Cyclization of (−)-Pregaliellalactone in the Fungus Galiella rufa

An intramolecular Diels–Alder reaction with inverse electron demand in a polyketide biosynthetic pathway is reported in which the rate of the cyclization is enhanced in vivo. Alive mycelia of the fungus Galiella rufa catalyze the cyclization of (−)-pregaliellalactone (1) to (+)-desoxygaliellalactone (2), while dead mycelia have no effect. No rate increase was observed with the unnatural enantiomer (+)-1.