Negative conjugation - donation from oxygen lone pairs to breaking bonds

5 Negative conjugation - donation from oxygen lone pairs to breaking bonds 

Oxyanionic assistance to Cope rearrangements In the previous example, we utilized acceptor substituents to stabilize bond breaking in the TS. In this section, we will illustrate how to use donors to impose large effects on pericycUc reactions. 

VAE leads to C4-C5 bond weakening it is aligned with the Claisen reaction coordinate 

Hetero Diels-Alder - intermolecular case - donor and acceptor are in different molecules (but interact via two orbital subsets) Intermolecular delocalization is not limited to the earlier discussed alkyne/ azide cycloadditions. It has been reported to provide significant contribution to the observed electronic effects in other cycloadditions. For example, Boger and coworkers reported that the Diels-Alder reaction of 

The two layers of donor-acceptor interactions in this reaction play distinctly different roles - the formation of new o-bonds in the cycle and the observed regioselectivity of cycloaddition are controlled by the aikene diene interactions whoteas the stereoselectivity is controlled by the interaction. The 

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