gave, in one step, the tetrahedral adduct (aza-cyclol)(13). The same adduct was obtained starting from (R)-5-oxothiomorpholin-3-ylcarbonyl-L-phenylalanyl-L-proline p-nitrophenyl ester (12). The conformationally rigid polycyclic skeleton stabilizes the structure of the aza-cyclol (13) which is an example of a tetrahedral adduct derived from amide–amide interaction. Relevant conformational details, as revealed