Direct anti and regio-specific aldol reactions of cyclododecanone catalyzed by alkali metal hydroxides: implications for supramolecular helical design

Abstract

The direct aldol condensation of benzaldehyde to cyclododecanone (CDD), catalyzed by NaOH is expected to yield bis-benzylidene cyclododecanone, but we interestingly ended up with novel β-hydroxy carbonyl compounds and monobenzylidene cyclododecanone derivatives in the cases of 2-halo substituted benzaldehydes. This exceptional behaviour is due to the capability of the CDD ring to exist in the zwitter ionic form. The formation of monobenzylidine derivatives can be due to less stable hydrogen bonds between –OH and –CO groups, electrostatic interaction between aldehyde substituents and metal enolates and the bulkiness of the benzaldehyde ring substituents. We extensively optimized the reaction conditions using different solvents and alkali metal hydroxide catalysts and we are getting high yield, regio-specificity and high diastereoselectivity (anti-aldol) in β-hydroxy carbonyl compounds. The representative crystal structures were examined and they suggest the tendency of anti-aldol compounds to form supramolecular helical motifs. The β-hydroxyl carbonyl derivatives form supramolecular helices via O–H⋯O hydrogen bonds with two anti-aldol units per turn and a pitch of ∼6 Å. The cyclododecanone ring adopts the minimum energy [3333] square conformation. The nucleophile will attack only from the less hindered side of the carbonyl group in the CDD ring.