V-acylimidazoles

Y/ .v-(3-Lactains have also been obtained by the Staudinger reaction carried out between divinylimine and /V-acylimidazoles possessing an electron-withdrawing group (EWG) in a position [111]. This latter were prepared by treatment of a-EWG substituted carboxylic acids with 1,1-carbonyldiimidazole. 

Hydrolysis of 2,4,5-triphenyl-V-acylimidazoles, as followed by UV-Vis measurements, was found to proceed through a concerted pathway that was accelerated by steric crowding around the V-acyl bond <1997JOC2872>. The pXa values were determined based on the kinetic data of the hydrolysis of 5-methyl-3,5-diarylimidazolidine-2,4-dione 101 yielding 102 <2005JHC899> (Scheme 20). 

Much work in the review period has concerned enantioselective substitution in five-membered heterocyclics. The enantioselective alkylation of some pyrroles by unsaturated 2-acylimidazoles catalysed by the bis(oxazolinyl)pyridine-scandium(in) triflate complex (31) has been reported.39 Compound (33) is formed in 98% yield and 94% ee from the 2-acylimidazole (32) and pyrrole at —40 °C. A series of enantiomer- ically pure aziridin-2-ylmethanols has been tested as catalysts in the alkylation of /V-mclhylpyrrolc and (V-methylindole by ,/l-unsalura(cd aldehydes.40 Enantiomeric excesses of up to 75% were observed for the alkylation of /V-mcthylpyrrole by ( >crotonaldehyde using (2.S ,3.S )-3-mclhylazirin-2-yl(diphenyl)methanol TFA salt as catalyst to form (34). 

Now the seven-membered ring must be built up. Here an azole-based reagent, CDI, carbonyl di-imidazole proves invaluable. Reaction with 232 gives the acylimidazole 233 and this acylates the potassium enolate of nitromethane in good yield to give 234 and hence, by reduction, the diamine 235. The /V-benzyl group can now be removed and the synthesis of the seven-membered ring is completed with the one-carbon electrophile HC(OEt)3. 

Reagents i, Oj ii, AcaO-NaOAc iii, HO(CH2)aOH-H+ iv, NaOH v, LKOBuOsAlH on acylimidazole vi, RuOa-NaI04 vii, NaOMe viii, CF3CO3H, Na,HP04 ix, C5HiiCOCH2PO(OMe)a-NaH... 

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