Oxazolones, 4-substituted, Michael

Oxidative dimerization of various 2-benzyloxy-2-thiazoline-5-ones (222) catalyzed by iodine and triethylamine is another example of the nucleophilic reactivity of the C-4 atom (469) (Scheme 112). Treatment of 212 with pyrrolidinocyclohexene yields the amide (223) (Scheme 113). The mechanism given for the formation of 223 is proposed by analogy with the reactivitx of oxazolones with enamines (4701. 4-Substituted 2-phenylthiazol-5(4Hi-ones react with A -morphoiino-l-cyclohexene in a similar manner (562j. Recently. Barret and Walker have studied the Michael addition products... 

The behavior of 2-substituted- or 2-unsubstituted-5(4//)-oxazolones is completely different. Reaction of electrophiles with 2-substimted-5(47/)-oxazolones usually occurs at C-4, whereas reaction of electrophiles with 2-unsubstituted-5(47/)-oxazolones affords the corresponding 5(2//)-oxazolones. The best example of the behavior of 2-unsubstituted-5(47/)-oxazolones involves 4-isopropyl-5(47/)-oxazolone 143, the anion of which can be considered as a formyl anion equiva-lent. ° ° Thus, reaction of 143 with a catalytic amount of triethylamine gives 144 that reacts with a,p-unsaturated carbonyl compounds to afford Michael adducts or with aldehydes to produce the corresponding aldol adducts. Mild acid hydrolysis of 145 yields the corresponding aldehydes (Scheme 7.42). 

Scheme 3.10 Michael reaction of 4-substituted oxazolones with a,P-unsaturated aldehydes catalyzed by 31c and further transformations carried out on the obtained adducts.

Scheme 4.13 Enantioselective Michael addition of racemic 4-substituted oxazolones to nitroalkenes.

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