A total synthesis of (+)-brazilin

Advances in the synthesis of rearranged homoisoflavonoids

Rearranged homoisoflavonoids constitute a unique group of natural products, renowned for their structural diversity and complexity. These compounds, derived from modifications in the 3-benzylchroman skeleton, are categorized into four subclasses: brazilin, caesalpin, protosappanin, and scillascillin homoisoflavonoids. This review examines the advancements in the total synthesis of these complex structures, aiming to highlight the challenges and opportunities encountered. A comparative analysis of the strategies employed thus far to synthesize these compounds provides a comprehensive understanding of the progress in this field.

Hydroxymethylation hydroxylation of 1,3-diarylpropene through a catalytic diastereoselective Prins reaction: cyclization logic and access to brazilin core

A catalytic diastereoselective Prins reaction for hydroxymethylation and hydroxylation of 1,3-diarylpropene was successfully utilized to prepare various 1,3-dioxanes 7 in 14-88% yields. Take advantage of the synthetic intermediate 7h, the key B/C rings in brazilin core could be constructed by the sequential of Friedel-Crafts/Ullmann-Ma rather than Ullmann-Ma/Friedel-Crafts reactions.

Chemoenzymatic formal synthesis of (+)-brazilin

Herein, the manuscript presents a chemoenzymatic formal synthetic route of (+)-brazilin, a homoisoflavonoid natural product with a chroman skeleton cis-fused with a 2,3-dihydro-1H-indene unit, which is isolated from the traditional Chinese medicine, Caesalpinia sappan L. The key feature of the synthetic strategy includes an enzyme-mediated desymmetrization by employing lipase from Candida antarctica type B (CALB) and a one-pot SN2/hydrolysis reaction.

Tetracyclic homoisoflavanoid (+)-brazilin: a natural product inhibits c-di-AMP-producing enzyme and Streptococcus mutans biofilms

The tenacious biofilms formed by Streptococcus mutans are resistant to conventional antibiotics and current treatments. There is a growing need for novel therapeutics that selectively inhibit S. mutans biofilms while preserving the normal oral microenvironment. Previous studies have shown that increased levels of cyclic di-AMP, an important secondary messenger synthesized by diadenylate cyclase (DAC), favored biofilm formation in S. mutans. Thus, targeting S. mutans DAC is a novel strategy to inhibit S. mutans biofilms. We screened a small NCI library of natural products using a fluorescence detection assay. (+)-Brazilin, a tetracyclic homoisoflavanoid found in the heartwood of Caesalpinia sappan, was identified as one of the 11 "hits," with the greatest reduction (>99%) in fluorescence at 100 µM. The smDAC inhibitory profiles of the 11 "hits" established by a quantitative high-performance liquid chromatography assay revealed that (+)-brazilin had the most enzymatic inhibitory activity (87% at 100 µM) and was further studied to determine its half maximal inhibitory concentration (IC50 = 25.1 ± 0.98 µM). (+)-Brazilin non-competitively inhibits smDAC's enzymatic activity (Ki = 140.0 ± 27.13 µM), as determined by a steady-state Michaelis-Menten kinetics assay. In addition, (+)-brazilin's binding profile with smDAC (Kd = 11.87 µM) was illustrated by a tyrosine intrinsic fluorescence quenching assay. Furthermore, at low micromolar concentrations, (+)-brazilin selectively inhibited the biofilm of S. mutans (IC50 = 21.0 ± 0.60 µM) and other oral bacteria. S. mutans biofilms were inhibited by a factor of 105 in colony-forming units when treated with 50 µM (+)-brazilin. In addition, a significant dose-dependent reduction in extracellular DNA and glucan levels was evident by fluorescence microscopy imaging of S. mutans biofilms exposed to different concentrations of (+)-brazilin. Furthermore, colonization of S. mutans on a representative model of enamel using suspended hydroxyapatite discs showed a >90% reduction with 50 µM (+)-brazilin. In summary, we have identified a drug-like natural product inhibitor of S. mutans biofilm that not only binds to smDAC but can also inhibit the function of smDAC. (+)-Brazilin could be a good candidate for further development as a potent therapeutic for the prevention and treatment of dental caries.IMPORTANCEThis study represents a significant advancement in our understanding of potential therapeutic options for combating cariogenic biofilms produced by Streptococcus mutans. The research delves into the use of (+)-brazilin, a natural product, as a potent inhibitor of Streptococcus mutans' diadenylate cyclase (smDAC), an enzyme crucial in the formation of biofilms. The study establishes (+)-brazilin as a non-competitive inhibitor of smDAC while providing initial insights into its binding mechanism. What makes this finding even more promising is that (+)-brazilin does not limit its inhibitory effects to S. mutans alone. Instead, it demonstrates efficacy in hindering biofilms in other oral bacteria as well. The broader spectrum of anti-biofilm activity suggests that (+)-brazilin could potentially serve as a versatile tool in a natural product-based treatment for combating a range of conditions caused by resilient biofilms.

Acid-Mediated Cascade Cyclization Pathway to Indeno[2,1-c]chromen-6(7H)-ones

Developing mild and effective synthetic strategies for producing significant molecules starting from readily available starting materials is indispensable in organic synthesis. Herein, we present a triflic acid-driven dual cyclization pathway to produce functionalized indeno[2,1-c]chromen-6(7H)-ones from simple 2-formyl (or 2-acyl) cinnamate esters and phenols. Notably, this protocol enabled the construction of two C-C bonds and one C-O bond under metal-free reaction conditions via the activation of the unreactive ester moiety in a single pot. The isolation of intermediate indenol-ester might suggest self-intramolecular cycloaddition by the proximate double bond of the enoate ester with the o-carbonyl moiety, followed by an electrophilic attack with phenol and a subsequent cyclocondensation pathway. In addition, the photophysical properties have also been examined.

Sharpless Asymmetric Dihydroxylation: An Impressive Gadget for the Synthesis of Natural Products: A Review

Sharpless asymmetric dihydroxylation is an important reaction in the enantioselective synthesis of chiral vicinal diols that involves the treatment of alkene with osmium tetroxide along with optically active quinine ligand. Sharpless introduced this methodology after considering the importance of enantioselectivity in the total synthesis of medicinally important compounds. Vicinal diols, produced as a result of this reaction, act as intermediates in the synthesis of different naturally occurring compounds. Hence, Sharpless asymmetric dihydroxylation plays an important role in synthetic organic chemistry due to its undeniable contribution to the synthesis of biologically active organic compounds. This review emphasizes the significance of Sharpless asymmetric dihydroxylation in the total synthesis of various natural products, published since 2020.

One-Pot Double Intramolecular Cyclization Approach to Tetralin-Based Cis-Fused Tetracyclic Compounds

Presented herein is a BF₃·OEt₂-mediated, diastereoselective one-pot double cyclization of 4-aryl-2-[(arylthio)methyl]butanals leading to the formation of cis-tetrahydro-6H-naphtho[2,1-c]thiochromenes for the first time. Mechanistically, the formation of the title products involves the one-pot intramolecular Friedel-Crafts hydroxyalkylation/intramolecular Friedel-Crafts alkylation cascade. This synthetic methodology is featured by its high atom economy, broad substrate scope, mild transition-metal-free reaction conditions, capability to assemble two new rings in one pot, and moderate to high yields (up to 94% yield). It was then applied in the synthesis of a thia analogue of brazilane and a chromeno[3,4-c]chromene derivative. Moreover, the methodology was successfully extended to the synthesis of cis-hexahydrobenzo[c]phenanthrenes. Specifically, 1,5-diarylpentan-3-ones were first subjected to the Corey-Chaykovsky reaction, and the resulting epoxides, without being chromatographically isolated, were treated with BF₃·OEt₂ to afford the cyclized products in high yields (up to 84% yield over two steps).

Scalable Total Synthesis of (+)- and (-)-Codonopiloneolignanin A via Ti(IV)/NHC Cooperative Control Highly Enantioselective Dimerization of Multisubstituted Cinnamaldehyde

The first gram-scale asymmetric total synthesis of (+)- and (-)-codonopiloneolignanin A has been achieved from multisubstituted cinnamaldehyde in four steps with 37% overall yield. The synthetically challenging tricyclic [5, 3, 0, 0^3,8] decane skeleton was efficiently constructed via a highly enantioselective dimerization of multisubstituted cinnamaldehyde, followed by a sequence of cascade reactions including Prins cyclization, cation mediated cyclization, and deprotection. Furthermore, the scope of NHC-catalyzed/Ti(IV)-mediated synergistic control multisubstituted cinnamaldehyde dimerization was investigated. Significantly, the bioactivity of codonopiloneolignanin A and its enantiomer, particularly scarce in nature, was tested and showed good anticancer activity.

Total synthesis of (-)-brazilane via a lipase-catalyzed desymmetrisation reaction

Herein, we described the asymmetric total synthesis of (-)-brazilane, an optically active natural product. The key steps of this synthetic approach are a lipase-catalyzed desymmetrisation reaction of a prochiral diol using vinyl acetate to prepare a chiral primary alcohol and a trifluoroacetic acid-catalyzed one pot intramolecular tandem Prins/Friedel-Crafts reaction used to construct the cis-fused chromane and indane framework.