T. W. Green, P. G. M. Wuts,
Protective Groups in Organic Synthesis,
Wiley-Interscience, New York, 1999, 308-322, 724-727.
|H2O:||pH < 1, 100°C||pH = 1, RT||pH = 4, RT||pH = 9, RT||pH = 12, RT||pH > 12, 100°C|
|Reduction:||H2 / Ni||H2 / Rh||Zn / HCl||Na / NH3||LiAlH4||NaBH4|
|Oxidation:||KMnO4||OsO4||CrO3 / Py||RCOOOH||I2, Br2, Cl2||MnO2 / CH2Cl2|
1,3-Dioxanes and 1,3-dioxolanes can easily be prepared from carbonyl compounds with 1,3-propanediol or 1,2-ethanediol in the presence of a Brönsted or a Lewis acid catalyst. 1,3-Diols give more stable compounds. A standard procedure for protection employs toluenesulfonic acid as catalyst in refluxing toluene, which allows the continuous removal of water from the reaction mixture using a Dean-Stark apparatus. A mixture of orthoesters or molecular sieves can also provide effective water removal through chemical reaction or physical sequestration.
Cyclic acetals offer stability against all types of nucleophiles and bases.
Deprotection is often performed by acid catalyzed transacetalization in acetone or hydrolysis in wet solvents or in aqueous acid. Some strong oxidation agents may cleave acetals such as HClO4 in CH2Cl2. Cyclic ketals and acetals as a rule are stable to mild high-valent chromium reagents (PCC, PDC, Jones, etc.), but strongly acidic reagents will oxidize them to the lactone, or related cleavage products. Addition of strong Lewis acids enhances the sensitivity towards oxidants such as KMnO4, and MCPBA.
Protection of Carbonyl Compounds
Acyclic and cyclic acetals of various carbonyl compounds were obtained in excellent yields in the presence of trialkyl orthoformate and a catalytic amount of tetrabutylammonium tribromide in absolute alcohol. This convenient, mild, chemoselective method allows acetalization of an aldehyde in the presence of ketone, unsymmetrical acetal formation, and tolerates acid-sensitive protecting groups.
R. Gopinath, Sk. J. Haque, B. K. Patel, J. Org. Chem., 2002, 67, 5842-5845.
Zirconium tetrachloride (ZrCl4) is a highly efficient and chemoselective catalyst for the acetalization, and in situ transacetalization of carbonyl compounds under mild reaction conditions.
H. Firouzabadi, N. Iranpoor, B. Karimi, Synlett, 1999, 321-323.
Various types of carbonyl compounds are efficiently converted to their 1,3-dioxanes by the use of 1,3-bis(trimethylsiloxy)propane (BTSP) and a catalytic amount of iodine under essentially neutral aprotic condition.
B. Karimi, B. Golshani, Synthesis, 2002, 784-788.
Using a photochemical method for acetalization of aldehydes under low-energy visible light irradiation, a broad range of aromatic, heteroaromatic, and aliphatic aldehydes have been protected under neutral conditions in good to excellent yields using a catalytic amount of Eosin Y as the photocatalyst. Even challenging acid-sensitive aldehydes and sterically hindered aldehydes can be converted, while ketones remain intact.
H. Yi, L. Niu, S. Wang, T. Liu, A. K. Singh, A. Lei, Org. Lett., 2017, 19, 122-125.
Various types of hydroxyacetophenones are efficiently converted into the corresponding cyclic acetals in the presence of a diol, triisopropyl orthoformate, and a catalytic amount of cerium(III) trifluoromethanesulfonate under mild reaction conditions.
F. Ono, H. Takenaka, T. Fujikawa, M. Mori, T. Sato, Synthesis, 2009, 1318-1322.
Aliphatic and aromatic ketones can be directly converted into their corresponding α-chloroketone acetals in very good yields using iodobenzene dichloride in ethylene glycol in the presence of 4 Å molecular sieves at room temperature.
J. Yu, C. Zhang, Synthesis, 2009, 2324-2328.
Other Syntheses of Cyclic Acetals
Ammonium salts that can act as hydrogen-bond donors exert a remarkable acceleration on the rates of the regioselective arylation of electron-rich olefins by aryl halides in ionic liquids and common solvents.
J. Mo, J. Xiao, Angew. Chem. Int. Ed., 2006, 45, 4152-4157.
The addition of bromomagnesium 2-vinyloxy ethoxide to various aldehydes in the presence of 10 mol% Sc(OTf)3 provides a broad range of functionalized protected aldol compounds. A Swern oxidation-CBS reduction sequence enables the preparation of chiral protected aldol products.
P. Quinio, L. Kohout, D. S. Roman, J. Gaar, K. Karahiosoff, P. Knochel, Synlett, 2016, 27, 1715-1719.
A new palladium(0)-catalyzed three-component reaction of salicylic aldehyde triflates, ethylene glycol vinyl ether, and various secondary nucleophilic amines involving an initial internal Heck arylation, iminium ion formation, and subsequent tandem cyclization gives tertiary 3-aminoindan acetals.
A. Arefalk, M. Larhed, A. Hallberg, J. Org. Chem., 2005, 70, 938-942.
Acetals and ketals are readily deprotected under neutral conditions in the presence of acetone and indium(III) trifluoromethanesulfonate as catalyst at room temperature or mild microwave heating conditions to give the corresponding aldehydes and ketones in good to excellent yields.
B. T. Gregg, K. C. Golden, J. F. Quinn, J. Org. Chem., 2007, 72, 5890-5893.
Er(OTf)3 is a very gentle Lewis acid catalyst in the chemoselective cleavage of alkyl and cyclic acetals and ketals at room temperature in wet nitromethane.
R. Dalpozzo, A. De Nino, L. Maiuolo, M. Nardi, A. Procopio, A. Tagarelli, Synthesis, 2004, 496-498.
A chemoselective method for the cleavage of acetals and ketals at room temperature in wet nitromethane by using catalytic cerium(III) triflate at almost neutral pH is presented. High yields and selectivity make this procedure particularly attractive for multistep synthesis.
R. Dalpozzo, A. De Nino, L. Maiuolo, A. Procopio, A. Tagarelli, G. Sindona, G. Bartoli, J. Org. Chem., 2002, 67, 9093-9095.
A convenient deprotection of acyclic and cyclic O,O-acetals and O,O-ketals is achieved in excellent yields within minutes under neutral conditions in the presence of a catalytic amount of iodine. Double bonds, hydroxyl groups, acetate groups, and highly acid-sensitive groups such as furyl, tert-butyl ethers, and ketoximes are tolerated.
J. Sun, Y. Dong, L. Cao, X. Wang, S. Wang, Y. Hu, J. Org. Chem., 2004, 69, 8932-8934.
Deprotection of acetals and ketals can be achieved by using a catalytic amount of sodium tetrakis(3,5-trifluoromethylphenyl)borate (NaBArF4) in water at 30 °C. For example, a quantitative conversion of 2-phenyl-1,3-dioxolane into benzaldehyde was accomplished within five minutes.
C.-C. Chang, B.-S. Liao, S.-T. Liu, Synlett, 2007, 283-287.
Conversion of Cyclic Acetals and Ketals to Other Functional Groups
Aldehydes and ketones were protected as their thioacetals in the presence of a catalytic amount of iodine. These mild reaction conditions were also applied in the transthioacetalization of O,O-acetals, O,O-ketals, O,S-acetals, and acylals.
H. Firouzabadi, N. Iranpoor, H. Hazarkhani, J. Org. Chem., 2001, 66, 7527-7529.
An efficient oxidation of various acetals, including open-chain acetals, 1,3-dioxanes and 1,3-dioxalanes, with molecular oxygen in the presence of catalytic amounts of N-hydroxyphthalimide (NHPI) and Co(OAc)2 as co-catalyst gave esters.
B. Karimi, J. Rajabi, Synthesis, 2003, 2373-2377.