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2838 SYNLETT Spotlight 106 SPOTLIGHT Oxalic Acid: A Very Useful Brønsted Acid in Organic Synthesis Compiled by Kovuru Gopalaiah This feature focuses on a reagent chosen by a postgraduate, highlighting the uses and preparation of the reagent in current research Kovuru Gopalaiah was born in Andhra Pradesh, India in 1976. He received his Master degree in organic chemistry from Sri Venkateswara University, Tirupati, India. In 2000 he joined the Department of Organic Chemistry, Indian Institute of Science, Bangalore, India for his PhD degree under the tutelage of Prof. S. Chandrasekhar. His research interests focus on the development of novel synthetic strategies, reaction mechanisms and study of stereoelectronic effects in the area of amide chemistry. Introduction The title compound oxalic acid is available in anhydrous and dihydrate forms and exhibits several features which have made it particularly attractive as a reagent in organic synthesis. Oxalic acid is a mild Brønsted acid, which finds application in the Beckmann reaction,1 protection and deprotection of carbonyl compounds,2–4 and various selective cleavage and hydrolytic reactions. It is frequently used as a mild acidic quench for a variety of reactions including oxidations.5 Oxalic acid has also been used as an acidic agent in a number of condensation processes such as condensation of allylic alcohols with aromatic rings,6 carbonyl compounds and hydrazines,7 aromatic amines and aldehydes,8 and it has been utilized as a bifunctional condensation partner in the synthesis of heterocyclic systems.9 Oxalic acid is the simplest of the dicarboxylic acids and is widely used in inorganic chemistry as a precipitant and chelating agent (oxalate as a bidentate ligand has been of great interest in coordination chemistry).10 Abstracts (A) Oxalic acid is an efficient reagent for the Beckmann reaction – a variety of ketoximes are converted into the corresponding secondary amides by classical anti-periplanar migration upon heating to ca. 100 °C, and aldoximes afford the corresponding nitriles. The procedure offers high yield of the desired products, gaseous by-products (CO + CO2) and the absence of reaction solvent.1 (B) Anhydrous oxalic acid is an excellent mediator for one-pot transformation of ketones to amides in the presence of hydroxylamine hydrochloride. The method is effective for various aromatic and aliphatic ketones, and provides excellent yield of the products.1 (C) In the presence of aqueous oxalic acid, enol ethers undergo expeditious hydrolysis to the corresponding ketones without concomitant migration of the double bond.11 In contrast, use of a mineral acid typically gives the conjugated ketone. This method offers considerable advantage in terms of regioselectivity of the product. SYNLETT 2004, No. 15, pp 2838–283907.12 04 Advanced online publication: 25.11.2004 DOI: 10.1055/s-2004-836028; Art ID: V10804ST © Georg Thieme Verlag Stuttgart · New York OH N (COOH)2 R-NH-CO-R1 80–130 °C, 3–15 h R1 R R, R1 = 64–95% aryl, alkyl O R R1 + NH2OH·HCI (COOH)2 R-NH-CO-R1 100–110 °C, 3.5–12.5 h 60–96% COMe Me COMe Me (COOH)2 H MeO H THF–H2O O This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. Department of Organic Chemistry, Indian Institute of Science, Bangalore – 560012, India E-mail: [email protected] SPOTLIGHT O MeO O O (COOH)2, MeCN r.t., 9 h, 86% N3 N3 MeO CN (COOH)2 OEt C (CH2)3 CH MeO OEt Acetone–H2O iPr MeO 91% MeO MeO (E) Oxalic acid on SiO2 facilitates mild acidic hydrolysis of diethyl b-dialkylamino vinylphosphonates (vinylogous phosphoramides) to b-keto phosphonates. b-Keto phosphonates are excellent reagents for homologation of aldehydes and ketones to a,b-unsaturated aldehydes and ketones via the Wadsworth–Emmons–Horner reaction.12 (F) Alcohols have been efficiently converted into alkenes either by heating with anhydrous oxalic acid or refluxing in aqueous oxalic acid.13 Appropriately inclined hydroxyl ketones have been cyclodehydrated to give cyclic ethers.14 HOCH2C(CH3)2CH2OH N R1 R PO(OEt)2 EtO Oxalic acid, SiO2, CH2Cl2 EtO 58–80% CN C (CH2)3-CHO iPr O O P R1 R R, R1 = H, alkyl, aryl, alkenyl O BnO OH Me H Oxalic acid CO2Me BnO CH2Cl2, reflux Me O CO2Me 57% (G) Oxalic acid has been used to perform selective protodestannylation and protodesilylation of dihydropyridine compounds.15 SnMe3 Oxalic acid N H O THF–H2O R 49–68% R1 N H O R R1 1 R, R = alkyl, aryl (H) Oxalic acid is an efficient reagent for the selective removal of the ester functionality of b-keto esters to the corresponding ketone by conventional acid hydrolysis and decarboxylation in aqueous media.16 EtO2C O O Oxalic acid N R CO2Et H2O, dioxane 100 °C 67–89% N R CO2Et R = alkyl, alkenyl, aryl References (1) Chandrasekhar, S.; Gopalaiah, K. Tetrahedron Lett. 2003, 44, 7437. (2) (a) Andersen, N. H.; Uh, H.-S. Synth. Commun. 1973, 3, 125. (b) Smith, A. B. III; Empfield, J. R.; Vaccaro, H. A. Tetrahedron Lett. 1989, 30, 7325. (c) Caine, D.; Venkataramu, S. D.; Kois, A. J. Org. Chem. 1992, 57, 2960. (d) Ziegler, F. E.; Becker, M. R. J. Org. Chem. 1990, 55, 2800. (3) (a) Pearson, W. H.; Poon, Y.-F. Tetrahedron Lett. 1989, 30, 6661. (b) Mitani, K.; Yoshida, T.; Morikawa, K.; Iwanaga, Y.; Koshinaka, E.; Kato, H.; Ito, Y. Chem. Pharm. Bull. 1988, 36, 367. (c) Huet, F.; Lechevallier, A.; Pellet, M.; Conia, J. M. Synthesis 1978, 63. (4) (a) Martin, V. A.; Murray, D. H.; Pratt, N. E.; Zhao, Y.-b.; Albizati, K. F. J. Am. Chem. Soc. 1990, 112, 6965. (b) Avery, M. A.; Chong, W. K. M.; Detre, G. Tetrahedron Lett. 1990, 31, 1799. (5) (a) Apparao, S.; Schmidt, R. R. Synthesis 1987, 896. (b) Liu, H.-J.; Nyangulu, J. M. Synth. Commun. 1989, 19, 3407. (6) Fieser, L. F. J. Am. Chem. Soc. 1939, 61, 3467. (7) Paquette, L. A.; Wang, T.-Z.; Vo, N. H. J. Am. Chem. Soc. 1993, 115, 1676. (8) Peesapati, V.; Pauson, P. L.; Pethrick, R. A. J. Chem. Res., Synop. 1987, 194. (9) Eweiss, N. F.; Bahajaj, A. A. J. Heterocycl. Chem. 1987, 24, 1173. (10) (a) Krishnamurty, K. V.; Harris, G. M. Chem. Rev. 1961, 61, 213. (b) Kim, D.-J.; Kroeger, D. M. J. Mater. Sci. 1993, 28, 4744. (11) (a) Burn, D.; Petrow, V. J. Chem. Soc. 1962, 364. (b) Weinstein, B.; Fenselau, A. H. J. Org. Chem. 1965, 30, 3209. (12) Boeckman, R. K.; Walters, M. A.; Koyano, H. Tetrahedron Lett. 1989, 30, 4787. (13) (a) Demyttenaere, J.; Syngel, K. V.; Markusse, A. P.; Vervisch, S.; Debenedetti, S.; Kimpe, N. D. Tetrahedron 2002, 58, 2163. (b) Carlin, R. B.; Constantine, D. A. J. Am. Chem. Soc. 1947, 69, 50. (c) Miller, R. E.; Nord, F. F. J. Org. Chem. 1950, 15, 89. (14) (a) Bartlett, P. A.; Meadows, J. D.; Ottow, E. J. Am. Chem. Soc. 1984, 106, 5304. (b) Lygo, B.; O’Connor, N. Tetrahedron Lett. 1987, 28, 3597. (15) (a) Comins, D. L.; Abdullah, A. H.; Mantlo, N. B. Tetrahedron Lett. 1984, 25, 4867. (b) Comins, D. L.; Hong, H. J. Am. Chem. Soc. 1991, 113, 6672. (16) Giles, M.; Hadley, M. S.; Gallagher, T. J. Chem. Soc., Chem. Commun. 1990, 1047. Synlett 2004, No. 15, 2838–2839 © Thieme Stuttgart · New York This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. (D) Oxalic acid has been used for both the preparation and the cleavage of acetal and ketal functionalities. Ketones and aldehydes react with ethylene glycol or ethanol in the presence of anhydrous oxalic acid to provide the corresponding ketals and acetals. Cleavage of ketals and acetals to regenerate the carbonyl group has been accomplished with the use of aqueous oxalic acid. Acid-sensitive functionality is well tolerated under these conditions.2–4 2839