Nonatrienes twist asynchronicity

For trienes like (5)-<7) with dienophile activating groups located at C(2) ( internally activated systems), however, the situation is reversed. The LUMO coefficient at C(l) is greater than at C(2) and so bonding between C(l) and C(9) should be more advanced early in the transition state. This form of twist asynchronicity, however, is probably less significant than that involving the C(2)—C(6) bond for terminally activated nonatrienes U,2) since the selectivity of the IMDA reactions of (5)-<7) is comparable with that obtained with the unsubstituted nonatriene (3) (also compare results with (4L Figure 2). 

The increased stereoselectivity of decatrien-3-ones relative to the n(matrien-3-ones may also be the consequence of cooperativity between twist asynchronicity and endo stabilization that both favor the c -fused product. The decatrien-3-one transitirm state is considerably less strained than the nonatrien-3-one transition state, and the uncatalyzed decatrienone cyclizations occur at or near ambient temperature. Consequently, one expects greater endo stabilization, and hence also greater cis stereoselectivity, in the decatrienone intramolecular Diels-Alder reactions. 

For (Z, , )-nonatrienes, however, twist asynchronicity favors the frani-fused adduct and Alder endo stabilization favors the c -fused product. These effects evidently cancel such that no significant change in selectivity occurs with increased dienophile activation. 

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