Hetero Diels-Alder reactions / Enamino ketones 1 High-pressure reactions / Pyrans
The hetero Diels-Alder reaction of enamino ketone 1 with the vinyl ethers 7-11 leading to the dihydropyrans 12a-e and 13ae is studied in dichloromethane under high pressures up to 7 kbar. The kinetics is measured by on-line FT-IR spectroscopy up to 3 kbar. For the first time individual rate coefficients kci, and k,,,,, have been determined. Pressure-averaged activation volumes, m, for the overall reaction leading to the formation of the two diastereomers in dichloromethane solution between -(19.1 i 1.7) and -(25.4 & 1.6) cm3 mol-' are measured. The corresponding activation enthalpies, AH*, are between (62.2 i 1.6) and (76.3 f 3.6) k J mol-l. The cycloaddition shows a remarkable increase in diastereoselectivity in favour of the cis adducts 12a-e toward higher pressure. The pressure-averaged differences in activation volume, -AAV*, for the two reaction pathways leading to cis and trans diastereomers are found to be between -(4.5 i 0.3) and -(6.9 i 0.7) cm3 mol-l. The corresponding differences in activation enthalpy, AAH*, are between -(4.7 f 1.4) and -(9.9 i 1.6) kJ mol-'. Combination of the overall rate coefficient k = kCi, + k, , , for the reaction leading to the two diastereomers with the measured product ratio, cCis/ctrans, which is identified with the ratio of the rate coefficients kCi,/kt,,,,, yields the individual coefficients k,, and k,,,,.
The rate coefficient k,, is strongly influenced by the steric demand of the substituents on both diene and dienophile whereas ktrans varies to a smaller extent, and the observed changes appear to be mainly due to the electronic properties of the substituents.
Previous investigations of the hetero Diels-Alder reaction of enamino ketones 1-3 with ethyl vinyl ether (4) have shown that the ratio of the diastereomeric products 5a-c and 6a-c is strongly dependent on
In this paper we present data on the kinetics and on the diastereoselectivity of the cycloaddition reaction of enamino ketone 1 with several vinyl ethers 7-11 to yield the new dihydropyrans 12a-e and 13a-e. The synthesis of 1 has already been publi~hed'~'. The structure of 12a-e and 13a-e has mainly been determined by using 'H-NMR spectroscopy. The data are similar to those obtained for 5a and 6a, which have recently been published. The conformation of dihydropyrans similar to 12a -e/13a -e, being governed by the anomeric and steric effects, has been discussed in detail elsewhereL6].
The cycloadditions have been studied by direct quantitative infrared spectroscopy under high pressures up to 3 kbar. They have also been performed on a preparative scale at ambient pressure (120°C, 24 h, in toluene solution) with good to excellent yields (85-99%). The experimental setup including the optical high-pressure cell has already been describedL7]. The concentrations of the starting compounds and products are directly derived from the IR spectra[3761.