Pyrazolones stereoselectivity

Hence, deazetization of the ketone 15 required flash-vacuum pyrolysis at 400 °C or photolysis at 185 nm. The cyclopropanone formed was unstable under these conditions and the only products obtained were 17 and 18. However, conversion of ketone 15 to its imine 19 and direct irradiation at 90°C gave an almost quantitative yield of the cyclopropanimine 20 343,366 When the photolysis was carried out in deoxygenated hydrocarbon solvent deazetization was accompanied by photoreduction, a reaction typical of these reluctant azo systems.The pyrazolones 16 were similarly converted to the corresponding cyclo-propanimines and in the deazetization step, there is high stereoselectivity (i.e. the cisjtrans disposition of the alkyl substituents is retained). 

Complete a-selectivity was observed in the isosteviol-derived amino-thiourea 24-catalyzed conjugate addition-aromatization reaction of azlactone with a,-P-unsaturated pyrazolones (Scheme 22) [39]. Although generally good to excellent chemical yield and stereoselectivity were obtained with C2-aryl azlactones, the reaction failed with C2-alkyl azlactone (Ar = u, = Pr) even at room temper-... 

Likewise, the application of a similar protocol to unsaturated pyrazolones 149 allowed Rios and coworkers to accomplish the synthesis of several spiropyrazolones 150 bearing four chiral centers (Scheme 2.44) [74]. The proposed Michael/ Michael/aldol reaction resulted to be quite efificient in terms of both yields (up to 65%) and stereoselectivity (dr up to >25 1, ee up to >99%) when the (S)-a,a-diphenylprolinol 70 was employed as catalyst together with benzoic acid as additive. 

The broad substrate scope of this reaction was demonstrated by using different aliphatic and aromatic enals along with several N-H-free oxindoles. In all the examples, excellent yields and stereoselectivities were obtained. Moreover, the substrate scope of the reaction was extended to other heterocycles such as pyrazolones... 

Other base-modifications. Hassan and co-workers reported a novel synthesis of pyrazolone nucleosides and their antimicrobial activities. Mata and his group reported the stereoselective AAglycosylation of 2-deo3ythioribosides. This facile synthesis (shown in Scheme 3) involves N-2-deo3yribosylation of a modified nucleobase with 2-deoiythioriboside in the presence of bis-(trimethylsilyl)acetamide, AT-iodosuccinimide and trimethylsilyl trifluoromethanesulfonate to afford stereoselective nucleosides (19) in excellent yields. 

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