This thesis describes work towards a range of novel nitrogen containing polyaromatic heterocycles for use as organic semiconductors. A route to benzo[b]carbazole end-capped oligothiophenes has been developed; relying on two key steps; namely a connective-Pummerer cyclisation and a SmI2-mediated cleavage–cyclisation cascade. A route to extended dibenzoindolo[3,2-b]carbazole-based aza-heptacenes has also been developed, by employing the same key steps in a two-directional manner. The physical and electronic properties of the resulting materials has been assessed, and the utility of the materials in OFET devices has been demonstrated.

Carbonyl reduction is a fundamental transformation that underpins synthetic chemistry. The re-routing of carbonyl reduction through less-conventional intermediates allows new selectivity and reactivity to be found in the resulting reaction space. We have shown for the first time that the unusual radical anions formed by electron transfer to the ester carbonyl group can be exploited in additions to alkenes. We have demonstrated that the reductive cyclisations of lactones, triggered by electron-transfer from SmI2-H2O, allow for highly decorated cyclopentane and cycloheptane ring systems to be constructed. Futhermore, the reductive cyclisations of lactones can be exploited in cyclisation cascades. The cascade sequences involve the generation and trapping of unusual radical anions formed from the ester carbonyl followed by the trapping of more conventional radical anions from the carbonyl groups of ketone intermediates.

The development of new methods for the reduction of carboxylic acid derivatives is described. The ability to reduce these carbonyl derivatives through radical intermediates provides an orthogonal approach as compared with hydride based reductions. Initial experiments focused on the development of the SmI2–H2O system, where we have shown that chelation effects can be utilised to facilitate reduction of cyclic esters. Furthermore, a revised mechanism for the SmI2–H2O mediated reduction of lactones is discussed, and the effective reduction potential of the system determined. Also described is the optimisation of barbituric acids using SmI2–H2O to give the corresponding hemiaminal product. Next, experiments towards the development of a more reactive SmI2-based system are presented; where we have demonstrated that the SmI2–amine–H2O system is capable of the reduction unactivated carboxylic acid derivatives. The reductions of carboxylic esters and acids are described with mechanistic discussions. In addition, the design of a new divalent lanthanide system based on thulium diiodide is described. The addition of proton sources to TmI2 increases the effective reduction potential and facilitates unprecedented reactivity with amides. An investigation into the preparation of the reagent is also described, which has been one of the key factors developing all of the chemistry presented.

An improved second generation approach to pestalotiopsin A and 6-epitaedolidol has been developed. A silicon group bonded directly to carbon has been used as a stereocontrol element in cyclisation substrates for a SmI2 – mediated 4-exo-trig radical cyclisation. The silicon groups acts as a steric block to set up three new stereocentres in high to complete diastereocontrol. Several approaches to the cyclisation substrates are outlined, including a silyl transfer/aldol cascade and a novel C2-symmetric N-heterocyclic carbene – copper catalyst to asymmetrically install a C-Si bond. The silicon group is a versatile handle for further manipulation and has been oxidised using Fleming oxidation conditions to install the hydroxyl functionality required in the target compounds. The silicon group also simplifies the alcohol protecting group strategy in the approach to pestalotiopsin A and 6‐epitaedolidol. The first application of N-heterocyclic carbene – copper catalysed silyl transfer in natural product synthesis and the first use of a SmI2 – mediated cyclisation directed by a C-Si stereocontrol element are described.

The development of selective SmI2-H2O-mediated mono-reductions of cyclic-1,3-diesters to the corresponding 3-hydroxy acids is described. The reaction proceeds with complete selectivity for cyclic-1,3-diesters over acyclic esters. Sequential one-pot conjugate reduction-ester reduction of alkylidene cyclic-1,3-diesters is also reported. Furthermore, we describe the exploitation of the unusual ketyl radical intermediates formed via single electron reduction of the ester carbonyl in unprecedented 5-exo-trig cyclisations providing access to highly substituted, stereo-defined, cyclopentanols and cyclopentanones. Also described is the use of a silicon control element to direct the stereochemical outcome of the SmI2-MeOH-mediated conjugate reduction-intramolecular aldol cyclisations of α,β-unsaturated lactones. These cyclisations generate two contiguous quaternary centres with complete diastereocontrol; the utility of the silicon-directing group as a synthetic handle for derivatisation of the cyclisation products has also been demonstrated. These cyclisations have been applied in a model approach to the anti-mitotic natural product pseudolaric acid B.

A highly regio- and diastereoselective approach towards complex 6-membered lactones was developed using allyl/propargyl benzyl ethers and delta keto esters. Crucially, the classical ET reagent SmI2 gave unsatisfactory results and it was necessary to develop and screen new Sm(II) cyclopentadienyl complexes to deliver high selectivity in the transformation. The methodology was extended to a one-pot approach to complex cycloheptanols using SmI2-H2O in a second stage of the process. Samarium(II) folding cascades were developed where simple, linear starting materials are converted to complex polycyclic architectures bearing multiple stereocentres. It was found that, depending on the sidechain in the starting material, it was possible to achieve four different pathways from the common radical intermediate. Crucially, transannular 1,5-HAT from tertiary and benzylic positions was observed to give diverse products. A proposed 1,5-HAT facilitated SmI2-mediated 6-membered lactone radical cyclisations for the first time without an activating proton donor additive. Enantioselective samarium(II) mediated cyclisation cascades were achieved, where simple beta keto esters are converted to complex polycyclic architectures bearing up to five contiguous stereocentres with high diastereo- and enantiocontrol. In the process, a simple and easy to prepare chiral aminodiol was employed which could be recycled after the reaction. Unprecedented, enantioselective transannular radical cascades allowed access to unique 3- dimensional scaffolds inaccessible by other synthetic methods.