The Asymmetric Formal Hetero-Diels-Alder Reaction of Methyl (E)-4-Aryl-2-oxo-3-butenoates Catalyzed by [Sc(OTf)3/pybox] Complexes

The complex between scandium triflate and pybox is a good enantioselective catalyst for the reaction between methyl (E)-4-aryl-2-oxo-3-butenoates (1) and enol silyl ethers (2). The main products with (4'S)-isopropyl-, (4'S)-phenyl-, and (4'S,5'S)-4-TIPS-pybox are methyl (4S,4aS,8aS)-4-aryl-8a-trialkylsiloxy-hexahydro-4H-chromen-2-carboxylates (7), which are obtained in good yield and enantioselectivities of more than 95?% ee. Because the reaction gives products with a trans ring junction that cannot derive from a conventional concerted hetero-DielsAlder pathway, the mechanism involved in the enantioselective catalytic cycle and the origin of the stereoinduction were investigated. The structures of two desilylated products, 8b and 9b, were determined by X-ray crystal analysis. Their absolute configuration was then related to that of 7 by stereospecific isomerization and/or desilylation reactions. If aryl-butenoates 1 are coordinated to the catalyst, forming reacting complexes characterized by the five-membered structure 11, these rigid reacting intermediates give a face discrimination that is determined by the configuration of the pybox 4'-substituent. The resulting face-selective attack of enol silyl ethers to coordinated 1 gives an enantioselective tandem MukaiyamaMichael addition/intramolecular ring closure reaction, which is an enantioselective formal hetero-DielsAlder reaction, which rationalizes the absolute configuration of the reaction products.