Applications of isothioureas in organocatalysis : kinetic resolution of secondary alcohols and intramolecular Michael addition-lactonisation

Abstract

Obtaining enantiomerically pure compounds is of major importance in modern organic chemistry, and this PhD thesis outlines new advances made in this area. Two new enantioselective synthetic methodologies have been designed using isothiourea based- organocatalysts. A brief introduction to the recent literature involving this catalyst architecture is presented. Then, the results of the structure-activity relationship study carried out on a range of isothiourea catalysts for the kinetic resolution of (±)-1-naphthylethanol 16 is described. Chiral isothiourea HBTM 2.1 58 was identified as the optimum catalyst and was further employed in the resolution of a range of secondary alcohols. Good levels of conversion (c~50%) and selectivity (S up to 100) were achieved delivering alcohols in high levels of enantioselectivity (up to 98% ee). The synthetic utility of this process was subsequently demonstrated through isolation of a range of enantiopure alcohols (>99% ee) on a preparative scale, using low catalyst loading of HBTM 2.1 (0.10 mol%). Secondly, a new organocatalytic asymmetric transformation involving the generation of a C1-ammonium enolate using isothiourea is described. An intramolecular Michael addition-lactonisation (IMAL) of enone-acid substrates was developed using (‒)-tetramisole 37 as a catalyst. The generality of this protocol was probed via structural variation of the enone-acid substrates, providing a range of highly functionalised syn-polycyclic carbo- and heterocycles in excellent yields, diastereo- and enantioselectivities (up to 99% yield, 99 : 1 dr and 99% ee). Whilst investigating this process, we discovered that using cinchona alkaloid derivatives as catalysts gave preferential access to the diastereomeric anti-products. Therefore, a range of anti-dihydrobenzofurans was synthesised from readily available starting materials in good yield and stereoselectivities (up to 73%, 20 : 80 dr and 99% ee) using OTMS-quinidine catalyst. These two complementary synthetic strategies allowed facile and controllable access to both syn- and anti-diasteroisomers of a product in high enantioselectivity via judicious choice of catalyst. Subsequently, two telescoped procedures combining the synthesis of the enone-acid through olefination and asymmetric functionalisation were designed, giving rapid access to stereodefined pyrrolidines in high levels of diastereo- and enantioselectivitiy (up to 99 : 1 dr and 99% ee). The last part of this thesis describes preliminary studies toward the development of a doubly diastereoselective intramolecular Michael addition-lactonisation process, opening new avenues for the construction of complex molecular architectures.