An <i>endo</i>-borneol based oxazolidinone 77 was used in a series of asymmetric transformations to assess its worth as a chiral auxiliary. In regard to titanium catalysed Diels-Alder reactions with cyclopentadiene, the acrylate and crotonate derivatives 87 and 101 exhibited poor selectivity; the acrylate displayed an optimum ratio of <i>endo</i> products of 2:1 and the crotonate of 3:1. Furthermore, the selectivity was shown to be dependent upon the order of addition of the reagents. The cinnamate derivative 102 reacted very sluggishly under the same conditions and only <i>ca.</i> 20% of the starting material was consumed after three days. When diethylaluminium chloride was employed as catalyst, the selectivity of 87 increased to 4:1 and that of the 101 increased to 6:1. Only 102 showed almost complete selectivity using this catalyse. The dienophile 87 reacted with isoprene to give a 2:1 mixture of products using titanium catalysts and 5:1 using diethylaluminium chloride catalyst. In regard to alkylation reactions, the lithium enolate 110 of the propionyl derivative of 77 condensed stereospecifically with benzyl bromide, but ethyl tosylate was inert to this nucleophile. Reaction of 110 with acetyl and propionyl chloride formed products with high selectivity, but small amounts of concomitant <i>O</i>-acylation occurred. Only reaction with methyl cyanoformate (Mander's reagent) occurred wholly at carbon, with a selectivity of 10:11. The reaction of this enolate with benzoyl chloride was stereospecific, but large amounts of enol product were also formed in this particular case. The enolate 10 underwent aldol condensation with isobutyraldehyde with 48% d.e. and 29% d.e. with acetaldehyde. However, the latter reaction was marred by the sluggishness of its reaction and the formation of dehydration products. The corresponding chlorotitanium enolate reacted with benzaldehyde to form a 6 : 5 ratio of <i>anti : syn</i> products. Use of excess Lewis acid furnished solely <i>anti</i> products. The analogous zinc enolate 132 was found to be completely unreactive towards even benzaldehyde complexed with diethylaluminium chloride and underwent chemical cleavage in boiling tetrahydrofuran.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:651712 |
| Date | January 1993 |
| Creators | Grant, Keith J. |
| Publisher | University of Edinburgh |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/1842/14933 |
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