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The work described in this thesis is an investigation into the reactivity and possible synthetic applications of aziridinylcarbinyl radicals. These radicals rapidly rearrange via β cleavage, which can proceed by breakage of either the CoN or C-C bond. Cleavage of the latter has been found when the molecule has a phenyl stabilising group attached to the aziridine ring. Chapter I is a review of the known radical reactions involving aziridines whilst chapter 2 discusses the various methods of aziridine syntheses. Chapter 3 outlines the project aims with reference to the potential of directed cleavage of aziridinylcarbinyl radicals in synthesis. Chapter 4, the main body of the work, describes the synthetic routes to aziridines derived from 3-phenyl-2-cyclohexen-I-one and indenone. The radical mediated β-cleavage reactions of these is reported and the selectivity of C-C v. CoN bond homolysis has been investigated. Two successful approaches to the target aziridines were involved. I) Conversion of 3-azido-3-phenylcyclohexan-I,2-diol, derived from the epoxide of 3- phenyl-2-cyc1ohexen-I-ol, to the aziridine via reaction with triphenylphosphine. Subsequent methylation and formation of the thiocarbonylimidazolide gave the radical precursor Nmethyl- 5-[imidazol-I-yl(thiocarbonyl)oxy]-I-phenyl-7-azabicylo[4.1.0]heptane. In the course of this work, several unusual cyclic thiocarbonates resulting from the reaction of 3-azido-3- phenylcyclohexan-I,2-diol and 2-azido-3-phenylcyclohexan-I,3-diol with I, I 'thiocarbonyl diimidazole were isolated. 2) Formation of N-(2-ethylquinazolinonyl)-I-phenyl-7-azabicyclo[4.1.0]heptan-5-01 from the reaction of 3-amino-2-ethyl-4(3H)-quinazolinone with 3-phenyl-2-cyc1ohexen-I-ol in the presence of lead tetraacetate. These aziridines show interesting acid-catalysed rearrangements to diazadioxabicyclo[2.2.2]octanes. Formation of the thiocarbonylimidazolide then gave the radical precursor. A number of other aziridines have been prepared using this methodology. It has been found that in all cases the precursors undergo CoN bond homolysis under radical conditions. These results are discussed and conclusions are drawn. Suitable future work is also suggested.

Ribonucleotide Reductases (RNRs) are crucial enzymes that catalyze reduction of ribonucleotides to deoxyribonucleotides, required for the biosynthesis of DNA. Vital role played by RNR in the biosynthesis of DNA and its control on cell growth made it one of the main targets for anticancer therapy. Several laboratories clarified the aspects of reaction cascades at active site of RNR. Biochemical studies of RNR by Stubbe for the inactivation of RDPR by 2'-chloro-2'-deoxyuridine-5'-diphosphate emphasizes departure of chlorine as an anion, while biomimetic studies by Robins with 6'-O-nitro-2'-chloro-homonucleosides emphasizes the elimination of chlorine substituent from 2'-position as a radical. To clarify the ambiguity in the mechanism of inhibition of RNR by 2'-chloro-2'-deoxyuridine, biomimetic reactions with model 6-O-nitro-1,5-dideoxyhomosugar derivatives were investigated. The study includes several modes: (i) synthesis of 6-O-nitro-1,5-dideoxyhomosugar derivatives with chlorine, bromine or tosyl substituent at the C2 position with ribo and arabino configurations, (ii) biomimetic studies of 6-O-nitro-1,5-dideoxyhomosugar derivatives with Bu3SnH/AIBN to provide chemical evidences to distinguish the nature of elimination of chlorine from 2'-chloro-2'-deoxyuridine upon its incubation with enzyme, and (iii) kinetic studies to differentiate between heterolytic or homolytic C2'-chlorine bond cleavage. In the second half of this dissertation, azido and sulfenamide modified nucleosides and 2-azidolyxofuranoside derivatives have been synthesized with the azido or sulfenamide substitution at a specific site in the sugar or in the base moiety. The electron-induced site specific formation of neutral aminyl radicals (RNH●) and their subsequent reactions have been investigated using ESR spectroscopy. In 2'-AZdC the RNH● site is attached to a 2o C-atom, where as in 4'-AZdC, the RNH● site is attached to a 3o C-atom, respectively. These studies elucidated how stereo and electronic environment affect formation and subsequent reactivity of various types of RNH● generated from azidonucleosides. To avoid the interaction of transient radical with nucleoside heterocyclic bases, 2-azidolyxofuranoside derivatives as a simpler abasic model were synthesized and studied with ESR spectroscopy. Aminyl radical generated from 2-azidolyxofuranoside derivatives subsequently abstracted hydrogen from C5 intramolecularly. These studies were designed to understand the mechanism of damage in various DNA model structures.

Two classes of C5 azido-modified pyrimidine nucleosides were synthesized and explored as radiosensitizers. The 5-azidomethyl-2'-deoxyuridine (AmdU) was prepared from thymidine and converted to its cytosine counterpart (AmdC). The 5-(1-azidovinyl) modified 2'-deoxyuridine (AvdU) and 2'-deoxycytidine (AvdC) were prepared employing regioselective Ag-catalyzed hydroazidation of 5-ethynyl pyrimidine substrates with TMSN3. AmdU and AmdC were converted to 5'-triphosphates AmdUTP and AmdCTP, and incorporated into DNA-fragments via polymerase-catalyzed reaction during DNA replication and base excision repair. Radiation-mediated prehydrated electrons formed in homogeneous aqueous glassy (7.5 M LiCl) systems in the absence of oxygen at 77 K led to site-specific formation of π-type aminyl radicals (RNH•) from AmdU, AmdC, AvdU, and AvdC. The ESR spectral studies and DFT calculations showed RNH• undergo facile conversion to thermodynamically more stable σ-type iminyl radicals, R=N•. For AmdU, conversion of RNH• to R=N• was bimolecular involving α-azidoalkyl radical as intermediate; however, for AvdU, RNH• tautomerized to R=N•. Our work provides the first evidence for the formation of RNH• attached to C5 position of azidopyrimidine nucleoside and its facile conversion to R=N• under reductive environment. These aminyl and iminyl radicals can generate DNA damage via oxidative pathways. The azido-nucleosides were successfully applied as radiosensitizers in EMT6 cancer cells in both hypoxic and normoxic conditions. To explore the generation and reactivity of 2'‑deoxyguanosin-N2-yl radical (dG(N2-H)•) postulated to generate from guanine moiety towards •OH, 2-azido-2'-deoxyinosine (2-N3dI) was prepared by conversion of 2-amino group in protected dG into 2-azido via diazotization with tert-butyl nitrite followed by displacement with azide and deprotection. The investigation of dG(N2-H)• generated from 2-N3dI and its subsequent reactions using ESR will be discussed. Cycloaddition between 5-ethynylpyrimidine or 8-ethynylpurine nucleosides and TMSN3 in the presence of Ag2CO3, CuI, or CuSO4/sodium ascorbate provided N-unsubstituted 1,2,3-triazol-4-yl analogues of the parental DNA bases (i.e. 5-TrzdU, 5‑TrzdC, 8-TrzdA, and 8-TrzdG). These novel triazolyl nucleosides showed excellent fluorescent properties: 8-TrzdA exhibits the highest quantum yield (ΦF) of 44% while 8‑TrzdG had ΦF of 9%. The 5-TrzdU and 5-TrzdC showed a large Stokes shift of ~110 nm. The application of these fluorescent nucleosides to cell imaging and DNA modifications will also be discussed.