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Title: Gas phase heterocyclisation reactions of free radicals
Author: MacPherson, A. P.
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1994
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A new strategy of the synthesis of benzofurans was extended to cases with substituents on the 2,3,5-and/or 6 positions. This involves pyrolysis of 2-allyloxycinnamate esters to generate oxygen centred radicals which cyclise with loss of carbon dioxide and an alkyl radical. Small amounts of substituted coumarin by-products were identified in some cases and their formation rationalised mechanistically. This work was extended in four ways. Firstly the use of 2-t-butylthiocinnamate ester, which generates a sulphur centred radical on pyrolysis, leads to benzothiophene in adequate yield. Secondly when the base benzene ring was replaced by thiophene the preparation of the unusual thieno[3,2-b]furan ring system was achieved. The attempted extension to seven membered ring formation by pyrolysis of 2-substituted-aryl pyrroleacrylates gave instead six membered ring products. Finally the attempted synthesis of pyrrolizine by cyclisation of N-pyrrolylmethyl radicals failed to give any products, although the precursors were successfully prepared. The intramolecular attack of phenoxyl, aminyl, benzyl and thiophenoxyl radicals on ortho disposed pyrrole rings was studied systematically. In the case of the unsubstituted pyrrole ring phenoxyl and aminyl radicals did not cyclise: products showed that 1,5-aryl shifts were taking place following hydrogen atom capture. Benzyl and thiophenoxyl radicals both led to good syntheses of fused tricyclic systems and an investigation of the chemistry of the little known pyrrolo[2,1-b]benzothiophene system was undertaken. 2,5-Dimethyl substituents on the pyrrole ring led to a variety of products in all cases studied (phenoxyl, benzyl and thiophenoxyl). These arose either by direct attack on the pyrrole ring or by an interesting radical transfer, cyclisation sequence from the methyl group. Finally, the radical transfer, cyclisation sequence was again observed when a phenoxyl radical reacted with 2,5-diphenyl substituents on the pyrrole ring. The corresponding thiophenoxyl radical gave a number of products which were separated and identified using a variety of NMR techniques, one of which was a dibenzopyrrolothiazepine, the first example of seven membered ring formation by this method.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available