The hetero‐Diels‐Alder reaction of the phenylthio‐activated N‐monoacyl‐ and N,N‐diacyl‐enamino ketones 8a‐c containing a methyl group at position 2 of the oxadiene with the electron‐rich dienophiles 9a‐c yields the exo/endo adducts 10a‐d and 11a‐d in 82–95% yield with the endo adducts as the main products. The enamino ketone 8d with a S‐ethyl group at C‐3 does not react. Hydrogenation of 11a with Raney nickel in THF leads to the desulfurized dihydropyran 12, whereas in MeOH gives stereoselectively the perhydrophthalimido derivative 13, which, after deprotection, yields the β‐methyl glycoside 14 of rac‐4‐deoxydaunosamine. Under similar conditions 10c is converted into the β‐ethyl glycoside 15 of rac‐N‐benzoyl‐4‐deoxyristosamine. Stereoselective hydroboration of 12 affords 17 and subsequent cleavage of the protecting group leads to the β‐methyl glycoside 18 of rac‐acosamine.

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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 12ae 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.

The intramolecular hetero Diels-Alder reaction of the benzylidene-isoxazolone 3 giving the adducts 4 and 5 is studied in dichlormethane under high pressure up to 5 kbar. The kinetics is measured by on-line FT-IR spectroscopy up to 3 kbar. The cycloaddition shows a clear pressure-induced increase in diastereoselectivity. For the difference in activation volume These k values are overall rate coefficients which are related to the individual rate coefficients for the pathways leading to the cis and trans products by k = k,.,, + k,,.a,,v Rate coefficients k for cycloadditions at 333 K and various pressures and for cycloadditions at 1000 bar and several temperatures are listed in Table 1.The ratio of the two diastereomeric adducts 4 and 5 was determined off-line by HPLC of the crude reaction mixture, since it was not possible to distinguish them on-line by IR

The Hetero Dieb-Alder reaction of the enamino ketones Sa-d and ethyl vinyl ether (2) to give the dihydropyrans 6a-d and 7n-d is studied in dichloromethane and in hept urene under high pressure up to 7 kbar at temperatures between 0.5 and 130°C. High pressure has been applied to many transformations in order to increase their rates2). Thus all reactions which display a negative A V * , can be accelerated under high pressure. Activation volumes of about -25 cm3/mol have been found for the intramolecular and of -25 to -45 cm3/mol for intermolecular Diels-Alder reactions2''. Because of the relevance of selective organic synthesis, increasing interest has been addressed to the influence of pressure on relative rates in the formation of stereoisomers. Earlier attempts, however, to induce diastereoselectivity by high p r e s~u r e~,~) were disappointing in that the observed absolute values of AAV' did not exceed 1 cm3/ mol. We now show, that a significant and synthetic useful increase in stereoselectivity may be obtained by applying high pressure.In some recent publications we investigated the hetero DielsAlder reaction" of enamine carbaldehydes 1 and enamino ketones 5 with vinyl ethers such as 2 leading to 3-amino sugars of the garosamine and daunosamine type6.'). The cycloaddition of 1 and 2 to give the dihydropyrans 3 and 4 was studied by direct quan- Werte konnte bei der Reaktion von 5a mit 2 zu 6a/7a eine Erhiihungder Selektivitiit von 1.67: 1.00 bei 90°C und Normaldruck auf 13.6: 1.Q bei 05°C und 6 kbar erreicht werdea Dieses Beispiel migt, da13 eine signifikante und synthetisch wertvolle Erhiihuag der Diastereoselektivit chemischer Reaktionen durch Anwendung von bohem Druck m6glich ist titative infrared spectroscopy under high pressure, and an activation volume of -(25.1 f 1.7) cm3/mol could be determined in dichlorornethane*'. This newly developed method allows a detailed study of organic reactions in compressed fluid phase.In this paper we describe thc kinctics and the diastereoselectivity of the cycloadditions of 5a-d and ethyl vinyl ether (2) to give the dihydropyrans 6a-d and 7a -d under high pressure. A part of this work (reaction of 5a and 2) has already been published as a communication ' ).Chem. Ber. 122 (1989) 11 79 -11 86 0 VCH Verlagsgesellschaft mbH, D