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Title: Synthetic applications of nitrile oxide/isoxazoline chemistry
Author: Boyd, Ewan Campbell
Awarding Body: University of Edinburgh
Current Institution: University of Edinburgh
Date of Award: 1992
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Three alkenes, (R/S)-2-phenyl-4-vinyl-4,5-dihydro-oxazole (11), (4R)-3-N-t-butoxycarbonyl-2,2-dimethyl-4-vinyloxazolidine (12) and (2R)-2-(N-t-butoxycarbonyl)aminobut-3-en-1-ol (13), have been prepared from (S)-serine and used in cycloaddition reactions with some common nitrile oxides, to investigate the effect of an allylic nitrogen at an α-chiral centre on π-facial selectivity. Alkene (11) was prepared in racemic form after an unexpected racemisation during its synthesis. Cycloaddition reactions with three nitrile oxides (benzonitrile oxide, ethoxycarbonylformonitrile oxide and bromonitrile oxide) furnished the isoxazoline/oxazoline adducts in poor to good yield. π-facial selectivity varied from 69.31 (BrCNO) to 82:18 (EtO_2CCNO) in favour of the (5R,4'S) erythro product. The stereoselectivity has been explained by both a steric transition state model and by the existence of a stereoelectronic contribution to the stability of the transition state. Alkene (12) afforded good yield of cycloadducts with the same three nitrile oxides, but only gave moderate π-facial selectivity (ca. 66:34); the (5R,4'S) erythro products were again favoured. The diastereomeric products were separated after partial deprotection of the β-amino-alcohol moiety (deacetonisation). The observed π-facial selectivity has been explained by a steric transition state model. The N-protected vinylamino-alcohol (13) has been reacted with benzonitrile oxide and ethoxycarbonylformonitrile oxide to give the corresponding cycloadducts in moderate yield. π-facial selectivity was poor, and the (5S,2'S) threo isomer was favoured. The reduction and reversal of π-facial induction has been explained by a hydrogen bonding interaction in the transition state. Two examples of chiral heterocyclic nitrile oxides (39) and (41) which correspond to the two heterocyclic alkenes have also been prepared, each of which represents a protected chiral β-amino-alcohol nitrile oxide.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available