Pybox/Lanthanide‐Catalysed Diels–Alder Reactions with an Unsaturated α‐Oxo Ester or 3‐Alkenoyl‐2‐oxazolidinone as Dienophile: The Sense of Stereoinduction in Five‐ or Six‐Membered Bidentate Reagent Coordination

The enantioselective Diels–Alder (DA) reactions of cyclopentadiene with 3‐cinnamoyl‐1,3‐oxazolidinone ( 3 ) and methyl ( E )‐2‐oxo‐4‐phenyl‐3‐butenoate ( 4 ) carried out in the presence of lanthanide triflate complexes of (4′ S ,5′ S )‐2,6‐bis[4′‐(triisopropylsilyloxymethyl)‐5′‐phenyl‐1′,3′‐oxazolin‐2′‐yl]pyridine ( 1 ) have been compared. The aim of this work was to compare the effect of the α‐ and β‐dicarbonyl derivatives, coordinated to the cationic site of the catalysts through a five‐ or six‐membered ring chelation, respectively, on the face‐selectivity of the reaction. Only the Sc III triflate complex of 1 was found to be a very selective catalyst in the reaction with cinnamoyloxazolidinone 3 , which allowed almost complete stereocontrol. The reaction between cyclopentadiene and methyl keto ester 4 was more complicated because, in addition to the expected normal DA adducts 7 and 8 , the less expected endo ‐ 9 product of the [4+2] hetero‐DA (HDA) reaction, which resulted from the α‐keto ester behaving as a heterodiene, was obtained. [3,3] Claisen rearrangement of HDA adduct 9 stereospecifically gave the endo DA product, which allowed a correlation to be made between the absolute configuration of these products. The scandium complex was found to be the best catalyst in both reactions: the DA reaction gave an endo ‐ 7 : exo ‐ 8 ratio of 95:5 and the ee of (2 R ,3 R )‐ 7 was 99.5 %, whereas the HDA reaction was strongly selective and gave a single stereoisomer: (4 R ,4a S ,7a R )‐ 9 with an ee > 99.5 %. The lanthanum complex furnished the opposite enantiomer in both reactions, albeit with low ee ; the lanthanides with intermediate ionic radius gave results that were in between those of scandium and lanthanum. A rationale of the stereochemical outcome is proposed. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)