The influence of high pressure on the diastereoselectivity of the intermolecular all-carbon Diels ± Alder reaction of the phenylbutadienes 1 a ± c with the dicyanoethylenes 2 a ± d to give the cyclohexenes 3 ± 8 is described. The differences in activation volume, DDV = , for the two pathways leading to cis and trans diastereomers range from À (0.7 AE 0.8) to À (6.4 AE 0.6) cm 3 mol À1, indicating a pressure-induced increase of diastereoselectivity in favour of the cis adducts 3 a ± d, 5 a ± d and 7 b ± d.

In our ongoing studies to develop asymmetric, catalytic methodology to functionalized acyclic compounds, we envisioned substituted cyclic enol ethers 1 as direct precursors to aldol and homoaldol products. Since a variety of catalytic,l as well as noncatalytic,2 methods to enantiomerically enriched dihydrofurans and dihydropyrans are known, we investigated the possibility of their oxidative ring opening by ozonolysis to develop a new access to aldol3 and homoaldo14 products.Although enol ethers play an important role in synthetic chemistry, their ozonolysis has not widely been ~t u d i e d .~ In particular, there are only a few accounts on the ozonolysis of cyclic enol ethers.6 Nevertheless, Danishef-sky7 et al. have noted that dihydro-y-pyrones can be unmasked as aldol compounds by ozonolysis. In this paper we wish to report that dihydropyrans 1 ( n = 1) are excellent equivalents for homoaldol as well as dihydrofurans 3 (n = 0 ) for aldol compounds.Our results are summarized in Table I. Since ozonolysis of 3,Cdihydro-W-pyran under reductive conditions leads to a mixture of products,6d it was desirable for the cleavage (1) (a) Hudlicky, T.; Reed,of 8-unsubstituted enol ethers to create a stable este9 rather than an aldehyde functionality. Therefore, the dihydropyrans Id-f, as well as the dihydrofurans 3 were ozonolyzed in a dichloromethane/methanol mixture under conditions developed by Schreiberg for carbocycles. According to this procedure the hydroperoxides, which are generated in situ, are dehydrated with triethylamine/acetic anhydride after azeotropic removal of methanol with benzene. These conditions could be successfully applied to cyclic enol ethers; however, it was discovered that the removal of methanol can be omitted with equally good results. This variation is especially useful for large-scale preparations,since the hydroperoxides which result from the ozonolysis of e g . 3a are very explosive and should only be handled at low temperatures. Thus, the 3-formyloxy ester 4a was conveniently prepared by this protocol (entry 10) as the sole product. Also, 3,4-dihydro-6-phenyl-2H-pyran (le) (entry 8) gave rise to the 4-formyloxy ester 2e in 91 % isolated yield accompanied by minor amounts (<3% as judged by IH NMR of the crude) of an unidentified byproduct. In an even better yield (94%) the 3-formyloxy ester 4b was obtained through ozonolysis of 5-phenyl-2,3-dihydrofuran 3b (entry 11). Cinnamylaldehyde was identified as the only byproduct (<3 % by lH NMR).Chiral dihydrofurans and dihydropyrans are opened under the applied conditions without loss of optical purity (entries 9 and 11). Multiple functionalized dihydropyran derivatives with usual protecting groups are also readily transformed into the corresponding acyclic compounds by ozonolysis (entries 4,5, and 9).l0 Moreover, the resulting products are stable to epimerization, since e.g. in 4b the formyl group prevents racemization via a retroaldol/aldol process.To test the limits of this fragmentation, a series of vinylsubstituted dihydropyrans were subjected...

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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...

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