Tungsten Hexachloride (WCl6) as a Mild and Efficient Reagent for Deprotection of Acetals and Ketals$ Habib Firouzabadi,* Nasser Iranpoor* and Babak Karimi Department of Chemistry, College of Sciences, Shiraz University, Shiraz 71454, Iran A variety of acetals and ketals are efficiently and rapidly converted to the corresponding carbonyl compounds by using WCl6 in dichloromethane or acetonitrile at room temperature. Acetals are one of the most useful and versatile protecting groups in organic syntheses.They Rnd widespread appli- cation for example in the protection of carbonyl, hydroxy, and diol functions.1 Therefore, regeneration of the parent carbonyl group from their masked form seems to be a useful synthetic process. There are several methods for the deprotection of acetals and ketals such as aqueous acid hydrolysis,1 and K-10 montmorillonite in aqueous methanol,2 and non-aqueous methods including those based on TeCl4,3 [Ru(CH3CN)3(triphos)](OTf )2,4 silica-supported guanidinium chloride�}acetyl chloride,5 SiCl4�}NaI,6 K-10 montmorillonite,7 DDQ,8 Ph3P�}CBr4,9 SnCl2 2H2O,10 CuSO4�}SiO2,11 CeCl3 7H2O,12 oxidation methods13 etc.1 But the synthetic application of this transformation is so important that the introduction of improved methods continues to attract attention.New applications of tungsten hexachloride (WCl6) have been of interest to us in recent years. Halo-dehydroxylation, dihalo-de-bisubstitution reac- tions and chemoselective dithioacetalization of carbonyl compounds and transthioacetalization of acetals have been reported.14 In continuation of our studies we have found that tungsten hexachloride eciently converts various types of acetals and ketals (dimethyl, diethyl and cyclic acetals) to the corresponding carbonyl compounds under mild reaction conditions (Scheme 1).As shown in Table 1 a variety of dimethyl acetals (entry 1) and diethyl acetals (entries 2�}6), of structurally di€erent carbonyl compounds as well as 1,3-dioxolanes of aldehydes (entries 7�}10) can be cleanly deprotected at room tempera- ture using 0.15�}0.2 equivalent of WCl6 in dry CH2Cl2.On the other hand, cleavage of 1,3-dioxolanes derived from aromatic and aliphatic ketones was achieved in dry CH3CN in the presence of 0.5�}0.8 equivalent of WCl6 (entries 11�}16). Nitro and methoxy groups are conserved in this method (entries 5, 8). In conclusion, mild reaction con- ditions, easy work-up, and excellent yields of the desired products are worthy of mention as advantages of the present method.J. Chem. Research (S), 1998, 664�}665$ Scheme 1 Table 1 Deprotection of acetals and ketals with WCl6 Entry R1 R2 XX Subst. :WCl6 ratio Solvent Time/min Yielda (%) 1 Ph H (OMe)2 1:0.15 CH2Cl2 5 92 2 Ph H (OEt)2 1:0.15 CH2Cl2 5 93 3 p-MeC6H4 H (OEt)2 1:0.15 CH2Cl2 3 94 4 p-ClC6H4 H (OEt)2 1:0.15 CH2Cl2 5 91 5 p-NO2C6H4 H (OEt)2 1: 0.2 CH2Cl2 7 98 6 (OEt)2 1: 0.2 CH2Cl2 5 98 7 p-MeC6H4 H -OCH2CH2O- 1: 0.2 CH2Cl2 5 90 8 p-MeOC6H4 H -OCH2CH2O- 1: 0.2 CH2Cl2 5 89 9 PhCH.CH H -OCH2CH2O- 1: 0.2 CH2Cl2 5 86 10 n-C6H13 H -OCH2CH2O- 1:0.25 CH2Cl2 15 81 11 Ph Me -OCH2CH2O- 1: 0.7 CH3CN 10 94 12 p-ClC6H4 Me -OCH2CH2O- 1: 0.8 CH3CN 10 89 13 p-PhC6H4 Me -OCH2CH2O- 1: 0.8 CH3CN 10 82 14 PhCH2CH2 Me -OCH2CH2O- 1: 0.6 CH3CN 10 91 15 -OCH2CH2O- 1: 0.6 CH3CN 20 75 16 -OCH2CH2O- 1: 0.8 CH3CN 15 90 aThe yields refer to isolated pure products. Experimental General Procedure for Deprotonation of Acetals with WCl6.DTo a solution of acetal 1 (2 mmol) in dry CH2Cl2 or CH3CN (10 ml), WCl6 (0.3�}1.6 mmol) was added.The solution was stirred at room $This is a Short Paper as deRned in the Instructions for Authors, Section 5.0 [see J. Chem. Research (S), 1998, Issue 1]; there is there- fore no corresponding material in J. Chem. Research (M). *To receive any correspondence. 664 J. CHEM. RESEARCH (S), 1998temperature, and the progress of the reaction was monitored by TLC.On completion (3±20 min), the reaction was quenched with NaOH aqueous solution (10%; 15 ml), and extracted with CH2Cl2 (330 ml). The organic layer was washed successively with saturated NaCl solution (215 ml), and water (15 ml) and dried over anhydrous Na2SO4. Evaporation of the solvent under reduced pressure gave almost pure product. Further puri®cation was achieved by column chromatography on silica gel or recrystalliza- tion from the appropriate solvent to give the desired product in good to excellent yields (Table 1).Financial support by the Shiraz University Research Council is gratefully acknowledged. Received, 27th May 1998; Accepted, 23rd June 1998 Paper E/8/03977A References 1 T. W. Greene and P. G. M. Wuts, Protective Groups in Organic Synthesis, Wiley, New York, 1991, pp. 178±186. 2 J. Asakura, M. J. Robins, Y. Asaka and T. H. Kim, J. Org. Chem., 1996, 61, 9026. 3 H. Tani, T. Inamasu, K. Masumoto, R.Tamura, H. Shimizu and H. Suzuki, Phosphorus, Sulfur, and Silicon, 1992, 67, 261. 4 S. Ma and L. M. Venanzi, Tetrahedron Lett., 1993, 34, 8071. 5 P. Gros, P. L. Perchec and J. P. Senet, J. Chem. Res. (S), 1995, 196. 6 S. S. Elmorsy, M. V. Bhatt and A. Pelter, Tetrahedron Lett., 1992, 33, 1657. 7 E. C. L. Gautier, A. E. Graham, A. McKillop, S. P. Standen and R. J. K. 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