New approaches to heterocyclic scaffolds using Diels-Alder chemistry
Isatin-derived 1,2,4-triazines were employed as electron deficient azadienes in inverse electron demand Diels-Alder (IEDDA) chemistry with tethered alkynyl dienophiles, leading to biologically interesting heterocyclic lactam- and lactone-annulated α-carbolines in excellent yields under microwave promotion. The chemistry scope was probed using various alkynyl amines and alcohols, and the impact of the tether length between the triazines and the dienophiles was also investigated. This IEDDA methodology was subsequently applied to the synthesis of an α-carboline library, producing eighty-eight members starting with various isatin-derived triazines, indole-derivatives, and propargylamine derivatives. In a related project, a regioselective Lewis acid catalyzed amidation of dimethyl 5H-pyridazino[4,5-b]indole-1,4-dicarboxylate was established to selectively direct the amidation to occur at either the C1 or C4 ester positions. This chemistry was then applied to tether dienophiles to the pyridazinoindole ring via an amide linkage. Subsequent IEDDA cycloadditions of these pyridazinoindole/dienophile pairs afforded carbazoles in excellent yields under thermal conditions. The scope of this chemistry scope was also thoroughly probed, leading to a library of one hundred and eighty-eight members. The synthesis of a third heterocyclic scaffold using Diels-Alder chemistry was also accomplished. Asymmetric cycloadditions of anthrone with various maleimides were successfully achieved using different chiral organocatalysts. The identity of the optimal catalytic system depended upon the dienophile, with good enantioselectivities achieved (78-83% ee). This chemistry was successfully applied as the stereocontrol element in a Diels-Alder/Functional Group Transformation/retro-Diels-Alder sequence to prepare alkaloidal scaffolds in an optically enriched form.