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Vanadyl(IV) acetate: A mild and efficient catalyst for the deprotection of acetals and ketals
Catalysis Communications 2 (2001) 301±304 www.elsevier.com/locate/catcom
Vanadyl(IV) acetate: A mild and ecient catalyst for the deprotection of acetals and ketals M. Lakshmi Kantam *, V. Neeraja, P. Sreekanth Inorganic Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500007, India Received 3 August 2001; received in revised form 5 October 2001; accepted 5 October 2001
Abstract Vanadyl(IV) acetate in acetonitrile eciently converts acetals and ketals into the corresponding carbonyl compounds in quantitative yields under mild conditions for the ®rst time in a novel heterogeneous medium. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Vanadyl(IV) acetate; Acetals; Ketals; Deprotection
1. Introduction The selective introduction and removal of protecting groups have great signi®cance in organic synthesis. The success of the methodology largely depends on the stability of the protecting groups towards dierent acidic or non-acidic reagents and also how easily they can be incorporated and removed. Protection of carbonyl groups into ketals or acetals, in particular, the 1,3-dioxolanes, as well as their cleavage to the corresponding carbonyl compounds, is a well recognised method [1] specially in the total synthesis of complex natural products. Acetal group is one of the most useful protective groups, which is stable under non-acidic
* Corresponding author. Tel.: +91-40-717-15-10; fax: +91-40717-09-21. E-mail address: [email protected] (M. Lakshmi Kantam).
conditions. The deprotection of acetals and ketals is usually accomplished by aqueous acid hydrolysis [1]. However, very often this method is incompatible for substrates containing acid sensitive groups. A number of methods using wet silica gel [2] or lithium tetra¯uoroborate in wet acetonitrile [3], phosphorous triiodide and diphosphorous tetraiodide [4], iodotrimethylsilane [5], chlorotrimethyl-silane/sodiumiodide [6], titanium(IV) chloride [7], triphenyl phosphine/carbon tetrabromide [8], trimethylsilyl bis(¯uorosulphuryl) imide [9], transition metals and other Lewis acids [10] and oxidative ethodologies [11] have been reported. However, these systems show some limitations including the use of inaccessible or expensive reagents, harsh or aqueous reaction conditions, and applicability for a limited range of substrates. Based on simple insolubility principle, we have recently designed a heterogeneous catalytic system, vanadyl(IV) acetate, for acylation of alcohols [12]
1566-7367/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 7 3 6 7 ( 0 1 ) 0 0 0 5 0 - 4
302
M. Lakshmi Kantam et al. / Catalysis Communications 2 (2001) 301±304
Table 1 Vanadyl(IV) acetate catalysed deprotection of acetals and ketals S. No.
Time (h)
Conversion (%)
1
0.5
98
2
2
90a
3
1
98
4
1
96
5
0.5
98, 96b
6
0.5
98
7
1
82
8
1
98
9
2
86a
10
1
95
11
0.5
96
12
2
80a
13
1
100
14
3
75
a
Substrate
Reaction carried out at 60 °C.
b
Yield after ®fth cycle.
M. Lakshmi Kantam et al. / Catalysis Communications 2 (2001) 301±304
303
Scheme 1.
and trans-esteri®cation of b-ketoesters [13]. In view of the resounding success in selective organic transformations by VO OAc2 , it is aimed to explore the possibility of using it in the deacetalisation reaction. Also the simple preparation, low cost and ease in handling this catalyst prompted us to study its application for the deprotection of acetals and ketals. 2. Experimental 2.1. Preparation of catalyst Vanadyl(IV) acetate was prepared according to the literature procedure [12]. 2.2. Preparation of acetals or ketals Acetals or ketals were prepared by reacting aldehydes or ketones with methanol in the presence of 1.5 equiv of trimethyl orthoformate and a catalytic amount of Y-Zeolite at re¯ux temperature. On completion of the reaction, the catalyst was ®ltered and the solvent was evaporated to dryness using a rotary evaporator. Then, the residue was puri®ed by ¯ash chromatography.
3H), 6.9±7.0 (d, 2H), 7.8±7.9 (d, 2H), 10.0 (s, 1H) identical to authentic samples. The rest of the products were characterised similarly. 3. Results and discussion Here, we report a mild and ecient method for the deprotection of acetals and ketals in almost to quantitative yields (Scheme 1) with a reusable catalyst, vanadyl(IV) acetate. A wide range of structurally varied dimethyl acetals or ketals and even 1,3-dioxolanes were deprotected using vanadyl(IV) acetate to their corresponding carbonyl compounds in quantitative yields (Table 1). Acetals and ketals of aromatic carbonyl compounds underwent deprotection in short time, whereas with aliphatic carbonyl compounds, the reaction is relatively slow (6±12 h) with low yields at room temperature. However, aliphatic dimethyl acetals are deprotected in a short time with excellent yields by raising the reaction temperature to 60 °C. The 1,3dioxolanes of benzaldehyde and cyclohexanone underwent only partial deprotection at room temperature but by increasing the temperature to 60 °C they aorded excellent yields. The 1,
2.3. Deprotection of acetals and ketals In a general procedure, a solution of dimethyl acetal (1 mmol) in acetonitrile (5 ml) was treated with vanadyl(IV) acetate (60 mg, 0.324 mmol) and stirred under nitrogen atmosphere at the appropriate temperature for the required time as mentioned in Table 1. The reaction was monitored by TLC. On completion of the reaction, the reaction mixture was ®ltered and the ®ltrate was concentrated under vacuum to give the desired product. The products were characterised by 1 H NMR and compared with the authentic samples. A representative example: Entry 5, 4-methoxy benzaldehyde dimethyl acetal: 1 H NMR: 3.9 (s,
Scheme 2. Plausible mechanism for deprotection of acetals and ketals.
304
M. Lakshmi Kantam et al. / Catalysis Communications 2 (2001) 301±304
3-dioxolanes of benzaldehyde and cyclohexanone were deprotected in good yields by using stoichiometric amounts of TMSN SO2 F2 [9]. The yields are very impressive when compared to some of the earlier reported methods. One such method being the SmCl3 and trimethylchlorosilane catalytic system reported by Ukaji et al. [14] which required long reaction times for good yields. The present catalytic system is very stable unlike iodotrimethylsilane reported by Olah et al. [6] which decomposes on prolonged storage. The catalyst was recovered by simple ®ltration and reused for ®ve cycles with consistent activity. A plausible mechanism for the deprotection of acetals or ketals is illustrated in Scheme 2. The vanadyl(IV) acetate 1 reacts with acetal or ketal to give vanadium complex 2, which loses CH3 OV OAc2 O species 3 to give oxonium ion 4. This CH3 OV OAc2 O species then attacks the methyl of the oxonium ion to give dimethyl ether and free carbonyl compound by regenerating the original catalyst as proposed in the case of TMSN SO2 F2 [9]. In case of 1,3-dioxolanes, one would expect ethylene oxide formation instead of dimethyl ether. 4. Conclusions Vanadyl(IV) acetate was observed to be an ef®cient catalyst for the deprotection of dimethyl acetals or ketals and 1,3-dioxolanes by simple insoluble principle. Other notable advantages offered by this procedure are the simplicity in operation, high catalytic activity and reusability of the catalyst. Acknowledgements This work was realised in the frame of an IndoFrench collaborative programme funded by IFCPAR (Project No. IFC/1106-2/96/2460). P.S. is thankful to CSIR, New Delhi, for providing Senior Research Fellowship. IICT Communication No. 4602.
References [1] C.B. Reese, in: J.F.W. McOmie (Ed.), Protecting Groups in Organic Chemistry, Plenum Press, London, 1973, Chapter 3; L.F. Fieser, M. Fieser, in: Reagents for Organic Synthesis, Wiley, New York, 1967, Vol. 1, p. 256; T.W. Greene, P.G.M. Wuts, second ed., Protecting Groups in Organic Synthesis, Vol. 31, Wiley, New York, 1991, Vol. 31; P.J. Kocienski, Protecting Groups, Thieme, Stuttgart, 1994, p. 256. [2] F. Huet, A. Lechevalier, M. Pellet, J.M. Conia, Synthesis (1978) 63. [3] B.H. Lipshutz, D.F. Harvey, Synth. Commun. 12 (1982) 267. [4] J.N. Denis, A. Krief, Angew. Chem. Int. Ed. Engl. 92 (1980) 1039; J.N. Denis, A. Krief, Angew. Chem. Int. Ed. Engl. 19 (1980) 1006. [5] M.E. Jung, W. Alex Andrew, P.L. Ornstein, Tetrahedron Lett. 48 (1977) 4175. [6] G.A. Olah, A. Hussain, B.P. Singh, A.K. Malhotra, J. Org. Chem. 48 (1983) 3667. [7] G. Balme, J. Gare, J. Org. Chem. 48 (1983) 3336. [8] G. Johnstone, W.J. Kerr, J.S. Scott, J. Chem. Soc. Chem. Commun. (1996) 341. [9] G. Kaur, A. Trehan, S. Trehan, J. Org. Chem. 63 (1998) 2365. [10] P. Gros, P. LePerchec, J.P. Senet, J. Chem. Res. Synop. (1995) 196; S.H. Wu, D.Z. Biao, Synth. Commun. 24 (1994) 2173; S. Ma, L.M. Venanzi, Tetrahedron Lett. 34 (1993) 8071; C. Chang, K.C. Chu, S. Yue, Synth. Commun. 22 (1992) 1217; A.K. Mandal, P.Y. Shrotri, A.D. Ghogare, Synthesis (1986) 221; K.S. Kim, Y.N. Song, B.H. Lee, C.S. Hahn, J. Org. Chem. 51 (1986) 404; B.H. Lipshutz, D. Pollart, J. Monforte, H. Kotsuki, Tetrahedron Lett. (1985) 705; B.H. Lipshutz, D.F. Harvey, Synth. Commun. 12 (1982) 267. [11] T. Nishiguchi, T. Ohasima, A. Nishida, S. Fujisaki, J. Chem. Soc. Chem. Commun. (1995) 1121; D.H.R. Barton, P.D. Magnus, G. Steckert, G. Smith, D. Zurr, J. Chem. Soc. Perkin Trans. 1 (1972) 542; D.H.R. Barton, P.D. Magnus, G. Smith, D. Zurr, J. Chem. Soc. Chem. Commun. (1971) 861. [12] B.M. Choudary, M.L. Kantam, V. Neeraja, T. Bandyopadhyay, P. Narsi Reddy, J. Mol. Catal. 25 (1999) 140. [13] M.L. Kantam, V. Neeraja, B. Bharathi, Ch. Venkat Reddy, Catal. Lett. 62 (1999) 67. [14] Y. Ukaji, N. Koumoto, T. Fujisawa, Chem. Lett. (1989) 1623.
Vanadyl(IV) acetate: A mild and ecient catalyst for the deprotection of acetals and ketals M. Lakshmi Kantam *, V. Neeraja, P. Sreekanth Inorganic Chemistry Division, Indian Institute of Chemical Technology, Hyderabad 500007, India Received 3 August 2001; received in revised form 5 October 2001; accepted 5 October 2001
Abstract Vanadyl(IV) acetate in acetonitrile eciently converts acetals and ketals into the corresponding carbonyl compounds in quantitative yields under mild conditions for the ®rst time in a novel heterogeneous medium. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Vanadyl(IV) acetate; Acetals; Ketals; Deprotection
1. Introduction The selective introduction and removal of protecting groups have great signi®cance in organic synthesis. The success of the methodology largely depends on the stability of the protecting groups towards dierent acidic or non-acidic reagents and also how easily they can be incorporated and removed. Protection of carbonyl groups into ketals or acetals, in particular, the 1,3-dioxolanes, as well as their cleavage to the corresponding carbonyl compounds, is a well recognised method [1] specially in the total synthesis of complex natural products. Acetal group is one of the most useful protective groups, which is stable under non-acidic
* Corresponding author. Tel.: +91-40-717-15-10; fax: +91-40717-09-21. E-mail address: [email protected] (M. Lakshmi Kantam).
conditions. The deprotection of acetals and ketals is usually accomplished by aqueous acid hydrolysis [1]. However, very often this method is incompatible for substrates containing acid sensitive groups. A number of methods using wet silica gel [2] or lithium tetra¯uoroborate in wet acetonitrile [3], phosphorous triiodide and diphosphorous tetraiodide [4], iodotrimethylsilane [5], chlorotrimethyl-silane/sodiumiodide [6], titanium(IV) chloride [7], triphenyl phosphine/carbon tetrabromide [8], trimethylsilyl bis(¯uorosulphuryl) imide [9], transition metals and other Lewis acids [10] and oxidative ethodologies [11] have been reported. However, these systems show some limitations including the use of inaccessible or expensive reagents, harsh or aqueous reaction conditions, and applicability for a limited range of substrates. Based on simple insolubility principle, we have recently designed a heterogeneous catalytic system, vanadyl(IV) acetate, for acylation of alcohols [12]
1566-7367/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 5 6 6 - 7 3 6 7 ( 0 1 ) 0 0 0 5 0 - 4
302
M. Lakshmi Kantam et al. / Catalysis Communications 2 (2001) 301±304
Table 1 Vanadyl(IV) acetate catalysed deprotection of acetals and ketals S. No.
Time (h)
Conversion (%)
1
0.5
98
2
2
90a
3
1
98
4
1
96
5
0.5
98, 96b
6
0.5
98
7
1
82
8
1
98
9
2
86a
10
1
95
11
0.5
96
12
2
80a
13
1
100
14
3
75
a
Substrate
Reaction carried out at 60 °C.
b
Yield after ®fth cycle.
M. Lakshmi Kantam et al. / Catalysis Communications 2 (2001) 301±304
303
Scheme 1.
and trans-esteri®cation of b-ketoesters [13]. In view of the resounding success in selective organic transformations by VO OAc2 , it is aimed to explore the possibility of using it in the deacetalisation reaction. Also the simple preparation, low cost and ease in handling this catalyst prompted us to study its application for the deprotection of acetals and ketals. 2. Experimental 2.1. Preparation of catalyst Vanadyl(IV) acetate was prepared according to the literature procedure [12]. 2.2. Preparation of acetals or ketals Acetals or ketals were prepared by reacting aldehydes or ketones with methanol in the presence of 1.5 equiv of trimethyl orthoformate and a catalytic amount of Y-Zeolite at re¯ux temperature. On completion of the reaction, the catalyst was ®ltered and the solvent was evaporated to dryness using a rotary evaporator. Then, the residue was puri®ed by ¯ash chromatography.
3H), 6.9±7.0 (d, 2H), 7.8±7.9 (d, 2H), 10.0 (s, 1H) identical to authentic samples. The rest of the products were characterised similarly. 3. Results and discussion Here, we report a mild and ecient method for the deprotection of acetals and ketals in almost to quantitative yields (Scheme 1) with a reusable catalyst, vanadyl(IV) acetate. A wide range of structurally varied dimethyl acetals or ketals and even 1,3-dioxolanes were deprotected using vanadyl(IV) acetate to their corresponding carbonyl compounds in quantitative yields (Table 1). Acetals and ketals of aromatic carbonyl compounds underwent deprotection in short time, whereas with aliphatic carbonyl compounds, the reaction is relatively slow (6±12 h) with low yields at room temperature. However, aliphatic dimethyl acetals are deprotected in a short time with excellent yields by raising the reaction temperature to 60 °C. The 1,3dioxolanes of benzaldehyde and cyclohexanone underwent only partial deprotection at room temperature but by increasing the temperature to 60 °C they aorded excellent yields. The 1,
2.3. Deprotection of acetals and ketals In a general procedure, a solution of dimethyl acetal (1 mmol) in acetonitrile (5 ml) was treated with vanadyl(IV) acetate (60 mg, 0.324 mmol) and stirred under nitrogen atmosphere at the appropriate temperature for the required time as mentioned in Table 1. The reaction was monitored by TLC. On completion of the reaction, the reaction mixture was ®ltered and the ®ltrate was concentrated under vacuum to give the desired product. The products were characterised by 1 H NMR and compared with the authentic samples. A representative example: Entry 5, 4-methoxy benzaldehyde dimethyl acetal: 1 H NMR: 3.9 (s,
Scheme 2. Plausible mechanism for deprotection of acetals and ketals.
304
M. Lakshmi Kantam et al. / Catalysis Communications 2 (2001) 301±304
3-dioxolanes of benzaldehyde and cyclohexanone were deprotected in good yields by using stoichiometric amounts of TMSN SO2 F2 [9]. The yields are very impressive when compared to some of the earlier reported methods. One such method being the SmCl3 and trimethylchlorosilane catalytic system reported by Ukaji et al. [14] which required long reaction times for good yields. The present catalytic system is very stable unlike iodotrimethylsilane reported by Olah et al. [6] which decomposes on prolonged storage. The catalyst was recovered by simple ®ltration and reused for ®ve cycles with consistent activity. A plausible mechanism for the deprotection of acetals or ketals is illustrated in Scheme 2. The vanadyl(IV) acetate 1 reacts with acetal or ketal to give vanadium complex 2, which loses CH3 OV OAc2 O species 3 to give oxonium ion 4. This CH3 OV OAc2 O species then attacks the methyl of the oxonium ion to give dimethyl ether and free carbonyl compound by regenerating the original catalyst as proposed in the case of TMSN SO2 F2 [9]. In case of 1,3-dioxolanes, one would expect ethylene oxide formation instead of dimethyl ether. 4. Conclusions Vanadyl(IV) acetate was observed to be an ef®cient catalyst for the deprotection of dimethyl acetals or ketals and 1,3-dioxolanes by simple insoluble principle. Other notable advantages offered by this procedure are the simplicity in operation, high catalytic activity and reusability of the catalyst. Acknowledgements This work was realised in the frame of an IndoFrench collaborative programme funded by IFCPAR (Project No. IFC/1106-2/96/2460). P.S. is thankful to CSIR, New Delhi, for providing Senior Research Fellowship. IICT Communication No. 4602.
References [1] C.B. Reese, in: J.F.W. McOmie (Ed.), Protecting Groups in Organic Chemistry, Plenum Press, London, 1973, Chapter 3; L.F. Fieser, M. Fieser, in: Reagents for Organic Synthesis, Wiley, New York, 1967, Vol. 1, p. 256; T.W. Greene, P.G.M. Wuts, second ed., Protecting Groups in Organic Synthesis, Vol. 31, Wiley, New York, 1991, Vol. 31; P.J. Kocienski, Protecting Groups, Thieme, Stuttgart, 1994, p. 256. [2] F. Huet, A. Lechevalier, M. Pellet, J.M. Conia, Synthesis (1978) 63. [3] B.H. Lipshutz, D.F. Harvey, Synth. Commun. 12 (1982) 267. [4] J.N. Denis, A. Krief, Angew. Chem. Int. Ed. Engl. 92 (1980) 1039; J.N. Denis, A. Krief, Angew. Chem. Int. Ed. Engl. 19 (1980) 1006. [5] M.E. Jung, W. Alex Andrew, P.L. Ornstein, Tetrahedron Lett. 48 (1977) 4175. [6] G.A. Olah, A. Hussain, B.P. Singh, A.K. Malhotra, J. Org. Chem. 48 (1983) 3667. [7] G. Balme, J. Gare, J. Org. Chem. 48 (1983) 3336. [8] G. Johnstone, W.J. Kerr, J.S. Scott, J. Chem. Soc. Chem. Commun. (1996) 341. [9] G. Kaur, A. Trehan, S. Trehan, J. Org. Chem. 63 (1998) 2365. [10] P. Gros, P. LePerchec, J.P. Senet, J. Chem. Res. Synop. (1995) 196; S.H. Wu, D.Z. Biao, Synth. Commun. 24 (1994) 2173; S. Ma, L.M. Venanzi, Tetrahedron Lett. 34 (1993) 8071; C. Chang, K.C. Chu, S. Yue, Synth. Commun. 22 (1992) 1217; A.K. Mandal, P.Y. Shrotri, A.D. Ghogare, Synthesis (1986) 221; K.S. Kim, Y.N. Song, B.H. Lee, C.S. Hahn, J. Org. Chem. 51 (1986) 404; B.H. Lipshutz, D. Pollart, J. Monforte, H. Kotsuki, Tetrahedron Lett. (1985) 705; B.H. Lipshutz, D.F. Harvey, Synth. Commun. 12 (1982) 267. [11] T. Nishiguchi, T. Ohasima, A. Nishida, S. Fujisaki, J. Chem. Soc. Chem. Commun. (1995) 1121; D.H.R. Barton, P.D. Magnus, G. Steckert, G. Smith, D. Zurr, J. Chem. Soc. Perkin Trans. 1 (1972) 542; D.H.R. Barton, P.D. Magnus, G. Smith, D. Zurr, J. Chem. Soc. Chem. Commun. (1971) 861. [12] B.M. Choudary, M.L. Kantam, V. Neeraja, T. Bandyopadhyay, P. Narsi Reddy, J. Mol. Catal. 25 (1999) 140. [13] M.L. Kantam, V. Neeraja, B. Bharathi, Ch. Venkat Reddy, Catal. Lett. 62 (1999) 67. [14] Y. Ukaji, N. Koumoto, T. Fujisawa, Chem. Lett. (1989) 1623.