咪唑的合成（共2篇）
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苯并咪唑的合成篇一
Weinreb amide as an efficientreagent in the one pot synthesis of benzimidazoles and benzothiazoles Yadaganahalli K. Bommegowda    a , Gejjalagere S. Lingaraju a , Saji Thamas b , Koravangala S. Vinay Kumar a , Challanayakanahally S. Pradeepa Kumara    a , Kanchugarakoppal S. Rangappa a , ⇑, Marilinganadoddi P. Sadashiva a , ⇑
a b
Department of Studies in Chemistry, University of Mysore, Manasagangotri, Mysore 570006, India Jubilant Life Sciences Ltd., C-26, Sector 59, Noida 201301, India
a r t i c l e i n f o a
b s t r a
c t
One pot synthesis of 2-substituted benzimidazoles/benzothiazolesthrough condensation is followed by cyclization of Weinreb amide with o -diaminoarene or o -aminothiophenol is reported. In the presence of boron trifluorideetherate in 1,4-dioxane solvent, a high yield (75–94%)was achieved within 60min. Weinreb amide shows high selectivity in the reaction, even in presence of other active functional groups such as carboxyl, halogen, cyano, and methoxy.
Article history:
Received 31December 2012Revised 9March 2013Accepted 13March 2013
Available online 23March 2013Keywords:
Weinreb amide Benzimidazoles Benzothiazoles
Boron trifluorideetherate o -Diaminoarene o -Aminothiophenol
Ó2013Elsevier Ltd. All rights reserved.
2-Substituted benzimidazoles and benzothiazoles have gained importance in medicinal chemistry. They have been in use as anti-inflammatory,1a antimicrobial, 1b anti-ulcer, 1c antihelmentic, 1c antihypertensive, 1d anti-analgesic, 1e
antivirus, 1f and anticancer agents. 1g The general method of synthesis of 2-substituted benzim-idazoles involves the reaction between o -diaminoarene and a car-boxylic acid or an acid chloride or nitrile or imadates or ortho esters. The condensation of arenealdehyde with 1,2-diaminoben-zene to form    a Schiff base which undergoes intramolecular cycliza-tion generating    a dihydrobenzimidazole which undergoes dehydrogenation by 1,4-benzoquinone. 2Alternative oxidants that can be used are 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ),3manganese dioxide, 4lead tetraacetate {Pb(OAC)4},5potassium-monopersulfate (oxone),6and sodium bisulfate. 7Synthesis ofreaction to a book or an article
Corresponding authors. T el.:+918212419661; fax:+918212500846.
E-mail addresses:************************.in, ********************(K.S.Rangappa),
*********************(M.P.Sadashiva).
0040-4039/$-see front matter Ó2013Elsevier Ltd. All rights reserved. ing all these problems.
N -Methoxy-N -methylamide is popularly known as Weinreb amide, which Nahm and Weinreb firstemployed in
the synthesis of ketones from organometallics and also used as acylating agent to synthesize ester en
olate. 22Nowadays, the versatility of this re-agent has been increased in organic synthesis for instance, in the preparation of aldehydes, 23a ketones by classical Wittig reaction, 23b Grignard reactions, 24a,b organolithium additions, 24c–elithium aluminum hydride reductions, 24f acylation of oxazole, 25pyrazoles, 26nitriles, 27pyrrole carbaldehyde and pyrrolidinones, 28trifluoromethylketones, 29etc. The four main reasons for its in-creased use by chemists as a reagent in organic synthesis are the stability of Weinreb amide functionality, ease of preparation, the scalability of reaction and predictability. As part of our research on Weinreb amide application, we tried one pot syntheses of 2-substituted benzimidazoles and benzothiazoles.
Weinreb amides were synthesized from the carboxylic acid by coupling with N , O -dimethyl-hydroxyl amine. 30Optimization of the reaction conditions and parameters were stabilized for the syn-thesis of 2-phenyl-1H -benzimidazole (3a ) by cyclization of N -methoxy-N -methylbenzamide (1a ) with o -diamino benzene (2a ) using various Lewis acids in 1,4-dioxane solvent (T able 1).
The reaction was carried out initially between 1a and 2a in the presence of zinc chloride as Lewis acid in 1,4-dioxane at room tem-perature, but product 3a was not obtained even after a prolonged time of 24h (T able 1, entry 1). When the reaction was continued at 100°C no improvement in yield was observed (T able 1, entry 2). Further, the reaction was done with various Lewis acids such as AlCl 3, F
eCl 3, SnCl 4, BF 3ÁOEt 2, Sc(OTf)3, InCl 3and PTSA to enhance the product yield (T able 1, entries 3–9).Among these reagents, only BF 3ÁOEt 2gave the highest product yield of 88%(T able 1, entry 6).
The solvent and temperature factors on the cyclization reaction between 1a and 2a in the presence of boron trifluorideetherate were optimized (T able 2). Although most of the solvents promoted the reaction, 1,4-dioxane and toluene were found suitable media at 100°C conditions (T able 2, entries 1and 4).
Finally, the optimized reaction conditions were used to study the generality of the protocol and scope of this cyclization reaction. We have studied the reactivity of various alkyl, alkenyl, aryl, and hetero aryl Weinreb amides and the selectivity of Weinreb amide

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