The indenyl roof over the chiral Lewis acid catalyst site of [(indenyl)Ru((S,S)-biphop-F)]+ (see picture) gives rise not only to high enantioselectivities in Diels–Alder reactions between enals and cyclopentadiene but also affects the endo/exo diastereoselectivity. The exo product is formed preferentially even with acrolein.
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Kündig et al. (2001) studied this question.