Total Synthesis of Selligueain A, a Sweet Flavan Trimer

Imidazole-catalyzed construction of bridged bicyclo [3.3.1] ketals via formal [3 + 3]-cycloaddition of naphthols and 2-hydroxyl chromene derivatives

Bridged bicyclo[3.3.1]nonanes having the O,O-ketal subunit are widely found in medicinal compounds. While several acid-catalyzed approaches have been reported for the construction of ketals, this study discloses an imidazole-catalyzed modular construction of bicyclo[3.3.1]nonanes from naphthols/phenols and 3-acyl-2-hydroxyl chromenes. The proposed reaction pathway follows a formal [3 + 3] cycloaddition of phenols and an oxonium dipolar intermediate. The reaction is efficient regarding scalability, environmentally benign conditions, and resulting in the products in good to excellent yields.

De novo synthesis of dimerization-ready flavan unit via intramolecular Pummerer/Friedel-Crafts cascade

We describe the de novo synthesis of a dimerization-ready flavan derivative via the Pummerer/Friedel-Crafts cascade. This method allowed facile access to oligomeric proanthocyanidin natural products. The total synthesis of procyanidin B4 was achieved by this strategy.

Total Synthesis of Parameritannin A2, a Branched Epicatechin Tetramer with Two Double Linkages

The first total synthesis of parameritannin A2 (1), a branched epicatechin (EC) tetramer is reported. The "phloroglucinol trick" was used to circumvent two synthetic issues encountered when assembling four EC units, namely, the steric constraint and the formation of the C4-C6 interflavan linkage. As a substructure of the middle EC unit, phloroglucinol enabled the single-step assembly of two EC units (top and side) through A-type linkages. The middle EC unit was constructed by conducting a newly developed three-carbon flavan annulation via a Pummerer/Friedel-Crafts cascade reaction to furnish a trimeric intermediate bearing a thio-leaving group at C4 position, which allowed the final installation of the bottom EC unit.

Acid-catalysed iterative generation of o-quinone methides for the synthesis of dioxabicyclo[3.3.1]nonanes: total synthesis of myristicyclins A-B

We report a practical and efficient method for the synthesis of bioactive flavanoids relying on the strategic use of o-quinine methide (o-QM) intermediates. This involves Brønsted acid-catalysed iterative generation of o-QMs/[4+2] cycloaddition/intermolecular Michael addition/cyclative acetalization in a cascade sequence for the synthesis of dioxabicyclo[3.3.1]nonanes. The 'one-pot', controlled cascade sequence successfully provided the shortest route amenable for gram scale synthesis of natural products (±)-myristicyclins A-B.

Proanthocyanidin Block Arrays (PACBAR) for Comprehensive Capture and Delineation of Proanthocyanidin Structures

Proanthocyanidins (PACs) are near-ubiquitous and chemically complex metabolites, prototypical of higher plants. Their roles in food/feed/nutrition and ethnomedicine are widely recognized but poorly understood. With the analysis of evidence that underlies this challenge, this perspective identifies shortcomings in capturing and delineating PAC structures as key factors. While several groups have forwarded new representations, a consensus method that captures PAC structures concisely and offers high integrity for electronic storage is required to reduce confusion in this expansive field. The PAC block arrays (PACBAR) system fills this gap by providing precise and human- and machine-readable structural descriptors that capture PAC metabolomic structural diversity. PACBAR enables communication of PAC structures for the development of precise structure-activity relationships and will assist in advancing PAC research to the next level.

Synthesis of A-Type Proanthocyanidins and Their Analogues: A Comprehensive Review

Proanthocyanidins (PACs) are oligomers or polymers composed of units of flavanols. A-type PACs are a subclass of PACs characterized by the presence of at least a double linkage between two consecutive monomers of flavanol. These A-type PACs are found in some fruits and spices and possess potential health benefits as a result of their interesting biological activities, and consequently, their isolation and synthesis have given rise to great interest in the past. This review summarizes the synthetic efforts made to obtain both naturally occurring A-type PACs and their structurally simplified analogues. Most of the synthetic protocols reported involve the addition of a π-nucleophilic molecule over a molecule with two electrophilic carbons, such as a chalcone, a flavylium salt, or a flavanol derivative, among others. Synthesis of A-type PACs remains an issue at a very early stage of development compared to that of PACs with single linkages between monomers (B-type PACs), but the advances that are taking place in the last few years point to a significant development of the subject in the near future.

Syntheses of doubly linked proanthocyanidins using free flavan units as nucleophiles: insight into the origin of the high regioselectivity of annulation

Reported herein is a viable synthesis route to procyanidins A1 and A2, doubly linked flavan dimers. The use of nascent nucleophilic partner, catechin or epicatechin, enabled facile regioselective annulation with an ethylenedioxy-bridged flavan unit.