School of Chemistry - Theses

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Subject: synthesis × Subject: organic chemistry ×

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    Towards the synthesis of the emestrin family of natural products
    Fisher, Brendan ( 2018)
    A Cope rearrangement of a vinyl pyrrole epoxide (397) was utilised to form the dihydrooxepino[4,3-b]pyrrole core (398) of the emestrin family of natural products which involved the first examples of the dearomatisation of pyrrole in this type of rearrangement. It was found that an electron withdrawing ester substituent on the C2 position of the epoxide was essential for the [3,3]-rearrangement to occur. The vinyl pyrrole epoxides were synthesised in an efficient manner by a vinylogous Darzens reaction. Density functional calculations showed lower transition state energies for Cope rearrangements of epoxides with C2 esters when compared to the unsubstituted substrates which agreed with the observed experimental results. Silyl substituted vinyl bromide esters also participated in the Darzens reactions to give the desired vinyl pyrrole epoxides in good to excellent yields. Only the triethoxysilyl vinyl epoxide 313c underwent Cope rearrangement to provide the fully substituted emestrin core dihydrooxepine. The anion derived from an aryl bromosulfone did not give the Darzens product but underwent a previously unobserved stereoselective trimerization to afford the cyclohexene 343 as a single diastereoisomer. A mechanistic rationale involving SN2’ additions, [3,3]-Cope rearrangements and a stereoselective intramolecular conjugate addition was proposed and this was supported by density functional theory (DFT) calculations. A four-step total synthesis of biaryl ether natural product violaceic acid (11) is described. The steps include an SNAr reaction to afford the biaryl ether 136, tin chloride-mediated chemoselective reduction of the nitro group to amine 135. A Cu-mediated Sandmeyer reaction of 135 gave violaceic acid methyl ester 374 which is hydrolysed to give pure violaceic acid 11. An improved synthesis of the known biaryl iodide 119 is also described via a Sandmeyer reaction of amine 135.
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    Exploring the Petasis reaction through amino acid synthesis
    Bradley, Lucie ( 2015)
    The Petasis reaction was reviewed and shown to be a versatile and efficient reaction for the synthesis of nitrogen containing compounds and α-amino acids. Many different amines and amine equivalents can be used in the Petasis reaction, in conjunction with a wide variety of aryl and vinyl boronic acids and esters, and a small selection of aldehydes. Chiral reagents can enforce stereochemical control in the reaction. Certain chiral amines and chiral amine equivalents give the highest selectivity. Several limitations remained for the Petasis reaction: yields were low with sterically small amines and the organoborons were largely limited to aryl, heteroaryl and vinyl derivatives. These limitations were addressed to make the Petasis reaction a more well-rounded and useful synthetic method. tert-Butyl sulfinamide was explored as an amine equivalent and the kinetics of the Petasis reaction with this reagent were investigated through the use of in situ FT-IR and 1H NMR spectroscopy analysis. tert-Butyl sulfinamide and glyoxylic acid both had rate orders of one, whereas styrenyl boronic acid had a rate order of two. This accounted for an observed dramatic increase in reaction rate. A mechanism for this reaction system was proposed, in which the boronic acid acts as both a reagent and as a Lewis acid catalyst. Allyl boronic acid pinacol esters were synthesised by palladium catalysed borylation of allyl alcohols, and then reacted with tert-butyl sulfinamide and glyoxylic acid to yield allyl glycine derivatives. Isolated yields of the final amino acids were excellent, but the diastereoselective ratios achieved were low to moderate. The addition of scandium(III) triflate to the allyl-Petasis reaction gave excellent control over the syn/anti configuration of the product, resulting in diastereomeric ratios in the order of >20:1. However, stereochemical control at the α-carbon was still moderate. A mechanism was devised to explain this observation and several supporting reactions were conducted. N-Methyl tert-butyl sulfinamide was synthesised racemically in a single step from the commercially available tert-butyl sulfinyl chloride and methylamine solution. The product was isolated in a pure yield of 98%. Racemic N-methyl tert-butyl sulfinamide was applied to a modified allyl-Petasis reaction, which employed molecular sieves to promote the formation of the initial iminium ion, to yield N-methyl amino acids in a quick and efficient manner. The use of scandium(III) triflate gave excellent control of the syn/anti configurations. Enantiopure N-methyl tert-butyl sulfinamide was also synthesised and applied to the Petasis reaction, resulting in excellent yields and stereochemical control. This work demonstrated the robust and widely applicable nature of the Petasis reaction as a method to synthesise α-amino acids in an efficient manner. The Petasis reaction can therefore be utilised in the chemical total synthesis of more complex natural products containing unusual amino acids residues.