Ph.D. / The intramolecular insertion of allylpalladium species into alkene and alkyne bonds ('metalloene' cyclisation) of acetoxy-2,7-dienes and acetoxy-2-en-7-ynes has evolved into a synthetically powerful process. Proceeding from the favourable results obtained with achiral analogues, the palladium-catalysed 'metallo-ene' reactions of acyclic carbohydrate-derived 1,1-diacetoxy-2,7-diene and 1,1-diacetoxy-2-en-7-yne compounds proceeded in a stereospecific fashion to furnish chiral, multi-functionalised five-membered carbo- and heterocyclic products. A valuable consequence of the reactions of these geminal diacetate starting materials is that the products contain enolacetate, or latent aldehyde moieties. Further elaboration of these functionalities enables the facile entry to an array of substituted five-membered ring compounds, some of which have been identified as synthons or substructures of important natural products. In the presence of carbon monoxide (1 atm) the corresponding cyclic carboxylic acid products were obtained under mild 'metallo-ene' reaction conditions. Cyclised chloro componds were obtained by exposure of the 1,1-diacetoxy-2-en-7-yne substrates to Wacker-like conditions in the presence of an excess of LiC1. The highly atom economical Trost-type cycloisomerisation ensued when these enyne starting materials were stirred in acetic acid in the presence of a palladium(0) catalyst. The mechanism of the palladium-catalysed 'metallo-ene' transformations is discussed with respect to the enantioselectivity of carbopalladation of prochiral alkenes as well as the transition states. The topological influence of pre-existing centres over developing stereogenic centres is evaluated. Following indications that n-allylpalladium complexes can be prepared by the oxidative addition of Pd(0) to an a,13-unsaturated acetal, our next strategy was based on the use of suitably substituted A 2'3-pyranoside rings as chiral templates for the construction of multi-functionalised fivemembered ring systems via Pd-catalysed 'metallo-ene' cyclisations. Enantiomerically pure bi- and/or tricyclic products of carbonyl insertion were obtained in fair to good yields when these 'metallo-ene' reactions were conducted in the presence of CO under mild reaction conditions. Subsequently, bicyclic diene products were prepared via the palladium-catalysed cyclisation of Az3-C-glycoside derivatives. Important revelations concerning the relative stereochemistry of substrates for palladium-catalysed 'metallo-ene' cyclisation/carbonylation of unsaturated C-glycoside derivatives were made. The consecutive introduction of unsaturated C-alkyl side-chains at C-4 and C-1 of selected pseudoglycals via palladium-catalysed nucleophilic allylic substitution reactions accomplished the formation of chiral dienyne compounds. Palladium-catalysed cascade cycloisomerisation of these substrates effected the smooth synthesis of enantiopure tricyclic products. Cascade cyclisation of these highly unsaturated starting materials under Wacker-like conditions afforded [5,6,6]-tricyclic dichloro compounds. A mechanism to account for the stereospecific incorporation of two chlorine atoms into, and the ring expansion of the substrate carbon framework, is proposed. Finally, the readily accessible tert-butyl 6-0-acety1-4-[bis(ethoxycarbonyl)but-3-eny1]-2,3,4- trideoxy-a-D-erythro-hex-2-enopyranoside was converted, via a domino process, into a chiral substituted dihydropyran in the presence of a Pd(II)-catalyst in acetic acid/acetonitrile. By conducting the reaction under an atmosphere of CO, a tetrahydropyran compound containing a carboxylic acid side-chain was obtained, while the corresponding chloro product was prepared when the pseudoglucal starting material was exposed to an excess of LiC1 and CuC1 2 under Wacker-like conditions.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:uj/uj:10147 |
| Date | 12 September 2012 |
| Creators | Marais, Lizel |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
| Type | Thesis |
Page generated in 0.0015 seconds