Aluminium triflate as a Lewis acid catalyst in some epoxide and aromatic transformations
- Authors: Lawton, Michelle Claire
- Date: 2012-03-14
- Subjects: Lewis acids , Aluminum , Alcoholysis , Epoxy compounds , Catalysts , Aromatic compounds , Substitution reactions
- Type: Thesis
- Identifier: http://ujcontent.uj.ac.za8080/10210/368554 , uj:2163 , http://hdl.handle.net/10210/4536
- Description: M.Sc. , Lewis acids play an important role in catalysis; they are associated with mild conditions, high selectivities and unique reactivities. Traditional Lewis acids such as AlCb and BF3 successfully catalyse such well known reactions as the Friedel-Crafts acylation reaction, Aldol condensation reactions and many more. These catalysts, however, must be used in a stoichiometric amount and are destroyed during the aqueous workup procedures. Lately, there has been a lot of interest in the role of metal triflate as Lewis acid catalysts. They were found to be effective in a wide range of reactions when used in catalytic amounts. They were also found to be recyclable and reusable without the loss of activity. Most of this research has been centred around the lanthanide triflates as well as scandium, bismuth and yttrium triflates. Very little research has been done using aluminium triflate and this triflate forms the focus ofthis study. The work contained in this dissertation demonstrates that Al(OTf)3 is an efficient catalyst for the ring opening of a variety of epoxides by alcohols when present in only ppm amounts. These reactions provided products in very high yields and selectivities. Simple acyclic and cyclic epoxides readily underwent ring opening reactions with a range of alcohols, typically providing the monoglycol ethers as single compounds (from the cyclic epoxides) or as mixtures of the two possible glycol monoethers (from the acyclic epoxides). In the case of styrene oxide, essentially a single compound was isolated. In contrast, the glycidyl ethers required slightly higher catalyst loadings before similar rates and conversions to product were observed. Additionally, an interesting selectivity was observed in the orientation of the attack of the alcohol onto the epoxide, which appeared to be chelation controlled. Similarly, the Al(OTf)3 also catalysed the aminolysis of a variety of epoxides. These reactions proceeded smoothly with catalytic amounts of the triflate present, and served to nicely highlight the role that steric and electronic factors played in these reactions. A preliminary study was carried out into the efficacy of Al(OTf)3 as a catalyst for Friedel-Crafts acylation and aromatic nitration reactions. From these studies it is evident that the Al(OTf)3 is indeed an effective catalyst for these reactions when present in substoichiometric levels and further studies will be carried out in this area in the future.
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Aluminium triflate-mediated reactions of glycals: towards chiral multicyclic products
- Authors: Simelane, Sandile Bongani
- Date: 2015-04-23
- Subjects: Substitution reactions , Organic compounds - Synthesis , Enantioselective catalysis , Lewis acids
- Type: Thesis
- Identifier: uj:13557 , http://hdl.handle.net/10210/13698
- Description: Ph.D. (Chemistry) , The bridged chiral benzopyrans were strategically ring opened via acetolysis to yield either galactose based chromenes or chromans, depending on the reaction conditions. A proposal relating to the mechanism of this selective ring opening acetolysis is discussed. The benzopyrans (chromenes, chromans and bridged chiral benzopyrans) were de-acetylated via triethyl amine catalysed transesterification. Interestingly, the chromenes did not yield the anticipated hydrolysis product (triol) but a new class of bridged chiral benzopyrans which were a result of intramolecular oxa-Michael addition. A chromene that formed during the selective ring opening of the bridged chiral benzopyrans was employed to develop a method for the synthesis of a carbohydrate derived oxepane. The oxepane synthesis was achieved, albeit in the face of numerous challenges from side reactions. The difficulties encountered in the synthesis are discussed...
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Glycal inspired synthesis of antimalarial thiochromans, biologically relevant pyranotetrahydroquinolines and chromenoquinolines
- Authors: Moshapo, Paseka Thendo
- Date: 2016
- Subjects: Substitution reactions , Organic compounds - Synthesis , Heterocyclic compounds - Therapeutic use , Heterocyclic chemistry
- Language: English
- Type: Doctoral (Thesis)
- Identifier: http://hdl.handle.net/10210/233947 , uj:23897
- Description: Ph.D. (Chemistry) , Abstract: The work presented in this thesis describes the transformation of readily available glycals into a series of novel S- and N-heterocycles, namely, thiochromans, pyranotetrahydroquinolines and chromenoquinolines. The thiochroman derivatives have been synthesised by an efficient and versatile substitution of the side-chain iodo substituent in 2-deoxy-2-C- iodomethyl glucosides by thiophenolate ions, and subsequent intramolecular Friedel-Crafts alkylation to generate a series of molecular analogues bearing electron donating aryl substituents. These were further oxidised to their sulfoxide and sulfone derivatives providing a diverse library that exhibited antimalarial activities at low IC50 micromolar concentrations. Structure activity relationship studies and further chemical manipulations of these compounds revealed that the oxidation state, aryl lipophilic substituents as well as the presence of benzyl ethers on the sugar moiety are some of the factors found to be crucial for the antimalarial properties of these compounds. The methodology developed for the synthesis of thiochromans was then successfully extended to the diastereoselective synthesis of novel pyranotetrahydroquinolines. However, the initial trial reactions proceeded with some unexpected rearrangements to yield α,β-unsaturated carbaldehydes and glycals via interesting reaction mechanisms that are also discussed. Reaction optimisation studies and substrate manipulation then successfully afforded a series of tetrahydroquinoline derivatives containing both electron donating and electron withdrawing substituents and these have been fully characterised. This thesis also describes the novel synthesis of fused 5-membered ring pyranotetrahydroquinolines through a scandium triflate [Sc(OTf)3] catalysed multicomponent reaction consisting of a glycal, aniline and salicylaldehyde variants. The formation of 4C1 and B1,4 product mixtures was proposed based on the 1-D and 2-D NMR studies of the formed reaction products. The reaction is also proposed to proceed via a domino Ferrier rearrangement and intramolecular cyclisation mechanism to afford the observed...
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