Prins cyclization-mediated stereoselective synthesis of tetrahydropyrans and dihydropyrans: an inspection of twenty years.

Functionalized tetrahydropyran (THP) rings are important building blocks and ubiquitous scaffolds in many natural products and active pharmaceutical ingredients (API). Among various established methods, the Prins reaction has emerged as a powerful technique in the stereoselective synthesis of the tetrahydropyran skeleton with various substituents, and the strategy has further been successfully applied in the total synthesis of bioactive macrocycles and related natural products. In this context, hundreds of valuable contributions have already been made in this area, and the present review is intended to provide the systematic assortment of diverse Prins cyclization strategies, covering the literature reports of the last twenty years (from 2000 to 2019), with an aim to give an overview on exciting advancements in this area and designing new strategies for the total synthesis of related natural products.

Organic semiconductor photocatalyst can bifunctionalize arenes and heteroarenes.

Photoexcited electron-hole pairs on a semiconductor surface can engage in redox reactions with two different substrates. Similar to conventional electrosynthesis, the primary redox intermediates afford only separate oxidized and reduced products or, more rarely, combine to one addition product. Here, we report that a stable organic semiconductor material, mesoporous graphitic carbon nitride (mpg-CN), can act as a visible-light photoredox catalyst to orchestrate oxidative and reductive interfacial electron transfers to two different substrates in a two- or three-component system for direct twofold carbon-hydrogen functionalization of arenes and heteroarenes. The mpg-CN catalyst tolerates reactive radicals and strong nucleophiles, is straightforwardly recoverable by simple centrifugation of reaction mixtures, and is reusable for at least four catalytic transformations with conserved activity.

Generation of All-Carbon Quaternary Stereocenters at the C-3 Carbon of Lactams via [3,3]-Sigmatropic Rearrangement and Revision of Absolute Configuration: Total Synthesis of (-)-Physostigmine.

A diastereoselective (up to >99%) route to all carbon quaternary stereocenters at the C-3 position of cyclic lactams has been developed via Johnson-Claisen rearrangement of γ-hydroxy-α,ß-unsaturated lactams. It has been observed that olefin geometry plays an important role in the development of the absolute stereochemistry of the product. Dependence of product configuration on the olefin geometry is explained by postulating probable transition states. The success of this method has been shown for multigram-scale synthesis of these substituted lactams from commercially available cheap starting materials. The synthetic usefulness of this method is also demonstrated by carrying out the total synthesis of (-)-physostigmine.

Asymmetric Total Synthesis of Eburnamine and Eucophylline: A Biomimetic Attempt for the Total Synthesis of Leucophyllidine.

The first enantiospecific total synthesis of (+)-6 has been achieved employing a Friedländer quinoline synthesis as a key step. Asymmetric synthesis of the architecturally complex eburnamine 5 has also been accomplished utilizing an intramolecular acid-mediated cyclization of a carbinol amine lactone moiety. Highlights of the effective modular synthetic strategy include development of the common precursor 4 for the construction of the privileged scaffolds 5 and 6 with an all-carbon quaternary stereocenter utilizing a Johnson-Claisen rearrangement strategy. Attempts have been made to synthesize 1 by the biomimetic coupling of 5 and (+)-6; however, regioisomeric 26 was formed.

An expeditious route to both enantiomers of all carbon quaternary stereocenters at C-3 carbon of lactams via [3,3]-sigmatropic rearrangement: total synthesis of (-)-physostigmine.

A diastereoselective route to all carbon quaternary stereocenters at the C-3 position of cyclic lactams has been developed via Johnson-Claisen rearrangement of γ-hydroxy-α, ß-unsaturated lactams. It has been observed that olefin geometry plays an important role in the development of the absolute stereochemistry of the product. The dependence of the product configuration on the olefin geometry is explained by postulating probable transition states. The success of this method has been shown for the multigram scale synthesis of these substituted lactams from commercially available cheap starting materials. The synthetic usefulness of this method is also demonstrated by carrying out the total synthesis of (-)-physostigmine.