Acid-catalysed lactonisation and iodolactonisation of norbornene-carboxylic acids

Abstract

The acid-catalysed lactonisation of 3-(norborn-5-en-2-yl)propionic acid and 4-(norborn-5-en-2-yl)butyric acid affords 3-(2-exo-hydroxynorborn-2-endo-yl)propionic acid spiro-γ-lactone and 4-(2-exo-hydroxynorborn-2-endo-yl)butyric acid spiro-δ-lactone, respectively. The corresponding spiro-iodo-δ-lactone is obtained on iodolactonisation of 4-(norborn-5-en-2-yl)butyric acid. These results, coupled with known results on lactonisation of other carboxylic acid derivatives of norbornene, point to common reaction pathways with a series of equilibrating norbornyl-type cations involved in product formation. Results with 3-exo-carboxy- and 3-exo-methylnorborn-5-en-2-endo-yl-carboxylic acids indicate that the relative importance of various intermediates in determining product formation is affected by the presence of substituents.