methodology to synthesize illudin M. However, these syntheses are long and not well suited for making acylfulvenes on a large scale.

Thus, a continuing need exists for improved methods for synthesizing acylfulvenes.

Summary of the Invention

The present invention provides compounds of formula I and pharmaceutically acceptable salts thereof: wherein in formula I:

• (a) R' is (C1-C4) alkyl, R1 and R2 together are ethylene dioxy, and R3 is H; or
• (b) R' is (C1-C4) alkyl, R1 is OH, and R2 and R3 are H.

The invention also provides pharmaceutical compositions comprising said compounds or salts, and the use of said compounds or salts in the manufacture of medicaments for inhibiting tumor cell growth.

The present invention also provides a method of synthesizing a compound of formula (XVII):    wherein R₁ is OH, R₂ is H, and R'is (C₁-C₄)alkyl, preferably methyl, i.e. a compond as defined under I(b) above.

According to the present invention, a method is provided of synthesizing a diketone of formula (XXV), a preferred intermediate in the synthesis of compounds of formula (XVII),:    comprising the steps of

• (a) cleaving the oxybridge in the compound of formula (XIV): to yield a diketone of formula (XXV).

The method further comprises the steps of

• (b) protecting the hydroxyl group in the compound of formula (XXV) with a removable hydroxyl protecting group X; and
• (c) introducing a double bond in the five-membered ring to yield a compound of the formula (XV):

   wherein R'₁ and R'₂ together are keto; and    X is a removable hydroxyl protecting group. Removable hydroxyl protecting groups may be introduced by reaction with a suitable reagent, such as a reagent of the formula ((C₁-C₄)alkyl)₃SiCl, including triethylsilyl (TES) chloride, trimethylsilyl (TMS) chloride, t-butyldimethylsilyl (TBDMS) chloride, dimethyl (1,2,2-trimethylpropyl)silyl chloride, or tris(isopropyl)silyl; and methoxymethyl chloride, β-methoxyethoxymethyl chloride, and isobutylene.

The method further comprises the steps of

• (d) reducing both keto groups to yield hydroxy groups under conditions that yield a compound of formula (XVI):
• (e) eliminating the cyclopentenol hydroxyl group; and
• (f) oxidizing the cyclohexanol hydroxyl group and removing hydroxyl protecting group X to yield a compound of formula (XVII):

wherein R₁ is OH and R₂ is H.

The method additionally comprises the step of

• (g) following step (d), treating the alcohol with mesyl chloride in the presence of a base to produce a mesylate of the formula (XVIII):

   wherein R"₁ is -OX, R"₂ is absent and R is H.

The present invention further provides a method of synthesizing compounds of the formula (XXIII): wherein R'₁ and R'₂ together are ethylenedioxy, and R'is (C₁-C₄)alkyl, preferably methyl i.e. a compound as defined under I(a) above.

According to the present method, the carbonyl group of the compound of formula (XIII) is converted to an acetal group to yield a compound of formula (XIX):

The method further comprises the steps of

• (b) protecting the hydroxyl group in the compound of formula (XIX) with a removable hydroxyl protecting group X; and
• (c) introducing a double bond in the five-membered ring to yield a compound of the formula (XX):

   wherein X is a removable hydroxyl protecting group.

The method further comprises the steps of

• (d) reducing the keto group to yield a hydroxy group under conditions that yield a compound of formula (XXI):
• (e) eliminating the cyclopentenol hydroxyl group;
• (f) removing hydroxyl protecting group X to yield a compound of formula (XXII): and
• (g) oxidizing the cyclohexanol hydroxyl group to yield a compound of formula (XXIII):

The method further comprises the step of

• (h) following step (d), treating the alcohol with mesyl chloride to produce a mesylate of the formula (XXTV):

With respect to both mesylates of formulas (XVIII) and (XXIV), the mesylates are relatively unstable and convert to fulvenes upon standing. Removal of the protecting group X and oxidation yield compounds of formulas (XVII) and (XXIII), respectively.

The invention also provides novel compounds of formula XV, XVI, XIX, XX, XXI, XXII and XXV, all of which are useful as intermediates in the synthesis of 6-substituted acylfulvene analogs (6-substituted acylfulvenes) as disclosed, for example, in Kelner et al., U.S. patent no. 5,523490, or which have antitumor or cytotoxic activity per se.

Brief Description of the Drawings

• Figure 1 is a schematic representation of the synthesis of compound of Formula (XXIII), specifically compound 35.
• Figure 2 is a schematic representation of the synthesis of compound of 25 Formula (XVII), specifically compound 42.

Detailed Description of the Invention

An acylfulvene analog of formula (XXIII) where R'₁ and R'₂ together are ethylenedioxy (compound 35), may be synthesized as shown in Figure 1. The oxybridge in the intermediate 7 is cleaved with K₂CO₃ in isopropanol at room temperature giving the diketone 27 (82%). Regioselective acetal formation (ethylene glycol, p-TsOH, C₆H₆, room temperature) gives in quantitative yield the monoacetal 28. Protection of the hydroxyl as the triethyl silyl ether (triethylsilylchloride, pyridine, 60°C) is quantitative. A double bond is introduced into compound 29, by treatment with benzene seleninic anhydride in chlorobenzene at 95°C, yielding cross conjugated ketone 30 (78%). Reduction of 30 (NaBH₄, CeCl₃. 7 H₂O in MeOH) gives alcohol 31. This compound on treatment with methane sulfonyl chloride and triethylamine gives the fulvene 33 (via the unstable mesylate 32). Removal of the silyl protecting group (p-TsOH, acetone-water 1:1) gives the alcohol 34, which upon oxidation with pyridinium dichromate in dichloromethane affords the acylfulvene 35 (60% yield for four steps).

Another analog of formula (XVII) where R₁ is OH and R₂ is H (compound 42) can be made from intermediate 27. As shown in Figure 2, compound 27 is converted to the triethylsilyl (TES) ether 36. A double bond is then introduced in the five membered ring by reaction with phenylseleninic anhydride giving 37 in good yield. Reduction of the diketone with sodium borohydride-ceric chloride gives the corresponding alcohols accompanied by rearrangement of the TES group, resulting in compound 38. Treatment of the latter with triethylamine and mesylchloride gives the unstable mesylate 39 which directly yields the fulvene 40. Oxidation of 40 with Dess-Martin reagent and removal of the silyl protecting group gives ± acylfulvene analog 42.

The compounds of formulas (XVII) and ((XXIII) and intermediates thereof are useful as antineoplastic agents, i.e., to inhibit tumor cell growth in vitro or in vivo, in mammalian hosts, such as humans or domestic animals, and are particularly effective against solid tumors and multi-drug resistant tumors. These compounds may be particularly useful for the treatment of solid tumors for which relatively few treatments are available. Such tumors include epidermoid and myeloid tumors, acute (AML) or chronic (CML), as well as lung, ovarian, breast and colon carcinoma. The compounds can also be used against endometrial tumors, bladder cancer, pancreatic cancer, lymphoma, Hodgkin's disease, prostate cancer, sarcomas and testicular cancer as well as against tumors of the central nervous system, such as brain tumors, neuroblastomas and hematopoietic cell cancers such as B-cell leukemia/lymphomas, myelomas, T-cell leukemia/lymphomas, and small cell leukemia/lymphomas. These leukemia/lymphomas could be either acute (ALL) or chronic (CLL).

The compounds may also be incorporated in a pharmaceutical composition, such as pharmaceutical unit dosage form, comprising an effective anti-neoplastic amount of one or more of the illudin analogs in combination with a pharmaceutically acceptable carrier.

The methods of the present invention may also be adapted to make pharmaceutically acceptable salts of compounds of formula (XVII) or (XXIII). Pharmaceutically acceptable salts include, where applicable, salts such as amine acid addition salts and the mono-, di- and triphosphates of free hydroxyl groups. Amine salts include salts of inorganic and organic acids, including hydrochlorides, sulfates, phosphates, citrates, tartarates, malates, maleates, bicarbonates, and the like. Alkali metal amine or ammonium salts can be formed by reacting hydroxyaryl groups with metal hydroxides, amines or ammonium.

The compounds can be formulated as pharmaceutical compositions and administered to a mammalian host, such as a human cancer patient, in a variety of forms adapted to the chosen route of administration, i.e., orally or parenterally, by intravenous, intraperitoneal, intramuscular or subcutaneous routes.

The subject can be any mammal having a susceptible cancer, i.e., a malignant cell population or tumor. The analogs are effective on human tumors in vivo as well as on human tumor cell lines in vitro.

Thus, the compounds may be orally administered, for example, in combination with a pharmaceutically acceptable vehicle such as an inert diluent or an assimilable edible carrier. They may be enclosed in hard or soft shell gelatin capsules, may be compressed into tablets, or may be incorporated directly with the food of the patient's diet. For oral therapeutic administration, the active compound may be combined with one or more excipients and used in the form of ingestible tablets, buccal tablets, troches, capsules, elixirs, suspensions, syrups, wafers, and the like. Such compositions and preparations should contain at least 0.1% of active compound. The percentage of the compositions and preparations may, of course, be varied and may conveniently be between 2 to about 60% of the weight of a given unit dosage form. The amount of active compound in such therapeutically useful compositions is such that an effective dosage level will be obtained.

The tablets, troches, pills, capsules and the like may also contain the following: A binder such as gum tragacanth, acacia, corn starch or gelatin; excipients such as dicalcium phosphate; a disintegrating agent such as corn starch, potato starch, alginic acid and the like; a lubricant such as magnesium stearate; and a sweetening agent such as sucrose, lactose, or saccharin or a flavoring agent such as peppermint, oil of wintergreen, or cherry flavoring may be added. When the unit dosage form is a capsule, it may contain, in addition to materials of the above type, a liquid carrier, such as a vegetable oil or a polyethylene glycol. Various other materials may be present as coatings or to otherwise modify the physical form of the solid unit dosage form. For instance, tablets, pills, or capsules may be coated with gelatin, wax, shellac or sugar and the like. A syrup or elixir may contain the active compound, sucrose as a sweetening agent, methyl and propylparabens as preservatives, a dye and flavoring such as cherry or orange flavor. Of course, any material used in preparing any unit dosage form should be pharmaceutically acceptable and substantially non-toxic in the amounts employed. In addition, the active compound may be incorporated into sustained-release preparations and devices.

The active compound may also be administered intravenously or intraperitoneally by infusion or injection. Solutions of the active compound can be prepared in water, optionally mixed with a nontoxic surfactant. Dispersions can also be prepared in glycerol, liquid polyethylene glycols, triacetin, and mixtures thereof and in oils. Under ordinary conditions of storage and use, these preparations contain a preservative to prevent the growth of microorganisms.

The pharmaceutical dosage forms suitable for injection or infusion use can include sterile aqueous solutions or dispersions or sterile powders comprising the active ingredient which are adapted for the extemporaneous preparation of sterile injectable of infusible solutions or dispersions. In all cases, the ultimate dosage form must be sterile, fluid and stable under the conditions of manufacture and storage. The liquid carrier or vehicle can be a solvent or liquid dispersion medium comprising, for example, water, ethanol, a polyol (for example, glycerol, propylene glycol, liquid polyethylene glycols, and the like), vegetable oils, nontoxic glyceryl esters, and suitable mixtures thereof. The proper fluidity can be maintained, for example, by the formation of liposomes, by the maintenance of the required particle size in the case of dispersion or by the use of surfactants. The prevention of the action of microorganisms can be brought about by various antibacterial and antifungal agents, or example, parabens, chlorobutanol, phenol, sorbic acid, thimerosal, and the like. In many cases, it will be preferable to include isotonic agents, for example, sugars, buffers or sodium chloride. Prolonged absorption of the injectable compositions can be brought about by the use in the compositions of agents delaying absorption, for example, aluminum monostearate and gelatin. Sterile injectable solutions are prepared by incorporating the active compound in the required amount in the appropriate solvent with various of the other ingredients enumerated above, as required, followed by filter sterilization. In the case of sterile powders for the preparation of sterile injectable solutions, the preferred methods of preparation are vacuum drying and the freeze drying techniques, which yield a powder of the active ingredient plus any additional desired ingredient present in the previously sterile-filtered solutions.

Useful dosages of compounds made according to the present methods can be determined by correlating the compounds' in vitro activity, and in vivo activity in animal models, such as murine or dog models as taught for illudin analogs such as those of U.S. Patent Nos. 5,439,936 and 5,523,490, to activity in higher mammals, such as children and adult humans as taught, e.g., in Borch at al. (U.S. Patent No. 4,938,949).

The therapeutically effective amount of analog necessarily varies with the subject and the tumor to be treated. However, it has been found that relatively high doses of the analogs can be administered due to the decreased toxicity compared to illudin S and M. A therapeutic amount between 30 to 112,000 µg per kg of body weight is especially effective for intravenous administration while 300 to 112,000 µg per kg of body weight is effective if administered intraperitoneally. As one skilled in the art would recognize, the amount can be varied depending on the method of administration.

The invention will be further described by reference to the following detailed examples.

EXAMPLES EXAMPLE I - Synthesis of Compound 35

General. Melting points are uncorrected. ¹H and ¹³C NMR spectra were measured at 300 and 75 MHz. High resolution mass spectra were determined at the University of Minnesota Mass Spectrometry Service Laboratory. All chromatography used silica gel (Davisil 230-425 mesh, Fisher Scientific) and solvent was ethyl acetate and hexanes. Analytical TLC was carried out on Whatman 4420 222 silica gel plates. Reactions were routinely monitored by TLC. Yield was calculated after recycling starting materials.

Compound 7. Compound 7 was made following literature as a white solid: mp 134-6 °C; IR (KBr) 2993, 2952, 1757, 1743, 1454 cm^-1; ¹H NMR (CDCl₃) δ 0.74 (m, 1H), 1.03 (m, 1H), 1.13 (m, 1H), 1.25 (s, 3H), 1.32 (m, 1H), 2.08 (m, 2H), 2.27 (m, 2H), 2.54 (d, J = 7.5 Hz, 1H), 2.92(m, 1H), 4.45 (s, 1H); ¹³C NMR (CDCl₃) δ 216.6, 211.4, 87.7, 87.4, 57.6, 41.3, 39.2, 38.3, 25.1, 14.1, 13.4, 11.9; MS m/z 206 (M⁺), 177,149,124; HRMS for C₁₂H₁₄O₃ calcd 206.0943, found 206.0941.

Compound 27. To a stirred solution of 7 (2.83 g, 13.7 mmol) and 2-propanol (500 ml) was added K₂CO₃ (8 g, 58.0 mmol) at 25 °C. The mixture was stirred for 7 days, then partitioned between EtOAc and water. The organic extract was washed with saturated NH₄Cl and dried over MgSO₄. Then the crude product was concentrated and chromatographed to give 1.88 g of 7 and 0.78 g of 27 (82.1%). 27 is a white solid: mp 183-5 °C; IR (KBr) 3369, 2995, 1696, 1616, 1407, 1367, 1226 cm^-1; ¹H NMR (CDCl₃) δ 1.24 (m, 1H), 1.38 (m, 1H), 1.68 (m, 1H), 1.88 (m, 1H), 2.00 (s, 3H), 2.16 (m, 2H), 2.46 (m, 2H), 3.21 (m, 1H), 4.06 (d, J = 2.7 Hz, 1H); ¹³C NMR (CDCl₃) 6 206.1, 204.8, 147.5, 128.0, 72.0, 42.2, 39.5, 32.1, 21.7, 19.4, 18.6, 11.7; MS m/z 206 (M⁺), 177, 150, 147; HRMS for C₁₂H₁₄O₃ calcd 206.0943, found 206.0944.

Compound 28. p-Tohmesulfonic acid (12 mg, 0.063 mmol) was added to a stirred solution of 27 (107 mg, 0.519 mmol) and ethylene glycol (3.04 g, 49 mmol) in benzene (10 ml) at 25 °C which was then stirred for 24 h. The mixture was partitioned between EtOAc and saturated NaHCO₃. The combined organic layers were washed with saline, dried over MgSO₄ and concentrated to an oil which was chromatographed to give 5 mg of 27 and 118 mg of 28 (95.3%) as colorless oil: IR (KBr) 3469, 2952, 2892, 1757, 1690, 1616, 1374, 1159, 1085 cm^-1; ¹H NMR (CDCl₃) δ 1.00 (m, 3H), 1.36 (m, 1H), 1.88 (d, J = 2.7 Hz, 3H), 1.96 (m, 2H), 2.36 (m, 2H), 3.19 (t, J = 3.9 Hz, 1H), 3.78 (t, J = 3.9 Hz, 1H), 4.00 (m, 4H); ¹³C NMR (CDCl₃) δ 205.4, 148.3, 128.3, 108.9, 67.9, 65.6, 64.5, 41.9, 39.3, 26.8, 20. 8, 12.8, 11.5, 6.22; MS m/z 250 (M⁺), 221, 193, 177; HRMS for C₁₄H₁₈O₄ calcd 250.1205, found 250.1201.

Compound 29. To a stirred solution of 28 (8.0 mg, 0.032mmol) and pyridine (0.5 ml) was added TESCl (0.1 ml, 0.25 mmol) under N₂. The reaction mixture was stirred at 60 °C for 30 min and then concentrated to an oil. The crude product was purified by chromatography to give 13 mg of 29 (quantitative) as a colorless oil: IR (KBr) 2959, 2885, 1710, 1610, 1454, 1414, 1381, 1219 cm^-1; ¹H NMR (CDCl₃) δ 0.62 (q, J = 7.8 Hz, 6H), 0.94 (m, 11H), 1.28 (m, 1H), 1.83 (m, 1H), 1.87 (d, J = 2.4 Hz, 3H), 2.35 (m, 2H), 3.13 (m, 2H), 3.75 (d, J = 3.3 Hz, 1H), 4.01 (m, 4H); ¹³C (CDCl₃) δ 205.6, 148.8, 128.8, 109.5, 69.1, 65.3, 64.7, 43.3, 39.5, 27.4, 21.5, 12.9, 11.6, 6.8, 6.5, 4.8; MS m/z 364 (M⁺), 336, 291, 219, 161; HRMS for C₂₀H₃₂O₄Si calcd 364.2070, found 364.2070.

Compound 30. A solution of 29 (13 mg, 0.0357 mmol) and phenylseleninic anhydride (13 mg, 0.0361 mmol) in chlorobenzene (0.5 ml) was stirred at 95 °C for 0.5 h under N₂. The solution was then concentrated and chromatographed to give 4.9 mg of 29 and 7.0 mg of 30 (78.2%) as colorless oil: IR (KBr) 2959, 2878, 1716, 1683, 1622, 1454, 1381, 1213 cm^-1; ¹H NMR (CDCl₃) δ 0.54 (q, J = 6.3 Hz, 6H), 0.89 (m, 10H), 1.27 (m, 2H), 1.57 (m, 1H), 1.93 (m, 3H), 3.79 (s, 1H), 4.00 (m, 4H), 6.30 (dd, J = 2.4, 6 Hz, 1H), 7.28 (dd, J = 2.1, 6 Hz, 1H); ¹³C NMR (CDCl₃) δ 195.9, 154.7, 146.9, 137.7, 127.5, 109.5, 69.2, 65.5, 64.6, 47.4, 28.0, 12.8, 11.1, 7.1, 6.7, 5.0; MS m/z 362 (M⁺), 333, 289, 187, 159, 87; HRMS for C₂₀H₃₀O₄Si calcd 362.1913, found 362.1919.

Compound 34. To the solution of 30 (20 mg, 0.055 mmol) and CeCl₃.7H₂O (35 mg, 0.094 mmol) in MeOH (1 ml) was added NaBH₄ (excess). The mixture was stirred for 15 min at 25 °C and then more NaBH₄ was added. After 15 min of stirring the mixture was partitioned between Et₂O and saturated NH₄Cl. The ether extract was dried over MgSO₄ and concentrated to give crude product 31 as pale yellow oil.

To the solution of the above crude product 31 in CH₂Cl₂ (1 ml) was added Et₃N (20 ml, 0.143 mmol) and MsCl (20 ml, 0.258 mmol) respectively at 25 °C. It was stirred for 5 min. Then the mixture was partitioned between Et₂O and saturated NaHCO₃. The ether extract was washed by saline and dried over MgSO₄. After concentration, it was chromatographed to give 33 and 34 as yellow gum.

To the solution of the above compound 33 in acetone (2 ml) and water (1 ml) was added some p-TsOH at room temperature. The mixture was set aside for 5 min and partitioned between Et₂O and saturated NaHCO₃. Then the ether extract was washed by saline and dried by MgSO₄. After concentration and chromatography, it was mixed with the above product 34 to give 10.5 mg of 34 as yellow gum: IR (KBr) 3456, 2912, 2885, 1730, 1636, 1441, 1367 cm^-1; ¹H NMR (CDCl₃) δ 0.75 (m, 1H), 1.10 (m, 2H), 1.24 (m, 1H), 1.88 (s, 3H), 2.34 (d, J = 6.9 Hz, 1H), 3.95 (m, 2H), 4.06 (m, 2H), 4.68 (d, J = 5.7 Hz, 1H), 6.34 (m, 1H), 6.42 (m, 2H); ¹³C NMR (CDCl₃) d 152.0, 139.8, 134.6, 130.5, 125.3, 117.9, 111.9, 71.3, 67.0, 66.1, 31.5, 16.4, 9.5, 6.6; MS m/z 232 (M⁺), 215, 189, 160, 145; HRMS for C₁₄H₁₆O₃ calcd 232.1099, found 232.1093.

Compound 35. A solution of 34 (7.3 mg, 31 mmol) and pyridinium dichromate (26 mg, 69 mmol) in CH₂Cl₂ (1 ml) was stirred for 1 h at 25 °C. The mixture was diluted by Et₂O and then filtered. The concentrated crude product was chromatographed to give 5.2 mg of 35 (71.9%) as yellow crystal: mp 138-140 °C; IR (KBr) 2959, 2892, 1683, 1616, 1549, 1441, 1360 cm^-1; 1H NMR (CDCl₃) δ 1.14 (m, 2H), 1.35 (m, 2H), 2.06 (s, 3H), 4.02 (m, 2H), 4.16 (m, 2H), 6.63 (dd, J = 2.4, 4.8 Hz, 1H), 6.76 (d, J = 4.8 Hz, 1H), 7.39 (s, 1H); ¹³C NMR (CDCl₃) δ 187.6, 159.6, 140.3, 135.4, 131.0, 127.9, 124.8, 106.2, 66.0, 33.4, 16.9, 12.9; MS m/z 230 (M⁺), 202, 158; HRMS for C₁₄H₁₄O₃ calcd 230.0942, found 230.0948; UV γ_max (methanol) 230 nm (e 6543), 330 (e 3484).

EXAMPLE II - Synthesis of Compound 42

Compound 36. To a solution of 27 (Example II) (37 mg, 0.18 mmol) in pyridine (3 ml) was added TESCl (0.25 ml, 0.624 mmol). The mixture was stirred at 60 °C for 0.5 h under N₂. After concentration and chromatography, it gave 50 mg of 36 (87%) as colorless oil: IR (KBr) 2952, 2872, 1703, 1622, 1461, 1414, 1226 cm^-1; ¹H NMR (CDCl₃) δ 0.58 (q, J = 7.8 Hz, 6H), 0.97 (m, 10H), 1.25 (m, 2H), 1.58 (m, 1H), 1.85 (m, 2H), 1.98 (s, 3H), 2.42 (m, 2H), 3.09 (b, 1H), 4.01 (d, J = 3 Hz, 1H); ¹³C NMR (CDCl₃) δ 206.0, 205.0, 147.0, 128.6, 72.6, 43.0, 39.6, 32.1, 21.4, 19.6, 18.0, 11.5, 6.5, 4.5; MS m/z 320 (M⁺), 291, 259, HRMS for C₁₈H₂₈O₃Si calcd 320.1808, found 320.1803.

Compound 37. The solution of 36 (278 mg, 0.869 mmol) and phenylseleninic anhydride (320 mg, 0.889 mmol) in chlorobenzene (2.5 ml) was stirred at 95 °C for 0.5 h under N₂. The mixture was then concentrated and chromatographed to give 58.7 mg of 36 and 131.2 mg of 37 (60.2%) as colorless gum: IR (KBr) 2952, 2878, 1730, 1690, 1636, 1454, 1240 cm^-1; ¹H NMR (CDCl₃) δ 0.52 (q, J = 7.8 Hz, 6H), 0.85 (t, J = 7.8 Hz, 9H), 1.20 (m, 1H), 1.36 (m, 1H), 1.69 (m, 1H), 1.82 (m, 1H), 2.06 (s, 3H), 3.58 (s, 1H), 4.26 (d, J = 2.4 Hz, 1H), 6.45 (dd, J = 2.1, 6 Hz, 1H), 7.33 (dd, J = 2.1, 6 Hz, 1H); ¹³C NMR (CDCl₃) δ 205.9, 195.3, 153.2, 144.3, 139.4, 127.7, 72.1, 47.3, 32.4, 20.1, 19.7, 11.4,6.4,4.4; MS m/z 318 (M⁺), 289, 261; HRMS for C₁₈H₂₆O₃Si calcd 318.1651, found 318.1658.

Compound 40. To a solution of 37 (9.5 mg, 0.0299 mmol), CeCl₃.7H₂O (58.5 mg, 0.157 mmol) in MeOH (0.3 ml) was added NaBH₄ (excess) at 25 °C. It was stirred for 30 min. Then the mixture was partitioned between Et₂O and saturated NH₄Cl. The ether extract was dried by MgSO₄ and concentrated to give crude product 38 as pale yellow oil.

To the solution of above 38 in CH₂Cl₂ (0.2 ml) was added Et₃N (5 ml, 0.036 mmol) and MsCl (5 ml, 0.965 mmol) at 25 °C. The mixture was stirred for 5 min and then separated between Et₂O and saturated NaHCO₃. Then the ether extract was washed by saline and dried by MgSO₄. After concentration, it was chromatographed to give 8.2 mg of 40 (90.3%) as yellow gum: IR (KBr) 3557, 3449, 2946, 2878, 1716, 1643, 1461, 1112 cm^-1; ¹H NMR (CDCl₃) δ 0.66 (q, J = 7.8 Hz, 6H), 0.87 (m, 2H), 0.98 (t, J = 7.8 Hz, 9H), 1.26 (m, 2H), 1.86 (s, 3H), 2.55 (d, J = 3.9 Hz, 1H), 3.24 (s, 1H), 4.94 (d, J = 2.1 Hz, 1H), 6.35 (m, 2H), 6.46 (m, 1H); ¹³C NMR (CDCl₃) δ 148.9, 140.0, 130.4, 117.8, 117.5, 77.0, 68.6, 61.9, 16.1,11.6, 7.8, 6.8, 5.0; MS m/z 304 (M⁺), 287, 275; HRMS for C₁₈H₂₈O₂Si calcd 304.1859, found 304.1860.

Compound 41. A solution of 40 (1.2 mg, 3.95 mmol) and Dess-Martin reagent (2.2 mg, 5.19 mmol) in CH₂Cl₂ (0.2 ml) was stirred for 30 min at 25 °C. The mixture was separated between Et₂O and 10% Na₂SO₃. Then the ether extract was washed by saline and dried by MgSO₄. After concentration, it was chromatographed to give 1.1 mg of 41 (92.3%) as yellow gum: IR (KBr) 2952, 2872, 1690, 1610, 1549, 1354, 1132 cm^-1; ¹H NMR (CDCl₃) δ 0.71 (q, J = 7.8 Hz, 6H), 0.85 (m, 1H), 0.97 (t, J = 7.8 Hz, 9H), 1.21 (m, 2H), 1.45 (m, 1H), 2.08 (s, 3H), 4.50 (s, 1H), 6.66 (dd, J = 2.4, 4.8 Hz, 1H), 6.72 (d, J = 5.1 Hz, 1H), 7.25 (s, 1H); ¹³C NMR (CDCl₃) δ 193.3, 161.2, 140.7, 131.8, 131.2, 128.3, 122.8, 32.9, 17.1, 12.5, 10.3, 6.9, 5.2; MS m/z 302 (M⁺), 273, 245; HRMS for C₁₈H₂₆O₂Si calcd 302.1702, found 302.1710; UV γ_max 227nm (e 15612), 323nm (e 10720).

Compound 42. To a solution of 41 (9.0 mg, 0.0298 mmol) in acetone (0.8 ml) and H₂O (0.4 ml) was added some p-TsOH. The mixture was stirred for 30 min. Then it was partitioned between Et₂O and saturated NaHCO₃. The ether extract was washed by saline and dried by MgSO₄. After concentration, it was chromatographed to give quantitative 42 as yellow gum: IR (KBr) 3449, 3013, 2925, 1663, 1609, 1441, 1367, 1260 cm^-1; ¹H NMR (CDCl₃) δ 0.81 (m, 1H), 1.25 (m, 1H), 1.36 (m, 1H), 1.44 (m, 1H), 2.12 (s, 3H), 3.82 (d, J = 2.4 Hz, 1H), 4.55 (d, J = 2.1 Hz, 1H), 6.70 (dd, J = 2.7, 5.1 Hz, 1H), 6.81 (t, 1H), 7.32 (s, 1H); ¹³C NMR (CDCl₃) δ 194.2, 162.2, 140.9, 132.7, 131.4, 126.5, 124.1, 74.6, 32.8, 17.0, 12.7, 10.3; MS m/z 188 (M⁺), 160, 145; HRMS for C₁₂H₁₂O₂ calcd 188.0837, found 188.0840; UV γ_max (methanol) 227 nm (e 13626), 323nm (e 7474).

The invention has been described with reference to various specific and preferred embodiments and techniques. However, it should be understood that many variations and modifications may be made while remaining within the scope of the invention.

Claims (18)

1. Compounds of formula I and pharmaceutically acceptable salts thereof: (wherein in formula I:

   (a) R' is (C₁-C₄) alkyl, R₁ and R₂ together are ethylene dioxy, and R₃ is H) or (b) R' is (C_1-4) alkyl, R₁ is OH and R₂, R₃ are both H.
2. A pharmaceutical composition comprising a compound of formula I as claimed in claim 1 or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier.
3. A compound of formula I as claimed in claim 1 or a pharmaceutically acceptable salt thereof for use in inhibiting tumor cell growth.
4. The use of a compound of formula I as claimed in claim 1 or a pharmaceutically acceptable salt thereof in the manufacture of a medicament for use in a method of inhibiting tumor cell growth.
5. A method of synthesizing compounds of formula I(a) as claimed in claim 1, said method comprising the steps of:

   (a) reducing the keto group in a compound of formula XX: (wherein R' is (C₁-C₄) alkyl, and R₁ and R₂ together are ethylene dioxy) to yield a hydroxy group under conditions that yield a compound of formula XXI: (wherein R' is (C₁-C₄) alkyl, R₁ and R₂ together are ethylene dioxy, and X is a removable hydroxyl protecting group);

   (b) eliminating the cyclopentenol hydroxyl group;

   (c) removing hydroxyl protecting group X to yield a compound of formula XXII: (wherein R' is (C₁-C₄) alkyl, and R₁ and R₂ together are ethylene dioxy); and

   (d) oxidizing the cyclohexanol hydroxyl group.
6. A method as claimed in claim 5, wherein the compound of formula XX is produced by:

   (a) protecting the hydroxyl group of a compound of formula XIX: (wherein R' is (C₁-C₄) alkyl, and R₁ and R₂ together are ethylene dioxy) with a removable hydroxyl protecting group X; and

   (b) introducing a double bond in the five-membered ring to yield a compound of formula XX.
7. A method as claimed in claim 6, wherein the compound of formula XIX is produced by converting the carbonyl group of a compound of formula XXV: (wherein R' is (C₁-C₄) alkyl) to an acetal group to yield a compound of formula XIX.
8. A method as claimed in any one of claims 5 to 7, wherein X is ((C₁-C₄)alkyl)₃Si-.
9. A compound of formula XIX, XX, XXI, XXII or XXV as defined in any one of claims 5 to 8.
10. Compounds of formula I as defined in claim 1 and pharmaceutically acceptable salts thereof wherein in formula I: R' is (C₁-C₄) alkyl, R₁ is OH, and R₂ and R₃ are H.
11. A pharmaceutical composition comprising a compound of formula I as claimed in claim 10 or a pharmaceutically acceptable salt thereof, together with a pharmaceutically acceptable carrier.
12. A compound of formula I as claimed in claim 10 or a pharmaceutically acceptable salt thereof for use in inhibiting tumor cell growth.
13. The use of a compound of formula I as claimed in claim 10 or a pharmaceutically acceptable salt thereof in the manufacture of a medicament for use in a method of inhibiting tumor cell growth.
14. A method of synthesizing compounds of formula I as claimed in claim 10, said method comprising the steps of:

    (a) reducing both keto groups of a compound of formula XV: (wherein R' is (C₁-C₄) alkyl, R₁ and R₂ together are a keto group, and X is a removable protecting group) to yield hydroxy groups under conditions that yield a compound of formula XVI: (wherein R' is (C₁-C₄) alkyl and X is a removable protecting group);

    (b) eliminating the cyclopentenol hydroxyl group; and

    (c) oxidizing the cyclohexanol hydroxyl group and removing hydroxyl protecting group X.
15. A method as claimed in claim 14, wherein the compound of formula XV is produced by:

    (a) protecting the hydroxyl group in a compound of formula XXV as defined in claim 7 with a removable hydroxyl protecting group X; and

    (b) introducing a double bond in the five-membered ring to yield a compound of formula XV.
16. A method as claimed in claim 15, wherein the compound of formula XXV is produced by:

    (a) cleaving the oxybridge in a compound of formula XIV: (wherein R' is (C₁-C₄) alkyl) to yield a diketone of formula XXV.
17. A method as claimed in any one of claims 14 to 16, wherein X is ((C₁-C₄)alkyl)₃Si-.
18. A compound of formula XV or XVI as defined in claim 14.