Biogenesis of Symbioimines

The unique structures of symbioimines, including a 6,6,6-tricyclic iminium ring, can be explained by the plausible biogenetic pathway shown in Scheme 6, [72,74]. An intramolecular exo transition-state Diels-Alder reaction followed by imine cyclization could form the carbon framework of 35 and 36 stereospecifically, as in the case of pinnatoxins (Fig. 2). Since the 6,6,6-tricyclic iminium ring moiety of neosymbioimine (36) is composed of 15 carbons including three methyl groups, a sesquiterpene biosynthesis pathway can be considered. However, due to its inconsistency with the products in the normal sequential head-to-tail condensation of two molecules of isopentenyl diphosphate with dimethylallyl diphosphate, we proposed the polyketide synthesis pathway for construction of the characteristic C1-C12 moiety in 35 and 36. From this point of view, the tricyclic ring in sym-bioimines can be considered to be biosynthesized by a cyclic imine formation, endo-transition state Diels-Alder reaction to provide cz's-6,6,6-tricyclic ring stereospecifically, followed by epimerization. Studies of the biosynthetic path ways of symbioimines using isotope-labeled precursor incorporation studies are currently underway.

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