Enantioselective Photochemistry
263
Scheme 5.
Scheme 6.
ity is by no means restricted to these reactions. Other photochemically
induced cycloaddition reactions successfully performed enantioselectively include, for example, the [4+4]-photocycloaddition of pyridone
(17) and cyclopentadiene (18) (Scheme 6) to give the diastereomeric
products exo-19 and endo-19 (Bach et al. 2001c).
The Diels-Alder [4+2]-cycloaddition reaction of the photochemically
generated ortho-quinodimethane from substrate 20 and acrylonitrile resulted in tricyclic product 21 (Scheme 7) (Grosch et al. 2003; Grosch et
al. 2004). In an analogous fashion the reactive diene could be trapped
by methyl acrylate or dimethyl fumarate. It was shown that the association constant of the corresponding products to the template was much
lower than that of the substrates, an observation that is in line with an
increasing ee upon increasing reaction time. This fact was also responsible for high enantioselectivities even at higher irradiation temperature.
The pressure dependence of the reactions was studied and it was found
that despite an increased association the enantioselectivity of the re-