Peri- and enantioselectivity of thermal, scandium-, and [pybox/scandium]-catalyzed diels-alder and hetero-Diels-Alder reactions of methyl (E)-2-oxo-4-aryl-butenoates with cyclopentadiene

The cycloaddition between methyl (E)-2-oxo-4-aryl-3-butenoates (2a-d) and cyclopentadiene, in addition to the expected normal Diels-Alder (DA) adducts endo-3a-d and exo-4a-d. gives the less expected endo-5a-d products of the [4+2] hetero-Diels-Alder (HDA) reaction in which the alpha-ketoester behaves as a hetero-diene. If a comparison is made between the thermal and the scandium(111) triflate-catalyzed conditions, the periselectivity changes and whereas under thermal conditions the main products are those from the DA reaction (3a-d), in the presence of Sc(OTf)(3) (OTf= triflate), the HDA products 5a-d become largely predominant. The reactions are enantioselectively catalyzed by the scandium(III) triflate complex of (4'S,5'S)-2,6-bis[4'-(triisopropylsilyl)-oxymethyl-5'-phenyl-1',3'-oxazolin-2'- yl]pyridine (1) and both the DA and the HDA products are obtained with excellent enantiomeric excess, up to > 99% ee. The X-ray crystallographic structure determination of 5c assigns it the 4R,4aS,7aR absolute configuration. ne thermal retro-Claisen rearrangement of 3c into (4R,4aS,7aR)-5c allows the correlation of their absolute configuration, and 3c has therefore the 2R,3R configuration. By analogy the same absolute configuration can be assigned to 3a,b,d and 5a,b,d, and the stereospecific thermal Claisen rearrangement of the optically active 5a,b,d into 3a,b,d completes the correlation between their absolute configuration. The [3,3]-sigmatropic rearrangements can be easily carried out under catalytic conditions with scandium(III) triflate, which promotes the equilibration between 3a-d and 5a-d, with a different degree of enantioselectivity characterizing the process starting from 3a-d or 5a-d. The unambiguous attributions of the configuration to the products allows us to propose a rationale of the stereochemical outcome of the catalyzed cycloaddition and to investigate the reaction mechanism of the competing DA and HDA reactions and shifts in products distribution by acid catalysis.