Unambiguous synthesis and spectral characterization of 1,8-dihydroxy-4-methylanthraquinone

1,8-Dihydroxy-4-methylanthraquinone was recently isolated as a red liquid from cyanobacterium. We have confirmed that assignment by synthesizing unambiguously the titled compound in a two-step process. This synthetic methodology consisted of treating 4-methoxy-3-cyanophthalide with 2-bromo-4-methylanisole under aryne-forming conditions then demethylating the1,8-dimethoxy-4-methylanthra-quinone so formed.


Introduction
Recently several new antibacterial metabolites were isolated from cyanobacterium Nostoc commune. 1 Among these metabolites was suggested to be 1,8-dimethoxy-4-methylanthraquinone (7), which presumably was the first anthraquinone to be isolated from a cyanobacterium.The structure of 7 was proposed on the basis of spectral analysis.

Results and Discussion
The finding that 7 existed as a red oil intrigued us.This is somewhat unusual since simple substituted anthraquinones are solid substances.We thus decided to confirm further the structure and physical form of 7 by preparing it unambiguously by a two-step reaction outlined in Scheme 1.
During the acidic aqueous workup, adduct 5 is converted to 1,8-dimethoxy-4methylanthraquinone 6. Treatment of 6, which was obtained as a yellow solid, with BBr 3 affords title compound 7.
The 1 H NMR, 13 C NMR, IR, and UV spectra of 6 were consistent with proposed structure.For example, the 1 H NMR spectrum of 6 exhibited five resonances characteristic of 5-aromatic hydrogen atoms and three resonances corresponding to the two methoxy and methyl groups, In addition, the 13 C NMR of 6 contained 14 signals for the aromatic carbons and two carbonyl carbon resonances at 183.9 and 186.4 ppm.Additionally, its IR spectrum exhibited a carbonyl stretching frequency of 1671 cm -1 and its UV spectrum showed a λ max at 389 nm as well as 271 and 259 nm.Final structural proof of 6 was obtained by single crystal X-ray diffraction.See ORTEP of compound 6 below Purification of 7 was accomplished by column chromatography and was found to be a bright orange solid (UV [CHCl 3 ], λ max 442, 285 and 253 nm), rather than a red oil as previously reported.The IR spectrum of 7 showed two separated carbonyl absorption bands at 1626 cm -1 belonging to the chelated carbonyl group at C-9 and at 1729 cm -1 belonging to the free carbonyl group at C-10.Interestingly, the IR of 6, in which chelation with neither carbonyl is possible, reveals only one absorption band at 1671 cm -1 .This value is consistent with aromatic carbonyl groups.
The 1 H NMR spectrum of 7, which was obtained in CDCl 3 revealed all characteristic signals of all hydrogen atoms including the 1-OH (δ 12.05) and 8-OH (δ 12.58).The relatively high chemical shifts of the OH hydrogens are indicative of H-bonding between the nearby OH and C=O groups. 4Other support for such intramolecular hydrogen bonding was the observation that 7 dissolved readily in the non-polar solvent CDCl 3 .Comparison of the 1 HNMR of the synthesized 7 with that reported for the red liquid (which was obtained in MeOH-d 4 ) could not be accomplished due to the insolubility of the former in MeOH-d 4 In any case, our data confirms clearly the structure of 7 to be 1,8-dihydroxy-4-methylanthraquinone.

Experimental Section
General Procedures.Melting points were taken on a Mel-Temp II capillary apparatus, and are uncorrected with respect to stem correction.IR spectra were recorded on a Nicolet Magna-IR TM 550 FTIR spectrometer and the 1 H and 13 C NMR spectra were recorded on a 400 MHz Bruker AVANCE DRX-400 Multi-nuclear NMR spectrometer; chemical shifts were referenced to TMS as internal standard.The UV/VIS spectra were recorded on a Beckman DU 660 Spectrometer.Elemental analyses were obtained from SMU Analytical Services Laboratories.3-Cyano-7-methoxyphthalide and 2-bromo-4-methylanisole were available from previous studies.LDA, BBr 3 , and THF were purchased from Aldrich Chemical Company.THF was distilled from Na/benzophenone immediately prior to use.The glassware was heated at 125 o C in an oven overnight prior to use.All reactions were done under an atmosphere of dry O 2 -free N 2 via balloon.

General procedure for the preparation of 6
To a flame-dried flask was added 20 mL of LDA (2.0 M, 40 mmol) at -70 o C.After stirring for 10 min, 7-methoxy-3-cyanophthalide (0.95 g, 5 mmol) was added, and the stirring continued for 20 min to ensure complete anion formation.2-Bromo-4-methylanisole (1.2g, 6 mmol) was added, the resulting solution was allowed to warm to room temperature, where it was stirred for 6 h.The reaction was then quenched with saturated NH 4 Cl solution (30 mL) and extracted with methylene chloride.The combined extracts were washed with dilute HCl then dried (Na 2 SO 4 ) and concentrated (rotary evaporator) to give a crude material.Chromatography of this material on silica gel (hexane/ethyl acetate, 4:1) gave the pure product 6 (180 mg, 15%).The physical and spectral properties for 6 are shown below.