Diversely Functionalised Cytochalasins through Mutasynthesis and Semi-Synthesis

Mutasynthesis of pyrichalasin H fromMagnaporthe grisea NI980 yielded a series of unprecedented 4 '-substituted cytochalasin analogues in titres as high as the wild-type system (approximate to 60 mg L-1). Halogenated,O-alkyl,O-allyl andO-propargyl examples were formed, as well as a 4 '-azido analogue. 4 '-O-Propargyl and 4 '-azido analogues reacted smoothly in Huisgen cycloaddition reactions, whereasp-Br andp-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 the 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-visualisation tools with filament-barbed end-binding specificity.