Cycloaddition Reactions with 1,3-Dipoles for the Formation of Complex Heterocycles

Nitrogen-containing 1,3-dipoles such as azomethine ylides and zwitterionic 1,3-dipoles were utilised in cycloaddition reactions with aromatic dipolarophile partners via inter- and intramolecular modes. In addition, the reactivity of the zwitterionic 1,3-dipoles was investigated in a proposed [3 + 3] cycloaddition process. Firstly, in Chapter 2, the reaction of in-situ generated azomethine ylides with pendent nitro-aromatics was developed for the construction of potentially bioactive fused heterocycles. The initial dearomatised cycloadducts rapidly lost nitrous acid with concomitant rearomatisation forming tetracyclic isoindoline containing systems. In Chapter 3, a [3 + 2] dearomatisation reaction of highly electron-deficient 3-nitroindoles with nitrogen-containing zwitterionic 1,3-dipoles was developed. This reaction proceeds via the Pd(0)-catalysed ring-opening of a N-tosylvinylaziridine to reveal the corresponding zwitterionic 1,3-dipole, delivering densely functionalised pyrroloindolines in a diastereoselective manner. The reactions favoured the trans diastereoisomers in most cases, however, if the 4-position of the nitro-indole is occupied, a switch in diastereoselectivity towards the cis diastereoisomer is observed. In Chapter 4, efforts in achieving [3 + 3] cycloaddition between two in-situ generated 1,3-dipoles derived from 3-membered rings are detailed. A series of vinyl-substituted cyclopropanes and aziridines were reacted directly with other donor-acceptor cyclopropanes and arylaziridines under Pd(0) and Lewis acid catalysed conditions. Unexpectedly, a preference in reactivity toward dimerization, lactone formation, and Friedel–Crafts substitution with electron-rich aromatic and heteroaromatics was uncovered instead of the proposed cycloaddition.