donor and acceptor properties alternate along the chain
as we move away from a carbonyl group.
This `natural reactivity' of carbonyl compounds explains why we find it easy to discuss ways of mak-
ing 1,3- and 1,5-difunctionalized compounds, because they arise from al+d2 and from a3+do
Reagents corresponding to syntbons like dl or aZ are rarer, and therefore compounds with 1,2一or
1,4- related functional groups require special consideration retrosynthetically.