Total syntheses of kelsoene and preussin

Total syntheses of the natural products kelsoene and preussin are comprehensively reviewed. Kelsoene is a sesquiterpene with a unique tricyclo[6.2.0.0^2,6]decane skeleton. It contains six stereogenic centers the selective construction of which has been addressed differently in the five syntheses known to date. Three syntheses employ an intermolecular [2+2]-photocycloaddition reaction as key step. One synthesis is based on a homo-Favorskii rearrangement and one on an intramolecular [2+2]- photocycloaddition. Preussin is a pyrrolidine alkaloid with three stereogenic centers which are all located within the central heterocyclic core (C-2, C-3, C-5). So far, 18 total syntheses of preussin have been completed. Seven syntheses include the nucleophilic attack on an l-phenylalanine derived electrophile as key step, five use α-amino- or α-hydroxycarboxylic acids as chiral pool building blocks. Two syntheses are based on sugars as chiral starting materials and two are based on the desymmetrization of meso-compounds. In addition, there are two syntheses which use a chiral auxiliary to establish the first stereogenic element en route to preussin.