Synthetic Approaches to Some Sulphur-Containing Heterocycles
This thesis is in two parts. The first part deals with metalation of aromatic compounds with organolithium reagents as a route to sulphur-containing heterocycles. As the studies are directed towards synthesis of new aromatic sultones: benzooxathins; comppounds with an ortho-B- hydroxy group contiguous to an aromatic sulphoramdes were required. Consequently N-t-butylbenzenesulphonamide was lithiated with n-BuLi to obtain the corresponding 2-litho species which were quenched with a variety of expoxides as electrophiles. The reactions gave the appropriate alcohols in low yields. The alcohols were converted to alkyl halides on which heterocyclisation were attempted with sodium hydride. Dehydrohalogenation products were obtained instead. Metalation of the tertiary sulphonomide:N-(benzene-sulphonyl) piperidine gave the corresponding anion but the desired corresponding compounds were not obtained. Benzylic metalation was considered as an alternative method of introduction of the B-hydroxy functionality. N-t-butyl-2-methylbenzene sulphonamide was lithiated and coupled with benzophenone to give a carbinol which on cyclisation did not give the desired sultone but gave a new benzothiazine. Benzylic lithiation of ethyl 2-methylbenzenesulphonate gave anions which were coupled smoothly with a range of electrophiles giving a variety of newly substituted benzene sulphonates. Metalations of ethyl 2,4-dimethylbenzenesulphonate gave the 2-lithiomethyl anion mainly, giving credence to a predominant coordination mechanism in benzylic lithiation. In attempts to obtain new pyridine-fused sultones, 2 and 4(N,N-dialkylamino) sulhonylpyridines were lithated with IDA to give a 3-lithio species in both cases. These were coupled with benzophenone to give pyridine carbionols. Thermal cyclisation of the six carbionols gave two new pyridine-fused sultones. N-t-butylpyridine-3-sulphonamide was also metalated with LDA to give 4-lithio compounds which were quenched with benzophenone and carbon dioxide furnishing a carbinol and an acid respectively in good yields. The latter acid was cyclised with PPA or phosphorous oxychloride to give isothiazolo (5,4-c) pyridines. Part II of the thesis deals with the synthesis of pyrido (1, 2-b) 1,2,4-benzothiadiazines and its substituted analogues via readily generated endocyclic iminium ions. Appropriate sulphonyl chlorides were condensed with piperidine-2-carboxylic acid. Five benzene analogues were obtained in good yields. N-(Arylsulphonyl) tetrahydropyridinium salts were obtained region-specifically and in high yield by smooth triflate-assisted decarbonylation of the corresponding N-(arylsulphony) piperidine-2-carboxylic acid chlorides at room temperature. These iminium salts were converted to the corresponding nitroamines. These compounds underwent a smooth reductive exo-tet cyclocondensation reaction to give corresponding new 9-substituted tricyclic azacycles: 1,2,3,4,11,11a-hexahydropyrido (1,2-b) - 1,2,4-benzothiadiazine-6,6-dioxides.