Divergent kinetic control of classical versus ozonolytic lactonization: Mechanism-based diastereoselection

A strategy has been developed in which mechanistically distinct lactonization reactions are used to prepare diastereomeric δ-lactones relevant to the C1-C9 fragment of (+)-peloruside A. Depending upon which of two reaction types is used, the central (C5) hydroxyl group can be directed to differentiate the C1 versus C9 termini of pseudosymmetric substrates to provide diastereomeric lactones. Thus, the 5-hydroxy-1,9-diester substrate 1 (an azelaic ester) cyclizes under classical (acid- or base-catalyzed) lactonization conditions to give a predominance of one diastereomer, whereas the 5-hydroxy-1,10-diene congener 2 provides the opposite sense of diastereoselectivity when subjected to ozonolytic lactonization (O₃, MeOH, NaOH, at -78 °C). Thus, this under-utilized oxidative transformation is mechanistically orthogonal to the classical reaction.