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

Abstract

AbstractThe 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 ScIII 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 (2R,3R)‐7 was 99.5 %, whereas the HDA reaction was strongly selective and gave a single stereoisomer: (4R,4aS,7aR)‐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)