Azlactones, aldol type reaction

Aldol-Type Reaction of Azlactones with Vinyl Ethers... 

They applied the intermediary oxocarbenium to a direct aldol type reaction of azlactones [78] via their oxazole tautomer and obtained the corresponding products with excellent enantio and diastereoselectivities (Scheme 3.35) [79]. The method enables efficient access to biologically and pharmaceutically intriguing p hydroxy a amino acid derivatives having a quaternary stereogenic center at the a carbon atom. 

Scheme 3.35 Aldol type reaction of azlactones via protonation of vinyl ethers.

Terada, M. Tanaka, H. Sorimachi, K. Enantioselective Direct Aldol-Type Reaction of Azlactone via Protonation of Vinyl Ethers by a Chiral Bronsted Acid Catalyst. /. Am. Chem. Soc. 2009,131,3430-3431. 

The direct aldol-type reaction of azlactone 106 with an oxocarbenium ion via a protonation of vinyl ethers (107) by the chiral phosphoric acid 95a as a catalyst provides (l-alkoxy-a-amino acid derivatives (108) bearing a quaternary stereogenic center with high diastereo- and enantioselectivity (Scheme 28.9). The ion pairs 109... 

---