Selective [3+2] and [3+3]-Cycloaddition Reactions of Nitrones
Doyle, Michael P
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Cationic chiral dirhodium(II,III) carboxamidates, obtained from the oxidation of the corresponding dirhodium(II,II) carboxamidates by nitrosonium salts, are efficient promoters in asymmetric Lewis acid catalyzed reactions. High regiocontrol and stereocontrol have been achieved with the cationic chiral dirhodium(II,III) carboxamidate whose ligand is (R)-menthyl (S)-2-oxopyrrolidine-5-carboxylate in 1,3-dipolar cycloaddition reactions of nitrones with α,β-unsaturated aldehydes. In addition, higher rates and selectivities have been obtained in hetero-Diels-Alder and carbonyl-ene reactions with the diastereomeric catalyst having the (S)-menthyl (S)-2-oxopyrrolidine-5-carboxylate ligand. Dramatic solvent influences on reaction rates and selectivities characterize the catalysis of cationic chiral dirhodium(II,III) carboxamidates, and these influences are explained by competitive coordination of solvent to catalyst and by the influenced coordination angle of the aldehyde substrate relative to catalyst by the solvent environment. Rhodium vinylcarbenes, generated from the reactions between vinyldiazoacetates and dirhodium catalysts, are highly reactive intermediates. Through reacting rhodium vinylcarbenes with nitrones, we have discovered a [3+3]-cycloaddition pathway; and by using chiral dirhodium carboxylates as the catalysts, a highly enantioselective [3+3]-cycloaddition of nitrones with vinyldiazoacetates has been achieved. The products of this [3+3]-cycloaddition are 3,6-dihydro-1,2-oxazines, which are versatile intermediates for the synthesis of α-substituted β-amino acids and related compounds that are not easily accessible by other methods. The broad scope of cyclic and acylic nitrones that are applied demonstrates the power of this methodology. The limitation of this [3+3]-cycloaddition methodology is the requirement of using the β-TBSO-substituted vinyldiazo compounds as the rhodium vinylcarbene precursors. Although vinyldiazoacetates without the β-TBSO substituent are not reactive for the [3+3]-cycloaddition with nitrones, we have discovered an alternative reaction pathway with an unsubstituted vinyldiazoacetate. The reaction occurs with a dirhodium vinylcarbene-induced [3+2] nitrone cycloaddition, followed by subsequent cascade carbenoid aromatic cycloaddition/N-O cleavage and rearrangement. In this cascade process, both the [3+2]-cycloaddition of nitrones with a rhodium vinylcarbene and the [1,7]-oxygen migration with N-O cleavage are unprecedented in the literature. The complexity of the reaction pathway and the uniqueness of the formed heterocyclic products are of great interest to synthetic chemists.