Experimental and Theoretical Studies on the Reactions of Aliphatic Imines with Isocyanates

In the context of a project aiming at the replacement of the 3-substituted β-lactam ring in classical β-lactam antibiotics by an N(3)-acyl-1,3-diazetidinone moiety, we have investigated the reaction of isocyanates with imines derived from allyl glycinate and differently substituted propionaldehydes. Imines of aromatic aldehydes with anilines have been reported to react with acyl isocyanates to give 1,3-diazetidinones or 2,3-dihydro-4H-1,3,5-oxadiazin-4-ones, via [2+2] or [4+2] cycloaddition, respectively. However, neither of these products was formed with imines derived from allyl glycinate and 2-(mono)methyl propionaldehydes. α,α-Dimethylation of the imine enabled the [4+2] cycloaddition pathway, but the desired 1,3-diazetidinone products were not observed. Surprisingly, the imines obtained from thioesters of 2,2-dimethyl 3-oxo propionic acid reacted with aryl isocyanates or with benzyl isocyanate to give 5,5-dimethyl-2,4-dioxo-6-(aryl-/alkylthio)tetrahydropyrimidines, via thiol displacement and re-addition to a putative six-membered iminium intermediate. These experimental results obtained for the reactions could be rationalized by DFT calculations. In addition, we have shown that N(3)-acyl-1,3-diazetidinone and 2,3-dihydro-4H-1,3,5-oxadiazin-4-one products can be distinguished based on experimental IR data in combination with theoretical reference spectra employing the IR spectra alignment (IRSA) algorithm. This discrimination was not possible by means of 1 H, 13 C, or 15 N NMR spectroscopy.

Catalytic templated length-controlled oligomerization

Templated synthesis is an intriguing strategy for the length-controlled synthesis of oligomers. Traditionally, such reactions require stoichiometric amounts of the template with respect to the product. Recently we reported catalytic macrocyclic templates that promote oligomerization of a small molecule substrate with a remarkable degree of length control. Herein we present our efforts toward creating linear templates for catalytic length-controlled oligomer synthesis.

Synthesis and Structure-Activity Relationship Studies of C(13)-Desmethylene-(-)-Zampanolide Analogs

We describe the synthesis and biochemical and cellular profiling of five partially reduced or demethylated analogs of the marine macrolide (-)-zampanolide (ZMP). These analogs were derived from 13-desmethylene-(-)-zampanolide (DM-ZMP), which is an equally potent cancer cell growth inhibitor as ZMP. Key steps in the synthesis of all compounds were the formation of the dioxabicyclo[15.3.1]heneicosane core by an intramolecular HWE reaction (67%-95% yield) and a stereoselective aza-aldol reaction with an (S)-BINOL-derived sorbamide transfer complex, to establish the C(20) stereocenter (24%-71% yield). As the sole exception, for the 5-desmethyl macrocycle, ring-closure relied on macrolactonization; however, elaboration of the macrocyclization product into the corresponding zampanolide analog was unsuccessful. Compared to DM-ZMP, all modifications led to reduced cellular activity and lowered microtubule-binding affinity, albeit to a different extent. For compounds incorporating the reactive enone moiety of ZMP, IC50's for cancer cell growth inhibition varied between 5 nM and 133 nM, compared to 1 nM to 12 nM for DM-ZMP. Reduction of the enone double bond led to a several hundred-fold loss in growth inhibition. The cellular potency of 2,3-dihydro-13-desmethylene zampanolide, as the most potent analog identified, remained within a 9-fold range of that of DM-ZMP.