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Synthetic applications of the meta photocycloaddition reaction

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posted on 2023-06-07, 15:25 authored by Simon Berritt
The meta photocycloaddition between a substituted aryl ring and alkene affords architecturally complex tricylic intermediates that have been utilised to investigate three different areas of synthetic organic chemistry. Firstly, a novel palladium catalysed oxidative fragmentation/cyclisation of the Z-but-2-ene-1,4-diol and anisole derived meta photocycloadduct 30 was investigated. It was proposed that a meso p-allyl palladium intermediate was generated during the course of the oxidative cyclisation process and it was hoped that a desymmetrisation reaction could be achieved by attaching a homochiral ligand to the palladium core and result in the enantioenrichment of the cyclised product. Unfortunately under the conditions used only a racemic mixture of products was obtained. Secondly, the use of nitrogen in exchange for oxygen in the oxidative cyclisation procedure was investigated with a view to preparing a conformationally constrained bicyclic amino acid. Installation of the amino group at the photocycloaddition stage failed and derivatisation of the hydroxyl group of the allyl alcohol/anisole derived photoadduct resulted in an undesired fragmentation. Acid induced fragmentation of the allyl alcohol/anisole derived photoadduct 25 and subsequent triflation of the hydroxyl followed by displacement with azide, reduction with Lindlar’s catalyst and protection, afforded the requisite amide. Cyclisation with phenylselenylchloride and concomitant oxidation furnished the cyclised amide. Finally, the total synthesis of gelsemine was investigated. Building on the results of a previous researcher in the group an advanced intermediate derived from the meta photocycloaddition of allyl alcohol/anisole failed to undergo the desired palladium catalysed alkoxycarbonylation to assemble the key quaternary centre of the oxindole group. Reaction of the allyl alcohol/anisole photoadduct with N-chlorosulfonyl isocyanate afforded an alternative route forward and subsequent synthetic modifications led to an attempted intramolecular Heck spirocyclisation reaction, which led to the formation of a spirooxindole centre that was epimeric to gelsemine. Utilisation of the diol photoadduct afforded functionality which would allow construction of the pyrrolidine ring prior to forming the spirooxindole centre. The same synthetic route was applied to afford the hydroxyl group, where epimerisation of the oxidised product, a further oxidation and Curtius rearrangement furnished the exo carbamate 154. The system now had the requisite functionality present to complete the total synthesis of gelsemine.


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