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S-0029-1216653.Pdf SPOTLIGHT 1353 SYNLETT Isoureas: Versatile Alkylation Spotlight 277 Reagents in Organic Chemistry Compiled by Yongwei Liu This feature focuses on a re- agent chosen by a postgradu- Yongwei Liu was born in Xinzhou, Shanxi Province, P. R. of China, ate, highlighting the uses and in 1983. He obtained his B.Sc. degree in 2007 from Datong Univer- sity, P. R. of China. Currently he is working towards his M.Sc. de- preparation of the reagent in gree under the supervision of Professor Z. Li at Dalian University. current research His research interest focuses on new reactions of O-alkyl isoureas. College of Environmental and Chemical Engineering, Dalian Uni- versity, 116622 Dalian, P. R. of China E-mail: [email protected] Introduction DIC and Cu(OTf)2/CuCl was stirred for 1 h at room tem- perature. The progress of the reaction can be monitored by The name of isoureas (alternatively named pseudoureas) IR spectroscopy by the disappearance of diimide absorp- derives from their isomeric relationship to ureas. Howev- tion at 2100 cm–1 and the appearance of isourea absorption er, their chemical reactivities are much different to that of at 1660 cm–1. Pure isoureas can be obtained after column ureas. Isoureas are important, sometimes commercial re- chromatography or distillation.1 agents in organic chemistry, mainly being used as alkyla- tion reagents in esterification, etherification and in the N Cu(OTf) /CuCl (1–5 mol%) 2 N NC C ROH synthesis of alkyl halides, etc. Isoureas can be convenient- + 1 h, r.t. O NH ly prepared from alcohol and N,N-dialkyl carbodiimide. R In most cases, the alkyl group is cyclohexyl or isopropyl. A typical procedure is as following: a mixture of alcohol, Scheme 1 Abstracts (A) The reaction of O-alkyl isoureas with carboxylic acids affords N CHO the corresponding esters.2 Even though ester can be conveniently 1. MeCN, 20 h, reflux N Cy N + 2. NaHCO , H O 2 prepared by a one-pot reaction from carboxylic acid, alcohol and 3 (CH ) O 2 2 O(CH ) CO H 3 2 carbodiimide. The method via O-alkyl isourea provides an inversion 2 HN Cy of the alkyl configuration in case of a chiral one.3 S-alkyl thioate can N 4 O be obtained from thioic S-acid. N (CH ) O 2 2 CHO (H C) O 2 3 Ph N Cy O O benzene O + Ph SH Ph S Ph HN Cy (B) The reaction of polymer-supported O-methyl, O-benzyl, and O OR Cy Cy O 1 N O-allyl isoureas with carboxylic acids provides the corresponding CNH Cu(OTf) R OH 2 N CN 1 R alkyl esters in high yields and purity. The reaction can be finished OR in 3 to 5 minutes with microwave heating, without compromising ROH yield, purity, or chemoselectivity.5 (R = Me, Bn, Allyl) O O -Pr (C) Phosphate esters could be obtained via the reaction of O-alkyl i O O N H HO 6 benzene, DMF H BnO + P N OBn isoureas with phosphoric acid. C P N OBn C i-Pr H H 8 h, 80 °C BnO N This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. HO n-Bu BnO n-Bu H (D) O-alkyl isoureas can be efficiently converted into alkyl -Pr bromides and iodides by treatment with one mol equivalent of tri- i N CF SO H 3 fluoromethanesulphonic acid in the presence of an excess of tet- 3 RX R i-Pr ( n-C H ) NX 4 9 4 N rabutylammonium bromide or iodide. The conversion can be O performed either with the pure isolated O-alkyl isourea or with H crude isourea without detriment to yield.7 SYNLETT 2009, No. 8, pp 1353–1354 xx.xx.2009 Advanced online publication: 07.05.2009 DOI: 10.1055/s-0029-1216653; Art ID: V28308ST © Georg Thieme Verlag Stuttgart · New York 1354 SPOTLIGHT (E) Linclau and co-workers reported that primary and secondary i-Pr R N alcohols were converted into the corresponding alkyl halide via the R NXS, THF -Pr corresponding O-alkyl isoureas. High yields could be obtained in the i Ph X Ph O N Δ or MW case of chlorides and bromides. This method tolerates a range of H 8 functional groups and does not rely on the use of phosphines. R = H, Me (F) Linclau also reported that N-(b-hydroxy)amides could be i-Pr i-Pr cyclized with diisopropylcarbodiimide (DIC) to give the corre- N HN sponding 2-oxazolines in high yields. The reaction requires only OH O O Δ O DIC Ph very mild Lewis acid catalysis [5mol% Cu(OTf)2] and can be ac- O Cu(OTf) Ph N N complished with conventional heating or under microwave irradia- 2 H N 9 Ph tion. H -Pr (G) The reaction of O-alkyl isoureas with N-acylsulfonamide gave i the N-alkylated products. The reaction can be carried out with poly- N 2 R O 10 O O O N O -Pr mer-bound sulfonamide. i 1 H S NH C R 1 S N C R O O O O 2 R NMe 2 (H) Phenol ether and thiophenolether can be prepared via the reac- Ph O N CCl 4 + O PhSH -Pr tion of -alkyl isoureas with phenol and thiophenol, respective- i 11,12 PhS HN ly. Ph NMe 2 i-Pr CH O CH CH CHO 2 2 (I) O-alkyl isoureas were reported as allyl reagents in C–C bond for- H DMF 13 H Cy C N mation reactions in the presence of a palladium catalyst. CHO + CH CH=CH 2 2 N Cy (J) The dehydration of secondary, tertiary and benzylic alcohols R N could be carried out in good yield in the presence of carbodiimide N HO R H and catalytic amount of CuCl. It is believed that the reaction pro- OMe 14 H ceeds via O-alkyl isourea intermediates. O OMe CuCl, DCC H 25 °C H O H O OMe 4 h, 90 °C 76% H O (K) 2-Allyl isourea acts as a starting material for palladium- Pd(0) O NCy + P Ph + PhCHO 3 Ph – (NHCy) CO catalyzed Wittig-type allylidenation of aldehydes to give the 2 NHCy 15 O – Ph P corresponding conjugated olefins in moderate to good yields. 3 References (1) (a) Mathias, L. J.; Fuller, W. D.; Nissen, D.; Goodman, M. (8) Li, Z.; Crosignani, S.; Linclau, B. Tetrahedron Lett. 2003, Macromolecules 1978, 11, 534. (b) Mathias, L. J. Synthesis 44, 8143. 1979, 561. (9) Crosignani, S.; Young, A. C.; Linclau, B. Tetrahedron Lett. This document was downloaded for personal use only. Unauthorized distribution is strictly prohibited. (2) Fraga-Dubreuil, J.; Bazureau, J. P. Tetrahedron Lett. 2001, 2004, 45, 9611. 42, 6097. (10) Zohrabi-Kalantari, V.; Heidler, P.; Larsen, T.; Link, A. Org. (3) Vowinkel, E. Chem. Ber. 1967, 100, 16. Lett. 2005, 7, 5665. (4) Nowicki, T.; Markowska, A.; Kiebasinski, P.; Mikoajczyk, (11) Jaeger, R. Synthesis 1991, 465. M. Synthesis 1986, 305. (12) Poelert, M. A.; Hulshof, L. A.; Kellog, R. M. Recueil Trav. (5) (a) Crosignani, S.; White, P. D.; Steinauer, R.; Linclau, B. Chim. Pays-Bas. 1994, 113, 365. Org. Lett. 2003, 5, 853. (b) Crosignani, S.; White, P. D.; (13) Inoue, Y.; Toyofuku, M.; Taguchi, M.; Okada, S.; Linclau, B. J. Org. Chem. 2004, 69, 5897. Hashimoto, H. Bull. Chem. Soc. Jpn. 1986, 59, 885. (6) Pícha, J.; Buděšínský, M.; Šanda, M.; Jiráček, J. (14) (a) Majetich, G.; Hicks, R.; Okha, F. New. J. Chem. 1999, Tetrahedron Lett. 2008, 49, 4366. 129. (b) Robben, U.; Lindner, I.; Gaertner, W. J. Am. Chem. (7) Collingwood, S. P.; Davies, A. P.; Golding, B. T. Soc. 2008, 130, 11303. Tetrahedron Lett. 1987, 28, 4445. (15) Inoue, Y.; Toyofuku, M.; Hashimoto, H. Bull. Chem. Soc. Jpn. 1986, 59, 1279. Synlett 2009, No. 8, 1353–1354 © Thieme Stuttgart · New York.
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