No abstract

A concise synthesis of three members of the withasomnine family of natural products is reported. The synthesis features a regioselective sydnone-alkynylboronate cycloaddition followed by Suzuki cross coupling and silyl group removal, and marks the first divergent approach to this family of pyrazole based natural products.

Cu-salts have been found to promote the cycloaddition reaction of sydnones and terminal alkynes, providing significant reduction in reaction times. Specifically, the use of Cu(OTf)2 is found to provide 1,3-disubstituted pyrazoles whereas simply switching the promoter system to Cu(OAc)2 allows the corresponding 1,4-isomers to be produced. The mechanism of the Cu-effect in each case has been investigated by experimental and theoretical studies, and they suggest that Cu(OTf)2 functions by Lewis acid activation of the sydnone whereas Cu(OAc)2 promotes formation of reactive Cu(I)-acetylides.

Two synthetic approaches to 4-trifluoromethylsydnones, a novel class of these mesoionic reagents, are reported. These compounds undergo regioselective alkyne cycloaddition reactions, thereby providing a general approach to 5-trifluoromethylpyrazoles. This method has been employed in a short formal synthesis of the herbicide fluazolate.

We report the synthesis and some structural studies of 4-trifluoromethyl, 4-difluoromethyl-, and 4-monofluoromethylsydnones. All but the latter compounds are stable and represent effective precursors to a range of pyrazoles after cycloaddition reactions with alkynes. The cycloadditions are generally highly regioselective and provide 5-fluoromethylpyrazole products, although we have observed that Bn-substituted sydnones can provide an unexpected alkyne insertion mode that generates the 3-fluoromethyl isomer.

Irradiation of N-acylindoles in the presence of olefins produces cyclobutanes by cycloaddition of the olefin to the indole 2,3-double bond.

No abstract

The React ions of Methyl benzenes with 2,3-D ichloro-5,6-d icyano-p-benzoquinone By R. Foster and 1. HormanThe rates of decay of the charge-transfer complexes formed between 2.3-dichloro-5.6-dicyano-p-benzoquinone and various methylated benzenes have been measured. The results are interpreted in terms of a monomolecular decomposition of the charge-transfer complex and/or a bimolecular reaction of free dichlorodicyanobenzoquinone with free hydrocarbon. The association constants for initial complex formation have been evaluated from the kinetic data. These agree with direct determinations from optical density measurements of initial absorption of solutions equimolar with respect to dichlorodicyanobenzoquinone and hydrocarbon, but there is disagreement with direct evaluations of association constant from optical measurements on solutions where the hydrocarbon is in large excess (Benesi Hildebrand method). The final products are 2,3-dichloro-5,6-dicyanoquinol mono-and di-ethers.