Chinese Chemical Letters  2014, Vol.25 Issue (09):1235-1239   PDF    
Bi(OTf)3-catalyzed tandem reaction of naphthols with β,γ-unsaturated α-ketoesters. Effi cient synthesis of functionalized 4H-chromenes
Hui-Jing Lia,b , Dong-Hui Luoa, Qin-Xi Wua, Chun-Yang Daia, Zhi-Lun Shena, Yan-Chao Wua     
a School of Marine Science and Technology, Harbin Institute of Technology at Weihai, Weihai 264209, China;
b Beijing National Laboratory for Molecular Sciences (BNLMS), and Key Laboratory of Molecular Recognition and Function, Institute of Chemistry Chinese Academy of Sciences, Beijing 100190, China
Abstract: An efficient synthesis of functionalized 4H-chromenes by the tandem reaction of β,γ-unsaturated α-ketoesters with 2-naphthols, 1-naphthols, and 1,3-dihydroxynaphthalenes has been accomplished with high selectivity and excellent yields in the presence of a catalytic amount of bismuth triflate [Bi(OTf)3, 5 mol%] under mild conditions. The functionalized 4H-chromene synthesis and our previous 2H-chromene hemiacetal synthesis could complement each other to enrich reaction diversity.
Key words: Bismuth triflate     4H-Chromenes     Tandem reaction     Selective synthesis    
1. Introduction

4H-Chromenes have been a subject of consistent interest due to the presence of their structural motifs in a large number of natural products [1] and their wide spectrum of pharmacological activities, such as antioxidant [2],antifungal [3],antimicrobial [4],antileishmanial [5],antiviral [6],antitubercular [7],antiproliferative [8],and anticancer [9] properties. Different 4H-chromenes are required in an increasing number of applications [2, 3, 4, 5, 6, 7, 8, 9, 10, 11],which makes the development of new 4H-chromene platforms a high priority. It is worth mentioning that a quinoline-2-carboxylic acid amide derivative of saframycin A was shown to possess single-digit picomolar potency against three human sarcoma cell lines,100 times more potent than Et-743 (yondelis,trabectedin) [12],that has received European approval for the treatment of soft tissue sarcoma and ovarian carcinoma [13]. We envisioned that 4Hchromene-2-carboxylic acid amide derivatives of saframycin A might be of some use for the structural-activity relationship studies of this natural antitumor/antibiotic product [14]. The required 4H-chromene-2-carboxylic acid esters 3 were thought to be prepared from naphthols 1 and β,γ-unsaturated α-ketoester 2 (Scheme 1).

We have continual interests in the development of functional heterocycles [15],and have developed several strategies for the selective synthesis of 2H-chromenes [16]. For example,we have developed a synthesis of 2H-chromene hemiacetals 4 by the tandem reaction of naphthols 1 and β,γ-unsaturated α-ketoester 2 in TFA [17] under reflux (Scheme 1,route I),in which trace 4Hchromene-2-carboxylic acid ester intermediates 3 could be detected [16a]. However,the formation of 3 (Scheme 1,route II) was not completely finished when the transformation of these 4Hchromenes 3 into the corresponding 2H-chromene hemiacetals 4 (Scheme 1,routes III-IV) took place [16a]. Although 3 could be obtained from1and 2 in either an AuCl3/3AgOTf-catalytic system [18] or a thiourea-tertiary-amine-catalytic system followed by dehydration with concentrated sulfuric acid (H2SO4) [19],the protocols are not attractive from an economic point of view. Thus, it is still desirable to develop an efficient and cost effective protocol for the synthesis of functionalized 4H-chromenes 3. To this end, herein we would like to report an expedient synthesis of3via Bi(OTf)3-catalyzed tandem reaction of naphthols 1 with β,γ-unsaturated α-ketoester 2 (Scheme 1,route V). It is note worthy that Bi(OTf)3is a powerful σ- and π-Lewis acid with low toxicity and low cost [20]. The Bi(OTf)3-catalyzed functionalized 4Hchromene synthesis and our previous 2H-chromene hemiacetal synthesis could complement each other to enrich reaction diversity.

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Scheme 1. Reaction of naphthols1and β,γ-unsaturated α-ketoester 2.
2. Experimental

The mixture of 2-naphthol (1a,98%,147.1 mg,1.0 mmol), β,γ-unsaturated α-ketoester 2a (209.2 mg,1.1 mmol) and Bi(OTf)3 (98%,33.5 mg,0.05 mmol) in dichloromethane (CH2Cl2,5 mL) was stirred at 50°C (sealed tube) for 8 h,cooled to room temperature, filtered through celite,and concentrated. The residue was purified by flash chromatography on silica gel (100-200 mesh) to afford 4H-chromene 3a (291.0 mg) in 92% yield. White solid; mp 177- 178°C; 1H NMR (400 MHz,CDCl3):δ 7.79 (d,2H,J= 6.8 Hz),7.64 (d, 1H,J= 2.8 Hz),7.41-7.17 (m,8H),6.46 (d,1H),5.35 (d,1H),3.85 (s, 3H); 13C NMR (100 MHz,CDCl3):δ162.3,149.1,144.4,139.1,131.3, 129.4,129.0,128.5,127.8,126.9,126.8,124.5,123.6,118.0,115.0, 113.1,52.4,39.1; FTIR (film,cm-1): 1725,1674,1622,1599,1516, 1443,1335,1231,1190,1113,1070,1037,764,748,705. Anal. Calcd. for C21H16O3:C,79.73; H,5.10. Found: C,79.64; H,5.37. See Supporting information for the experimental details and copies of NMR spectra of 4H-chromenes 3a-p. 3. Results and discussion

The tandem reaction of 2-naphthol (1a) withβ,γ-unsaturated α-ketoester 2a was used as a probe for evaluating the reaction conditions,and the representative results are summarized in Table 1. Tandem reaction of 1a with 2a in the presence of 10 mol% of FeCl3 and Fe(OTf)3in 1,2-dichloroethane (DCE) could take place at 25°C to afford 4H-chromene 3a in 18%-21% yields within 4 h (entries 1-2). However,the above reaction could not be completed at 0-50°C within many days,and degradation was observed when the reaction was performed at a higher temperature. In contrast to FeCl3 and Fe(OTf)3,this reaction in the presence of Bi(OTf)3, Hf(OTf)4and AuCl3did not afford 3a at 25°C,in which reaction intermediates were detected (entries 3-5). To our delight, 3a was obtained in 90% yield within 4 h when the reaction was performed at 50°C in the presence of Bi(OTf)3(10 mol%,entry 6). However, the reaction still did not generate 3a at 50°C in the presence of Hf(OTf)4and AuCl3(entries 7-8). LiOTf,PdCl2,Cu(OTf)2,Yb(OTf)3, Mg(OTf)2,Zn(OTf)2,Al(OTf)3,and AgOTf were not effective promoters for this reaction. Treatment of 1a with 2a in DCE at 50°C for 4 h in the presence of these salts did not generate any products and the starting materials were recovered (entries 9-16). Compound 3a was also not obtained when the reaction was performed at 50°C in the presence of Sc(OTf)3,in which reaction intermediates were detected (entry 17). Bi(OTf)3 is the most efficient promoter for this reaction in all screened Lewis acids (entries 1-17). The reaction still went smoothly when the loading of Bi(OTf)3was decreased from 10 mol% to 1 mol%,albeit with a longer reaction time (entries 18-19 and 6). With the use of methanol (MeOH),N,N-dimethylformamide (DMF),toluene (PhCH3) and acetonitrile (CH3CN) in comparison to DCE,relatively lower yields were observed (entries 18-23). The reaction went smoothly under solvent-free conditions at 60°C,whereas the mixture of 1a and 2a is solid at less than 60°C (entry 24). When dichloromethane (CH2Cl2) was used as the reaction solvent (sealed tube),a relatively higher yield was observed (entries 18 and 25). Furthermore,when scaling up1ato 1.45 g,the reaction provided an excellent yield (entry 26).

Table 1
Survey of conditions for tandem reaction of 2-naphthol 1a with β,γ-unsaturated α ketoester 2a.a

With the optimized reaction conditions in hand,the scope of the reaction was subsequently investigated,and the representative results are summarized in Table 2. With theparaposition of their aromatic moiety bearing a hydrogen atom (entry 1),a weak electron-donating group (entry 2),a strong electron-donating group (entries 3 and 4),a weak electron-withdrawing group (entries 5-7) and a strong electron-withdrawing group (entries 8 and 9), β,γ-unsaturated α-ketoester 2a -ireacted smoothly with 2-naphthol (1a) in dichloromethane at 50°C to afford 4Hchromenes 3a-e in excellent yields within 8 h (entries 1-9), indicating that the electronic factor of the aromatic moiety of β,γ-unsaturated α-ketoesters 2 has little effect on these tandem reactions.β,γ-Unsaturated α-ketoesters 2 with different substitution patterns reacted with1auneventfully (entries 6 and 10-12).

Table 2
Bi(OTf)3-catalyzed tandem reaction of naphthols 1 withb,β,γ-unsaturated α-ketoester 2.a

In testing substitution of a methoxycarbonyl group to an ethoxycarbonyl group,β,γ-unsaturated α-ketoester 2m reacted equally well with 1a under the standard conditions to afford 4Hchromene 3i in 88% yield (entries 6 and 13). The tandem reaction of 6-bromo-2-naphthol (1b) with β,γ-unsaturated α-ketoester 2a went smoothly under the standard conditions to generate 4Hchromene 3n in a good yield (entry 14). 1-Naphthol (1c) and 1,3-dihydroxynaphthalene (1d) have also been investigated,which reacted smoothly withβ,γ-unsaturated α-ketoester 2a under the standard conditions,albeit with a relatively higher temperature (entries 15-16).

Possible reaction pathways for the above tandem reaction are outlined in Scheme 2. Friedel-Crafts alkylation of naphthols1with β,γ-unsaturated α-ketoesters 2 in the presence of Bi(OTf)3 generates Friedel-Crafts adducts7(Scheme 2,routes A and B) [21],which in turn undergo cyclodehydration to afford 4Hchromenes 3 (Scheme 2,routes C and D). As the ketone functionalized moieties of 2a re activated by the ester groups, another possibility for the formation of 4H-chromenes 3 involves processes of hemiacetalization (Scheme 2,route E),intramolecular Friedel-Crafts alkylation,and tautomerization (Scheme 2,routes F-G).

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Scheme 2. Possible reaction pathways for the tandem reaction.
4. Conclusion

In summary,we have developed a Bi(OTf)3-catalyzed tandem reaction of β,γ-unsaturated α-ketoesters with 2-naphthols,1-naphthols,and 1,3-dihydroxynaphthalenes under mild conditions. The process provided a cheap,convenient,efficient,and selective approach for the synthesis of functional 4H-chromenes,a structural motif for a large number of natural products,pharmaceuticals,and functionalized materials. Applications of this protocol to the selective synthesis of bioactive molecules are in progress in our research group. Acknowledgments

This work was supported by the Science and Technology Development Project of Weihai (Nos. 2011DXGJ13,2012DXGJ02), the Natural Science Foundation of Shandong Province (No. ZR2012BM002),and the National Natural Science Foundation of China (Nos. 21202028,21372054). Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version,at http://dx.doi.org/10.1016/j.cclet.2014.05.023.

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