High Pressure Applications in Hetero Diels-Alder Reactions

High Pressure Applications in Hetero Diels-Alder Reactions 

Comparable results were also obtained in the hetero Diels-Alder reaction of 8-1 and 8-5. Again, the ratio of the products 8-6 and 8-7 was not changed under high pressure (Fig. 8-2) [541]. 

In a similar fashion, Achmatowicz et al. have studied the cycloaddition of 1-acetoxy- and l-trimethylsilyloxy-3-methyl-l,3-butadiene with diethyl oxoma-lonate and isopropyl glyoxylate under thermal and high pressure conditions [542], 

Vandenput et al. applied high pressure to perform hetero Diels-Alder reactions with unactivated 1-oxa-1,3-butadienes such as 8-8 and enol ethers 8-9 in the presence of a weak Lewis acid. The cycloadditions led to the dihydropyrans 8-10 as a mixture of diastereomers in 23 to 85% yield (Fig. 8-3) [543]. 

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. 

---