Trienes boat-like transition states

Relative diastereoselection is an issue that is confrtHited most frequently in the IMDA reactions of trienes with one or more stereocenters on the chain connecting the diene and dienophile. As a general rule, trienes with saturated amnecting chains cyclize via transition states with chair-like conformations, while boat-like transition states are frequently encountered with the 1,7,9-decatrienones as well as with... 

The tandem Cope-Cltdsen rearrangement of triene (96 equation 14) creates a product with three new stereogenic centers. Only two of the four possible stereoisomers are formed, namely, aldehydes (100) and (98) in a 78 22 ratio. The factors that control the stereochemistry are the chair-like versus boat-like transition state of the Cope rearrangement (96 giving 97) and the formation of the carbon-carbon bond in... 

Decatrienones and Other Trienes with Unsaturated Connecting Chains Intervention of Boat-like Transition States... 

Conformational effects also play an important role in determining IMDA diastereoselectivity. For example, trienes like (49) and (50) with two sp hybridized atoms within the connecting chain cyclize preferentially to ci s-fused rather than the trans-fuscd products that would be expected based tm the presence of the C(8)-methyl substituent (Figure 15). The benzo fusion forces the bridging chain to adopt boat-like conformations in the transition states, and the 1,3-diaxial interactions that the C(8)-methyl ordinarily experiences in the cif-fused, chair-like transition state are thus relieved (refer to Figure 12). The preferential formation of the cif-fused diastereomer may then be the consequence of a favored skew butene rotamer at C(6)—C(7) in (51) compared with a less favorable gauche butene rotamer at this position in the trans-fused transition state. This effect is discussed in more detail in Section 4.4.3.2 concerning relative diastereoselection in the IMDA reactions of substituted decatrienones. 

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