Lactone Reduction Using Samarium Diiodide: Biocatalysis, Radical Cyclisations and Ester Migration A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy (PhD) in the Faculty of Science and Engineering 2019 Charlotte V Morrill School of Chemistry Table of Contents Abbreviations 4 Abstract 6 Declaration 7 Copyright statement 8 Acknowledgements 9 1. Introduction 10 1.1. Introduction to Samarium Diiodide 10 1.2. Activation of Carbonyls by SmI2 10 1.3. Activation of Carboxylic Acid Derivatives 11 1.3.1. Reduction of Aromatic Carboxylic Acid Derivatives 11 1.3.2. Reduction of Aliphatic Carboxylic Acid Derivatives 12 1.3.2.1. Mechanism of Lactone Reduction 13 1.3.2.2. Ring-Size Selectivity in Lactone Reductions 14 1.3.3. Reductive Radical Cyclisations of Carboxylic Acid Derivatives 16 1.3.3.1. Monocyclisations of Six-Membered Lactones 16 1.3.3.2. Cyclisations of Seven-Membered Lactones 19 1.3.3.3. Lactone Cyclisation Cascades 20 1.3.4. Activation of Other Carboxylic Acid Derivative by SmI2 22 1.3.4.1. Reduction of Carboxylic Acid Derivatives 22 1.3.4.2. Cyclisations of Carboxylic Acid Derivatives 23 1.3.5. Role of Additives 26 1.4. Enantioselective Cyclisation Reactions Using SmI2 28 2. SmI2-Mediated Reductive Radical Cyclisations of Five-Membered Lactones 30 2.1. Project Proposal 30 2.2. Optimisation of the Cyclisation 30 2.3. Scope of the Cyclisation 32 2.4. Cyclisation Mechanism and Rationale for Diastereoselectivity 34 2.5. Summary 35 3. Biocatalytic Conversion of Cyclic Ketones Bearing α-Quaternary Stereocentres to Lactones in an Enantioselective Radical Approach to Medium-Sized Carbocycles 36 3.1. Project Proposal 36 3.2. The Biocatalytic Baeyer-Villiger Reaction 37 3.2.1. Substrate Scope 38 3.2.2. Cofactor Regeneration Systems 39 3.3. Development of the BVMO-mediated Oxidation of Cyclic Ketones Bearing α- Quaternary Stereocentres 41 3.3.1. Synthesis of Racemic Starting Materials 41 3.3.2. Expression and Purification of Cyclohexanone Monooxygenase 42 3.3.3. Initial Biotransformations Using Model Substrates 43 3.3.4. Scope of the Biotransformation 44 3.3.5. Determination of Absolute Configuration 48 3.3.6. Computational Modelling Using CHMORhodo 50 3.4. SmI2-Mediated Cyclisations of Enantiomerically Enriched Substrates 51 2 3.4.1. Cyclisation of Enantiomerically Enriched Lactones Using SmI2-H2O via a Carbonyl-Alkene Coupling 51 3.4.2. Cyclisation of Enantiomerically Enriched Ketone Substrates Using SmI2-HMPA via a Ketone-Alkene Coupling 52 3.5. Summary 54 4. SmI2-Mediated 1,4-Ester Migration in Lactone Substrates 56 4.1. Project Proposal 56 4.2. Synthesis of Starting Materials 56 4.3. Initial Investigations of the Lactone Cyclisation 58 4.4. Identification of Unknown Compound 119 60 4.5. Optimisation of the 1,4-Ester Migration 61 4.6. Scope of the Transformation 63 4.6.1. Scope with Respect to the R Substituent 63 4.6.2. Scope with Respect to the Styrenyl Aromatic Substituent 65 4.7. Development of a One-Pot Procedure 69 4.8. Mechanism of the 1,4-Ester Migration 71 4.9. Rationale for the Origin of Diastereoselectivity 75 4.10. Summary 76 5. Overall Summary and Future Work 77 5.1. Summary 77 5.2. Future Work 79 5.2.1. Biocatalytic Baeyer-Villiger Reaction 79 5.2.2. SmI2-Mediated 1,4-Ester Migration 80 5.2.3. Biocatalytic Processes for the Construction of Lactone Substrates for Reactions with SmI2 81 6. Experimental Section 84 6.1. General Information 84 6.2. Preparation of Samarium Diiodide 85 6.3. Protein Production and Purification 85 6.4. CHMO Sequence 86 6.5. CHMO Modelling 86 6.6. Experimental Data 87 6.6.1. Experimental Data for SmI2-Mediated Reductive Radical Cyclisations of Five-Membered Lactones 87 6.6.2. Experimental Data for Biocatalytic Conversion of Cyclic Ketones Bearing α-Quaternary Stereocentres to Lactones in an Enantioselective Radical Approach to Medium-Sized Carbocycles 100 6.6.3. Experimental Data for SmI2-Mediated 1,4-Ester Migration in Lactone Substrates 149 7. References 194 Word count 49024 3 Abbreviations ADH alcohol dehydrogenase APCI atmospheric pressure chemical ionisation BLAST basic local alignment search tool BV Baeyer-Villiger BVMO Baeyer-Villiger monooxygenase CDMO cyclododecanone monooxygenase CHMO cyclohexanone monooxygenase CPMO cyclopentanone monooxygenase Cy cyclohexyl dba dibenzylideneacetone DMAP 4-dimethylaminopyridine DMF dimethylformamide DMP Dess-Martin periodinane dr diastereoisomeric ratio ee enantiomeric excess eq. equivalents er enantiomeric ratio ES electrospray FAD flavin adenine dinucleotide G6PDH glucose-6-phosphate dehydrogenase GC gas chromatography GDH glucose dehydrogenase h hours HFIP 1,1,1,3,3,3-hexafluoro-2-propanol HG(II) Hoveyda-Grubbs second generation catalyst HMPA hexamethylphosphoramide HPLC high performance liquid chromatography HRMS high resolution mass spectrometry 4 IPTG isopropyl-β-D-thiogalactoside IR infrared LB lysogeny broth mCPBA meta-chloroperoxybenzoic acid min minutes mp melting point MWCO molecular weight cut off NADP+ nicotinamide adenine dinucleotide phosphate NADPH nicotinamide adenine dinucleotide phosphate (reduced form) NMR nuclear magnetic resonance nOe nuclear Overhauser effect PHOX phosphinooxazoline ppm parts per million pTSA para-toluenesulfonic acid rac racemic rpm revolutions per minute rt room temperature SCE standard carbon electrode SDS-PAGE sodium dodecyl sulfate polyacrylamide gel electrophoresis SET single electron transfer TBDMS tert-butyl dimethyl silyl Tf trifluoromethylsulfonate THF tetrahydrofuran THP tetrahydropyran TTN total turnover number 5 Abstract Samarium diiodide (SmI2, Kagan’s reagent) has shown itself to be a highly versatile reducing agent since its introduction to the synthetic community in 1977. Although SmI2 was initially thought to be unable to reduce carboxylic acid derivatives, the discovery and development of the SmI2-H2O reagent system has rendered such reactivity possible and has opened new avenues for exploration. The reductive cyclisation of six-membered lactone substrates has previously been reported by the Procter group. Herein, the analogous cyclisations of five-membered lactones, substrates which exhibit lower reactivity towards Sm(II), are described. By the use of appropriate additives, such lactones can be used to access substituted cyclohexanone motifs through diastereoselective radical cyclisation. A synthetic route towards enantiomerically enriched cycloheptan- and cyclooctanols, structural motifs present in various biologically relevant molecules, is also disclosed. The strategy exploits Baeyer-Villiger monooxygenase-mediated biocatalysis in order to access lactone substrates with high enantioenrichment. A kinetic resolution process which transforms cyclic ketones bearing α-quaternary stereocentres into the corresponding lactones has been developed. The products of the kinetic resolution are suitable substrates for diastereoselective Sm(II)-mediated cyclisations. Overall the process gives access to diverse enantiomerically enriched carbocyclic scaffolds from simple racemic starting materials. Finally, a novel reactivity mode of Sm(II) is reported, with the development of a 1,4-ester migration. The reaction proceeds through an unusual radical species formed at an acyclic ester moiety, rather than at the lactone carbonyl as observed in previous work. The reactivity appears to be determined by the conformation of the lactone substrate, where an alkyl substituent at the 5-position plays a key role in determining lactone conformation and thus the course of the reaction. 6 Declaration No portion of the work referred to in this thesis has been submitted in support of an application for another degree or qualification of this or any other university or other institute of learning. Part of this work has been published in peer-reviewed journals: X. Just-Baringo, C. Morrill, D. J. Procter Tetrahedron, 2017, 72, 7691. C. Morrill, C. Jensen, X. Just-Baringo, G. Grogan, N. J. Turner, D. J. Procter Angew. Chem. Int. Ed. 2018, 57, 3692. H-M. Huang, M. Garduño-Castro, C. Morrill, D. J. Procter, Chem. Soc. 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