published_or_final_version / abstract / toc / Chemistry / Doctoral / Doctor of Philosophy

Thesis (Ph. D.)--University of Hong Kong, 2004. / Also available in print.

No description available.

Dipolar (3+2) cycloadditions are extensively utilized by synthetic chemists for accessing important 5-membered heterocyclic structures. After the pioneering work by Rolf Huisgen in the early 1960s, the field greatly matured and found applications in a variety of fields of chemistry. Worthy of mention, the discovery by Meldal, Sharpless, and Folkin of copper-catalyzed azide alkyne cycloadditions (CuAAC), also referred to as a “Click” reaction, was awarded the Nobel Prize in 2022. The finding of this ideal CuAAC reaction originated from the reliability of dipolar (3+2) cycloaddition reactions, whose transformation was rendered extremely kinetically favorable and stereospecific with the use of copper-catalysis. It is therefore of high importance to continue finding novel (3+2) cycloadditions, despite the apparent maturity of the field. The research described in this thesis presents the efforts towards the synthesis of fused pyrroles and thiophenes by means of (3+2) cycloaddition cascades using ynamides and alkynyl sulfides as isoelectronic species to 1,3-dipoles. In Chapter 2, the exploration of different strategies to bridge the in-situ synthesis of alkyne tethered ynamide and our group’s previously described thermally induced (3+2) cycloaddition cascade was investigated. Many challenges were faced when attempting to design one-pot procedures including the unprecedented degradation of yne-ynamides under metal-containing reaction conditions. This impeded the use of copper-catalyzed cross-coupling reactions as a general retrosynthetic disconnection for the in-situ formation of the ynamide functionality. Even an attempt to functionalize an ynamide precursor containing a tethered terminal alkyne by a Sonogashira cross-coupling was unsuccessful. With the aim to find an efficient way of synthesizing these diynes while limiting the use of stoichiometric reagents, the use of a previously unreported ynamide substituted propynal building block was explored. These aldehyde synthons were easily synthesized from accessible ynamide substituted propargyl alcohols using Dess-Martin Periodinane as the oxidant. Upon mixing these propynal derivatives with primary propargyl amines, a rapid condensation reaction takes place as long as the removal of water is done. These in-situ formed yne-ynamides then undergo (3+2) cycloaddition cascades towards fully substituted fused pyrroles at temperatures ranging from 60 to 100 oC. While the method was found to be limited to [3.3.0] fused pyrroles and moderate yields were observed (22-55% yields, 8 examples), this one-pot method permitted an extremely rapid growth of molecular complexity. Collectively, the work described in this chapter further accentuates the utility of ynamides as building blocks for densely functionalized pyrrole heterocycles. In Chapter 3, the reactivity of analogous alkyne tethered alkynyl sulfides (thioalkynes) was investigated. Alkynyl sulfides are an important class of heteroatom-substituted alkynes, whose alkynyl carbons are weakly polarized in contrast to ynamines (N-alkynyl amines) derivatives. While thioalkynes display superior stability in contrast to ynamides, both X-alkynyl species share similar reactivities. Upon heating of S-ester substituted yne-alkynyl sulfides, fully substituted thiophenes were obtained indicating that the reactivity observed with ynamides (as 4 cycloaddition partner) was transferable to thioalkynes. When S-alkyl substituted yne-thioalkynes are used, 5-unsubstituted thiophenes are formed instead. The use of S-tert-butyl substituted alkynyl sulfides enabled a broad scope of 5-unsubstituted fused thiophenes to be obtained via an intramolecular (3+2) cycloaddition and dealkylation cascade. The transient thiophenium ylide cycloadducts formed as a result of (3+2) cyclization were also efficiently trapped with electrophiles generating complex functionalized thiophenes. The use of S-n-propyl substituted yne-alkynyl sulfide was necessary in this case to provide control over product selectivity and to permit the electrophilic trapping to occur before dealkylation. Collectively, the reactivity cascades of thermally formed thiophenium ylides cycloadducts were studied in detail revealing a modulable and controllable reactivity by judicious choice of alkynyl sulfide substitution and reaction condition. In Chapter 4 the use of coinage metals for catalyzing the (3+2) cycloaddition of yne-alkynyl sulfides at room temperature was presented. Our group established that metal-induced low-energy pathways are accessible when alkynyl sulfides are tethered with terminal alkynes. Application of the new set of reaction conditions to an S-phenyl substituted yne-thioalkyne substrate revealed the formation of a thiophenium cycloadduct intermediate. The screening of alternative reaction conditions enabled the successful isolation of this S-phenyl thiophenium cycloadduct by precipitation from the reaction crude enabling structure confirmation by NMR and X-ray crystallography. The reactivity of this previously undescribed S-phenyl thiophenium salt was also evaluated under thermolysis and (metallo)photoredox conditions. The synthesis of S-(hetero)aryl yne-thioalkynes derivatives was first tackled revealing an incompatibility of the current methods described in the literature for a broad range of (hetero)aryl substituted alkynyl sulfides. Despite the numerous challenges encountered, the synthesis of para-substituted electron-poor and rich phenyl derivatives was successfully achieved using sulfur umpolung methods. A one-pot strategy was applied to these S-phenyl derivatives involving the in-situ formation of thiophenium cycloadducts which readily underwent a [1,5]-sigmatropic rearrangement and aromatization upon mild heating (70 oC) towards 2-aryl substituted fused thiophenes. Lastly, the compatibility of the S-phenyl thiophenium cycloadduct in (metallo)photoredox transformations for new CPh-C bond formation was evaluated. In contrast to electrophilic S-aryl sulfonium reagents commonly employed, this first generation of thiophenium salt was not efficient in providing high yields for the desired cross-coupled products. It was postulated that undesired HAT side reactivity was detrimental to the reaction efficacy. These preliminary studies allowed us to gain crucial insight into the inherent reactivity of an S-phenyl thiophenium salt with the hope to guide the next generation of potentially useful electrophilic reagents.

No description available.

In the first part of this study, four new chiral ligands (FAM) were synthesized and used in catalytic amounts in asymmetric 1,3-dipolar cycloaddition reactions of azomethine ylides. This method leads to the synthesis of chiral pyrrolidines, which are found in the structure of many biologically active natural compounds and drugs. It was found that using 10 mol% of one of these chiral ligands with different dipolarophiles (dimethyl maleate, dimethyl fumarate, methyl acrylate, tert-butyl acrylate, and Nmethylmaleimide), pyrrolidine derivatives could be synthesized in up to 94% yield and 95% ee. In the second part of the study, the catalytic activity of these chiral ligands were tested with titanium in asymmetric alkynylzinc addition reactions to aldehydes. By this method, chiral propargylic alcohols, which are important precursors for the natural products and pharmaceuticals can be synthesized. Using our catalyst, chiral propargylic alcohols were obtained in up to 96% yield and 98% ee. Although, most of the catalyst systems in the literature worked only with aromatic or aliphatic aldehydes and phenylacetylene, the catalyst system developed in this study worked with four different types of aldehydes (aromatic, aliphatic, heteroaromatic and a,b-unsaturated) and two different aliphatic acetylenes very successfully. Additionally, chiral ligand can be recovered in more than 90% yield and reused without losing its activity.

In this study new twelve phosphorus based chiral ligands were synthesized and characterized. Then the catalytic activity of these chiral ligands was tested with Cu(II) and Ag(I) salts in asymmetric 1,3-dipolar cycloaddition reactions of azomethine ylides. This method provides the synthesis of different pyrrolidine derivatives with up to four stereogenic centers. Pyrrolidine derivatives are found in the structure of many biologically active natural compounds and drugs. Therefore the asymmetric synthesis of these compounds is highly important and many groups are involved in this area. As the precursor of the azomethine ylides, N-benzyliden-glycinmethylester, N-(4-methoxy benzyliden)-glycinmethylester, N-(naphthalene-1-ylmethylene)-amino-acetic acid methyl ester, and N-(naphthalen-2-ylmethylene)-amino-acetic acid methyl ester were synthesized and used. As the dipolarophiles, methyl acrylate, dimethyl maleate and N-methyl maleimide were used. Using these imines and dipolarophiles with 6 mol % of one of the P-FAM chiral ligands in the presence of Ag(I) salt, pyrrolidine derivatives were synthesized in up to 95% yield and 89% enantioselectivity. Additionally, chiral ligand was recovered in more than 80% yield and reused without losing its activity.

In this thesis, different cycloaddition reactions, such as the enantioselective 1,3-dipolar-cycloaddition, which takes place between in situ generated stabilized azomethine ylides, and electrophilic alkenes, and the diastereoselective multicomponent reactions Amine-Aldehyde-Dienophile (AAD) or Phosphoramidate-Aldehyde-Dienophile (PAD) are described. In Chapter 1, an asymmetric 1,3-dipolar cycloaddition reaction involving an imino ester with tert-butyl acrylate was carried out using a silver(I) complex with double chirality, formed from a chiral phosphoramidite and chiral silver binolphosphate(I). The goal of this reaction is to synthesize key enantiomerically enriched structures to access the GSK-third generation of HCV inhibitors. In Chapter 2, the synthesis of polysubstituted cyclohex-2-enylamines using the multicomponent Amine-Aldehyde-Dienophile reaction involving benzyl or 4-methoxybenzylamine, is described. The study the diastereoselective version, employing commercially available chiral benzylic amines, or even a maleimide with the chiral information at the nitrogen atom, are also reported. In Chapter 3, the synthesis of polysubstituted cyclohex-2-enylamines derivatives using the multicomponent Phosphoramidate-Aldehyde-Dienophile (PAD), is described. Several series of N-substituted phosphoramidates reacted with α,β-unsaturated aldehydes, bearing hydrogen atoms at the γ-position, in good yields.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal.

Thèse numérisée par la Division de la gestion de documents et des archives de l'Université de Montréal