Advances in Heterocyclic Aromatic Construction
Rubén Vicente and Luis A. López at the University of Oviedo in Spain reported (Angew. Chem. Int. Ed. 2012, 51, 8063. DOI: 10.1002/anie.201203914) the synthesis of cyclopropyl furan 2 from alkylidene 1 and styrene by way of a zinc carbene intermediate. The same substrate 1 was also converted (Angew. Chem. Int. Ed. 2012, 51, 12128. DOI: 10.1002/anie.201207300) to furan 3 via catalysis with tetrahydrothiophene in the presence of benzoic acid by J. Stephen Clark at the University of Glasgow. Xue-Long Hou at the Shanghai Institute of Organic Chemistry discovered (Org. Lett. 2012, 14, 5756. DOI: 10.1021/ol302586m) that palladacycle 6 catalyzes the conversion of bicyclic alkene 4 and alkynone 5 to furan 7. A silver-mediated C-H/C-H functionalization strategy for the synthesis of furan 9 from alkyne 8 and ethyl acetoacetate was developed (J. Am. Chem. Soc. 2012, 134, 5766. DOI: 10.1021/ja301153k) by Aiwen Lei at Wuhan University.
Ning Jiao at Peking University and East China Normal University found (Org. Lett. 2012, 14, 4926. DOI: 10.1021/ol302270z) that azide 10 and aldehyde 11 could be converted to either pyrrole 12 or 13 with complete regiocontrol by judicious choice of metal catalyst. Meanwhile, Michael A. Kerr at the University of Western Ontario developed (Angew. Chem. Int. Ed. 2012, 51, 11088. DOI: 10.1002/anie.201206177) a multicomponent synthesis of pyrrole 16 involving the merger of nitrone 14 and the donor-acceptor cyclopropane 15. The pyrrole 16 was subsequently converted to an intermediate in the synthesis of cholesterol-lowering drug compound Lipitor.
A robust synthesis of the ynone trifluoroboronate 17 was developed (Org. Lett. 2012, 14, 5354. DOI: 10.1021/ol302418b) by James D. Kirkham and Joseph P. A. Harrity at the University of Sheffield, which thus allowed for the ready production of trifluoroboronate-substituted pyrazole 18. An alternative pyrazole synthesis via oxidative closure of unsaturated hydrazine 19 to produce 20 was reported (Org. Lett. 2012, 14, 5030. DOI: 10.1021/ol3022353) by Yu Rao at Tsinghua University.
A unique fluoropyrazole construction was developed (Angew. Chem. Int. Ed. 2012, 51, 12059. DOI: 10.1002/anie.201206946) by Junji Ichikawa at the University of Tsukuba that involved nucleophilic substitution of two of the fluorides in 21 to form pyrazole 22. Yunfei Du and Kang Zhao at Tianjin University demonstrated (Org. Lett. 2012, 14, 5480. DOI: 10.1021/ol3025583) that β-acyloxylation of enamine 23 with acid 24 followed by dehydrative cyclization led to the formation of oxazole 25. Amide formation between acid chloride 26 and propargyl amine (27) followed by iron(III) chloride-promoted cyclization led to oxazole 28 as reported (Org. Lett. 2012, 14, 4478. DOI: 10.1021/ol301980g) by Jeh-Jeng Wang at Kaohsiung Medical University in Taiwan. Ning Jiao at Peking University discovered (Angew. Chem. Int. Ed. 2012, 51, 11367. DOI: 10.1002/anie.201206382) that phenylacetaldehyde (29) and amine 30 were converted to oxazole 31 by a copper-mediated oxygenation/annulation procedure.
An interesting approach to indole synthesis involving the cascade combination of aniline derivative 32 and sulfur ylide 33 was reported (Angew. Chem. Int. Ed. 2012, 51, 9137. DOI: 10.1002/anie.201203657) by Wen-Jing Xiao at Central China Normal University and Lanzhou University. Finally, Jieping Zhu at the EPFL in Switzerland developed (Angew. Chem. Int. Ed. 2012, 51, 12311. DOI: 10.1002/anie.201205596) a method to couple ortho-alkynylaniline 35 with 3-butyn-1-ol to produce the complex indole 36.