Total Synthesis and Biological Evaluation of an Antifungal Tricyclic o-Hydroxy-p-Quinone Methide Diterpenoid

Abstract

A convergent route has been developed to synthesize an antifungal tricyclic o-hydroxy-p-quinone methide diterpenoid and analogs. The Li/naphthalene mediated reductive alkylation was employed for coupling β-cyclocitral and the corresponding benzyl chloride while the BBr₃–mediated one-pot bis-demethylation and intramolecular Friedel Crafts alkylation was used to assemble the tricyclic molecular skeleton. The structure-activity relationship of the diterpenoid was assessed based on anti-proliferation assays of the natural product and analogs against strains of pathogenic yeasts and filamentous fungi.

INTRODUCTION

Quinone methides are typically reactive electrophilic species that are formed as transient intermediates in chemical transformations.¹ However, in the presence of stabilizing structural elements, such as extended conjugation, relatively stable quinone methides may be formed. Indeed, a number of o-hydroxy-p-quinone methide diterpenoids and triterpenoids, such as fuerstion (1), taxodione (2) and its dimer (4), pristimerin (5), celastrol (6), tingenone (7), many of which show interesting biological activities, have been isolated from Nature (Figure 1).² As part of our efforts in investigating natural products with useful pharmacological properties,³ we became interested in 3, an unnamed quinone methide diterpenoid isolated from the root bark of Bobgunnia madagascariensis.⁴ In particular, we were intrigued by its reported potent anti-fungal activity, which even rivals that of the clinical drugs amphotericin B and fluconazole. Our interest was partially fueled by the increasing demand for new antifungal drugs.⁵ The increasing incidence of invasive fungal infections associated with substantial mortality and high financial burden, especially among immunocompromised patient populations, rendered it increasingly important to develop new classes of drugs to combat fungal infection.

Figure 1.

Some o-hydroxy-p-quinone methide natural products

RESULTS AND DISCUSSION

Whereas a number of approaches have been reported for synthesis of taxodione and related compounds,⁶ many employing linear synthetic routes starting from natural precursors, no total synthesis had been reported for 3 since its isolation in 2000.^5a To fully explore the biomedical potential of 3, we envisioned a convergent synthetic approach that not only would enable access of the natural product, but also allow preparation of analogs for structure-activity relationship (SAR) and mode-of-action studies. In this regard, we envisioned that the o-hydroxy-p-quinone methide moiety of 3 could be prepared by oxidation of catechol 8 (Scheme 1). An intramolecular Friedel-Crafts alkylation would be relied upon to build the tricyclic core of 8. The corresponding precursor 9 would be assembled by coupling of benzyl chloride 10 and commercially available β-cyclocitral. Herein we describe the results of our study, which led to the first synthesis of (±)–3 and elucidation of its preliminary SAR.

Scheme 1.

Synthetic design

Our synthesis started from the known bromobenzene 11, available in 4 steps from commercial 3,4-dimethyoxytoluene (Scheme 2).⁷ Stille coupling of 11 and tributylvinyltin was envisioned for introducing the ethylene group as a precursor to the 2-hydroxylethyl functionality of 3. After extensive screening of reaction conditions, the coupling was achieved using Pd₂(dba)₃ and P(t-Bu)₃ to give 12 in 98% yield.⁸ The reduction of ester 12 with DIBAL-H yielded benzyl alcohol 13, which was converted to benzyl chloride 14 upon treatment with SOCl₂. Whereas complex mixtures were obtained when Li/naphthalene was used for coupling benzyl chloride 14 and β-cyclocitral,^6c no desired product (i.e. 15) could be isolated from our attempts of coupling through the corresponding Grignard and organolithium reagents of 14.

Scheme 2.

Synthesis of 14

We speculated that the extended conjugation of 14 might be responsible for the difficulty of the coupling. Thus, we explored the possibility of furnishing the 2-hydroxylethyl group prior to the coupling reactions. However, attempts to introduce the 2-hydroxylethyl group of 17 through hydroboration/oxidation of 12 was only partially successful as the undesired regio-isomer (i.e. 16) was formed as the major product under all the conditions except when BH₂I·Me₂S was used,⁹ which gave the desired regio-isomeric product 17 somewhat preferentially (2:1) (Scheme 3).

Scheme 3.

Hydroboration/oxidation of 12

In order to improve the efficiency of the synthesis, we turned to the allyl-substituted benzyl chloride 19, which was expected to be compatible with the coupling reaction (Scheme 4). The previously employed Stille reaction conditions proved to be also effective for coupling of 11 and allyltributyltin to give 18. DIBAL-H reduction followed by chlorination with SOCl₂ converted 18 into benzyl chloride 19. As we had expected, the coupling of benzyl chloride 19 and β-cyclocitral went smoothly to give 20 (79% yield), which was converted to α, β-unsaturated ketone 21 through oxidation with IBX. After oxidative cleavage of the alkene of 21 through OsO₄-catalyzed dihydroxylation and subsequent oxidative cleavage of the vicinal diol with NaIO₄, aldehyde 22 was obtained in 85% yield. Selective reduction of the aldehyde in the presence of α, β-unsaturated ketone was achieved with NaBH(OAc)₃ to give primary alcohol 23.

Scheme 4.

Synthesis of 23

The generation of the tricyclic molecular skeleton of 3 by the intramolecular Friedel-Crafts alkylation reaction is challenging because it requires simultaneous dearomatization and the formation of an all-carbon quaternary center. Thus, to evaluate the feasibility and to elucidate the nuance of this transformation, we used 25 as a model substrate to test various conditions for the cyclization. This substrate (i.e. 25) was synthesized through Li/naphthalene mediated coupling of the known benzyl chloride 24 and β-cyclocitral followed by oxidation with IBX (Scheme 5).^6j

Scheme 5.

Synthesis of 25

A number of achiral and chiral Brønsted/Lewis acids were evaluated for the intramolecular Friedel-Crafts alkylation of 25. The combination of formic acid and phosphoric acid was reported to be suitable for cyclizing 25 into 26 in 41% yield at 115 °C (not shown).^6f Interestingly, extended reaction under such conditions led to formation of the undesired cis-isomeric 28 as the major product (Table 1, entry 1), suggesting its thermodynamic nature. The BINOL-based chiral phosphoric acid 31c was also effective for the cyclization (entry 2), but no enantioselectivity was observed under the harsh conditions necessary for the reaction to proceed. A number of Lewis acids were also tested (see Supporting Information for a complete list). Treatment of 25 with BBr₃ led to one-pot bis-demethylation and the intramolecular Friedel-Crafts alkylation to give 27 (entry 3),¹⁰ which could be oxidized with Ag₂O to give taxodione (i.e. 2, not shown).^6a On the other hand, the starting material was recovered when 25 was treated with BF₃·Et₂O (entry 4). Surprisingly, the one-pot demethylation and Friedel-Crafts cyclization of 25 with TMSI led to diastereoselective formation of the cis-isomeric product 30, with only one of the methoxyl groups hydrolyzed (entry 5). In order to impose enantioselection during the cyclization, the oxazaborolidine-based Lewis acid 32 was also used.¹¹ However, the cyclization proceeded with low yield and low enantioselectivity (entry 6). A series of chiral Brønsted and Lewis acidic conditions were also tested (see Supporting Information). However, no enantioselection was observed for the cyclization.

Table 1.

The intramolecular Friedel-Crafts alkylation of 25

entry	conditions	T (°C)	time	product	% yield (ee)
1	HCO2H, H3PO4	115	12 h	26+28	18 + 38
2	HCO2H, 31c	110	96 h	26+28	43 + 14
3	BBr3, CH2Cl2	−78 to 0	30 min	27	45
4	BF3·Et2O, CH2Cl2	−78 to rt	2h	27	0
5	TMSI, toluene	70	4h	30	54
6	32, BBr3, CH2Cl2	0	20 min	27	20 (< 20)

The application of the BBr₃-mediated one-pot reaction conditions allowed the completion of the synthesis of 3. Thus, treatment of 23 with BBr₃ led to one-pot bis-demethylation and cyclization to form a tricyclic catechol, which was subjected to oxidation with Ag₂O to give 3 (Scheme 6). Comparison of the ¹H, ¹³C NMR and MS spectra with those reported for the natural product confirmed the identity of synthetic 3. Selective mono-etherificaiton of 3 with dimethoxymethane gave analog 33.

Scheme 6.

Synthesis of 3 and 33

To evaluate the effect of the 2-hydroxyethyl group of 3 over the antifungal activity, we also prepared analog 37 from the known carboxylic acid 34⁷ (Scheme 7). Thus, reduction of 34 with borane dimethyl sulfide complex followed by chlorination with SOCl₂ provided benzyl chloride 35. The coupling of 35 and β-cyclocitral went smoothly with Li/naphthalene to give the corresponding allyl alcohol, which was oxidized with IBX to give enone 36. Compound 37 was obtained by BBr₃-mediated one-pot bis-demethylation and intramolecular Friedel-Crafts alkylation followed by oxidation of the resulting catechol with Ag₂O.

Scheme 7.

Synthesis of 37

With the compounds synthesized above in hand, we evaluated their anti-fungal activity against strains of pathogenic yeasts (Candida spp. and Cryptococcu spp.) and the mold Aspergillus fumigatus. Candida, Cryptococcus, and Aspergillus species are the major fungal pathogens of global significance (Table 2).¹² The strains were chosen because of existing data on their susceptibility to fluconazole or itraconazole,^13,14 two azole drugs that are commonly used in clinic to treat infections caused by yeast and mold respectively. Consistent with the original report, the synthetic o-hydroxy-p-quinone methide 3 indeed was found to be potently cytotoxic towards all the yeast strains tested, with MIC₁₀₀ values ranging from 0.4 to 6.4 mg/L. Interestingly, the values of MIC₁₀₀ were found to be similar to those of MFC (minimal fungicidal concentration). By contrast, MFCs are often much higher than MICs for fungistatic drugs such as fluconazole (Table 2). This finding indicates the fungicidal rather than fungistatic nature of these compounds. Compounds 3 and 37 are particularly effective against all the yeast strains, including the Candida glabrata, Candida krusei, and Candida parapsilosis strains that are resistant to the commonly used antifungal fluconazole (Table 2).¹⁴ The lack of any apparent correlation between the fungal susceptibility against these new compounds and that against azole drugs suggests that these compounds might differ in their mode of action against fungi from the known drug fluconazole. Taxodione (2) showed potent activity as well. However, the methoxymethyl ether of 3 (i.e. 33) significantly diminished the growth inhibition activity. None of these compounds were found to be effective against the mold A. fumigatus. It appears that the cytotoxicity of these compounds is correlated with the fungal growth mode (yeast versus mold) rather than their evolutionary relatedness, given that Candida (yeast) and Aspergillus (mold) spp. are closely related and their common ancestors diverged from that of Cryptococcus (yeast) spp. about one billion years ago. The cause of the differential cytotoxicity of these compounds against yeasts and the filamentous fungal stains is currently unknown.

Table 2.

Taxodione (2), 3, and analogs exhibit fungicidal effect (mg/L)^*

Yeast Strains	MIC100	MFC*	MIC100	MFC	MIC100	MFC	MIC100	MFC	MIC90	MFC
	3	3	37	37	2	2	33	33	Azole	Azole
Cryptococcus gattii R265	0.4	0.4	0.8	0.8	0.8	0.8	6.4	12.8	4	-
Cryptococcus neoformans H99α	0.4	0.8	0.8	0.8	1.6	1.6	6.4	12.8	1	8
Candida albicans SC5314	6.4	12.8	3.2	6.4	6.4	>12.8	>12.8	>100	0.2	64
Candida glabrata PAT2ISO3	1.6	6.4	1.6	3.2	3.2	>6.4	6.4	>12.8	8	>64
Candida krusei DUMC132.91	1.6	1.6	1.6	3.2	3.2	6.4	12.8	>12.8	64	64
Candida parapsilosis MMRL1594	3.2	6.4	1.6	3.2	6.4	12.8	12.8	>100	>64	>64
Aspergillus fumigatus Af293	25.6	25.6	17.1	17.1	>100	>100	100	>100	0.4	12.8
Aspergillus fumigatus CEA10	25.6	25.6	17.1	17.1	>100	>100	100	>100	0.5	-

^*MFC is defined as the lowest drug concentration at which at least 99% of cells were killed compared to the original inocula. Suspensions from the microdilution assays after 24 h (for Candida species and Aspergillus fumigatus strains) or 48 h (for strains of Cryptococcus species) of incubation were plated on drug-free medium for obtaining the values of colony forming units (CFU) to determine the minimal fungicidal concentration (MFC). Successive 2× serial dilutions of the compounds were used. The data on the susceptibility of these strains against azole drugs (itraconazole for the mold A. fumigatus and fluconazole for all yeast species) were obtained from previous studies.^13,14,15

CONCLUSION

In summary, we completed the first total synthesis of the antifungal tricyclic o-hydroxy-p-quinone methide diterpenoid 3. The Stille reaction was used to introduce the allyl group as a masked 2-hydroxyethyl side chain whereas the Li/naphthalene mediated reductive alkylation was employed for coupling benzyl chloride 19 and β-cyclocitral. We developed a BBr₃-mediated one-pot bis-demethylation/intramolecular Friedel-Crafts alkylation to assemble the tricyclic molecular skeleton and completed the synthesis of 3. Taxodione 2 and analogs of 3 were also synthesized. Compounds 3, 37, and 2 showed potent cytotoxicity against various strains of pathogenic yeasts tested, but etherification of the 2-hydroxyethyl led to significantly attenuated activity. Surprisingly, these compounds were found to be ineffective against the fungal strain Aspergillus fumigatus Af293. The cause of this discrepancy of activity against the yeasts versus filamentous fungi is currently unknown.

EXPERIMENTAL SECTION

General Information

All moisture sensitive reactions were carried out in flame-dried flasks under nitrogen atmosphere. Dichloromethane and diethyl ether were dried using an activated molecular sieve solvent purification system. Tetrahydrofuran was freshly distilled over sodium and benzophenone. All other commercial reagents were used as received. Reactions were monitored by TLC performed on pre-coated glass-backed TLC plates, Silica Gel 60 F254 (EMD 250 µm thickness). Spots were visualized with UV or through staining with an ethanolic solution of phosphomolybdic acid. Flash column chromatography was performed using 60 Å silica gel (230–400 mesh) as the stationary phase. ¹H NMR chemical shifts are reported as δ values in ppm relative to CDCl₃ (7.26 ppm) or acetone-d₆ (2.05 ppm), coupling constants (J) are reported in Hertz (Hz), and multiplicity follows normal convention. CDCl₃ (77.0 ppm) or acetone-d₆ (29.84 ppm) served as the internal standard for ¹³C NMR spectra. Infrared spectra (IR) resonance frequencies are given as wavenumbers in cm⁻¹. HRMS spectra were recorded using a tandem TOF spectrometer.

Methyl 4,5-dimethoxy-2-methyl-3-vinylbenzoate (12)

To a suspension of ester 11 (1.57 g, 5.45 mmol), Pd₂dba₃ (113 mg, 0.11mmol), tributylvinyltin (1.75 mL, 6.0 mmol) in toluene (11 mL) was added P(t-Bu)₃ (0.22 mL, 0.22 mmol, 1 M in toluene) under nitrogen. The reaction was stirred at 60 °C overnight. When the reaction was complete based on TLC, the solution was treated with KF (4 g), Et₂O (30 mL), and activated carbon (4 g). The mixture was stirred for 5 min before it was filtered through a short pad of silica gel (washed with ethyl acetate). After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/5) to provide 12 (1.26 g, 98 %) as yellow oil. IR (film, cm⁻¹) 2999, 2955, 2842, 1723, 1590, 1478, 1427, 1324, 1199, 1167, 1110, 983; ¹H NMR (500 MHz, CDCl₃) δ 7.32 (s, 1H), 6.71 (dd, J = 17.9, 11.6 Hz, 1H), 5.61 (dd, J = 11.6, 2.0 Hz, 1H), 5.52 (dd, J = 17.9, 2.0 Hz, 1H), 3.90 (s, 3H), 3.89 (s, 3H), 3.79 (s, 3H), 2.47 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 168.4, 150.0, 149.7, 133.8, 131.2, 131.1, 126.3, 121.1, 112.9, 60.3, 55.9, 52.0, 17.6; HRMS: molecular ion not observed.

(4,5-Dimethoxy-2-methyl-3-vinylphenyl)methanol (13)

To a stirred solution of ester 12 (1.24 g, 5.25 mmol) in THF (30 mL) was added DIBAL-H (1 M in hexanes, 15.75 mL, 15.75 mmol) at 0 °C under nitrogen. The mixture was stirred at 0 °C for 2 h before it was quenched with ethyl acetate (3 mL) and saturated aqueous Rochelle salt (30 mL). The mixture was stirred for another 1 h at room temperature. The organic layer was separated and aqueous phase was extracted with ethyl acetate. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/1) to provide 13 (992 mg, 91 %) as colorless oil. IR (film, cm⁻¹) 2934, 1587, 1475, 1318, 1235, 1116, 974, 921, 944; ¹H NMR (500 MHz, CDCl₃) δ 6.92 (s, 1H), 6.74 (dd, J = 17.9, 11.7 Hz, 1H), 5.58 (dd, J = 11.7, 2.1 Hz, 1H), 5.54 (dd, J = 17.9, 2.1 Hz, 1H), 4.67 (s, 2H), 3.86 (s, 3H), 3.75 (s, 3H), 2.25 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 150.4, 146.3, 134.6, 132.9, 131.5, 126.6, 120.5, 111.2, 63.9, 60.2, 55.9, 15.1; HRMS (ESI): calculated for C₁₂H₁₆O₃ [M+Li⁺] 215.1259, found 215.1266.

1-(Chloromethyl)-4,5-dimethoxy-2-methyl-3-vinylbenzene (14)

To a solution of alcohol 13 (832 mg, 4.0 mmol) in CH₂Cl₂ (20 mL) was added SOCl₂ (0.35 mL, 4.8 mmol) dropwise at 0 °C under nitrogen. The reaction was stirred at room temperature for 30 min before it was carefully quenched with saturated aqueous NaHCO₃ and taken into CH₂Cl₂. The organic layer was separated, washed with water, brine, and dried over Na₂SO₄. After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/20) to provide 14 (832 mg, 92 %) as yellow oil. IR (film, cm⁻¹) 3005, 2970, 2934, 2837, 1590, 1478, 1318, 1238, 1116, 1045, 980, 927, 853; ¹H NMR (500 MHz, CDCl₃) δ 6.84 (s, 1H), 6.73 (dd, J = 17.9, 11.7 Hz, 1H), 5.60 (dd, J = 11.7, 2.0 Hz, 1H), 5.54 (dd, J = 17.9, 2.1 Hz, 1H), 4.62 (s, 2H), 3.88 (s, 3H), 3.76 (s, 3H), 2.34 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 150.5, 147.4, 133.4, 131.3, 131.2, 128.2, 120.7, 113.0, 60.2, 55.9, 45.8, 15.4; HRMS: molecular ion not observed.

Ethyl 3-allyl-4,5-dimethoxy-2-methylbenzoate (18)

To a suspension of ester 11 (2.88 g, 10 mmol), Pd₂dba₃ (275 mg, 0.30 mmol), and allyltributyltin (3.72 mL, 12 mmol) in toluene (15 mL) was added P(t-Bu)₃ (1 M in toluene, 0.60 mL, 0.60 mmol) under nitrogen. The reaction was stirred at 60 °C for overnight. When the reaction was complete (monitored by TLC, ethyl acetate/petroleum ether = 1:10 for 2~3 times), KF (5 g) was added followed by Et₂O (30 mL) and activated carbon (5 g). The mixture was stirred for 5 min, and then it was filtered through a short pad of silica (washed with ethyl acetate). After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/5) to provide 18 (2.59 g, 98%) as yellow oil. IR (film, cm⁻¹) 2949, 2842, 1717, 1596, 1484, 1330, 1211, 1099, 1045, 989, ¹H NMR (500 MHz, CDCl₃) δ 7.29 (s, 1H), 5.95-5.89 (m, 1H), 5.00 (dd, J = 10.2, 1.8 Hz, 1H), 4.86 (dd, J = 17.1, 1.8 Hz, 1H), 3.88 (s, 3H), 3.87 (s, 3H), 3.83 (s, 3H), 3.49 (dt, J = 5.6, 1.8 Hz, 2H), 2.41 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 168.6, 150.2, 150.0, 136.0, 133.3, 132.1, 126.1, 115.1, 112.2, 60.9, 55.7, 51.9, 30.8, 16.1; HRMS (ESI): calculated for C₁₄H₁₈O₄ [M+H⁺] 251.1283, found 251.1274.

3-Allyl-1-(chloromethyl)-4,5-dimethoxy-2-methylbenzene (19)

To a solution of ester 18 (2.50 g, 10 mmol) in THF (50 mL) was added DIBAL-H (1 M in hexanes, 30 mL, 30 mmol) at 0 °C under nitrogen. The mixture was stirred at 0 °C for 4 h before it was treated with ethyl acetate (5 mL) and saturated aqueous Rochelle salt (50 mL). The mixture was stirred for another 1 h at room temperature. The organic layer was separated and the aqueous phase was extracted with ethyl acetate. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/1) to provide allyl alcohol S1 (2.01 g, 90 %) as colorless oil. IR (film, cm⁻¹) 3414, 2937, 1640, 1599, 1487, 1309, 1232, 1111, 1042, 906, 850; ¹H NMR (500 MHz, CDCl₃) δ 6.87 (s, 1H), 5.92 (ddt, J = 17.1, 10.1, 5.8 Hz, 1H), 4.99 (dd, J = 10.2, 1.8 Hz, 1H), 4.90 (dd, J = 17.1, 1.9 Hz, 1H), 4.64 (d, J = 4.5 Hz, 2H), 3.84 (s, 3H), 3.78 (s, 3H), 3.47 (dt, J = 5.8, 1.8 Hz, 2H), 2.18 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 150.4 146.7, 136.5, 134.4, 132.6, 127.5, 115.0, 110.5, 63.9, 60.9, 55.7, 30.9, 13.9; HRMS (ESI): calculated for C₁₃H₁₈O₃ [M+Li⁺] 229.1416, found 229.1406.

To a solution of S1 (1.85 g, 8.32 mmol) in CH₂Cl₂ (40 mL) was added SOCl₂ (0.73 mL, 9.99 mmol) dropwise at 0 °C under nitrogen. The reaction was maintained at room temperature for 30 min before it was carefully quenched with saturated aqueous NaHCO₃. The mixture was taken into CH₂Cl₂. The organic layer was separated, washed with water, brine, and dried over Na₂SO₄. After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/10) to provide 19 (1.93 g, 97 %) as white amorphous solid. IR (film, cm⁻¹) 2940, 1602, 1484, 1330, 1226, 1108, 1045, 986, 912; ¹H NMR (500 MHz, CDCl₃) δ 6.79 (s, 1H), 5.92 (ddt, J = 17.1, 10.2, 5.8 Hz, 1H), 5.01 (dd, J = 10.2, 1.8 Hz, 1H), 4.89 (dd, J = 17.1, 1.8 Hz, 1H), 4.60 (s, 2H), 3.86 (s, 3H), 3.80 (s, 3H), 3.48 (dt, J = 5.8, 1.8 Hz, 2H), 2.26 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 150.5, 147.7, 136.2, 133.1, 131.0, 129.1, 115.1, 112.2, 60.9, 55.7, 45.9, 31.0, 14.3; HRMS (ESI): calculated for C₁₃H₁₇ClO₂ [M+H⁺] 241.0995, found 241.1003.

2-(3-Allyl-4,5-dimethoxy-2-methylphenyl)-1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethanol (20)

A mixture of naphthalene (3.61 g, 28.13 mmol) and lithium (196 mg, 28.13 mmol) in THF (30 mL) was stirred at room temperature under nitrogen for 1.5 h. The mixture was then treated with a solution of β-cyclocitral (1.26 g, 8.25 mmol) and benzyl chloride 19 (1.80 g, 7.50 mmol) in THF (10 mL) dropwise via syringe at 0 °C. After stirring at room temperature for 2 h, the mixture was diluted with ether and treated with saturated aqueous NH₄Cl. The organic phase was separated. The aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/4) to provide 20 (2.13 g, 79%) as colorless liquid. IR (film, cm⁻¹) 3500, 2931, 1593, 1484, 1291, 1229, 1111, 1048, 989, 906; ¹H NMR (500 MHz, CDCl₃) δ 6.70 (s, 1H), 5.94 (dd, J = 17.1, 10.1 Hz, 1H), 5.01 (dq, J = 10.1, 1.7 Hz, 1H), 4.93 (dq, J = 17.1, 1.8 Hz, 1H), 4.45 (dd, J = 10.0, 4.3 Hz, 1H), 3.87 (s, 3H), 3.80 (s, 3H), 3.49 (ddd, J = 5.8, 3.9, 1.9 Hz, 2H), 3.19 (dd, J = 14.2, 10.0 Hz, 1H), 2.88 (dd, J = 14.2, 4.3 Hz, 1H), 2.24 (s, 3H), 2.03-1.99 (m, 2H); 2.02 (s, 3H), 1.65-1.54 (m, 2H), 1.46-1.44 (m, 2H), 1.11 (s, 3H), 0.93 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 150.3, 146.0, 139.3, 136.7, 133.3, 132.4, 132.0, 128.3, 114.9, 113.0, 71.2, 60.9, 55.8, 40.5, 40.0, 34.8, 34.2, 31.2, 28.5, 28.3, 21.4, 19.3, 15.2; HRMS (ESI): calculated for C₂₃H₃₄O₃ [M+Na⁺] 381.2406, found 381.2388.

2-(3-Allyl-4,5-dimethoxy-2-methylphenyl)-1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethanone (21)

A solution of alcohol 20 (910 mg, 2.54 mmol) and IBX (1.07 g, 3.81 mmol) in DMSO (5 mL) was stirred at room temperature for 3 h. The reaction was quenched with H₂O (5 mL) at 0 °C. The mixture was filtered and the aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration in vacuo, the residue was purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/1) to provide 21 (780 mg, 86%) as yellow oil. IR (film, cm⁻¹) 2937, 1691, 1596, 1487, 1285, 1226, 1113, 1048, 995; ¹H NMR (500 MHz, CDCl₃) δ 6.56 (s, 1H), 5.93 (ddt, J = 17.1, 10.2, 5.7 Hz, 1H), 4.99 (dq, J = 10.1, 1.7 Hz, 1H), 4.91 (dq, J = 17.1, 1.9 Hz, 1H), 3.87 (s, 2H), 3.82 (s, 3H), 3.78 (s, 3H), 3.48 (dt, J = 5.7, 1.8 Hz, 2H), 2.11 (s, 3H), 1.99 (t, J = 6.5 Hz, 2H), 1.72-1.67 (m, 2H), 1.63 (s, 3H), 1.48-1.46 (m, 2H), 1.11 (s, 6H); ¹³C NMR (125 MHz, CDCl₃) δ 207.8, 150.3, 146.4, 143.1, 136.6, 132.4, 129.5, 129.1, 128.4, 114.9, 112.9, 60.8, 55.8, 50.8, 39.0, 33.4, 31.3, 31.2, 28.8, 21.2, 18.9, 15.4; HRMS (ESI): calculated for C₂₃H₃₂O₃ [M+H⁺] 357.2430, found 357.2423.

2-(2,3-Dimethoxy-6-methyl-5-(2-oxo-2-(2,6,6-trimethylcyclohex-1-en-1-yl)ethyl)phenyl)acetaldehyde (22)

To a solution of alkene 21 (530 mg, 1.49 mmol) in acetone/H₂O (4/1, 15 mL) was added NMO (262 mg, 2.23 mmol) and OsO₄ (2.5 wt% solution in t-BuOH, 758 mg, 7.45% mmol). After being stirred at room temperature overnight, the mixture was concentrated in vacuo. The residue was taken into EtOAc, washed with H₂O, brine, and dried over Na₂SO₄. After concentration in vacuo, the reaction crude was dissolved in CH₃CN/H₂O (1/1, 16 mL) and treated with NaIO₄ (478 mg, 2.24 mmol) at 0 °C. The mixture was allowed to room temperature and stirred for 1 h before it was filtered through a short pad of silica gel (eluted with EtOAc). The filtrate was washed with H₂O, brine, and dried over Na₂SO₄. After concentration in vacuo, the residue was purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/5) to provide 22 (446 mg, 84% for 2 steps) as white amorphous solid. IR (film, cm⁻¹) 2937, 1720, 1697, 1484, 1279, 1229, 1108, 1066; ¹H NMR (500 MHz, CDCl₃) δ 9.68 (s, 1H), 6.62 (s, 1H), 3.89 (s, 2H), 3.84 (s, 3H), 3.81 (d, J = 2.0 Hz, 2H), 3.78 (s, 3H), 2.07 (s, 3H), 2.00 (t, J = 6.5 Hz, 2H), 1.72-1.66 (m, 2H), 1.64 (s, 3H), 1.49-1.46 (m, 2H), 1.12 (s, 6H); ¹³C NMR (125 MHz, CDCl₃) δ 207.6, 199.7, 150.2, 146.6, 143.0, 129.7, 129.2, 128.8, 125.7, 114.2, 60.5, 55.8, 50.8, 42.7, 38.9, 33.5, 31.3, 28.8, 21.2, 18.8, 16.1; HRMS (ESI): calculated for C₂₂H₃₀O₄ [M+Li⁺] 365.2304, found 365.2312.

2-(3-(2-Hydroxyethyl)-4,5-dimethoxy-2-methylphenyl)-1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethanone (23)

To a solution of aldehyde 22 (423 mg, 1.18 mmol) in benzene (10 mL) was added NaBH(OAc)₃ (376 mg, 1.77 mmol) and acetic acid (0.70 mL). After being stirred at room temperature under nitrogen for 6 h, the reaction was quenched with saturated aqueous NaHCO₃ at 0 °C. The aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration in vacuo, the residue was purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/1) to provide 23 (370 mg, 87%) as white amorphous solid. IR (film, cm⁻¹) 3446, 2934, 1694, 1596, 1492, 1300, 1229, 1116, 1048; ¹H NMR (500 MHz, CDCl₃) δ 6.55 (s, 1H), 3.88 (s, 2H), 3.83 (s, 3H), 3.82 (s, 3H), 3.77 (t, J = 6.7 Hz, 2H), 3.00 (t, J = 6.8 Hz, 2H), 2.16 (s, 3H), 2.00 (t, J = 6.4 Hz, 2H), 1.72-1.67 (m, 2H), 1.64 (s, 3H), 1.49-1.46 (m, 2H), 1.12 (s, 6H); ¹³C NMR (125 MHz, CDCl₃) δ 207.8, 150.1, 146.5, 143.0, 131.1, 129.6, 128.8, 128.8, 113.1, 62.7, 60.5, 55.7, 50.9, 38.9, 33.5, 31.3, 30.7, 28.8, 21.2, 18.8, 15.7; HRMS (ESI): calculated for C₂₂H₃₂O₄ [M+Li⁺] 367.2461, found 367.2469.

2-(3-Isopropyl-4,5-dimethoxyphenyl)-1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethanone (25)

A mixture of naphthalene (4.92 g, 38.35 mmol) and lithium (266 mg, 38.35 mmol) in THF (40 mL) was stirred at room temperature for 1.5 h before it was treated with a solution of β-cyclocitral (1.28 g, 8.44 mmol) and benzyl chloride 24 (1.75 g, 7.67 mmol) in THF (10 mL) dropwise at 0 °C. The resulting mixture was stirred at room temperature for 2 h, diluted with ether and then treated with saturated aqueous NH₄Cl. The organic phase was separated and the aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/5) to provide allyl alcohol S2 (2.26 g, 85%) as light yellow liquid. ¹H NMR (500 MHz, CDCl₃) δ 6.70 (d, J = 1.9 Hz, 1H), 6.66 (d, J = 1.9 Hz, 1H), 4.42 (dd, J = 10.1, 3.6 Hz, 1H), 3.87 (s, 3H), 3.80 (s, 3H), 3.34 (dt, J = 13.9, 6.9 Hz, 1H), 3.06 (dd, J = 13.9, 10.1 Hz, 1H), 2.82 (dd, J = 13.9, 3.5 Hz, 1H), 2.05-1.94 (m, 2H), 1.96 (s, 3H), 1.62-1.53 (m, 2H), 1.46-1.42 (m, 2H), 1.23 (s, 3H), 1.22 (s, 3H), 1.10 (s, 3H), 0.97 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 152.5, 144.9, 142.4, 138.9, 135.5, 131.8, 118.9, 110.7, 72.3, 60.9, 55.7, 43.4, 39.9, 34.8, 34.1, 28.6, 28.1, 26.8, 23.6, 23.5, 21.4, 19.3.

To a solution of S2 (370 mg, 1.07 mmol) in DMSO (4 mL) was added IBX (449 mg, 1.60 mmol) at room temperature. The resulting mixture was stirred at room temperature for 3 h before it was treated with H₂O (5 mL) at 0 °C. The mixture was filtered and the aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration, the residue was purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/5) to provide 25 (285 mg, 77%) as yellow amorphous solid. ¹H NMR (500 MHz, CDCl₃) δ 6.66 (d, J = 2.0 Hz, 1H), 6.64 (d, J = 2.0 Hz, 1H), 3.85 (s, 3H), 3.80 (s, 3H), 3.79 (s, 2H), 3.33 (dt, J = 13.9, 6.9 Hz, 1H), 1.99-1.96 (m, 2H), 1.69-1.67 (m, 2H), 1.56 (s, 3H), 1.47-1.44 (m, 2H), 1.20 (s, 3H), 1.19 (s, 3H), 1.08 (s, 6H); ¹³C NMR (125 MHz, CDCl₃) δ 152.3, 145.1, 143.1, 142.0, 129.7, 129.6, 119.7, 111.3, 60.9, 55.7, 52.2, 38.9, 33.4, 31.2, 28.8, 26.7, 23.5, 21.2, 18.8.

o-Hydroxy-p-quinone methide diterpenoid 3

To a solution of alcohol 23 (301 mg, 0.84 mmol) in dichloromethane (8 mL) was added a freshly solution of BBr₃ (1 M in dichloromethane, 5.04 mL, 5.04 mmol) dropwise at −78 °C under nitrogen. The reaction was stirred for 30 min, allowed to 0 °C and stirred for another 2 h. The reaction was carefully quenched with saturated aqueous NaHCO₃. The aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration in vacuum, the residue was taken into CHCl₃ (10 mL) and treated with Ag₂O (223 mg, 0.90 mmol). The mixture was stirred at 50 °C for 30 min and filtered through a cotton plug. The filtrate was concentrated and purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/1) to provide 3 (103 mg, 37% for 2 steps) as orange amorphous solid. IR (film, cm⁻¹) 3325, 2925, 1670, 1610, 1377, 1223, 1185, 1146, 1045; ¹H NMR (500 MHz, acetone-d₆) δ 8.32 (s, 1H), 6.50 (s, 1H), 3.75 (t, J = 5.7 Hz, 1H), 3.63-3.59 (m, 2H), 3.05-3.02 (m, 1H), 2.85-2.81 (m, 2H), 2.65 (s, 1H), 2.34 (s, 3H), 1.76-1.70 (m, 2H), 1.59-1.54 (m, 1H), 1.40-1.36 (m, 1H), 1.29-1.26 (m, 1H), 1.27 (s, 3H), 1.24 (s, 3H), 1.10 (s, 3H); ¹H NMR (500 MHz, acetone-d₆+D₂O) δ 6.52 (s, 1H), 3.59 (t, J = 7.1 Hz, 2H), 3.03 (d, J = 11.8 Hz, 1H), 2.83 (t, J = 7.2 Hz, 2H), 2.66 (s, 1H), 2.33 (s, 3H), 1.73-1.66 (m, 2H), 1.56-1.53 (m, 1H), 1.37 (d, J = 11.7 Hz, 1H), 1.26 (s, 3H), 1.23 (s, 3H), 1.10 (s, 3H); ¹³C NMR (125 MHz, acetone-d₆) δ 201.1, 182.3, 146.4, 145.1, 142.0, 134.8, 131.3, 127.0, 62.3, 61.2, 43.0, 42.9, 37.7, 33.4, 33.3, 31.0, 22.2, 21.6, 19.3, 15.9; HRMS (ESI): calculated for C₂₀H₂₆O₄ [M-H⁺] 329.1753, found 329.1738.

(4bS,8aS)-4-Hydroxy-2-(2-(methoxymethoxy)ethyl)-1,4b,8,8-tetramethyl-4b,5,6,7,8,8a-hexahydrophenanthrene-3,9-dione (33)

To a solution of alcohol 3 (9.9 mg, 0.03 mmol) in dichloromethane (2 mL) was added dimethyoxymethane (0.027 mL, 0.3 mmol) and P₂O₅ (43 mg, 0.15 mmol). The solution was stirred at room temperature for 2 h before it was diluted with dichloromethane, washed with saturated aqueous NaHCO₃, and then dried over Na₂SO₄. After concentration, the residue was purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/5) to provide 33 (7.4 mg, 66%) as yellow liquid. IR (film, cm⁻¹) 3322, 2934, 1670, 1611, 1380, 1152, 1116, 1025; ¹H NMR (500 MHz, CDCl₃) δ 7.53 (s, 1H), 6.51 (s, 1H), 4.60 (s, 2H), 3.62 (t, J = 6.9 Hz, 2H), 3.33 (s, 3H), 2.98-2.95 (m, 1H), 2.93-2.88 (m, 2H), 2.59 (s, 1H), 2.29 (s, 3H), 1.74-1.70 (m, 2H), 1.62-1.59 (m, 2H), 1.44-1.40 (m, 1H), 1.28 (s, 3H), 1.26 (s, 3H), 1.13 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 201.2, 181.5, 146.1, 143.7, 141.1, 133.5, 131.1, 126.6, 96.3, 65.9, 62.2, 55.2, 42.6, 42.4, 37.1, 33.2, 32.7, 27.2, 21.9, 21.7, 18.6, 15.8; HRMS (ESI): calculated for C₂₂H₃₀O₅ [M+Li⁺] 381.2248, found 381.2262.

(4bS,8aR)-4-Hydroxy-2-isopropyl-3-methoxy-4b,8,8-trimethyl-4b,5,6,7,8,8a-hexahydro phenanthren-9(10H)-one (30)

A mixture of hexamethyldisilizane (0.25 mL, 1.2 mmol) and iodine (305 mg, 1.2 mmol) was heated at 65 °C for 20 min under nitrogen. After cooling down to room temperature, a solution of ketone 25 (69 mg, 0.2 mmol) in toluene (1 mL) was added. The reaction mixture was stirred at 70 °C for 4 h before it was concentrated in vacuo. The residue was purified by flash column chromatography (silica gel, ethyl acetate/petroleum ether, 1/5) to provide 30 (35.9 mg, 54 %) as yellow amorphous solid. IR (film, cm⁻¹) 3414, 2955, 2931, 1694, 1448, 1418, 1300, 1229, 1057, 1033, 998; ¹H NMR (500 MHz, CDCl₃) δ 6.67 (s, 1H), 5.77 (s, 1H), 3.80 (s, 3H), 3.65 (d, J = 22.9, 1.1 Hz, 1H), 3.47 (d, J = 22.9 Hz, 1H), 3.27 (dt, J = 13.9, 6.9 Hz, 1H), 3.03-2.99 (m, 1H), 1.98 (s, 1H), 1.61-1.15 (m, 5H), 1.26 (d, J = 6.9 Hz, 3H), 1.24 (d, J = 6.9 Hz, 3H), 1.24 (s, 3H), 0.94 (s, 3H), 0.30 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 213.1, 145.8, 144.9, 133.7, 130.4, 126.4, 122.3, 68.5, 61.9, 44.0, 42.2, 40.1, 37.4, 34.4, 32.4, 31.9, 27.2, 22.8, 22.5, 22.4, 20.2; HRMS (ESI): calculated for C₂₁H₃₀O₃ [M+H⁺] 331.2268, found 331.2264.

1-(Chloromethyl)-4,5-dimethoxy-2-methylbenzene (35)

To a solution of carboxylic acid 34 (1.0 g, 5.10 mmol) in THF (9 mL) was added a solution of borane dimethylsulfide complex (2 M in THF, 4.08 mL, 8.15 mmol) at 0 °C. The solution was allowed to room temperature and stirred overnight. The reaction was carefully quenched with H₂O and the resulting mixture was diluted with EtOAc. The organic phase was separated, washed with H₂O, and dried over Na₂SO₄. After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/1) to provide the benzyl alcohol (836 mg, 90 %) as white amorphous solid.

The benzylic alcohol (1.82 g, 10 mmol) was taken into CH₂Cl₂ (20 mL) and treated with SOCl₂ (0.87 mL, 12 mmol) dropwise at 0 °C under nitrogen. The mixture was stirred at room temperature for 30 min before it was carefully quenched with saturated aqueous NaHCO₃. The organic phase was separated and the aqueous phase was extracted with CH₂Cl₂. The combined organic layers were dried over Na₂SO₄ and concentrated. The residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/5) to provide 35 (1.90 g, 95 %) as yellow oil. IR (film, cm⁻¹) 2963, 2934, 1611, 1519, 1466, 1338, 12821226, 1105, 998; ¹H NMR (500 MHz, CDCl₃) δ 6.84 (s, 1H), 6.71 (s, 1H), 4.60 (s, 2H), 3.89 (s, 6H), 2.38 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 149.1, 147.0, 129.8, 127.3, 113.7, 113.0, 56.0, 55.9, 45.2, 18.4; HRMS (ESI): calculated for C₁₀H₁₃O₂Cl [M+H⁺] 201.0682, found 201.0692.

2-(4,5-Dimethoxy-2-methylphenyl)-1-(2,6,6-trimethylcyclohex-1-en-1-yl)ethanone (36)

A mixture of naphathalene (5.77 g, 45 mmol) and lithium (313 mg, 45 mmol) in THF (50 mL) was stirred at room temperature under nitrogen for 1.5 h. To this mixture was added a solution of β-cyclocitral (1.51 g, 9.9 mmol) and benzyl chloride 35 (1.80 g, 9 mmol) in THF (10 mL) via syringe at 0 °C. The mixture was then stirred at room temperature for 2 h before it was diluted with ether and treated with saturated aqueous NH₄Cl. The organic phase was separated and the aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration, the residue was purified by flash column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/5) to provide allyl alcohol S3 (2.17 g,76%) as light yellow amorphous solid. IR (film, cm⁻¹) 3538, 2934, 1608, 1513, 1466, 1270, 1226, 1102, 998; ¹H NMR (500 MHz, CDCl₃) δ 6.74 (s, 1H), 6.69 (s, 1H), 4.44 (dd, J = 10.2, 4.1 Hz, 1H), 3.87 (s, 3H), 3.86 (s, 3H), 3.14 (dd, J = 14.2, 10.2 Hz, 1H), 2.79 (dd, J = 14.2, 4.1 Hz, 1H), 2.33 (s, 3H), 2.03-1.94 (m, 2H), 2.00 (s, 3H), 1.62-1.53 (m, 2H), 1.45-1.42 (m, 2H), 1.10 (s, 3H), 0.94 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) 147.3, 147.0, 139.3, 131.9, 129.6, 128.8, 113.8, 113.7, 71.4, 56.1, 55.9, 40.0, 39.5, 34.8, 34.2, 28.5, 28.3, 21.4, 19.6, 19.3; HRMS (ESI): calculated for C₂₀H₃₀O₃ [M+Li⁺] 325.2355, found 325.2339.

A solution of S3 (960 mg, 3.03 mmol) in DMSO (6 mL) was treated with IBX (1.27 g, 4.55 mmol) and stirred at room temperature for 2 h. The reaction mixture was treated with H₂O (6 mL) at 0 °C and filtered. The aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine, and dried over Na₂SO₄. After concentration in vacuo, the residue was purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/3) to provide 36 (701 mg, 73%) as light yellow amorphous solid. IR (film, cm⁻¹) 2931, 1700, 1516, 1460, 1270, 1229, 1108, 1039, 998; ¹H NMR (500 MHz, CDCl₃) δ 6.71 (s, 1H), 6.61 (s, 1H), 3.86 (s, 3H), 3.83 (s, 3H), 3.82 (s, 2H), 2.23 (s, 3H), 1.99 (t, J = 6.4 Hz, 2H), 1.71-1.66 (m, 2H), 1.62 (s, 3H), 1.48-1.46 (m, 2H), 1.11 (s, 6H); ¹³C NMR (125 MHz, CDCl₃) δ 208.0, 147.8, 146.8, 143.0, 129.5, 129.4, 124.5, 113.7, 113.6, 56.0, 55.8, 49.7, 38.9, 33.4, 31.2, 28.8, 21.1, 19.6, 18.8; HRMS (ESI): calculated for C₂₀H₂₈O₃ [M+H⁺] 317.2117, found 317.2126.

(4bS,8aS)-4-Hydroxy-1,4b,8,8-tetramethyl-4b,5,6,7,8,8a-hexahydrophenanthrene-3,9-dione (37)

To a solution of ketone 36 (95 mg, 0.30 mmol) in CH₂Cl₂ (3 mL) was added a freshly prepared solution of BBr₃ (1 M in CH₂Cl₂, 1.80 mL, 1.80 mmol) dropwise at −78 °C under nitrogen. The reaction was stirred for 30 min, warmed to 0 °C, and further stirred for 3 h. The reaction was carefully quenched with saturated aqueous NaHCO₃. The aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration in vacuo, the residue was taken into CHCl₃ (10 mL) and treated with Ag₂O (104 mg, 0.45 mmol). The mixture was stirred at 50 °C for 30 min and filtered through a plug of cotton. The filtrate was concentrated and purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/4) to provide 37 (35.9 mg, 42% for 2 steps) as orange amorphous solid. IR (film, cm⁻¹); 3432, 2925, 1673, 1626, 1413, 1380, 1356, 1223, 1146, 773; ¹H NMR; δ 7.42 (s, 1H), 6.43 (s, 1H), 6.42 (d, J = 1.2 Hz 1H), 2.97-2.94 (m, 1H), 2.59 (s, 1H), 2.26 (d, J = 1.3 Hz, 3H), 1.77-1.66 (m, 2H), 1.62-1.57 (m, 1H), 1.43-1.39 (m, 1H), 1.27 (s, 3H), 1.26 (s, 3H), 1.24-1.16 (m, 1H), 1.12 (s, 3H); ¹³C NMR (125 MHz, CDCl₃) δ 201.1, 181.6, 150.3, 144.5, 140.9, 131.3, 126.9, 126.4, 62.4, 42.9, 42.4, 37.1, 33.2, 32.8, 21.9, 21.7, 19.8, 18.6; HRMS (ESI): calculated for C₁₈H₂₂O₃ [M-H⁺] 285.1496, found 285.1501.

Taxodione (2)

To a solution of ketone 25 (103 mg, 0.30 mmol) in CH₂Cl₂ (3 mL) was added a freshly prepared solution of BBr₃ (1 M in CH₂Cl₂, 1.80 mL, 1.80 mmol) at −78 °C under nitrogen. The reaction was stirred for 30 min, warmed to 0 °C, and stirred for another 30 min. The reaction was carefully quenched with saturated aqueous NaHCO₃. The aqueous phase was extracted with EtOAc. The combined organic layers were washed with brine and dried over Na₂SO₄. After concentration in vacuo, the residue was taken into CHCl₃ (10 mL) and treated with Ag₂O (104 mg, 0.45 mmol). The mixture was stirred at 50 °C for 30 min before it was filtered through a plug of cotton. The filtrate was concentrated and purified by column chromatography (silica gel, eluted with ethyl acetate/petroleum ether = 1/4) to provide 2 (36.8 mg, 39% for 2 steps) as orange amorphous solid.