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Substitution and Addition Reactions of Methyl (R)-2-tert-Butyl-Δ4-1,3-oxazoline-3-carboxylates
Stucky, Gerhard,Seebach, Dieter
, p. 2365 - 2376 (2007/10/02)
Enantiomerically pure Δ4-1,3-oxazolines (1-5) bearing a tert-butyl group in the position 2 are prepared on a 100-g scale from serine or threonine using an electrochemical key step.They contain highly reactive double bonds, amenable to stereoselective electrophilic (1-3) or nucleophilic attack (4, 5).Thus, Vilsmeier and Friedel-Crafts-type reactions (--> 21, 22) and Michael additions (--> 11, 13) occur at C-5.Furthermore, substituents may be introduced in position 4 (next to the carbamate group) by lithiation and reaction with electrophiles (products 4, 10).Finally, dienolate 16 (from 5) reacts with aldehydes at the exocyclic position (--> 18).Hydrogenations of the tri- or tetrasubstituted double bonds in the oxazolines thus obtained are highly stereoselective (--> 14, 19, 24, 26).In the most cases, the 2-tert-butyl substituent directs reactions to the opposite face of the five-membered ring.The overall transformations achieved are discussed with regard to the bonds of serine and threonine. - Key Words: EPC syntheses / Stereogenic centers, self-regeneration of / Δ4-1,3-Oxazoline, derivatives / Electrochemistry, oxidative decarboxylation (Hofer-Moest)
