Twist asynchronicity

The currently accepted interpretation of these data involves the concept of ccHicerted but asynchronous transition states. Boeckman was first to apply this idea to IMDA reactions, but shortly thereafter White,Taber S and Roush S also invoked this model to raticmalize stereochemical results. Houk further expanded this theory by introducing the concept of twist asynchronicity to ratitmalize differences between pairs of similarly substituted nona- and deca-trienes. ... 

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). 

Similar trends, albeit less pronounced in several instances, are apparent in the data summarized in Table 2 for ( , , )-decatrienes. Houk has attributed the smaller effect of the EtaN substituent in the deca- compared to the nona-triene series to the smaller requirement for twist asynchronicity in the deca-triene transition state. It is argued that this transition state is essentially unstrained by the four-carbon connecting chain and that less torque is applied to the developing C(2)—C(7) bond. - Although the... 

For terminally activated ( , , )-trienes in both series, twist asynchronicity and endo stabilization of the transition state are cooperative and increased selectivity for the (ra/u-fused product occurs with increased dienophile activatitm. The dienophile activating group occupies an endo position in the transititm state as long as the dienophile geometry is ( ). 

For (Z, , )-n(Miatrienes, however, twist asynchronicity favors the (ra/u-fused adduct and Alder endo stabilization favors the cir-fused product. These effects evidently cancel such that no significant change in selectivity occurs with increased dienophile activation. 

Increased endo stabilization of the c(.r-fused transition state must be invoked to account for the increased selectivity in the Lewis acid catalyzed cyclization of the (Z, , )-decatrienoate recorded in Table 2, entry 8. Because twist asynchrrmicity is believed to be less important in the decatriene transition state, Alder endo stabilization apparently dominates twist asynchronicity in this case. 

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. 

---