Diastereoselectivity pressure-induced increase

A synthetically useful pressure-induced increase of diastereoselectivity was also found for normal Diels-Alder reactions such as the cycloaddition of the phenyl butadienes (93a-c) with the dicyano ethylenes (94-97) to yield the ds-adducts 98a-d, lOOa-d and 102a-d as well as the trous-adducts 99a-d, lOla-d and 103a-d (Scheme 8.24) [54]. 

In Scheme 14 the effect of pressure on Diels-Alder reactions with acyclic heterodienophiles or heterodienes is presented. The application of high pressure leads also in these reactions to an enhancement of rates and improvement of yields. The hetero-Diels-Alder reaction (entry 3) is a good example of the interplay between pressure and temperature. At high pressure the rate of reaction as well as the diastereoselectivity are increased. The pressure-induced acceleration allows the temperature of reaction to be lowered, which leads to a further increase of diastereoselectivity. 

As the first example of a pressure induced change in selectivity we have found a significant increase in diastereoselectivity by applying high pressure for the cycloaddition of the 1-oxa-l,3-butadienes 8-14 and ethyl vinyl ether 8-12. The difference in the activation volume of the transition structures leading to the cis-and trans-cycloadducts 8-15 and 8-16, respectively depends on the size of the substituent R at the 2-position of the 1-oxa-l,3-butadiene. Thus, for 8-14c with the small ester moiety a AAV = 2.4 0.2 cm3 mol-1, for 8-14b with the trifluoro-methyl group a AAV =3.8 0.1 cm3 mol-1 and with the even bigger trichloro-methyl moiety a AAV =5.9 0.5 cm3 mol-1 was observed. 

It is thus only within a series with the diene 93a remaining unchanged that the AAV value increases towards larger steric hindrance which corresponds to a pressure-induced enhancement of diastereoselectivity. This was observed when varying the substituent R in the dienophile 94-97 by increasing its steric size. The measured differences in activation volume are fairly large in this series, with —AAV up to (6.4 0.7) cm mol" , which may allow for applications in selective synthesis. 

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