Iterative synthesis of 1,3-polyboronic esters with high stereocontrol and application to the synthesis of bahamaolide A

Polyketide natural products often contain common repeat motifs, for example, propionate, acetate and deoxypropionate, and so can be synthesized by iterative processes. We report here a highly efficient iterative strategy for the synthesis of polyacetates based on boronic ester homologation that does not require functional group manipulation between iterations. This process involves sequential asymmetric diboration of a terminal alkene, forming a 1,2-bis(boronic ester), followed by regio- and stereoselective homologation of the primary boronic ester with a butenyl metallated carbenoid to generate a 1,3-bis(boronic ester). Each transformation independently controls the stereochemical configuration, making the process highly versatile, and the sequence can be iterated prior to stereospecific oxidation of the 1,3-polyboronic ester to yield the 1,3-polyol. This methodology has been applied to a 14-step synthesis of the oxopolyene macrolide bahamaolide A, and the versatility of the 1,3-polyboronic esters has been demonstrated in various stereospecific transformations, leading to polyalkenes, -alkynes, -ketones and -aromatics with full stereocontrol.

Feedstock Reagents in Metal-Catalyzed Carbonyl Reductive Coupling: Minimizing Preactivation for Efficiency in Target-Oriented Synthesis

Use of abundant feedstock pronucleophiles in catalytic carbonyl reductive coupling enhances efficiency in target-oriented synthesis. For such reactions, equally inexpensive reductants are desired or, ideally, corresponding hydrogen autotransfer processes may be enacted wherein alcohols serve dually as reductant and carbonyl proelectrophile. As described in this Minireview, these concepts allow reactions that traditionally require preformed organometallic reagents to be conducted catalytically in a byproduct-free manner from inexpensive π-unsaturated precursors.

Organic Synthesis: New Vistas in the Brazilian Landscape

In this overview, we present our analysis of the future of organic synthesis in Brazil, a highly innovative and strategic area of research which underpins our social and economical progress. Several different topics (automation, catalysis, green chemistry, scalability, methodological studies and total syntheses) were considered to hold promise for the future advance of chemical sciences in Brazil. In order to put it in perspective, contributions from Brazilian laboratories were selected by the citations received and importance for the field and were benchmarked against some of the most important results disclosed by authors worldwide. The picture that emerged reveals a thriving area of research, with new generations of well-trained and productive chemists engaged particularly in the areas of green chemistry and catalysis. In order to fulfill the promise of delivering more efficient and sustainable processes, an integration of the academic and industrial research agendas is to be expected. On the other hand, academic research in automation of chemical processes, a well established topic of investigation in industrial settings, has just recently began in Brazil and more academic laboratories are lining up to contribute. All these areas of research are expected to enable the future development of the almost unchartered field of scalability.

The asymmetric syntheses of cryptocaryols A and B

The recent total syntheses of cryptocaryols A and B are reviewed. These efforts include the correction of the initially assigned absolute and relative stereochemistry of this class of natural products. In addition to enabling the initial structure activity relationships for this class of natural products, these syntheses demonstrated the practical utility of several novel synthetic approaches.

A Concise Synthesis of Rhodanthpyrone A and B, Natural 4-(Hydroxyphenyl)-substituted α-Pyrones

A concise synthesis of rhodanthpyrone A and B was accomplished via a Suzuki coupling reaction. To find the conditions appropriate to install hydroxyphenyl moieties to the α-pyrone skeleton, a model study was conducted using commercially available boronic acids. It was revealed that the hydroxy moiety of the phenylboronic acids should be concealed when reacted with labile 4-tosyl α-pyrone. Consequently, rhodanthpyrone A and B could be synthesized in high yields by Suzuki reaction using TIPS-protected arylboronic acids. This procedure provided a concise and versatile route for the synthesis of rhodanthpyrones and their 4-aryl substituted α-pyrone analogs.

Versatile process for the stereodiverse construction of 1,3-polyols: iterative chain elongation with chiral building blocks

A versatile process for the construction of 1,3-polyols, a key structural element of polyketide-type natural products, is presented. The modular synthesis strategy involves the iterative chain elongation with novel four-carbon building blocks to access all possible stereoisomers of a growing 1,3-polyol chain. These chiral building blocks are designed to install four carbon atoms with two stereogenic centres by performing only four experimentally simple steps per elongation cycle, thus making these building blocks attractive for the realization of a universal platform from which to access a diverse range of polyketidic molecules.

Enantioselective Alcohol C-H Functionalization for Polyketide Construction: Unlocking Redox-Economy and Site-Selectivity for Ideal Chemical Synthesis

The development and application of stereoselective and site-selective catalytic methods that directly convert lower alcohols to higher alcohols are described. These processes merge the characteristics of transfer hydrogenation and carbonyl addition, exploiting alcohols and π-unsaturated reactants as redox pairs, which upon hydrogen transfer generate transient carbonyl-organometal pairs en route to products of C-C coupling. Unlike classical carbonyl additions, stoichiometric organometallic reagents and discrete alcohol-to-carbonyl redox reactions are not required. Additionally, due to a kinetic preference for primary alcohol dehydrogenation, the site-selective modification of glycols and higher polyols is possible, streamlining or eliminating use of protecting groups. The total syntheses of several iconic type I polyketide natural products were undertaken using these methods. In each case, the target compounds were prepared in significantly fewer steps than previously achieved.

Total Synthesis of Cryptocaryol A by Enantioselective Iridium-Catalyzed Alcohol C-H Allylation

The polyketide natural product cryptocaryol A is prepared in 8 steps via iridium catalyzed enantioselective diol double C-H allylation, which directly generates an acetate-based triketide stereodiad. In 4 previously reported total syntheses, 17-28 steps were required.