Highly Diastereoselective One-Pot Synthesis of Spiro{cyclopenta[a]indene-2,2'-indene}diones from 1-Indanones and Aromatic Aldehydes

摘要

Treatment of 1-indanones with aromatic aldehydes and NaOEt in THF affords complex spiropolycyclic compounds through a four-component reaction in which two molecules of each starting compound are combined with formation of four new carbon-carbon bonds,leading to the elaboration of a new five-membered ring that bears five contiguous stereogenic centers with a well-defined relative configuration.Different amounts of a minor epimer of the main product are also formed.The presence of methoxy substituents in the indanone component and the use of aldehydes derived from pi-excedent heterocycles make the dimerization step a slower transformation.In these cases,better yields of spirodimers are obtained starting from the preformed enones.The reaction seems to take place by cross-aldol condensation,dehydration,and dimerization of the thus formed enones.The molecular mechanism of the dimerization reaction of enone 5g has been studied using DFT methods at the B3LYP/6-31G* level.The dimerization takes place through a process involving a Michael addition of a carbanion,obtained by deprotonation of 5g at the 3-position,to a second molecule of 5g,followed by an intramolecular Michael addition in the corresponding intermediate.The final protonation of the resulting anion accounts for the formation of the cis-fused pentacyclic system.