Synthesis and Reactivity of 1,4-Dicarbonyl Compounds Related to the Natural Product Onchidal

Abstract

Onchidal is the secondary metabolite produced by the opisthobranch mollusc Onchidella binneyi. The sesquiterpene acetate natural product possesses the ability to irreversibly bind to, and inhibit, acetylcholinesterase and the ability to inhibit the growth of the Gram-positive bacterium Staphylococcus aureus. Although literature has reported the bioactivities of onchidal, its reaction mechanism is not yet known. To better understand the reaction mechanism of onchidal, the synthetic and reactivity studies of analogues of onchidal and its 1,4-dialdehyde-containing hydrolysis product were undertaken. The preparation of para-substituted phenyl ethers 1.37, 1.38, 1.40, 1.41 and 1.42 proceeded smoothly via the Mitsunobu reaction (68–89% yield), the Horner-Wadsworth-Emmons reaction (19–43% yield), reduction (28–92%), Dess-Martin oxidation (60–98% yield) and acetylation (30–58% yield). Preliminary reactivity studies of 1,4-dialdehydes 1.40, 1.41 and E-1.42 towards model nucleophiles suggested that these compounds are not reactive to thiols but are highly reactive to amines, forming Paal Knorr and Michael addition products (1.72, 1.73 and 1.76). Further investigation, using lysozyme as a model protein, demonstrated that incubation with 1,4-dialdehydes 1.40, 1.41 and E-1.42 gave pyrrole adducts similar to those previously described for the n-pentylamine reactions. As up to seven additions of dialdehyde to lysozyme were observed, it was proposed that lysine residues and the N-terminus amine were responsible for the first nucleophilic attack at the 1,4-dialdehyde moiety to form a pyrrole. Furthermore, the addition of H2O onto dialdehyde:lysozyme adducts suggested that hydroxyl species are reactive for Michael addition and thus serine residues may be responsible for the second nucleophilic attack. In contrast, incubation of lysozyme with enol esters 1.37 and 1.38 gave exclusively Michael addition products, confirming that these compounds are less reactive compared to the dialdehydes and cannot undergo pyrrole synthesis. Dialdehyde 1.41 was examined as an alkyne partner in the Huisgen’s 1,3-dipolar cycloaddition “click” reaction and was proven to react with fluorescent azide 1.77, forming a triazole product 1.80. The reaction of fluorescently-tagged triazole 1.80 with lysozyme was found to be poor yielding and thus required product yield optimisation. Finally, the lysozyme adducts of 1,4-dialdehydes 1.40, 1.41 and E-1.42 were analysed by SDS-PAGE, which identified 1.41 and E-1.42 as protein cross-linkers.