Diversely Functionalised Cytochalasins via Mutasynthesis and Semi-Synthesis.

Mutasynthesis of pyrichalasin H from Magnaporthe grisea NI980 yielded a series of unprecedented 4'-substituted cytochalasin analogs in titres as high as the wild-type system ( ca 60 mg · L -1 ). Halogenated, O -alky, O -allyl and O -propargyl examples were formed, as well as a 4'-azido analog. 4'- O -Propargyl and 4'-azido analogs reacted smoothly in Huisgen cycloaddition reactions, while p -Br and p -I compounds reacted in Pd-catalysed cross-coupling reactions. A series of examples of biotin-linked, dye-linked and dimeric cytochalasins was rapidly created. In vitro and in vivo bioassays of these compounds showed that 4'-halogenated and azido derivatives retained their cytotoxicity and antifungal activities; but a unique 4'-amino analogue was inactive. Attachment of larger substituents attenuated the bioactivities. In vivo actin-binding studies with adherent mammalian cells showed that actin remains the likely intracellular target. Dye-linked compounds revealed visualisation of intracellular actin structures even in the absence of phalloidin, thus constituting a potential new class of actin-visualization tools with filament barbed end-binding specificity.