Note H 3 PW 12 O 40 /SiO 2 : An Eco-friendly Alternative for the Stereo-, Regio- and Chemoselective Claisen-Schmidt Condensation BiBi Fatemeh Mirjalili* and Zahra Zaghaghi Department of Chemistry, College of Science, Yazd University, P.O. Box 89195-741, Yazd, Iran H 3 PW 12 O 40 /SiO 2 or (PW/SiO 2 ) promotes the regio-, stereo- and chemoselective Claisen-Schmidt condensation with improved yields. Keywords: Dodecatungstophosphoric acid; H 3 PW 12 O 40 /SiO 2 ; Aldol condensation; Claisen-Schmidt condensation; a,b-Unsaturated ketones; Heteropoly acids. INTRODUCTION Claisen-Schmidt condensation as a type of crossed aldol condensation is an effective pathway for the prepara- tion of a,b-bis(substituted benzylidene)cycloalkanones as precursors for the synthesis of bioactive pyrimidine deriva- ties or nikkomycine. 1 Crossed aldol condensation is always carried out in the presence of acids or bases such as silica sulfuric acid, 2 Mg(HSO 4 ) 2 , 3 TiO 2 , 4 RuCl 3 , 5 TiCl 3 (SO 2 CF 3 ), 6 KHSO 4 , 7 [Cp*Rh(h 6 -C 6 H 6 )](BF 4 ) 2 , 8 NaOAc/HOAc, 9 ZrCl 4 , 10 FeCl 3 .6H 2 O, 11 LiOH.H 2 O, 12 I 2 , 13 Al 2 O 3 -MgO 14 acid-base functionalized catalyst 15 and polymer supported sulphonic acid, 16 among others. The use of toxic and ex- pensive reagents, low yields, long reaction times and for- mation of a mixture of products are among the drawbacks of the reported procedures. Recently, more attention has been paid to Claisen-Schmidt condensation. RESULTS AND DISCUSSION In continuation of our studies on the application of solid acids in organic synthesis 17-22 we report a simple, effi- cient and selective protocol for the Claisen-Schmidt con- densation in the presence of (H 3 PW 12 O 40 ) or (PW). It is ob- vious that bulk PW has a low surface area. The high surface area can be achieved by dispersing the PW on some solid supports with high surface areas. Thus, we have used silica, titania or zirconia supported H 3 PW 12 O 40 for Claisen-Schmidt condensation (Table 1). The results have indicated that the PW/SiO 2 was the best one among others. In the preparation of PW/SiO 2 , we have used 0.2 mL of HCl (0.5 M) to prevent the PW disso- 694 Journal of the Chinese Chemical Society, 2008, 55, 694-697 * Corresponding author. Fax: +98 351 821 0644; E-mail: fmirjalili@yazduni.ac.ir Table 1. Claisen-Schmidt condensation of 2 mmol of 3-nitro- benzaldehyde and 1 mmol of cyclohexanone in various conditions a Entry Catalyst Condition/solvent Yield a (%) 1 PW(0.06 g) Reflux/toluene 65 2 PW(0.06 g) Reflux/acetonitrile 52 3 PW(0.06 g) Reflux/THF 50 4 PW(0.06 g) Reflux/HOAc 70 5 PW(0.06 g) Reflux/EtOH 75 6 PW(0.06 g) Reflux/CH 2 Cl 2 80 7 PW(0.06 g) 50-60 °C/- 75 8 PW(0.06 g) Ultrasound/CH 2 Cl 2 35 9 SiO 2 50-60 °C/- - 10 60% PW/SiO 2 (0.06 g) 50-60 °C/- 90 11 60% PW/TiO 2 (0.06 g) 50-60 °C/- 62 12 60% PW/ZrO 2 (0.06 g) 50-60 °C/- 85 13 40% PW/SiO 2 (0.06 g) 50-60 °C/- 80 14 50% PW/SiO 2 (0.06 g) 50-60 °C/- 85 15 70% PW/SiO 2 (0.06 g) 50-60 °C/- 90 16 60% PW/SiO 2 (0.04 g) 50-60 °C/- 82 17 60% PW/SiO 2 (0.05 g) 50-60 °C/- 85 18 60% PW/SiO 2 (0.08 g) 50-60 °C/- 90 19 60% PW/SiO 2 (0.06 g) b 50-60 °C/- 88 20 60% PW/SiO 2 (0.06 g) b 50-60 °C/- 82 a Isolated yield. b Catalyst reused in three successive runs; the catalyst was filtered, washed with dry acetone and dried in a domestic microwave oven for 20 min at a power of 100.