Diene Substitution on the Geometry of Ring Fusion

WDson [30] has pointed out that free rotation around the single bond connecting the diene to the bridge can lead to a mixture of cis and trans products. A substituent at the 3-position of the diene can destabilize one of the rotamers, leading to improved selectivity in the cyclization (see p. 47). 

As the bridge between the diene and the dienophile is shortened from four to three atoms, the geometric outcome of the cyclization shifts, although the direction of the shift cannot yet generally be predicted [31-33]. It is apparent that inclusion of sp centers in the bridge in conjugation with the diene or dienophile will increase the proportion of cis product [34, 35]. This outcome is predicted by molecular models if it is assumed that overlap between adjacent n orbitals is maintained. 

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