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Cyclization of Pregaliellalactone in the Fungus Galiella rufa



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COMMUNICATIONS Cyclization of Pregaliellalactone in the Fungus Galiella rufa Martin Johansson B rbel K pcke Heidrun Anke and Olov Sterner The Diels Alder pericyclic 4 2 cycloaddition between a diene and a dienophile is one of the most powerful carbon carbon bond forming reactions in organic chemistry 1 and it has been utilized in the total syntheses of many natural products 2 However Nature does not appear to have appreciated the reaction to the same extent as synthetic chemists only few examples of enzyme catalyzed Diels Alder reactions in biosynthetic pathways have been reported 3 Some secondary metabolites may be Diels Alder adducts although it has been difficult to determine whether they were formed through an actual 4 2 cycloaddition or through an ionic or stepwise process 4 In reaction pathways which comprise a Diels Alder reaction the question whether the reaction is spontaneous or catalyzed for example by a Diels Alderase still remains The problems to demonstrate the action of a Diels Alderase in a biosynthetic pathway are associated with the difficulties to establish such a pathway unambiguously and with the low availability of biosynthetic precursors for testing So far only two enzyme catalyzed biosynthetic Diels Alder reactions have been reported Oikawa et al showed in several experiments that cell free extracts from the fungus Alternaria solani catalyze the conversion of the linear triene precursor prosolanapyrone III to the bicyclic product solanapyrone A 5 but they did not characterize the enzyme Recently the group of Vederas showed that isolated lovastatin nonaketide synthase LNKS involved in the biosynthesis of the fungal metabolite lovastatin catalyzes the exoselective cyclization of a hexaketide triene 6 The remarkable effect of the enzyme was not that it increased the reaction rate but that it stabilized the exo transition state giving the opposite diastereomer compared to the spontaneous cyclization The exo isomer could not be formed by chemical



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