Development and Application of Methods for Enantioselective Synthesis of Amines and Alcohols

Abstract

Homoallylic amines and alcohols, particularly those amenable to further functionalization, are among the most widely used building blocks in chemical synthesis and are typically accessed by addition of an allyl-metal compound to an electrophile. Studies discussed herein have focused on advancing stereoselective synthesis of versatile allylboron compounds and their utilization in catalytic regio-, diastereo-, and enantioselective addition to various electrophiles. Mechanistic principles have been central to the investigations described in this thesis, and it has been on this basis that catalytic strategies for practical synthesis of bioactive molecules were developed. Chapter One. Vicinal amino alcohols are ubiquitous in natural products and serve as versatile synthetic intermediates and we envisaged that diastereo- and enantioselective additions of O-substituted allyl boronates offers an attractive option to access these motifs. In the presence of zinc (II) methoxide as co-catalyst, it will be demonstrated that sequence of events may occur, wherein isomerization of an initially formed allyl complex occurs prior to addition of an aldimine with kinetic selectivity. As will be described, through the use of an optimal catalyst, N-protecting/activating group and appropriate reaction conditions, differentially protected vicinal amino alcohol derivatives may be synthesized in high enantiopurity. The utility of the approach is highlighted through synthesis of an NK1 agonist. Chapter Two. It will be demonstrated that additions of various organoboron compounds converts readily accessible and easy-to-handle silyl-substituted -tertiary amines. Contrary to additions to aldimines, isomerization of the initially generated aminophenol-allyl complex is preempted, suchthat linear products are favored. DFT analysis suggests that high enantioselectivity likely originates from attractive electrostatic interaction between a trifluoromethyl group and the catalyst’s ammonium moiety. The synthetic utility was highlighted through concise, enantioselective synthesis of a key intermediate of a recently reported BACE-1 inhibitor on gram scale. Chapter Three. This section details the development of a method for direct synthesis of homoallylic alcohols bearing a Z-alkenyl chloride, which may be directly subjected to stereo-retentive cross coupling without wasteful protection/deprotection or redox operations. Products were obtained in high regio- and enantioselectivity for aliphatic, alkenyl- and heteroaryl-substituted aldehydes. The approach was utilized in a concise, protecting group-free synthesis of anti-tumor agent mycothiazole. Chapter Four. A method for stereoselective synthesis of fluorine-containing trisubstituted allyl boronates will be presented. It will then be illustrated that in the presence of an appropriate aminophenol catalyst, a large assortment of homoallylic alcohols containing a quaternary fluoro- and trifluoromethyl-substituted stereogenic center may be obtained with efficiency and high diastereo- and enantioselectivity. The obtained products were then elaborated to the furanose core of Sofosbuvir, a recently approved treatment for hepatitis C.