α-exo-Alkylidene γ-lactones and γ-lactams via 2-alkoxycarbonyl allylboronates: mechanistic studies, diversity-oriented synthesis and target-oriented synthesis

Abstract

Allylboration reactions have been thoroughly utilized in organic chemistry since it was discovered that they could add in a nucleophilic fashion to aldehydes and ketones in 1964. Modification of allylboronates and the substrates that they can react with has been the focus of many research groups over the past three decades. Recent works have made use of catalysis to promote the addition of allylboronates that are generally otherwise unreactive toward various electrophiles. Chapter 2 will discuss the discovery that Brønsted acids can catalyze the addition of unreactive 2-alkoxycarbonyl allylboronates to aldehydes and that the diastereoselectivity of the reaction is determined by the electronic nature of the aldehyde.

Ketones and imines are much less reactive than aldehydes towards allylboronates due to steric and electronic factors. As a result, new conditions are often required to promote the allylboration reaction of ketones and imines. Chapter 3 will briefly discuss the challenges that ketones present as substrates for allylboration reactions and show my attempts at achieving this transformation. Chapter 4 will describe imines and their associated challenges as substrates for allylboration reactions. However, once harnessed, these substrates provide easy access to α-methylene γ-lactones when a 2-alkoxycarbonyl allylboronate is used as the allylating reagent.

The modification of important or interesting molecules by making major or minor changes to a common core structure is the basis of diversity-oriented synthesis of combinatorial libraries. α-Alkylidene γ-lactones and α-alkylidene γ-lactams are biologically interesting compounds present in numerous natural products. Chapter 5 will discuss how the title compounds were modified by various metal-catalyzed coupling reactions to provide a diversity-oriented combinatorial library of γ-lactones and γ-lactams. Since γ-lactones are prevalent in many natural products, the application of 2-alkoxycarbonyl allylboronates to a target-oriented synthesis was intriguing. Unlike diversity-oriented synthesis, target oriented synthesis aims at synthesizing a single compound through any number of controlled steps, arriving at one specific product that is obtained as a pure isomer. Access to highly complex γ-lactones is often tedious, however, Chapter 6 will discuss how a simple, one-step allylboration reaction of a complex aldehyde with a 2-alkoxycarbonyl allylboronate can lead to a highly substituted γ-lactone. This γ-lactone can be further modified and transformed into chinensiolide B, a biologically active natural product isolated from a plant found in various locations in China.