The hetero Diels-Alder reactions of enamino ketones 1 and 2 with the vinyl ethers 3-5a are studied in dichloromethane solution under high pressures up to 5 kbar. The kinetics is measured by on-line FT-IR spectroscopy up to 3 kbar. The cycloaddition reactions of enamino ketones 1 and 2 with vinyl ethers 3 and 4 yield the diastereomeric dihydropyrans 7a-d and 8a-d, respectively, whereas in the cycloaddition of enamino ketone 1 to vinyl ether 5a the constitutional isomers 9 and 10 are formed. The unexpected reaction of 1 with 5a to give the spiro compound 9 is explained by isomerisation of 5a to 5b under the reaction conditions. The kinetics of the reactions is discussed in terms of both overall and individual rate coefficients, activation energies, and activation volumes.Previous investigations into the pressure and temperature dependence of intramolecular ['] and hetero Diels-Alder reactions showed that the kinetics and the diastereoselectivity are strongly influenced both by the electronic properties of the substrates and the steric demand of substituents at the diene and dienophile. The cycloadditions of enamino ketones 1 and 2 to several alkyl vinyl ethers (ethyl, isopropyl, isobutyl, and tert-butyl vinyl ether) were quantitatively studiedw up to 2800 bar. It seemed worthwhile to extend these studies to hetero Diels-Alder reactions of enamino ketones 1 and 2 with vinyl ethers in which the steric demand at the reactive site is further increased by substituents at the C=C bond.In this paper we present data on the kinetics and on the selectivity of the cycloadditions of enamino ketones 1 and 2 to the substituted vinyl ethers 3-5a. As expected, the Diels-Alder reactions of 1 and 2 with vinyl ethers 3 and 4 gave the diastereomeric cis-and trans-substituted dihydropyrans 7a-d and 8a-d (see Scheme 1). In the reaction of 1 with 5a, however, the cis cycloadduct 10 was found as a single diastereomer together with the spiro compound 9 which actually was the main product. For the formation of 9 it was assumed that under reaction conditions the cyclic vinyl ether 5a undergoes isomerisation to 5b. The existence of the isomeric species 5b, however, could not be proven by analytical means in the solution of 5a.The cycloaddition reactions were performed in dichloromethane solution with the initial concentration of the vinyl ether (1.0 mol kg-') clearly exceeding that of the diene (0.003 mol kg-'). The reaction mixture was contained within a closed thin-walled poly(tetrafluoroethy1ene) (Teflon) bag which transmits IR light. This technique prevents the reaction from being influenced by the metal walls of the high-pressure cell. The reaction kinetics was monitored by direct quantitative infrared spectroscopy under high pressure. The experimental technique including the design of the high-pressure cell is described in Spectral series in the region of the C=O fundamental were measured during each experiment at constant pressure and temperature. Overall rate coefficients k for the parallel reaction pathways affording the diastereome...
