WO2009026206A1 - Method for synthesizing xanthohumol - Google Patents

Abstract

A method is provided for synthesizing xanthohumol by inserting a prenyl-group onto the aryl-ring via a para-Claisen rearrangement after using a Mitsunobu reaction to establish the key prenylether precursor. A Claisen-Schmidt condensation is used to construct the chalcone scaffold followed by removal of MOM protecting groups under acidic conditions that are optimized to prevent concomitant cyclization to the flavone.

Description

TITLE

METHOD FOR SYNTHESIZING XANTHOHUMOL

CROSS-REFERENCE TO RELATED APPLICATIONS AND STATEMENT REGARDING SPONSORED RESEARCH

[0001] The present invention claims the benefit of the provisional patent application Ser. No. 60/965,593 filed August 21, 2007. This invention was made with government support under USDA Grant No. 512115. The government has certain rights in this invention.

BACKGROUND OF THE INVENTION

[0002] Xanthohumol is the principal prenylated chalcone present in the female inflorescences of the hop plant, Hamulus lupulus L. (Cannabaceae). The latter are commonly used as an ingredient to add bitterness and flavor to beer wherein xanthohumol readily undergoes thermally-driven cyclization to the flavone isoxanthohumol during the brewing process.¹'² Xanthohumol has been found to have a range of interesting biological properties in vitro that may have therapeutic utility including hormonal (for relief of 'hot flashes' and treatment of osteoporosis),³ antioxidant (for treating atherosclerosis)⁴ and inhibition of HIV-I,⁵ as well as its multi-mechanism-based classification as a potential 'broad spectrum' anticancer and cancer prevention agent (applicable to both breast and prostate cancers).^6"10 As a result, studies directed toward dietary supplements

1 1 1 ^ having increased xanthohumol content ^" and further biological characterization of purified xanthohumol at the in vivo level are underway.¹⁴ The latter have been limited, however, by the need to obtain pure material from natural sources using a multi-step process.¹'¹⁵'¹⁶

[0003] Previous supplies of xanthohumol have had to rely upon extraction from natural sources, namely from the common hops. Although hops can serve as an abundant source of xanthohumol, the latter' s isolation in a pure form is tedious and involves four distinct steps: extraction; chromatography; precipitation; and, crystallization. The overall yield for this process is about 0.1 % and the chromatography step, in particular, does not lend itself to preparing large quantities of xanthohumol. In spite of such difficulties, there is still a great interest in the intracellular and in vivo disposition and activity of natural products such as xanthohumol, particularly since xanthohumol is believed to have potential anticancer or cancer preventive properties.¹⁷'¹⁸

[0004] While the inventors herein have had success in the synthesis of the pterocarpanoid family,¹⁹'²⁰ until the present invention, there has not been any efficient and relatively high-yielding method for the synthesis of xanthohumol. Rather, until the present invention, after its initial isolation by Power et al.,²¹ xanthohumol' s structure was elucidated by Verzele et al. and later confirmed by two independent groups using partial synthesis and chemical degradation methods.²³'²⁴ Although both a 4,4'-O,O-dimethylated analog and a 6'-O- desmethyl analog have subsequently been synthesized,²⁵'²⁶ a formal construction of xanthohumol itself has not been previously reported. The reason that xanthohumol has not been previously synthesized owes, in part, to the composite of three, subtle yet stubborn synthetic challenges within the context of a deceptively simple-looking, small molecule. These three challenges involve: (i) Insertion of a prenyl-group into a sterically crowded aryl-system; (ii) Deployment of a polyhydroxy-containing aryl-keto-partner that can quickly run astray when attempting to follow the otherwise well-trodden path to the chalcone scaffold via a Claisen-Schmidt condensation; and, (iii) Prevention of spontaneous cyclization to the flavone at the end of the synthesis.

[0005] Thus, what is needed is an improved method for the production of xanthohumol. In particular, there is a need for a method of production of xanthohumol that can be achieved using a synthetic route rather than the tedious and very low yielding extraction of xanthohumol from common hops. The present invention provides an improved method for synthesizing xanthohumol.

SUMMARY OF THE INVENTION

[0006] In one aspect, there is provided a method for method for preparing xanthohumol, comprising:

[0007] inserting a prenyl-group onto the aryl-ring via a para-Claisen rearrangement after using a Mitsunobu reaction to establish the key prenylether precursor,

[0008] using a Claisen-Schmidt condensation to construct the chalcone scaffold,

[0009] and removing MOM protecting groups under acidic conditions that are optimized to prevent concomitant cyclization to the flavone.

[0010] Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiment, when read in light of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Figure 1 is a schematic illustration of one embodiment of the synthetic method for making xanthohumol.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

[0012] In one aspect, there is provided a method to synthesize xanthohumol in an efficient and cost-effective manner.

[0013] In one particular aspect, there is provided a method for preparing xanthohumol, comprising:

[0014] a) treating phloracetophenone (1) so as to produce an intermediate (2) wherein both the 2- and 4-position hydroxyl- groups are simultaneously protected with identical R groups;

[0015] b) derivatizing the di-protected intermediate (2) to form the 3-position prenylated intermediate (4) by a two-step process of:

[0016] i) treating the intermediate (2) with 3-methyl-2-butene-l-ol according to Mitsunobu reaction conditions to first form the 6-position ether intermediate (3), and

[0017] ii) rearranging the 6-position ether intermediate (3) to the 3-position prenylated intermediate (4) according to pαrα-Claisen rearrangement conditions; [0018] c) methylating the 3-position prenylated intermediate (4) to form intermediate (5) having a methyl ether at the 6-position;

[0019] d) independently treating 4-hydroxybenzaldehyde (6) so as to produce an intermediate (7) wherein its 4-position hydroxyl- group is protected with the same type of R group;

[0020] e) coupling of intermediate (5) to independently prepared intermediate (7) according to Claisen-Schmidt condensation conditions so as to produce the protected chalcone intermediate (8); and,

[0021] f) simultaneously deprotecting all of the R-masked hydroxyl- groups present in the chalcone intermediate (8) by using acidic conditions which do not allow the xanthohumol product (9) to undergo cyclization to the undesired side- product isoxanthohumol (10);

[0022] wherein independent step (d) can be done at any point prior to the coupling step (e); and,

[0023] wherein intermediates (1) to (8), final product (9) and the undesired side-product (10) correspond to the labeled Formulae as respectively shown in the scheme as shown in Figure 1.

[0024] In another particular aspect, the protecting groups R can be at least one of : methoxymethyl groups, tetrahydropryan groups, methoxyethoxymethyl groups, or tertiary butyl-dimethylsilyl groups.

[0025] In one embodiment, the protecting groups R are methoxymethyl groups. [0026] Also, in one particular method, the methylation of intermediate (4) is accomplished by treatment with dimethyl sulfate under basic conditions. [0027] Referring now in particular to the embodiment shown in Figure 1, there is shown the scheme for synthesizing xanthohumol, indicated as compound 9 and as shown by the following structure

[0028] which includes:

[0029] a) MOMCl (3 equiv), Diisopropyl ethyl amine (3 equiv), CH₂Cl₂, RT

(60%);

[0030] b) (i) 3-Methyl-2-butene-l-ol (1.5 equiv), Diethylazodicarboxylate (1.6 equiv), PPh₃ (1.2 equiv), Toluene:THF, RT (80%);

[0031] b) (ii) N,N-Dimethylaniline, reflux, 200⁰C, (64%);

[0032] c) (CH₃O)SO₂ (2 equiv), K₂CO₃ (2equiv), Acetone, reflux (82%);

[0033] d) MOMCl (3 equiv), Diisopropyl ethyl amine (3 equiv), CH₂Cl_2, RT,

(90%);

[0034] e) Aq NaOH, MeOH, reflux (60%); and,

[0035] f) Cone. HCl (pH 1), MeOH:H₂O, RT (72%).

[0036] Note that undesired conversion to isoxanthohumol (dotted line) can occur spontaneously upon heating and by acid or base catalysis.

[0037] In the present inventive method, while the enlistment of robust protecting groups is advantageous for a Claisen-Schmidt condensation that ultimately produces a polydhydroxylated chalcone,²⁶ the inventors herein determined that their removal must be accomplished under very mild conditions in order to avoid prompting spontaneous ring-closure to the unwanted flavone.²⁵

[0038] The methoxymethyl (MOM) group was chosen in order to achieve both stability under basic conditions and reactivity under a range of adjustable acidic conditions.¹⁹'²⁰ Thus, the first step ("a" in the scheme in Figure 1) involved partial

MOM-protection of 2',4',6'-trihydroxyacetophenone (phloracetophenone; 1). After further grooming with a prenyl-substituent, the protected phloractephone (1) serves as the ketone partner for the condensation reaction.

[0039] One of the two ortho-hydroxy groups present in (1) is less reactive than the other two remaining hydroxyl- groups; while not wishing to be bound by theory, the inventors believe that this lesser reactivity is due to a strong hydrogen- bonding relationship with the ketone's carbonyl. This relationship enables the synthetic method to proceed and to regio selectively protect the para- and just one ortho-hydroxyl-group in (1).

[0040] Steps "b (i)" and "b (H)" involve insertion of a prenyl-group. In contrast to other research that has shown that direct alkylation of systems related to (1) and (2) often runs the risk of giving poor yields,²⁶'²⁷ the inventive method herein uses a Claisen rearrangement. The present inventive method also takes advantage of the free phenolic hydroxyl- group that had been intentionally set-up in (2).

[0041] The inventors attempt to form the prenylether by refluxing in acetone with prenyl bromide and potassium carbonate did not provide any significant formation of the desired product; again, while not wishing to be bound by theory, the inventors herein believe that this again reflects the inherent stability of this hydrogen-bond arrangement as now present within (2). Alternatively, a Mitsunobu reaction with prenyl alcohol was used to deliver (3) in 80 % yield. Returning to follow analogous methods from the literature, however, again met with difficulty. For example, no desired product was obtained after heating (3) neat at 200 °C²⁹ even though there was an observable loss of the prenylether. Refluxing in decalin produced an intractable mixture while no reaction was observed after refluxing in chloroform for up to two days even when attempts were made to inspire the process by also adding Eu(fod)₃ as a europium catalyst.³⁰

[0042] The inventors then discovered that, when (3) was heated at 200 °C in N,N-dimethylaniline,²⁸ the desired para-C-prenylated product (4) was obtained i about 65 % yield. Furthermore, the inventors devised an efficient method to remove the high-boiling solvent by adding ethyl acetate and washing with aqueous acid followed by column chromatography to obtain pure material. [0043] Step "c" involves introducing the desired methyl-functionality onto the re-exposed ortho-hydroxy- group on (4). This was accomplished by using dimethylsulfate in the presence of potassium carbonate to produce the ketone partner (5) fully-groomed, appropriately protected and thus perfectly poised for its part in the upcoming Claisen-Schmidt condensation. [0044] The MOM-protected, aldehyde partner (7) was obtained uneventfully from 4-hydroxbenzaldehyde (6) in step "d" by using standard conditions. Refluxing these partners in methanol and 10 % aqueous sodium hydroxide (step "e") gave the fully-protected chalcone (8) in 60 % yield

[0045] In the final step "f" the inventive method carefully avoids concomitant cyclization to isoxanthohumol (10) (dotted line) during MOM-deprotection. The unwanted cyclization is due to the ready propensity of the exposed ortho-hydroxy group to engage in a nucleophilic Michael addition reaction with the alpha,beta- unsaturated carbonyl system that becomes formalized in the target chalcone (9). [0046] The heating of (8) at pH 5-6 largely resulted in the cyclized product as did subsequent heating at pH 4-5 and pH 3-4 when attempting to use acidity to advantage.³² Alternatively, lowering pH to slightly less than 1 and avoiding elevated temperature altogether¹'² by stirring at room temperature for 12 hours allows for the gradual removal of the MOM groups without cyclization even though the ortho-MOM group which leads to the culprit hydroxyl-nucleophile appeared to be the first to depart (as evidenced by TLC).

[0047] The optimized yield for this final step became a surprising 72 % and the overall yield for the entire six step synthesis became about 10 %. Unlike the isolation and chromatographic procedures used to obtain material from natural sources, the inventive synthetic method can be scalable to the gram and multi- gram level. The present inventive method can help to alleviate the need for such quantities to conduct in vivo efficacy and toxicity studies. In this same regard, the present method can be readily modified for instillation of stable- and radioisotopes that can be additionally useful for in vivo disposition, metabolism and pharmacokinetic (PK) studies.

[0048] In one embodiment, the present inventive method, the protecting groups R are methoxymethyl-groups which can be instilled by using methoxymethyl chloride under basic conditions. Further, in certain embodiments, the methylation of intermediate (4) is accomplished by treatment with dimethylsulfate under basic conditions. [0049] Examples

[0050] All chemical reactions were conducted under nitrogen in anhydrous solvents unless stated otherwise. Reagents obtained from commercial suppliers were used without further purification. Acetone was dried over 4 A molecular sieves. THF was distilled under nitrogen over sodium-benzophenone. Thin-layer chromatography (TLC) was done on 250 μ fluorescent plates and visualized by using UV light or iodine vapor. Normal phase flash column chromatography was performed using silica gel (200-425 mesh 60 A pore size) and ACS grade solvents. Melting points (mps) are uncorrected. NMR spectra were recorded on either a 600 MHz or 400 MHz instrument and were referenced using either tetramethyl silane (TMS) or residual non-deuterated solvent as an internal standard. Proton coupling constants are expressed in Hertz and in some cases overlapping signals occurred in the ¹³C NMR spectra. Spectroscopic data is in agreement with all known compounds. Abbreviations: DCM = Dichloromethane; EtOAc = Ethyl Acetate; MeOH = Methanol; MOM = Methoxymethyl; DEAD = Diethylazodicarboxylate; RT = Room Temperature (ambient). [0051] 2'-Hydroxy-4',6'-dimethoxymethyl-acetophenone (2) [0052] To an ice-cooled suspension of 2',4',6'-trihydroxyacetophenone monohydrate (1.86 g, 10 mmol) in 20 mL DCM was added N,N- diisopropylethylamine (5.3 mL, 30 mmol). MOMCl (2.3 mL, 30 mmol) was added dropwise and the reaction was gradually allowed to reach RT. The reaction was stirred for 6 hours. After disappearance of most of the starting material (TLC), the reaction was quenched with aqueous saturated ammonium chloride solution (20 mL). The resulting mixture was extracted with DCM:water [2 x 40 mL (1: 1)]. The organic layers were combined, dried over sodium sulfate filtered and evaporated. The brownish residue was chromatographed over silica using hexanes:EtOAc (5: 1) to obtain 1.53 g (60%) of known 2³³ as a colorless oil which on standing at room temperature produced a white solid: mp 49-52⁰C [Lit (33) 42 ⁰C]; TLC R_/0.46 in hexanes:EtOAc (5: 1), ¹H NMR (600 MHz, CDCl₃) δ 6.27 (d, IH, J = 2.4 Hz, Ar-H), 6.24 (d, IH, J = 2.4 Hz, Ar-H), 5.26 (s, 2H, 0-CH₂-O), 5.17 (s, 2H, 0-CH₂-O), 3.52 (s, 3H, OCH₃), 3.47 (s, 3H, OCH₃), 2.66 (s, 3H, COCH₃); ¹³C NMR (100 MHz, CDCl₃) δ 203.5, 167.1, 163.7, 160.6, 107.1, 97.3, 94.7, 94.2, 56.9, 56.7, 33.3; HRMS CaIc. [M⁺ +Na] 279.0845, found: 279.0839. [0053] 4',6'-Dimethoxymethyl-2'-(3-methylbut-2-en-l-yloxy)- acetophenone (3)

[0054] To an ice-cooled solution of 2 (0.512 g, 2 mmol) in 10 mL of THF was added triphenyl phosphine (0.628 g, 2.4 mmol) and 3-methyl-2-buten-l-ol (0.3 mL, 3 mmol)._Diethyl-azodicarboxylate, i.e., DEAD, (1.5 mL of 40% solution in toluene, 3.2 mmol) was added drop- wise and the resulting yellow solution warmed to RT and stirred for 12 hours. The solvent was evaporated and the residue suspended in ether. A solid precipitated and was filtered. The filtrate was evaporated and the residue chromatographed over silica using hexanes:EtOAc (2: 1). Evaporation of the organic fractions provided 0.518 g (80%) of known 3²⁹ as a clear oil: R₇0.29 in hexanes:EtOAc (5: 1), ¹H NMR (600 MHz, CDCl₃) δ 6.44 (d, IH, J = 1.8 Hz, Ar-H), 6.31 (d, IH, J = 1.8 Hz, Ar-H), 5.4 (t, IH, J = 5.4 Hz, CH = ), 5.16 (s, 2H, 0-CH₂-O), 5.13 (s, 2H, 0-CH₂-O), 4.49 (d, 2H, J = 5.4 Hz, CH₂ ), 3.48 (s, 3H, OCH₃ ), 3.45 (s, 3H, OCH₃ ). 2.47 (s, 3H, COCH₃), 1.76 (s, 3H, CH₃), 1.71 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 202.0, 159.7, 157.5, 155.4, 138.2, 119.6, 116.5, 96.2, 95.2, 94.9, 94.7, 65.8, 56.5, 32.8, 26, 18.5, 18.0; HRMS CaIc. [M⁺ +Na] 347.1471, found: 347.1481. [0055] 2',4'-Dimethoxymethyl-6'-hydroxy-3'-(3-methylbut-2-en-l-yloxy)- acetophenone (4)

[0056] Intermediate 3 (0.324 g, 1 mmol) was dissolved in 10 mL of dimethylaniline and heated at 200⁰C for 4 hrs. Ethyl acetate (50 mL) was added and the mixture washed with IN HCl (2 x 50 mL). The organic layer was dried over sodium sulfate, filtered and evaporated. The residue was chromatographed over silica using hexanes:EtOAc (20: 1 gradient to 5: 1). Evaporation of the organic fractions provided 0.207 g (64%) of known 4²⁹ as a yellowish oil: TLC R_/ 0.55 in hexanes:EtOAc (5: 1); ¹R NMR (600 MHz, CDCl₃) δ 6.47 (s, IH, Ar-H), 5.21 (s, 2H, 0-CH₂-O), 5.14 (t, IH, J = 6.6 Hz, CH = ), 4.95 (s, 2H, 0-CH₂-O), 3.51 (s, 3H, OCH₃ ), 3.45 (s, 3H, OCH₃ ) 3.31 (d, 2H, J = 6.6 Hz, CH₂ ), 2.69 (s, 3H, COCH₃), 1.76 (s, 3H, CH₃), 1.56 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 204.2, 163.7, 161.8, 160.7, 131.9, 123.2, 116.4, 111.2, 101.6, 99.1, 94.1, 58.6, 56.6, 31.7, 25.9, 23.3, 18.1; HRMS CaIc. [M⁺ +Na] 347.1471, found: 347.1470 [0057] 2',4'-Dimethoxymethyl-6'-methoxy-3'-(3-methylbut-2-en-l-yloxy)- acetophenone (5)

[0058] To a stirred solution of 4 (0.162 g, 0.5 mmol) in 10 mL of acetone was slowly added oven-dried potassium carbonate (0.138 g, 1 mmol) and dimethyl sulfate (0.1 mL, 1 mmol). The reaction mixture was refluxed for 6 hours, allowed to cool to RT and quenched with 10 mL of ammonium hydroxide. It was then extracted with DCM: water [3 x 20 mL (1: 1)]. The organic layers were combined, dried over sodium sulfate and evaporated. The residue was chromatographed over silica using hexane:EtOAc (3: 1). Evaporation of the organic fractions provided 0.138 g (82%) of 5 as a yellow oil: R_/0.34 in hexanes:EtOAc (5: 1), ¹H NMR (600 MHz, CDCl₃) δ 6.54 (s, IH, Ar-H), 5.21 (s, 2H, 0-CH₂-O), 5.13 (t, IH, J = 6.6 Hz, CH = ), 4.90 (s, 2H, 0-CH₂-O), 3.78 (s, 3H, OCH₃), 3.47 (s, 6H, 2 x OCH₃ ) 3.30 (d, 2 H, J = 6.6 Hz, CH₂), 2.49 (s, 3H, COCH₃), 1.76 (s, 3H, CH₃), 1.65 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 201.9,157.5, 155.8, 153.5, 131.2, 123.1, 120.2, 117.1, 101.1, 94.7, 94.3, 57.4, 56.1, 32.6, 25.7, 23.3, 17.8; HRMS CaIc. [M⁺ +Na] 361.1627, found: 361.1628. [0059] 4-Methoxymethylbenzaldehyde (7)

[0060] To an ice-cooled suspension of 4-hydroxybenzaldehyde (1.22 g, 10 mmol) in 15 mL DCM was added N,N-diisopropylethylamine (5.3 mL, 30 mmol). MOMCl (2.3 mL, 30 mmol) was added drop- wise. The reaction was allowed to reach to RT and stirred for 12 hrs. The reaction was quenched with aqueous saturated ammonium chloride solution, followed by extraction with DCM:water [50 mL (1: 1)]. The organic layer was dried over sodium sulfate, filtered and evaporated to give an oily residue. The residue was passed through a short silica column using hexanes: EtOAc (1: 1) as eluant. Evaporation of the organic fractions provided 1.52 g (90%) of known 7³¹ as a brownish oil: R_f 0.44 in hexanes:EtOAc (5: 1), ¹R NMR (600 MHz, CDCl₃) δ 9.89 (s, IH, CHO), 7.84 (d, 2H, J = 8.4 Hz, Ar-H2/6) 7.14 (d, 2H, J = 8.4 Hz, Ar-H3/5), 5.25 (s, 2H, 0-CH₂- O), 3.48 (s, 3H, OCH₃).¹³C NMR (100 MHz, CDCl₃) δ 191.2, 162.4, 132.1, 130.9, 116.5, 94.3, 56.6; HRMS CaIc. [M⁺ +Na] 189.0528, found: 189.0531. [0061] 2',4',4-Trimethoxymethyl-6'-methoxy-3-(3-methylbut-2-en-l- yloxy)-chalcone (8)

[0062] To a stirred solution of 5 (0.170 g, 0.5 mmol) and 7 (0.082 g, 0.5 mmol) in 20 mL of methanol was added 1 mL of 10% aqueous NaOH solution, after which the reaction mixture was refluxed for 4 hours. After completion of reaction (TLC), the solution was extracted with EtOAc:water [2 x 30 mL (1: 1)]. The organic layers were combined, dried over sodium sulfate, filtered and evaporated. The residue was chromatographed over silica using hexanes:EtOAc (3: 1). Evaporation of the organic fractions provided 0.145 g (60%) of 8 as yellowish oil: R₇0.19 in hexanes:EtOAc (2: 1), ¹H NMR (600 MHz, CDCl₃). δ 7.48 (d, 2H, J = 9 Hz, Ar-H2/6), 7.37 (d, IH, J = 15.6, CH = ), 7.03 (d, 2H, J = 9 Hz, Ar-H3/5), 6.90 (d, IH, J = 15.6 Hz, CH = ), 6.58 (s, IH, Ar-H4'), 5.23 (s, 2H, 0-CH₂), 5.19 (s, 3H, O-CH₂/CH = ), 4.90 (s, 2H, 0-CH₂), 3.75 (s, 3H, OCH₃), 3.5 (s, 3H, OCH₃), 3.47 (s, 3H, OCH₃), 3.42 (s, 3H, OCH₃), 3.35 (d, 2H, CH₂), 1.76 (s, 3H, CH₃), 1.67 (s, 3H, CH₃); ¹³C NMR (100 MHz, CDCl₃) δ 194.4, 159.2, 157.8, 156.5, 154.3, 144.3, 131.5, 130.3, 128.9, 127.3, 123.4, 118.5, 117.4, 116.6, 100.9, 95.1, 94.6, 94.4, 57.7, 56.4, 25.9, 23.3, 18.1; HRMS CaIc. [M⁺ +Na] 509.2151, found: 509.2139.

[0063] 2',4',4-Trihydroxy-6'-methoxy-3-(3-methylbut-2-en-l-yloxy)- chalcone (9; Xanthohumol)

[0064] To a stirred solution of 8 (0.148 g, 0.3 mmol) in methanol:water (18 mL:2 mL) was added cone. HCl until the pH dropped to less than 1. The mixture was stirred at RT for ca. 12 hours. A gradual removal of the MOM groups was observed by TLC. After 12 hours, the reaction mixture was extracted with EtO Ac: water [2 x 40 mL (1: 1)]. The organic layers were combined, dried over sodium sulfate, filtered and evaporated at 20⁰C. The residue was chromatographed over silica using hexanes:EtOAc (2: 1) as eluant. Evaporation of the organic fractions provided 0.076 g (72%) of xanthohumol as a yellow solid: mp 148-151 ⁰C [Lit (34) 157-159⁰C, Chloroform]; R₇0.41 in hexanes: EtOAc Qi I)-¹H NMR (600 MHz, methanol-^)- δ 7.78 (d, IH, J = 15.6, CH = ), 7.67 (d, IH, J = 15.6, CH = ), 7.49 (d, 2H, J = 8.4 Hz, Ar-H2/6), 6.82 (d, 2H, J = 9 Hz, Ar-H3/5), 6.0 (s, IH, Ar-H4'), 5.19 (t, IH, CH = ), 4.9 (s, H₂O), 3.89 (s, 3H, OCH₃), 3.22 (d, 2H, CH₂), 1.75 (s, 3H, CH₃), 1.64 (s, 3H, CH₃); ¹³C NMR (100 MHz, methanol-^) δ 194.2, 166.3, 163.9, 162.6, 161.3, 143.5, 131.5, 131.4, 128.6, 126.0, 124.4, 117.0, 109.5, 106.7, 91.7, 56.3, 26.1, 22.4, 18.0; analysis calcd. for C₂₁H₂₂O₅ • 0.4 H₂O: C 69.75, H 6.36, found: C 69.90, H 6.32; HRMS CaIc. [M⁺ +Na] 377.1365, found: 377.1354.

[0065] In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope. [0066] References

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Claims

CLAIMSWhat is claimed is:

1. A method for preparing xanthohumol, comprising: inserting a prenyl-group onto the aryl-ring via a para-Claisen rearrangement after using a Mitsunobu reaction to establish the key prenylether precursor, using a Claisen-Schmidt condensation to construct the chalcone scaffold, and removing hydroxyl-protecting groups under acidic conditions that are optimized to prevent concomitant cyclization to the flavone.

2. A method for preparing xanthohumol, comprising: a) treating phloracetophenone (1) so as to produce an intermediate (2) wherein both the 2- and 4-position hydroxyl- groups are simultaneously protected with identical R groups; b) derivatizing the di-protected intermediate (2) to form the 3-position prenylated intermediate (4) by a two-step process of: i) treating the intermediate (2) with 3-methyl-2-butene-l-ol according to Mitsunobu reaction conditions to first form the 6-position ether intermediate (3), and ii) rearranging the 6-position ether intermediate (3) to the 3-position prenylated intermediate (4) according to pαrα-Claisen rearrangement conditions; c) methylating the 3-position prenylated intermediate (4) to form intermediate (5) having a methyl ether at the 6-position; d) independently treating 4-hydroxybenzaldehyde (6) so as to produce an intermediate (7) wherein its 4-position hydroxyl- group is protected with the same type of R group; e) coupling of intermediate (5) to independently prepared intermediate (7) according to Claisen-Schmidt condensation conditions so as to produce the protected chalcone intermediate (8); and, f) simultaneously deprotecting all of the R-masked hydroxyl- groups present in the chalcone intermediate (8) by using acidic conditions which do not allow the xanthohumol product (9) to undergo cyclization to the undesired side- product isoxanthohumol (10); wherein independent step (d) can be done at any point prior to the coupling step (e); and, wherein intermediates (1) to (8), final product (9) and the undesired side- product (10) correspond to the labeled Formulae as respectively shown in the following scheme:

Xanthohumol Isoxanthohumol

3. The method of claim 2, wherein the protecting groups R are either methoxymethyl groups, tetrahydropryan groups, methoxyethoxymethyl groups, or tertiary butyl-dimethylsilyl groups.

4. The method of claim 3, wherein the protecting groups R are methoxymethyl groups.

5. The method of claim 3, wherein the methylation of intermediate (4) is accomplished by treatment with dimethylsulfate under basic conditions.