Abstract
This thesis describes the results of a mechanistic investigation of norbornenone based, pH-triggered carbon monoxide prodrugs (oCOms) recently reported by Larsen and co-workers. To inform the design of oCOms, it is important to know the mechanism of CO release so that control of stereoelectronic and physicochemical factors can be used to optimise the oCOm’s CO release profile. It is known that CO release is triggered by the elimination of HBr to unmask an unstable norbornadienone, which then undergoes rapid decomposition to give CO and a substituted phthalimide byproduct (BP). To investigate the rate of HBr elimination, a series of oCOms (oCOms-63, -64 and -67) were synthesised by Diels-Alder cycloadditions between N-functionalised bromomaleimides and dimethyl-diphenyl cyclopentadienone 4.
By comparison of the relative reactivity of oCOm-63, -64 and -67 to base it was shown that more electron-deficient oCOms had more acidic methine protons and this increased the rate of CO release. The mechanism of CO release was therefore determined to proceed via an E1cb(irr)-type elimination of HBr where proton abstraction is the rate limiting step. Thus, control of the rate of this mechanism can be achieved by increasing or decreasing the acidity of the methine proton abstracted in the rate-limiting step.
In order to compare half-lives of these oCOms with others previously reported and to enable measurement of the effect of the EWG in a biologically relevant system, the synthesis of aqueous-soluble analogues of oCOms-63 and -64 was undertaken, by synthesis of the ethanolamine- or PEG ester- functionalionalised bromomaleimides, then cycloadditions with 4. While oCOm-63 analogues could be not accessed here, it was found that oCOms-68 and -70 and -71 were significantly more soluble in aqueous media than the parent compound oCOm-64. The half-lives of oCOms-68 and -70 and -71 in pH 7.4 PBS were measured by monitoring of the conversion of oCOm to BP by LCMS. It was found that oCOm-68 was the second-most rapidly releasing oCOm described to date (T½ ≈ 60 min), but that oCOms-70 and -71 were significantly longer (T½ ≈ 180 min). The increased half-life of the PEGylated oCOms was found to have been likely caused by the formation of aggregates in aqueous media which shield the methine proton from base.
Finally, the synthesis of the first R1 and R2-functionalised oCOms is also reported. Reaction of diestercyclopentadienone 44 and bromomaleimide gave only BP-73 due to the high reaction temperatures necessitated by the poor Diels-Alder favourability and the sensitivity of the resulting oCOm. The unsymmetrical cycloaddition of 57 gave only the anti oCOm-74 regioisomer, indicating that this may be a successful way of installing EWG at R2, where inductive electron-withdrawal has the greatest impact.