Portulal 1 is a perhydroazulenoid diterpene which has clerodane substitution and was originally isolated as a plant growth regulator from Portula grandiflora. The studies aimed at the total synthesis of this interesting compound will be presented in this lecture. The Diels-Alder reaction of chloromethyl maleic anhydride with 1-vinylcyclohexene proceeds stereospecifically to afford an adduct 13 which has the desired configuration in the three of four asymmetric centers present in 1. Prior to the construction of the remaining center at the angular position, 13 was transformed to an intermediate 25 with perhydroazulenoid skeleton in highly efficient manner including 8 steps. The introduction of the angular substituent was performed by the intramolecular alkylation and the following cleavage of the newly formed ring. The product 30 was converted via 32 to the dilactone 33, which was found to be identical with the one, derived from natural portulal via 37 in 9 steps. Thus our synthetic intermediate is confirmed to possess the correct stereochemistry in all diastereomeric centers. The synthetic path to give back 1 from 37 has been also elaborated. Therefore, provided that the missing link between 32 and 37 is connected, a formal total synthesis of portulal will be completed. The work in this line is in progress.
