Synthesis of Spiro Dihydrofurans and 1,8-Dioxo-xanthenes via DABCO Catalyzed Tandem Reaction of Aldehyde with Cyclohexane-1,3-dione and Dimedone

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1 CHEM. RES. CHINESE UNIVERSITIES 2011, 27(1), Synthesis of Spiro Dihydrofurans and 1,8-Dioxo-xanthenes via DABCO Catalyzed Tandem Reaction of Aldehyde with Cyclohexane-1,3-dione and Dimedone CHEN Jiao, SHI Jian and YAN Chao-guo * College of Chemistry & Chemical Engineering, Yangzhou University, Yangzhou , P. R. China Abstract The 1,4-diazabicyclo[2.2.2]octane(DABCO) catalyzed reaction of cyclohexane-1,3-dione or dimedone with various aldehydes in acetonitrile resulted in the polysubstituted tetraketones, spiro dihydrofurans or 1,8-dioxo-xanthenes derivatives as main products respectively according to the structure of reactants and reaction conditions. Keywords Tandem reaction; Dimedone; Spiro dihydrofuran; Tetraketone; 1,8-Dioxo-xanthene Article ID (2011) Introduction As typical reactive 1,3-dicarbonyl compounds, cyclohexane-1,3-dione and its analogy 5,5-dimethyl cyclohexane-1,3-dione(dimedone) have been widely used in versatile synthetic reactions [1,2]. Dimedone is not only a typical reagent for Knoevenagel condensation, but also adds easily to electron-deficient alkenes via Michael addition. On the other hand, its one or two carbonyl groups could take part in substitution and cyclization reactions through the tautomerized enolate form. Thus the cascade reactions of addition, elimination and substitution could be achieved in many reactions involving dimedone. The reactions of cyclohexane-1,3-dione or dimedone with aldehydes have been extensively studied in the past years, from which several types of compounds have been produced according to the reaction conditions [3,4]. The normal Knoevenagel condensation of cyclohexane-1,3-done or dimedone with aldehydes have been conducted with numerous methods including promotion via amines [5], Lewis acids [6], surfactants [7], zeolites [8], ionic liquids [9]. The use of environmentally benign methods like in aqueous medium [10] or in the absence of solvents [11] and the usage of ultrasound or microwave heating [12,13] have also been developed in recent years. The reactions usually proceed further through Michael addition reaction of the second molecule of dimedone to yield tetraketones as main products [14]. On the other hand, tetraketones could be easily converted to 9-substituted 1,8-dioxo-xanthenes by dehydration step [15]. 1,8-Dioxo-xanthenes can also be directly prepared from the condensation of two molecules of dimedone with aromatic aldehydes in the presence of different kinds of catalysts [16 18] such as Amberlyst [19], triethylbenzylammonium chloride [20], diammonium hydrogen phosphate [21], sulfonic acid and ionic liquid under ultrasonic irradiation [22,23]. In our continued interest in the design of new multicomponent reactions and the *Corresponding author. cgyan@yzu.edu.cn Received May 10, 2010; accepted July 20, Supported by the National Natural Science Foundation of China(No ). application in the synthesis of heterocyclic compounds [24,25], we found some unprecedented reaction patterns in the reaction of aromatic aldehydes with cyclohexane-1,3-dione or dimedone. Here we wish to report the very interesting results of 1,4-diazabicyclo[2.2.2]octane(DABCO) catalyzed tandem reaction of aldehydes with cyclohexane-1,3-dione or dimedone. 2 Experimental 2.1 General Procedure for the Reaction of Aldehyde with Dimedone A mixture of aldehyde(1.0 mmol), dimedone(2.0 mmol, g) and DABCO(2.0 mmol) in 15 ml of acetonitrile was stirred at room temperature for 8 10 h. TLC analysis showed the reaction had been finished. The solvent was evaporated in vacuum and the residue was recrystallized from methanol to give the pure product for analysis. Compound 3a: R=CH 2 CH 3, yield 61%. m.p C. IR(KBr), /cm 1 : 3432(m), 2960(s), 2875(m), 1588(vs), 1386(s), 1249(s), 1161(m), 908(m), 726(w). 1 H NMR(600 MHz, CDCl 3 ), δ: 12.49(s, 1H, OH), 3.82(t, J=7.8 Hz, 1H, CH), (m, 8H, CH 2 ), (m, 2H, CH 2 ), 1.07(d, J=6.6 Hz, 12H, CH 3 ), 0.84(t, J=7.2 Hz, 3H, CH 3 ). 13 C NMR (150 MHz, CDCl 3 ), δ: 190.0, 189.4, 116.4, 46.9, 46.1, 31.6, 31.1, 29.9, 26.5, 22.2, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 19 H 28 O 4 : C 71.22, H 8.81; found: C 70.86, H Compound 3b: R=n-C 3 H 7, yield 67.8%. m.p C. IR(KBr), /cm 1 : 2978(m), 2865(w), 1648(s), 1590(vs), 1350(s), 1200(s), 1145(m), 879(m). 1 H NMR(600 MHz, CDCl 3 ), δ: 12.45(s, 1H, OH), 3.45(d, J=11.4 Hz, 1H, CH), (m, 1H, CH), (m, 8H, CH 2 ), 1.07(d, J=12.0 Hz, 12H, CH 3 ), 0.84(t, J=6.0 Hz, 6H, CH 3 ). 13 C NMR (150 MHz, CDCl 3 ), δ: 190.4, 189.4, 116.4, 47.0, 46.2, 38.0,

2 50 CHEM. RES. CHINESE UNIVERSITIES Vol , 30.0, 26.8, 25.7, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 20 H 30 O 4 : C 71.82, H 9.04; found: C 72.15, H Compound 3c: R=p-t-BuC 6 H 4, yield 57%. m.p C. IR(KBr), /cm 1 : 3443(m), 2961(m), 1593(vs), 1466(w), 1373(s), 1255(w), 1154(w), 833(w). 1 H NMR(600 MHz, CDCl 3 ), δ: 11.92(s, 1H, OH), 7.28(d, J=7.8 Hz, 2H, ArH), 7.02(d, J=7.8 Hz, 2H, ArH), 5.50(s, 1H, CH), (m, 8H, CH 2 ), 1.29(s, 9H, 3CH 3 ), 1.24(s, 6H, 2CH 3 ), 1.10(s, 6H, 2CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 190.4, 189.3, 148.5, 134.8, 126.4, 125.2, 115.6, 47.0, 46.4, 34.3, 32.3, 31.3, 29.7, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 27 H 36 O 4 : C 76.38, H 8.55; found: C 76.50, H Compound 4a: R=C 6 H 5, yield 61%. m.p C. IR(KBr), /cm 1 : 2955(m), 2874(m), 1713(m), 1641(vs), 1514(w), 1467(w), 1394(s), 1320(m), 965(m); 1 H NMR(600 MHz, CDCl 3 ), δ: (m, 3H, ArH), 7.15(d, J=6.6 Hz, 2H, ArH), 4.44(s, 1H, CH), 3.07(d, J=14.4 Hz, 1H, CH 2 ), 2.66(s, 2H, CH 2 ), 2.54(d, J=15.0 Hz, 1H, CH 2 ), 2.22(d, J=16.2 Hz, 1H, CH 2 ), 2.14(t, J=15.0 Hz, 2H, CH 2 ), 1.95(d, J=14.4 Hz, 1H, CH 2 ), 1.16(s, 6H, 2CH 3 ), 1.11(s, 3H, CH 3 ), 0.83(s, 3H, CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 199.4, 199.1, 193.2, 176.4, 159.7, 129.7, 128.0, 114.4, 113.7, 103.7, 55.2, 54.6, 53.8, 51.2, 50.1, 37.4, 34.2, 30.5, 28.9, 28.4, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 23 H 26 O 4 : C 75.38, H 7.15; found: C 75.15, H Compound 4b: R=p-CH 3 C 6 H 4, yield 69%. m.p C. IR(KBr), /cm 1 : 3439(w), 2940(m), 1713(s), 1641(vs), 1514(w), 1430(s), 1300(w), 1198(w), 1145(m), 833(w); 1 H NMR(600 MHz, CDCl 3 ), δ: 7.10(d, J=7.2 Hz, 2H, ArH), 7.03(d, J=7.2 Hz, 2H, ArH), 4.41(s, 1H, CH), 3.07(d, J=14.4 Hz, 1H, CH 2 ), 2.65(s, 2H, CH), 2.53(d, J=15.0 Hz, 1H, CH 2 ), 2.30(s, 3H, CH 3 ), 2.22(d, J=16.2 Hz, 1H, CH 2 ), 2.14(d, J=16.2 Hz, 2H, CH 2 ), 1.99(d, J=14.4 Hz, 1H, CH 2 ), 1.15(s, 6H, 2CH 3 ), 1.12(s, 3H, CH 3 ), 0.83(s, 3H, CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 199.4, 199.0, 193.1, 176.5, 138.4, 133.0, 129.7, 128.4, 113.7, 103.9, 54.9, 53.8, 51.2, 50.1, 46.1, 37.3, 34.2, 30.5, 28.9, 28.4, 26.3, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 24 H 28 O 4 : C 75.76, H 7.42; found: C 75.82, H Compound 4c: R=p-CH 3 OC 6 H 4, yield 52.7%. m.p C. IR(KBr), /cm 1 : 3433(w), 2957(m), 1613(m), 1514(s), 1464(vs), 1389(m), 1274(s), 1204(m), 966(w). 1 H NMR(600 MHz, CDCl 3 ), δ: 7.07(d, J=7.2 Hz, 2H, ArH), 6.82(d, J=7.8 Hz, 2H, ArH), 4.41(s, 1H, CH), 3.77(s, 3H, ArH), 3.06(d, J=15.0 Hz, 1H, CH 2 ), 2.65(s, 2H, CH 2 ), 2.54(d, J=14.4 Hz, 1H, CH 2 ), 2.22(d, J=16.2 Hz, 1H, CH 2 ), (m, 2H, CH 2 ), 2.00(d, J =14.4 Hz, 1H, CH 2 ), 1.15(s, 6H, 2CH 3 ), 1.12(s, 3H, CH 3 ), 0.84(s, 3H, CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 199.4, 199.1, 193.3, 176.4, 159.6, 129.6, 127.9, 114.4, 113.7, 103.7, 55.2, 54.5, 53.8, 51.1, 50.1, 37.3, 34.2, 30.5, 28.9, 28.4, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 24 H 28 O 5 : C 72.70, H 7.12; found: C 72.55, H Compound 4d: R=p-CH 3 CH 2 C 6 H 4, yield 69.9%. m.p C. IR(KBr), /cm 1 : 3435(m), 2960(m), 2874(w), 1740(w), 1712(s), 1641(vs), 1500(m), 1464(w), 1390(s), 1320(w), 1233(w), 1200(w), 842(w); 1 H NMR(600 MHz, CDCl 3 ), δ: 7.12(d, J=8.4 Hz, 2H, ArH), 7.05(d, J=8.4 Hz, 2H, ArH), 4.42(s, 1H, CH), 3.08(d, J=15.0 Hz, 1H, CH 2 ), 2.66(s, 2H, CH 2 ), 2.59(q, J=7.8 Hz, 2H, CH 2 ), (m, 1H, CH 2 ), (m, 1H, CH 2 ), (m, 2H, CH 2 ), 1.99(d, J=14.4 Hz, 1H, CH 2 ), 1.20(t, J=7.8 Hz, 3H, CH 3 ), 1.16(s, 6H, 2CH 3 ), 1.12(s, 3H, CH 3 ), 0.83(s, 3H, CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 198.9, 193.1, 176.8, 135.2, 132.2, 130.1, 122.8, 113.5, 103.4, 54.2, 53.9, 51.0, 49.9, 37.2, 30.6, 30.5, 28.9, 28.4, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 25 H 30 O 4 : C 76.11, H 7.66; found: C 75.73, H Compound 4e: R=p-(CH 3 ) 2 CHC 6 H 4, yield 62.6%. m.p C. IR(KBr), /cm 1 : 3430(w), 2959(m), 2873(w), 1737(m), 1711(s), 1644(vs), 1509(w), 1466(w), 1388(s), 1300(w), 1142(w), 941(w). 1 H NMR(600 MHz, CDCl 3 ), δ: 7.14(d, J=7.8 Hz, 2H, ArH), 7.05(d, J=7.8 Hz, 2H, ArH), 4.42(s, 1H, CH), 3.08(d, J=14.4 Hz, 1H, CH 2 ), (m, 1H, CH), 2.66(s, 2H, CH 2 ), 2.53(d, J=14.4 Hz, 1H, CH 2 ), (m, 4H, CH 2 ), 1.97(d, J=14.4 Hz, 1H, CH 2 ), 1.20(d, J=6.6 Hz, 6H, CH 3 ), 1.16(s, 6H, 2CH 3 ), 1.12(s, 3H, CH 3 ), 0.83(s, 3H, CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 199.4, 193.3, 176.5, 149.2, 133.2, 128.4, 127.0, 113.7, 103.9, 54.8, 53.7, 51.1, 50.0, 37.3, 34.2, 33.7, 30.5, 28.9, 28.4, 26.3, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 26 H 32 O 4 : C 76.44, H 7.90; found: C 76.62, H Compound 4f: R=p-BrC 6 H 5, yield 66.4%. m.p C. IR(KBr), /cm 1 : 3438(m), 2962(w), 2871(w), 1741(w), 1711(s), 1639(vs), 1504(w), 1487(m), 1391(s), 1202(w), 1148(w), 835(w); 1 H NMR(600 MHz, CDCl 3 ), δ: 7.44(d, J=6.6 Hz, 2H, ArH), 7.04(d, J=6.6 Hz, 2H, ArH), 4.39(s, 1H, CH), 3.05(d, J=14.4 Hz, 1H, CH 2 ), 2.66(s, 2H, CH 2 ), 2.55(d, J=15.0 Hz, 1H, CH 2 ), (m, 2H, CH 2 ), (m, 1H, CH 2 ), 1.97(d, J=14.4 Hz, 1H, CH 2 ), (m, 9H, 3CH 3 ), 0.84(s, 3H, CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 190.4, 189.2, 148.4, 134.8, 126.4, 125.1, 115.6, 47.0, 46.4, 34.2, 32.3, 31.3, 29.7, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 23 H 25 BrO 4 : C 62.03, H 5.66; found: C 62.52, H Compound 4g: R=m-ClC 6 H 4, yield 62%. m.p C. IR(KBr), /cm 1 : 2959(s), 2871(m), 1745(s), 1715(s), 1642(vs), 1573(m), 1470(m), 1429(m), 1390(s), 1189(m), 1167(m), 696(m). 1 H NMR(600 MHz, CDCl 3 ), δ: (m, 2H, ArH), 7.16(s, 1H, ArH), 7.04(d, J=7.2 Hz, 1H, ArH), 4.39(s, 1H, CH), 3.06(d, J=9.0 Hz, 1H, CH 2 ), 2.67(s, 2H, CH 2 ), (m, 1H, CH 2 ), (m, 3H, CH 2 ), 1.99(d, J=13.2 Hz, 1H, CH 2 ), (m, 9H, 3CH 3 ), 0.85(s, 3H, CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 199.4, 193.2, 176.5, 144.6, 133.1, 128.4, 113.7, 103.9, 54.8, 53.7, 51.1, 50.0, 37.3, 34.2, 30.5, 28.9, 28.4, 26.3, MS(ESI ), m/z: Elemental anal.(%) calcd. for C 23 H 25 ClO 4 : C 68.91, H 6.29; found: C 68.54, H General Procedure for the Reaction of Aldehyde with Cyclohexane-1,3-dione A mixture of aldehyde(1.0 mmol), cyclohexane-1,3-dione (2.0 mmol) and DABCO(2.0 mmol) in 15 ml of acetonitrile

3 No.1 CHEN Jiao et al. 51 was stirred at room temperature for 8 10 h. TLC analysis showed the reaction had been finished. The solvent was evaporated in vacuum and the residue was recrystallized from methanol to give the pure product for analysis. Compound 5a: Ar=C 6 H 5, yield 61%. m.p C. IR(KBr), /cm 1 : 3313(w), 2960(w), 1721(m), 1639(s), 1603(vs), 1490(w), 1453(w), 1376(w), 1231(w), 1105(w), 1033(w), 995(w). 1 H NMR(600 MHz, CDCl 3 ), δ: 7.30(d, J=7.2 Hz, 2H, ArH), 7.22(t, J=7.8 Hz, 2H, ArH), 7.11(t, J=7.2 Hz, 1H, ArH), 4.81(s, 1H, CH), (m, 2H, CH 2 ), (m, 2H, CH 2 ), (m, 4H, CH 2 ), (m, 4H, CH 2 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 196.5, 192.1, 190.9, 163.9, 137.8, 128.4, 128.2, 126.5, 125.8, 116.4, 37.0, 33.5, 32.9, 27.2, MS(ESI+)(M+Na + ), m/z: Compound 5b: Ar=p-CH 3 C 6 H 5, yield 67.4%. m.p C. IR(KBr), /cm 1 : 3447(w), 2887(w), 1652(vs), 1492(w), 1451(w), 1232(s), 1132(s), 1055(s), 907(m). 1 H NMR (600 MHz, CDCl 3 ), δ: 7.19(d, J=7.2 Hz, 2H, ArH), 7.02(d, J=7.2 Hz, 2H, ArH), 4.77(s, 1H, CH), (m, 2H, CH 2 ), (m, 2H, CH 2 ), (m, 4H, CH 2 ), 2.25(s, 3H, CH 3 ), (m, 4H, CH 2 ). 13 C NMR (150 MHz, CDCl 3 ), δ: 196.4, 164.8, 143.0, 132.1, 129.8, 128.2, 116.5, 36.9, 31.3, 27.1, MS(ESI+)(M+Na + ), m/z: Compound 5c: Ar=p-CH 3 OC 6 H 5, yield 58.4%. m.p C. IR(KBr), /cm 1 : 3448(w), 2958(w), 1658(vs), 1509(m), 1459(w), 1359(s), 1234(s), 1202(s), 861(w). 1 H NMR (600 MHz, CDCl 3 ), δ: 7.21(d, J=8.4 Hz, 2H, ArH), 6.76(d, J=8.4 Hz, 2H, ArH), 4.75(s, 1H, CH), 3.73(s, 3H, OCH 3 ), (m, 2H, CH 2 ), (m, 2H, CH 2 ), (m, 4H, CH 2 ), (m, 4H, CH 2 ). 13 C NMR (150 MHz, CDCl 3 ), δ: 196.6, 163.7, 158.0, 129.3, 117.0, 113.5, 55.1, 37.0, 30.8, 27.1, MS(ESI+)(M+Na + ), m/z: Compound 5d: Ar=p-(CH 3 ) 2 CHC 6 H 5, yield 66.6%. m.p C. IR(KBr), /cm 1 : 3426(w), 2963(m), 1688(vs), 1509(w), 1458(w), 1358(s), 1199(s), 1175(m), 1014(m), 903(w). 1 H NMR(600 MHz, CDCl 3 ), δ: 7.18(br, 2H, ArH), 7.05(d, J=6.0 Hz, 2H, ArH), 4.79(s, 1H, CH), 2.81(d, J=6.0 Hz, 1H, CH 2 ), (m, 4H, CH 2 ), (m, 4H, CH 2 ), (m, 4H, CH 2 ), 1.18(d, J=3.6 Hz, 6H, CH 3 ). 13 C NMR(150 MHz, CDCl 3 ), δ: 196.5, 163.8, 146.6, 141.7, 128.1, 126.2, 117.1, 37.0, 33.6, 31.1, 27.2, 23.9, MS(ESI+)(M+Na + ), m/z: Compound 5e: Ar=p-ClC 6 H 5, yield 70.6%. m.p C. IR(KBr), /cm 1 : 3436(w), 2957(w), 1667(vs), 1459(w), 1420(m), 1201(s), 1171(m), 1012(m), 907(m). 1 H NMR(600 MHz, CDCl 3 ), δ: 7.24(d, J=7.2 Hz, 2H, ArH), 7.18(d, J=7.8 Hz, 2H, ArH), 4.77(s, 1H, CH), (m, 4H, CH 2 ), (m, 4H, CH 2 ), (m, 4H, CH 2 ). 13 C NMR (150 MHz, CDCl 3 ), δ: 196.5, 163.8, 141.6, 135.9, 128.8, 128.3, 117.1, 37.0, 31.2, 27.2, MS(ESI+)(M+Na + ), m/z: Compound 5f: Ar=p-BrC 6 H 5, yield 69%. m.p C. IR(KBr), /cm 1 : 3321(w), 2943(w), 1658(vs), 1599(m), 1504(s), 1423(m), 1358(m), 1212(m), 921(m). 1 H NMR(600 MHz, CDCl 3 ), δ: 7.33(d, J=7.8 Hz, 2H, ArH), 7.18(d, J=7.8 Hz, 2H, ArH), 4.76(s, 1H, CH), (m, 2H, CH 2 ), (m, 2H, CH 2 ), (m, 4H, CH 2 ), (m, 2H, CH 2 ), (m, 2H, CH 2 ). 13 C NMR (150 MHz, CDCl 3 ), δ: 196.6, 163.9, 129.9, 127.9, 116.8, 116.4, 115.0, 114.8, 36.9, 33.5, 32.4, 31.0, 27.1, MS(ESI+) (M+Na + ), m/z: Results and Discussion In an initial experiment, the reaction of dimedone with an aromatic aldehyde in the presence of a catalytic amount of 1,4-diazabicyclo[2.2.2]octane(DABCO) was examined(scheme 1). In a typical procedure, a mixture of benzaldehyde(1.0 mmol), dimedone(2.0 mmol) and DABCO(0.5 mmol) in acetonitrile was stirred overnight at room temperature. After workup we were very surprised to find that the resulting product was not the expected tetraketones(3). A new spiro dihydrofuran(4a) was formed in a yield of 61% as a sole product. Then various aromatic aldehydes were also tested under the same conditions. It is interesting to find that only p-t-butylbenzaldehyde and aliphatic aldehydes such as propionaldehyde and butyraldehyde gave the tetraketones 3a 3c as main products. Other substituted aromatic aldehydes with alkyl, alkoxy, chloro and bromo groups all yield the corresponding spiro dihydrofurans 4b 4g in high yields(table 1). The structures of tetraketones 3a 3c and spiro dihydrofurans 4a 4g were fully characterized by 1 H and 13 C NMR, MS, IR spectra and elemental analysis and were further confirmed by single X-ray diffraction study performed for an representative compound 4a(Fig.1). 1 H NMR spectra display that tetraketones 3a 3c existed in enol form with the sign of hydroxyl group at about δ From Fig.1 it is clearly seen that the spiro cyclohexane ring exists in chair conformation and the cyclohexene ring shows a twisted Scheme 1 Synthetic route of compounds 3a 3c and 4a 4c Table 1 Syntheses of tetraketones and spiro dihydrofurans Entry R Compd. Yield(%) 1 C 6 H 5 4a 61 2 p-ch 3 C 6 H 4 4b 69 3 p-ch 3 OClC 6 H 4 4c 53 4 p-ch 3 CH 2 C 6 H 4 4d 70 5 p-(ch 3 ) 2 CHC 6 H 4 4e 63 6 p-brc 6 H 4 4f 66 7 m-clc 6 H 5 4g 62 8 CH 3 CH 2 3a 61 9 p-ch 3 CH 2 CH 2 3b p-(ch 3 ) 3 CC 6 H 4 3c 57

4 52 CHEM. RES. CHINESE UNIVERSITIES Vol.27 Fig.1 Molecular structure of spiro dihydrofuran(4a) conformation. This unprecedented result is of value to us not only because we are interested in the design of the new multicomonent reaction, but also because we are unable to find examples of other methods allowing for such convenient synthesis in the related literature. Very recently Wang et al. [26] have described the synthesis of spiro dihydrofurans via the oxidative addition reaction of various aldehydes with dimedone promoted by molecular iodine and 4-dimethylaminopyridine under mechanical milling conditions. They proposed a possible reaction mechanism based on the α-iodonated tetraketones as the key intermediate. The similar spirodihydrofurans were facilely generated by the CAN mediated oxidative addition of dimedone to olefins [27,28]. Here the spiro dihydrofurans clearly come from the further reactions of the in situ formed tetraketones. The reactions of cyclohexane-1,3-dione with aromatic aldehydes under DABCO as base catalyst were also examined. Under similar reaction conditions, all the tested aromatic aldehydes gave 9-aryl-1,8-dioxo-xanthenes(5a 5f) as main products instead of above mentioned tetraketones(3) or spiro dihydrofurans(4, Scheme 2). The structures of 1,8-dioxo-xanthenes (5a 5f) were established via spectroscopic method and were confirmed by the determination of single crystal of an representative compound 5d(Fig.2). In the literature this kind of products was prepared by the depronation of previously tetraketones under weak acidic condition [15] and the acid catalyzed condensation of cyclohexane-1,3-dione with aromatic aldehydes [16 21]. Here our results show that 1,8-dioxo-xanthenes can be directly formed in one step from the base catalyzed condensation reaction of aromatic aldehydes with cyclohexane-1,3-dione. In this DABCO catalyzed condensation reaction of aromatic aldehydes with dimedone and cyclohexane-1,3-dione, three different kinds of products were formed in different cases. Though the exact reaction mechanism is now not well understood, a plausible reaction course for this tandem reaction can be proposed, which is illustrated in Scheme 3. The first step is the base catalyzed Knoevenagel condensation of dimedone with aromatic aldehyde to form 2-arylidene dimedone(a). The second step is Michael addition of excess dimedone to 2-arylidene dimedone(a) to form tetraketone(3). The in situ formed tetraketone could be transformed further according to two different paths. In the first path, a carbanion(b) was formed by the deprotonation of tetraketone with stronger base DABCO, which in turn intramolecularly attacked the hydroxyl group of enol group to give spiro dihydrofuran(4). In the second path, the two carbonyl groups in tetraketone tautomerized to enol form, which was dehydrated to give 1,8-dioxo-xanthene(5). This tandem reaction could be stopped at the stage of the formation of tetraketone, which could also proceeds further to give cyclization product according to the structure of the employed reaction components and the reactivity of base catalyst. Here cyclohexane-1,3-dione and dimedone showed different reactivities and gave different kinds of products. At present the reason for this difference is not very clear and needs further experimental and structural computational investigation. Scheme 2 Synthetic route of 1,8-dioxo-xanthenes(5a 5f) 5a 5f: Ar=C 6 H 5, p-ch 3 C 6 H 5, p-ch 3 OC 6 H 5, p-(ch 3 ) 2 CHC 6 H 5, p-clc 6 H 5, p-brc 6 H 5, respectively. Scheme 3 Reaction mechanism for the formation of three products Fig.2 Molecular structure of 1,8-dioxo-xanthene(5d) 4 Conclusions In summary, we have described an interesting DABCO

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