Two-Step Synthesis of a Possible Novel Benzotripyran from

Cinnamyl Alcohol and Phloroglucinol

Yin HuangDavid Thaisrivongs, Teaching Assistant

CHEM 136

March 21, 2008

1 Abstract

A compound tentatively identified as a novel benzotripyran was synthesized in a two-step reaction sequence.In the first step, cinnamyl alcohol was oxidized by sodium dichromate to produce cinnamaldehyde. Inthe second step, diaminocyclohexane diacetate was used to catalyze the reaction of cinnamaldehyde withphloroglucinol to give the product. In both steps, products were obtained at relatively high purity but lowyield because of imperfections in the synthetic process.

2 Introduction

Benzopyrans are compounds whose structures include a benzene ring fused to a pyran ring. They playimportant roles in biological systems and often have significant impacts on human health. Tocopherol (Fig.1), also known as vitamin E, is one of many examples. One of its most important functions is to neutralizehydroxyl radicals in vivo to prevent them from initiating deleterious reactions such as lipid peroxidation. Inhumans, tocopherol deficiency can lead to impaired sperm production and other consequences [1].

Figure 1: Many variations on the tocopherol backbone can be generated by attaching different R-groups.The resulting molecules are collectively known as vitamin E.

Three other examples of biologically active benzopyrans are gallocatechol, bruceol, and xyloketal A (Fig.2). Gallocatechol, also known as epigallocatechin, belongs to class of polyphenolic, substituted benzopyransknown as catechins. Found in green tea, it has been shown to slow the development of tumors by inhibitingthe enzyme urokinase, which contributes to the invasive growth of cancer cells [4]. Bruceol is a cannabinoidthat may have anti-allergic properties. Xyloketal A is an inhibitor acetylcholineesterase, an enzyme necessaryfor nervous control of muscle contraction. The complete inhibition of this enzyme leads to immediate death,while its gradual degradation contributes to Alzheimer’s disease [9].

Figure 2: Naturally occurring benzopyrans.

This experiment was motivated in general by the prevalence and biological regulatory activity of ben-zopyrans and in particular by the desire to synthesize a compound with structural similarities to naturallyoccurring benzopyrans. As a result, the products shown in Figure 3 were chosen as synthetic targets. Al-though they are structurally distinct, all three are accessible by the same reaction between cinnamaldehydeand phloroglucinol. Previous studies by Chambers et al. [2], Lee and Kim [5], and Pettigrew et al. [8] suggestthat the stoichiometry of the reaction can control which of the three products is formed. Specifically, a largeexcess of cinnamaldehyde is expected to favor the formation of the benzotripyran, with successively smalleramounts leading to the benzodipyran and benzopyran, respectively. The targets have structural similaritiesto the naturally occurring benzopyrans described above: the first resembles gallocatechol, the second resem-bles bruceol, and the third resembles xyloketal A. As such, they may help to elucidate how these naturalregulators perform their functions.

A two-step synthesis of the targets was attempted. In the first step, the oxidation of cinnamyl alcohol tocinnamaldehyde was attempted using three methods: with sodium dichromate in a solvent-free system [6];with a mixture of iodine, potassium iodide, and potassium carbonate in aqueous solution [3]; and with ferricnitrate on a kieselguhr support [7]. In the second step, the addition of cinnamaldehyde to phloroglucinol wasattempted using two methods: with a diaminocyclohexane-diacetate-catalyzed reaction based on a procedureoutlined by Lee and Kim [5] and with a phenylboronic-acid-mediated reaction described by Chambers et al.[2].

It was found in the completed study, however, that only the sodium-dichromate oxidation of cinnamylalcohol produced cinnamaldehyde. Furthermore, the diaminocyclohexane-diacetate-catalyzed reaction of thesecond step appeared to produce only the benzotripyran in Figure 3, while the alternative method gave aproduct that was not readily identified.

3 Results and Discussion

Although the synthetic approach needs to be optimized to improve yields, it nonetheless achieved the syn-thesis of a complex structure using mild conditions and only two steps.

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Figure 3: Cinnamaldehyde reacts with various amounts of phloroglucinol to give the corresponding benzopy-ran (top), benzodipyran (middle), and benzotripyran (bottom).

3.1 Oxidation of Cinnamyl Alcohol

3.1.1 Yield and Purity

The oxidation of cinnamyl alcohol produced crude cinnamaldehyde at low yield and purity. Despite that Louet al. claim to have achieved yields of around 90% for a variety of alcohols including cinnamyl alcohol, suchwas not the case in this experiment. Cinnamaldehyde (157.3 mg, 1.190 mmol) was obtained at only 11.90%yield on the first attempt, with little improvement in subsequent attempts. The incomplete conversionprobably stems from two factors. First, Lou et al. used the oxidation of benzyl alcohol as the exemplar forthe solvent-free system. Because benzyl alcohol is liquid at room temperature, it is more conducive to beingmixed with sodium dichromate through shaking. By contrast, the melting point of cinnamaldehyde is slightlyhigher than room temperature, at 30–33◦C. That cinnamaldehyde was a solid probably prevented it frommixing optimally with sodium dichromate, thus lowering the frequency of productive molecular collisionsbetween the reactants and reducing the yield.

Attempts to overcome the problem of mixing two solids by melting the alcohol appeared to bring little

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improvement. Although holding the reaction vial in one’s hand provided enough heat to melt the cinnamylalcohol, this approach was unable to maintain the alcohol in liquid form reliably. The use of a heating mantle,however, created other problems. Specifically, high temperatures appeared to promote the formation of atenacious black residue. Although this substance has not been characterized because of its insolubility andapparent heterogeneity, it may be the product of a side reaction that wastefully consumes cinnamyl alcohol.

The lack of specialized equipment may also have contributed to the problem. Whereas Lou et al. used anoscillator to shake the reaction mixture at a rate of 220 oscillations per minute, the rate of manual shakingwas slower and far less consistent. Inadequate shaking could therefore have impeded mixing of the reactantsand lowered the yield.

In an attempt to circumvent the challenges of the solvent-free system, the reaction was attempted inmethylene chloride. Unfortunately, this approach showed no appreciable improvement over the strictlysolvent-free system. Although cinnamyl alcohol and sodium dichromate were mixed for 48 hours, the yieldwas not significantly higher than that obtained in the solvent-free system. One factor that may have con-tributed to this result is reactant solubility. Cinnamyl alcohol, which is mostly non-polar, prefers non-polarsolvents. Sodium dichromate, however, prefers polar solvents because it is an ionic compound. The useof methylene chloride as the solvent represents a compromise between these opposing demands. As such,neither reactant dissolved very well, and the resulting lack of contact between them could have impeded thereaction.

Peak (cm−1) Significance3450.1 Alcohol O–H stretch3060.2 Aromatic C–H stretch3028.2 Aromatic C–H stretch2916.3 Alkyl C–H stretch2818.2 Alkyl C–H stretch2743.9 Alkyl C–H stretch1677.8 Ketone C=O stretch1625.3 Olefin C=C stretch1600.1 Aromatic C–C stretch1575.9 Aromatic C–C stretch1494.0 Aromatic C–C stretch1450.4 Aromatic C–C stretch1125.7 Alcohol C–O stretch971.8 Olefin C–H bend748.5 Aromatic C–H bend690.5 Aromatic C–H bend

Table 1: IR absorption peaks of crude cinnamaldehyde.

Despite the low yield, the IR spectrum of the crude product (Fig. 1) shows that at least some cinnamalde-hyde was formed, though it also reveals impurities. The carbonyl peak at 1677.8 cm−1 is diagnostic of analdehyde. The peak’s lower-than-usual wavenumber is consistent with the conjugation of the carbon–oxygendouble bond with carbon–carbon double bonds in cinnamaldehyde. Aromatic carbon–carbon stretches ap-pear at 1600.1, 1575.9, 1450.4, and 1125.7 cm−1. A major indication of impurity is the peak at 3450.1 cm−1,whose location and width is consistent with the presence of an alcohol. This observation is supported by thepresence of an alcohol carbon–oxygen stretch at 1125.7 cm−1. Together, they strongly suggest that excesscinnamyl alcohol was present in the crude product.

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δ (ppm) Integration Significance Multiplicity Coupling (Hz)9.723–9.703 1 d (aldehyde) proton doublet 3Jdc = 37.595–7.434 5 a (phenyl) protons multiplet —6.760–6.699 2 b and c protons quartet 3J = 6.14.128–4.109 — residual ethyl acetate quartet —2.188–2.157 — unknown impurities — —2.043–2.041 — residual ethyl acetate triplet —

Table 2: 1H NMR peaks of purified cinnamaldehyde. Peaks corresponding to residual ethyl acetate arevisible.

The 1H NMR spectrum (Table 2) of the refined product is consistent with the structure of cinnamaldehydeeven though it exhibits some unexpected yellow coloration. The doublet at 9.723–9.703 ppm is diagnosticof the aldehyde proton. The b and c protons reside in sufficiently similar electronic environments to giveoverlapping peaks, a feature seen in literature spectra. A triplet-quartet pattern, comprising the peaks at2.043–2.041 ppm and 4.128–4.109 ppm, is diagnostic of residual ethyl acetate. The peak at 2.043–2.041 ppmdoes not appear to correspond to any protons in cinnamaldehyde, cinnamyl alcohol, or solvent, so it mayarise from side products that were not completely removed by flash chromatography. All integration valuescorrespond to the expected proton counts.

The two remaining methods of oxidizing cinnamyl alcohol performed poorly. The I2–KI–K2CO3 systemdetailed by Gogoi and Kanwar gave a brown sludge from which cinnamaldehyde could not be isolated. Theferric-nitrate reaction may have given cinnamaldehyde, but the presence of multiple UV-active side productsmade it difficult to identify the cinnamaldehyde by TLC, thereby complicating any attempts at purificationusing flash chromatography.

3.1.2 Mechanism and Driving Force

In both the solvent-free reaction and the reaction in methylene chloride, the avoidance of water is necessaryto prevent the overoxidation of cinnamyl alcohol to cinnamic acid. This undesirable reaction occurs whencinnamaldehyde is exposed to water in the presence of sodium dichromate. Water reacts with cinnamaldehydeto form the corresponding geminal diol, which is then oxidized to give the carboxylic acid. The driving forceof this reaction is twofold. First, the reduction of chromium from the +6 oxidation state to the +3 oxidationstate is thermodynamically very favorable. Furthermore, the exchange of the carbon–oxygen single bondin cinnamyl alcohol for the carbon–oxygen double bond in cinnamaldehyde is also favorable because thecarbon–oxygen double bond has a higher bond energy. These two forces make the reaction exergonic.

3.2 Synthesis of the Benzotripyran

3.2.1 Yield, Purity, and Identity

The reaction of cinnamaldehyde with phloroglucinol in the diaminocyclohexane-diacetate-catalyzed reactionappeared to give the benzotripyran shown in Figure 3, which was isolated as a flaky, light-orange solid.Unfortunately, premature sample destruction precluded analysis by GCMS and determination of the yield,which is estimated to be less than 5%. The low quality of the 1H NMR spectrum (Table 3) complicatesdata interpretation. Nonetheless, the integration values are consistent with the three-fold symmetry of thetentatively identified product. Although fifteen d protons and three each of the a, b, and c protons are

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present on the benzotripyran (Fig. 4), the d signal integrates to only five protons while the a, b, and c signalsintegrate to only a single proton each. This three-fold difference between the actual proton counts and thecorresponding integration values could be explained by symmetry.

The region between 2.371 ppm and 0.184 ppm consists of numerous dense signals that probably arise fromresidual hexanes. A quartet diagnostic of ethyl acetate appears at 4.145–4.073 ppm. The identifiable peaksare comparatively small, suggesting that little product was obtained. As expected, the complex couplinginteractions of the phenyl protons give rise to a dense multiplet at 7.445–7.217 ppm. The b and a protonsgive signals at 6.761–6.728 and 5.834, respectively, though their splitting patterns appear to be inconsistentwith expectations. Specifically, the b proton should give rise to a quartet because of coupling to the a and cprotons, yet only a doublet is observed. Likewise, the a proton is expected to give a doublet via coupling tothe b proton, but only a singlet is observed. These discrepancies could be explained by insufficient resolution;the splitting patterns may simply be too fine to be distinguishable. On the other hand, the c protons givesrise to a doublet through coupling to the b proton, though the signal is surrounded by numerous small peaks.

δ (ppm) Integration Significance Multiplicity Coupling (Hz)7.445–7.217 5 d (phenyl) protons multiplet —6.761–6.728 1 b proton doublet 3Jab = 9.95.834 1 a proton singlet —5.640–5.608 1 c proton doublet 3Jcb = 9.64.145–4.073 — residual ethyl acetate quartet —2.371–0.184 — other residual solvents — —

Table 3: 1H NMR peaks of the product, tentatively identified as a benzotripyran.

Figure 4: Proton designations for 1H NMR spectrum interpretation. Only one third of the protons in thenovel benzopyran are labeled. The remaining protons are related to these protons by symmetry.

The product given by the phenylboronic-acid-mediated reaction detailed by Pettigrew et al. contained awhite, non-polar, crystalline substance and a flaky, light-orange substance similar to the product discussedabove. Neither, however, gave an NMR spectrum with identifiable peaks. Several side products were alsoisolated during flash chromatography, but they were impossible to analyze by NMR spectroscopy due toexcess solvent. Hence, it is unclear whether the phenylboronic-acid-mediated reaction is useful in synthesizing

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any of the three benzopyrans from Figure 3.

3.2.2 Mechanism and Driving Force

Although the mechanism of the diaminocyclohexane-diacetate-catalyzed reaction has yet to be elucidatedcompletely, a plausible series of elementary steps (Fig. 5) has been established by the work of Lee and Kim[5]. That the authors recommend ethylene diamine and acetic acid for production of the catalyst in situ,as opposed to an arbitrary base and acid, has yet to be explained. Furthermore, the effect of substitutingdiaminocyclohexane for ethylene diamine is difficult to determine without more-detailed knowledge of thereaction mechanism. The reaction is favorable because several carbon–hydrogen bonds are exchanged forlower-energy carbon–carbon bonds during the cyclization reaction. The product also contains conjugated πbonds, but the impact of the stabilization is difficult to determine since the reactants also exhibit conjugation.That the benzotripyran was formed selectively is somewhat difficult to explain on the basis of stoichiometryalone, as the ratio of only one equivalent of cinnamaldehyde to five equivalents of phloroglucinol is expectedto be insufficient for the formation of the observed product.

Figure 5: One possible mechanism by which cinnamaldehyde and phloroglucinol could react to form abenzopyran in the presence of diaminocyclohexane diacetate. Further electrophilic attack of cinnamaldehydeat one or both of product’s hydroxyl groups will presumably form the benzodipyran and benzotripyran,respectively.

The mechanism of the reaction described by Chambers et al., however, is more clearly understood. Itcan be divided into two parts. Phloroglucinol initiates the reaction by attacking phenylboronic acid to forman adduct. The adduct then undergoes the [4 + 2] cycloaddition to cinnamaldehyde, which establishes thebackbone of the pyran ring. A further series of intramolecular rearrangements then gives a quinone-likeintermediate, which cyclizes to form the product with the concomitant elimination of a boron-containingbyproduct. Because water is produced in a reversible step of the mechanism, the continuous removal ofwater by heating drives the reaction forward. The effectiveness of this reaction in the synthesis of the targetcompounds, however, has yet to be demonstrated.

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Figure 6: Cinnamaldehyde reacts with phloroglucinol in the presence of phenylboronic acid to form a ben-zopyran. Water must be continuously removed from the reaction mixture to drive the reaction forward.

4 Conclusions

A compound tentatively identified as a novel benzotripyran was synthesized from cinnamyl alcohol andphloroglucinol in a two-step reaction sequence. The oxidation of cinnamyl alcohol to cinnamaldehyde wassuccessful, though the the product was formed at low yield due to less-than-optimal reaction conditions.The reaction of cinnamaldehyde with phloroglucinol gave the product, which appears to have three-foldsymmetry. More studies and reaction optimization are needed to determine the product’s structure withcertainty.

5 Experimental

5.1 Oxidation of Cinnamyl Alcohol

5.1.1 Solvent-Free System

Cinnamyl alcohol (1.34 g, 10.0 mmol) and sodium dichromate (2.98 g, 10.0 mmol) were combined in a vial.The mixture was shaken by hand at approximately 200 oscillations per minute. The progress of the reaction

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was monitored by TLC, using a 20% mixture of ethyl acetate in hexanes as the liquid phase. When thereaction was complete, the mixture was dissolved in methylene chloride, vacuum-filtered through anhydrousmagnesium sulfate and diatomaceous earth, and brought to dryness using a rotary evaporator. The crudeproduct was then purified using flash chromatography using 20% solution of ethyl acetate in hexanes as theliquid phase.

5.1.2 In Methylene Chloride

Cinnamyl alcohol (1.34 g, 10.0 mmol) and sodium dichromate (2.98 g, 10.0 mmol) were combined in anErlenmeyer flask and dissolved in a minimal amount of methylene chloride. The mixture was stirred at roomtemperature for 48 hours, after which the product was isolated as above.

5.2 Synthesis of Benzopyran

5.2.1 Catalysis by Diaminocyclohexane Diacetate

Diaminocyclohexane (25.8 mg, 0.226 mmol) and glacial acetic acid (27.2 mg, 0.452 mmol) were combinedin a round-bottom flask. Cinnamaldehyde (150 mg, 1.13 mmol), phloroglucinol (715 mg, 5.68 mmol),and methylene chloride (5 ml) were added to the mixture. The resulting solution was stirred at roomtemperature for 48 hours. More methylene chloride (25 ml) was then added to the mixture, and the solutionwas transferred to a separatory funnel and washed with water (20 ml). The organic phase was dried withmagnesium sulfate, filtered, and concentrated. The crude product was purified by flash chromatographyusing a 5% solution of ethyl acetate in hexanes.

5.2.2 Mediation by Phenylboronic Acid

Cinnamaldehyde (157.3 mg, 1.19 mmol), phloroglucinol (50.03 mg, 0.40 mmol), and phenylbenzoic acid (96.75mg, 0.733 mmol) were dissolved in toluene (2 ml) in a three-necked flask. An addition funnel containingextra toluene was attached to the flask’s top neck, and one side neck was stoppered. The remaining sideneck was left open to facilitate the removal of water from the reaction mixture. The reaction was refluxed forapproximately 90 minutes, during which reaction progress was monitored by TLC using a mixture of 5% ethylacetate in hexanes as the liquid phase. Toluene was replenished from the addition funnel as needed. Whenthe reaction was complete, the remaining toluene was allowed to evaporate. The solid residue remaining atthe bottom of the flask was dissolved in methylene chloride to the furthest extent possible. The resultingsolution was purified by flash chromatography using 10% ethyl acetate in hexanes as the liquid phase.

References

[1] Berg, J. M. et al.; Biochemistry ; Freeman: New York, NY, 2007.

[2] Chambers, J. D. et al. Can. J. Chem. 1992, 70, 1717–1732.

[3] Gogoi, P.; Konwar, D. Org. Biomol. Chem. 2005, 3, 3473–3475.

[4] Jankun, J. et al. Nature. 1997, 387, 561.

[5] Lee, R. Y.; Kim, J. H. Syn. Lett. 2007, 14, 2232–2236.

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[6] Lou, J. D. et al. Tetrahedron Lett. 2006, 47, 311–313.

[7] Lou, J. D. et al. Syn. Lett. 2006, 36, 3061–3064.

[8] Pettigrew, J. D. et al. Org. Lett. 2005, 7, 467–470.

[9] Soreq, H.; Seidman, S. Nat. Rev. Neuro. 2001, 2, 294–302.

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