2-alkenyl -5-substituted oxazoles

TABLE 1.73. 2-ALKENYL(ARYL)OXAZOLES OR 2-ALKENYL(ARYL)-5-SUBSTITUTED OXAZOLES FROM PALLADIUM-CATALYZED CROSS-COUPLING OF OXAZOL-2YLZINC CHLORIDES"... 

Wenkert and co-workers prepared a series of 5-alkenyl-substituted oxazolium salts and investigated their intramolecular Diels-Alder reactions as a means to construct alkaloids (Scheme 1.427). The starting oxazoles 1662a-c, 1663, and 1664 were prepared from an acyclic methyl ester and lithiomethyl isocyanide. Methylation of 1662a-c, 1663, and 1664 with methyl trillate then afforded the corresponding iV-methyloxazolium salt quantitatively. The oxazolium salts 1665, 1666, and 1667 were stable for up to 3 h at 90°C. 

To functionalize the C5 position, Williams and Fu developed a 2-phenylsulfonyl substituted oxazole. The C5 position of this oxazole can be cleanly deprotonated with LDA and trapped with either NIS or NBS to form the 5-iodo- or 5-bromo-2-phenylsulfonyloxazole in good yield. The same report details that the 2-phenylsulfonyl group can subsequently be displaced with alkyl, alkenyl, or aryl lithium reagents to form 2,5-disubstituted oxazoles efficiently. A triflate at the C5 position can be prepared from the corresponding oxazolone however, the oxazolone decomposes at room temperature, and Kelly reported that attempted Stille coupling with C5 triflates failed due to decomposition of the triflate. ... 

Zinc Derivatives Oxazole and 5-substituted oxazoles are lithiated in the 2-position. Subsequent zincation gives the corresponding 2-oxazolylzinc chloride 112 for alkenylation (Scheme 48). Excess of zinc chloride was used in the zincation. Subsequent cross-coupling with a 1-butenyl iodide yields 2-alkenyl derivatives. The Pd-catalyst was pregenerated by reduction with DlBALH.t The alkenylated oxazole 113 was formed in a corresponding coupling reaction with a cyclic vinyl triflate. ... 

C4-Alkenylation of 2,5 -substituted oxazoles was reported by Antilla and coworkers [59] (Scheme 18.61). The alkenylated products can be converted to functionalized amino alcohol and amino acid derivatives. 

Daugulis and Piguel s groups also developed the Cu-catalyzed versions. Thus, perfluoroarenes [70] and 5-substituted oxazoles [71] undergo alkenylation in the... 

Of the many substituted and functionalized alkenes that have been combined with diazo dipoles to give A -pyrazolines or products derived from them (i.e., A -pyrazolines, pyrazoles, cyclopropanes), only a selection will be mentioned. These include ot-alkylidene-cycloalkanones (62), -flavanones, -thioflavanones, -chroma-nones, and thiochromanones (63,64) a-arylidene-indanones and -indolones (65) diarylideneacetones (66) l-benzopyran-2(77)-ones (coumarins) (67,68) 4-nitro-1,2-oxazoles (69) 2-alkylidene-2-cyanoacetates (70) dimethyl 2,3-dicyanofuma-rate (71) tetracyanoethylene (72) tetraethyl ethylenetetracarboxylate (72) 1,4-quinones (35,73-75) 2-X-l,l,l-trifluoro-2-propene [X = Br, (76), SPh, SOPh, S02Ph (77)] nitroalkenes (78) including sugar nitroalkenes (79) 1-diethoxyphos-phoryl-1-alkenyl-sulfoxides (80) methyl 2-(acetylamino)cinnamate and -acrylate... 

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