An Intramolecular Wittig Approach toward Heteroarenes: Synthesis of Pyrazoles, Isoxazoles, and Chromenone-oximes
α-Halohydrazones/ketoximes are transformed into trisubstituted pyrazoles/disubstituted isoxazoles by treatment with phosphine, acyl chloride, and a base. Mechanistic investigations revealed the in situ formation of azo/nitroso olefin intermediates which underwent a tandem phospha-Michael/N- or O-acylation/intramolecular Wittig reaction to afford the heteroarenes in moderate to good yields. Further, proper functionalization of α-haloketoximes and a change of conditions allowed the chemoselective synthesis of chromenone-oximes as well as rearranged isoxazoles, thereby realizing a diversity-oriented synthesis.
'Click synthesis' of 1H-1,2,3-triazolyl-based oxiconazole (=(1Z)-1-(2,4-dichlorophenyl)-2-(1H-imidazol-1-yl)ethanone O-[(2,4- dichlorophenyl)methyl]oxime) analogs
The 'click synthesis' of some oxiconazole analogs 5a-5v having 1H-1,2,3-triazolyl residues by Huisgen cycloaddition was achieved in four steps (Scheme 1). Oximation of phenacyl chloride (1) followed by azidation of 2-chloro-1-phenylethanone oxime (2) provided azido ketoxime 3. The CuI-catalyzed Huisgen cycloaddition of 3 with terminal alkynes gave the 4-substituted (at the triazole) 2-(1H-1,2,3-triazol-1-yl)-1-phenylethanone oximes 4a-4i. The O-alkylation of 4a-4i with various alkyl halides resulted in the formation of the target molecules 5a-5v in good yields. Copyright
Soltani Rad, Mohammad Navid,Asrari, Zeinab,Behrouz, Somayeh,Hakimelahi, Gholam Hossein,Khalafi-Nezhad, Ali
experimental part
p. 2194 - 2206
(2012/01/31)
Synthesis of 6-ethoxy-6H-1,2-oxazines by hetero Diels-Alder reaction of 1-bromo-2-ethoxyethene with α-nitroso alkenes
A variety of 6H-1,2-oxazines 10 could efficiently be prepared by hetero Diels-Alder reaction of α-nitroso alkenes 2 with 1-bromo-2-ethoxyethene (8) and consecutive elimination of HBr by DBU. Heterodienes 2 were generated in situ from α-halogen oximes 6. Primary cycloadducts 9 were isolated in singular cases, however, an attempt to substitute bromide in 9e by an azido group gave the desired compound 11 only as minor component together with elimation product 10e. 6H-1,2-oxazines 10 are valuable precursors for addition reactions which open new synthetic possibilities.