文摘
A new strategy for the synthesis of 3-aryl-4-methyl-5-O-substituted isoxazolines via tandem catalytic isomerization of O-allyl systems to O-(1-propenyl) systems?,3-dipolar cycloaddition (1,3-DC) to nitrile oxides is presented. The influence of the heteroatom in Ph-X-CHCHCH3 (X=O, S, or Se) on the regio- and stereoselectivity of ArCNO 1,3-cycloaddition to these dipolarophiles is analyzed as well. The dipolarophiles were obtained via [RuClH(CO)(PPh3)3]-, [RuH2(CO)(PPh3)]- or base (KOH/18-crown-6)-catalyzed double bond migration in corresponding allyl ethers, O-allyl acetals, PhS- and PhSe-allyl systems. Cycloadditions of nitrile oxides to O-(1-propenyl) systems were fully regioselective whereas in the reactions of ArCNO with the PhS-(1-propenyl) and PhSe-(1-propenyl) systems both possible regioisomers were formed. It was established that within the majority of dipolarophiles of ROCHCHCH3 type 1,3-DC is concerted, while for some dipolarophiles of RXCHCHCH3 (X=O and R=Ph3C, 2,3-dihydroxypropyl, tetrahydropyran-2-yl; X=S or Se and R=Ph) type 1,3-DC turns into a two-step reaction with simultaneous rotation about C-C bond. The results of the experiments have been analyzed theoretically using DFT calculations. The results of these calculations agreed well with the experimental data.
