Advances Towards the Synthesis of Aporphine Alkaloids: C-Ring Formation via Approaches Based on One- and Two-Bond Disconnections

Aporphine compounds constitute a class of substances with important pharmacological properties, including anticancer, antiviral, anti-HIV, anti-inflammatory, and leishmanicidal activities. Consequently, several strategies to obtain the aporphine core have been reported. Herein this review, we provide an overview of two relevant approaches used to construct the C-ring in the synthetic routes developed. The first approach, which is based on a one-bond disconnection, allows C-ring formation using a 1-benzyl-1,2,3,4-tetrahydroisoquinoline intermediate (mainly) employing cyclization reactions catalyzed by metals or promoted by light. The second approach, which is derived from a two-bond disconnection, leads to C-ring formation via a sequence of reactions starting with [4+2] cycloadditions. Through these approaches, aporphinoids with a diverse range of substitution patterns and biological activities can be synthesized.

Supramolecular Halogen Bonds in Asymmetric Catalysis

Halogen bonding has received a significant increase in attention in the past 20 years. An important part of this interest has centered on catalytic applications of halogen bonding. Halogen bond (XB) catalysis is still a developing field in organocatalysis, although XB catalysis has outgrown its proof of concept phase. The start of this year witnessed the publication of the first example of a purely XB-based enantioselective catalytic reaction. While the selectivity can be improved upon, there are already plenty of examples in which halogen bonds, among other interactions, play a crucial role in the outcome of highly enantioselective reactions. This paper will give an overview of the current state of the use of XBs in catalytic stereoselective processes.

O-Benzyl Xanthate Esters under Ni/Photoredox Dual Catalysis: Selective Radical Generation and Csp3-Csp2 Cross-Coupling

Alkyl xanthate esters are perhaps best known for their use in deoxygenation chemistry. However, their use in cross-coupling chemistry has not been productive, which is due, in part, to inadequate xanthate activation strategies. Herein, we report the use of O-benzyl xanthate esters, readily derived from alcohols, as radical pronucleophiles in Csp³-Csp² cross-couplings under Ni/photoredox dual catalysis. Xanthate (C-O) cleavage is found to be reliant on photogenerated (sec-butyl) radical activators to form new carbon-centered radicals primed for nickel-catalyzed cross-couplings. Mechanistic experiments support the fact that the key radical components are formed independently, and relative rates are carefully orchestrated, such that no cross reactivity is observed.