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Synthesis and Transformations of 2-Thiocarbohydrates : a Practical

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Synthesis and Transformations of 2-Thiocarbohydrates : a Practical Institut für Chemie Organische Chemie Synthesis and Transformations of 2-Thiocarbohydrates: A Practical Approach for Functionalization of Thiosugars Dissertation zur Erlangung des akademischen Grades "doctor rerum naturalium" (Dr. rer. nat.) in der Wissenschaftsdisziplin "Organische Chemie" eingereicht an der Mathematisch-Naturwissenschaftlichen Fakultät der Universität Potsdam von Prashant Pavashe From Ahmednagar, India Potsdam, im Januar 2017 Published online at the Institutional Repository of the University of Potsdam: URN urn:nbn:de:kobv:517-opus4-397739 http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-397739 Publication based on this thesis ‘‘Synthesis of 2-Thiocarbohydrates and Their binding to concanavalin A’’ Prashant Pavashe, Elangovan Elamparuthi, Cornelia Hettrich, Heiko M. Möller, Torsten Linker, J. Org. Chem. 2016, 81, 8595–8603. Table of contents Table of contents Table of contents………….................................................................................................. i Acknowledgements………….............................................................................................. iii Abstract…………................................................................................................................. v Zusammenfassung…………............................................................................................... viii 1. Introduction………….............................................................................................. 1 2. Research background…………................................................................................ 3 3. Aim of the studies…………....................................................................................... 14 4. Results and discussion…………............................................................................... 15 Chapter 1. Ceric ammonium nitrate (CAN) mediated thiocyanate radical additions to glycals…………..................................................................................................... 15 4.1.1. Synthesis of glycals…………..................................................................................... 15 4.1.2. Screening of the substrates for radical additions to glycals…………......................... 16 4.1.3. Thiocyanate radical addition to glycals…………....................................................... 17 4.1.4. Reaction mechanism………….................................................................................... 19 4.1.5. Reductions of the 2-thiocyanates to thiols…………................................................... 21 4.1.6. Summary…………...................................................................................................... 25 Chapter 2. Synthetic transformations of the 2-thiocarbohydrates……………............. 26 4.2.1. Synthesis of disulfides…………................................................................................. 26 4.2.2. S-Alkylations of 2-thiocarbohydrates………….......................................................... 29 i) S-Methylations ii) Sulfa-Michael addition (SMA) iii) (2→1) Linked thiodisaccharides 4.2.3. Trapping of the oxocarbenium cation by water…………........................................... 34 4.2.4. Lewis acid mediated transformations at the anomeric center…………...................... 36 4.2.5. Transformations of 2-thiocyanates………….............................................................. 37 i) (3 + 2) Cycloaddition ii) Trapping of the anomeric center by water 4.2.6. Summary…………...................................................................................................... 39 Chapter 3. Synthesis of thiodisaccharides via thiol-ene coupling………………............ 41 i Table of contents 4.3.1. Screening of the substrates for thiol-ene coupling…………....................................... 41 4.3.2. Radical additions of the 2-thiols to exo-glycals…………........................................... 45 i) Syntheses of the exo-glycals ii) TEC between the 2-thiols and exo-glycals 4.3.3. Proposed reaction mechanism for the thiol-ene coupling…………............................ 49 4.3.4. Summary…………...................................................................................................... 51 5. Future outlook …………........................................................................................... 52 6. Experimental section …………................................................................................ 54 7. References…………................................................................................................... 120 8. List of abbreviations………….................................................................................. 126 9. Compounds Index…………...................................................................................... 128 10. Curriculum vitae........................................................................................................ 131 ii Acknowledgements Acknowledgements I would like to express my deep sense of gratitude to my research supervisor Prof. Dr. Torsten Linker for giving me this opportunity to work in his research group. Also his generous support, guidance and motivation during the research activities were extremely helpful to pursue the studies. Especially, I would like to thank Prof. Dr. Bernd Schmidt and Prof Dr. László Somsák for agreeing to review my thesis and Prof. Dr. Heiko Möller for accepting to be the chairman of the examination committee. I do express my sincere appreciations to Prof. Dr. I.N.N. Namboothiri (IIT Bombay) for giving me an opportunity to spend a pre-doctoral stay in his research group. Special thanks are owed to Dr. Werner Fudickar for the valuable discussions and suggestions during the entire time of my research. Also, I thank all my present and former colleagues including Elangovon, Tukaram, Matthias, Sumaira, Marcel, Yasemin, Tobias, Melanie, Andreas and Marlis for their generous support. Technical discussions with Dr. M. Heydenreich and Angela Krtischka for the NMR analysis, Dr. I. Starke and Sylvia Fürstenberg for the Mass analysis, Prof. Dr. U. Schilde and Alexandra Kelling for the X-ray analysis were extremely helpful. I do express my appreciations to Andrea Romo for her valuable support, encouragement and motivation all the time. Friends are as important as my family is and therefore I owe deepest thanks to all of them including Shaveen, Rahul, Divya, Tarun, Vipul, Srikant, Amol, Kishor, Vardu, Ramdas, Suvendu, Tukaram, Narayanan, Varsha, Kaushalendra, Abhijit, Sanjay, Praveen, Tarique, Saurabh, Fiaz, Vikas, Sagar, Harshal, Satish, Vikram, Richa, Manaswita, Preeti, Olga, Tasca, Rocío, Kevin, Florian and many more… I am very grateful for the generous support from the University of Potsdam, which offered the research facilities, housing and the timely assiatance to foreign students. iii Acknowledgements Above all, I would like to thank my parents, my brother and all well-wishers who helped me directly or indirectly to achieve the success in my life. iv Abstract Abstract I. Ceric ammonium nitrate (CAN) mediated thiocyanate radical additions to glycals In this dissertation, a facile entry was developed for the synthesis of 2-thiocarbohydrates and their transformations. Initially, CAN mediated thiocyanation of carbohydrates was carried out to obtain the basic building blocks (2-thiocyanates) for the entire studies. Subsequently, 2- thiocyanates were reduced to the corresponding thiols using appropriate reagents and reaction conditions. The screening of substrates, stereochemical outcome and the reaction mechanism are discussed briefly (Scheme I). Scheme I. Synthesis of the 2-thiocyanates II and reductions to 2-thiols III & IV. An interesting mechanism was proposed for the reduction of 2-thiocyanates II to 2-thiols III via formation of a disulfide intermediate. The water soluble free thiols IV were obtained by cleaving the thiocyanate and benzyl groups in a single step. In the subsequent part of studies, the synthetic potential of the 2-thiols was successfully expanded by simple synthetic transformations. II. Transformations of the 2-thiocarbohydrates The 2-thiols were utilized for convenient transformations including sulfa-Michael additions, nucleophilic substitutions, oxidation to disulfides and functionalization at the anomeric position. The diverse functionalizations of the carbohydrates at the C-2 position by means of the sulfur linkage are the highlighting feature of these studies. Thus, it creates an opportunity to expand the utility of 2-thiocarbohydrates for biological studies. v Abstract Reagents and conditions: a) I2, pyridine, THF, rt, 15 min; b) K2CO3, MeCN, rt, 1 h; c) MeI, K2CO3, DMF, 0 °C, 5 min; d) Ac2O, H2SO4 (1 drop), rt, 10 min; e) CAN, MeCN/H2O, NH4SCN, i rt, 1 h; f) NaN3, ZnBr2, PrOH/H2O, reflux, 15 h; g) NaOH (1 M), TBAI, benzene, rt, 2 h; h) ZnCl2, CHCl3, reflux, 3 h. Scheme II. Functionalization of 2-thiocarbohydrates. These transformations have enhanced the synthetic value of 2-thiocarbohydrates for the preparative scale. Worth to mention is the Lewis acid catalyzed replacement of the methoxy group by other nucleophiles and the synthesis of the (2→1) thiodisaccharides, which were obtained with complete -selectivity. Additionally, for the first time, the carbohydrate linked thiotetrazole was synthesized by a (3 + 2) cycloaddition approach at the C-2 position. vi Abstract III. Synthesis of thiodisaccharides by thiol-ene coupling. In the final part of studies, the synthesis of thiodisaccharides by a classical photoinduced thiol- ene coupling was successfully
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