Prostaglandin Total Synthesis Enabled by the Organocatalytic Dimerization of Succinaldehyde

A concise and scalable chemoenzymatic synthesis of prostaglandins

Prostaglandins have garnered significant attention from synthetic chemists due to their exceptional biological activities. In this report, we present a concise chemoenzymatic synthesis method for several representative prostaglandins, achieved in 5 to 7 steps. Notably, the common intermediate bromohydrin, a radical equivalent of Corey lactone, is chemoenzymatically synthesized in only two steps, which allows us to complete the synthesis of prostaglandin F2α in five steps on a 10-gram scale. The chiral cyclopentane core is introduced with high enantioselectivity, while the lipid chains are sequentially incorporated through a cost-effective process involving bromohydrin formation, nickel-catalyzed cross-couplings, and Wittig reactions. This cost-efficient synthesis route for prostaglandins holds the potential to make prostaglandin-related drugs more affordable and facilitate easier access to their analogues.

Neurotrophic Natural Products

Neurotrophins (NGF, BDNF, NT3, NT4) can decrease cell death, induce differentiation, as well as sustain the structure and function of neurons, which make them promising therapeutic agents for the treatment of neurodegenerative disorders. However, neurotrophins have not been very effective in clinical trials mostly because they cannot pass through the blood-brain barrier owing to being high-molecular-weight proteins. Thus, neurotrophin-mimic small molecules, which stimulate the synthesis of endogenous neurotrophins or enhance neurotrophic actions, may serve as promising alternatives to neurotrophins. Small-molecular-weight natural products, which have been used in dietary functional foods or in traditional medicines over the course of human history, have a great potential for the development of new therapeutic agents against neurodegenerative diseases such as Alzheimer's disease. In this contribution, a variety of natural products possessing neurotrophic properties such as neurogenesis, neurite outgrowth promotion (neuritogenesis), and neuroprotection are described, and a focus is made on the chemistry and biology of several neurotrophic natural products.

Organocatalyst-mediated, pot-economical total synthesis of latanoprost

The enantioselective total synthesis of latanoprost, an antiglaucoma agent, has been accomplished with excellent diastereo- and enantioselectivities in a pot-economical manner using six reaction vessels. An enantioselective Krische allylation was conducted in the first pot. In the second pot, olefin metathesis, silyl protection, and hydrogenolysis proceeded efficiently. In the third pot, an organocatalyst-mediated Michael reaction proceeded with excellent diastereoselectivity. The fourth pot involved a substrate-controlled Mukaiyama intramolecular aldol reaction and elimination of HNO2 to afford a methylenecyclopentanone, also with excellent diastereoselectivity. The fifth pot involved a Michael reaction of vinyl cuprate. In the sixth pot, three reactions, a cis-selective olefin metathesis, diastereoselective reduction, and deprotection, afforded latanoprost. Nearly optically pure latanoprost was obtained, and the total yield was 24%.

De Novo Synthesis of Dihydrobenzofurans and Indolines and Its Application to a Modular, Asymmetric Synthesis of Beraprost

Dihydrobenzofurans and indolines are important constituents of pharmaceuticals. Herein, we describe a novel strategy for their construction in which the aromatic ring is created de novo through an inverse-electron demand Diels-Alder reaction and cheletropic extrusion sequence of a 2-halothiophene-1,1-dioxide with an enol ether/enamide, followed by aromatization. Unusually, the aromatization process proved to be highly challenging, but it was discovered that treatment of the halocyclohexadienes with a base effected an α-elimination-aromatization reaction. Mechanistic investigation of this step using deuterium-labeling studies indicated the intermediacy of a carbene which undergoes a 1,2-hydrogen shift and subsequent aromatization. The methodology was applied to a modular and stereoselective total synthesis of the antiplatelet drug beraprost in only 8 steps from a key enal-lactone. This lactone provided the core of beraprost to which both its sidechains could be appended through a 1,4-conjugate addition process (lower ω-sidechain), followed by de novo construction of beraprost's dihydrobenzofuran (upper α-sidechain) using our newly developed methodology. Additionally, we have demonstrated the breadth of our newly established protocol in the synthesis of functionalized indolines, which occurred with high levels of regiocontrol. According to density-functional theory (DFT) calculations, the high selectivity originates from attractive London dispersion interactions in the TS of the Diels-Alder reaction.

A unified strategy to prostaglandins: chemoenzymatic total synthesis of cloprostenol, bimatoprost, PGF2α, fluprostenol, and travoprost guided by biocatalytic retrosynthesis

Development of efficient and stereoselective synthesis of prostaglandins (PGs) is of utmost importance, owing to their valuable medicinal applications and unique chemical structures. We report here a unified synthesis of PGs cloprostenol, bimatoprost, PGF_2α, fluprostenol, and travoprost from the readily available dichloro-containing bicyclic ketone 6a guided by biocatalytic retrosynthesis, in 11-12 steps with 3.8-8.4% overall yields. An unprecedented Baeyer-Villiger monooxygenase (BVMO)-catalyzed stereoselective oxidation of 6a (99% ee), and a ketoreductase (KRED)-catalyzed diastereoselective reduction of enones 12 (87 : 13 to 99 : 1 dr) were utilized in combination for the first time to set the critical stereochemical configurations under mild conditions. Another key transformation was the copper(ii)-catalyzed regioselective p-phenylbenzoylation of the secondary alcohol of diol 10 (9.3 : 1 rr). This study not only provides an alternative route to the highly stereoselective synthesis of PGs, but also showcases the usefulness and great potential of biocatalysis in construction of complex molecules.

Time and Pot Economy in Total Synthesis

We would all like to make or obtain the materials or products we want as soon as possible. This is human nature. This is true also for chemists in the synthesis of organic molecules. All chemists would like to make their target molecules as soon as possible, particularly when their interest is in the physical or biological properties of those molecules.As demonstrated by today's COVID-19 (SARS-CoV-2) pandemic, rapid synthesis is also crucial to enable chemists to deliver effective therapeutic agents to the community. Several concepts are currently well-accepted as important for achieving this: atom economy, step economy, and redox economy. Considering the importance of synthesizing organic molecules rapidly, I recently proposed adding the concept of time economy.In a multisep synthesis, each step has to be completed within a short period of time to make the desired molecule rapidly. The development of rapid reactions is important but also insufficient. After each step, frequent and repetitive workup operations such as quenching the reaction, extraction, separation of water and organic phases, drying the organic phase, filtration, evaporation, and purification may be required, and the time necessary for these processing operations must be taken into account. Indeed, some of the most time-consuming operations in most syntheses are the purification stages.On the other hand, one-pot reactions are processes in which several sequential reactions are conducted in a single reaction vessel, which avoids the need to purify intermediates. One-pot reactions are a useful way to shorten the total synthesis time, and the approach generally leads to an increase in the yield and a reduction in the amount of chemical waste formed. Thus, I also propose the importance of pot economy.On the basis of these concepts of time and pot economy, we have accomplished efficient syntheses of several natural products and medicines. The key to the success of these syntheses is the use of diphenylprolinol silyl ether as an effective catalyst in a one-pot reaction, in which it does not disturb the subsequent reactions. Our strategy is (1) to construct the chiral key skeletons and/or key components of natural products and medicines directly using organocatalyst-mediated one-pot reactions and (2) to conduct the subsequent transformations to the final molecules in a small number of pots utilizing the internal quench method. By means of this strategy, PGE₁ methyl ester, estradiol methyl ether, and clinprost were synthesized in three, five, and seven pots, respectively. Furthermore, (-)-oseltamivir, ABT-341, baclofen, and Corey lactone were synthesized in a single reaction vessel. Further optimization of the reactions in terms of time economy allowed (-)-oseltamivir and Corey lactone to be synthesized within 60 and 152 min, respectively. These syntheses will be highlighted as case studies. Although the organocatalyst is a key compound in this Account, pot- and time-economical syntheses can be expanded to organometallic chemistry and, indeed, to organic chemistry in general.

Pot-Economical Total Synthesis of Clinprost

The pot-economical synthesis of clinprost is reported, in which the core bicyclo[3.3.0]octenone structure was synthesized by two key steps: an asymmetric domino Michael/Michael reaction catalyzed by diphenylprolinol silyl ether and an intramolecular Horner-Wadsworth-Emmons reaction. The trisubstituted endocyclic alkene was selectively introduced by 1,4-reduction followed by trapping of the generated enolate with Tf₂NPh and subsequent utilization of the Suzuki-Miyaura coupling reaction. Chiral, nonracemic clinprost was synthesized in seven pots with a 17% total yield and excellent enantioselectivity.

Total Synthesis of Thromboxane B2 via a Key Bicyclic Enal Intermediate

A 12-step asymmetric synthesis of thromboxane B₂ (TxB₂) from 2,5-dimethoxytetrahydrofuran is described. The synthesis employs our organocatalytic aldol reaction of succinaldehyde to give a key bicyclic enal intermediate. From here, the synthetic strategy involves a conjugate addition of an alkenyl side chain to the bicyclic enal, Baeyer-Villiger oxidation, and a highly Z-selective Wittig olefination of a hemiacetal. Key to success was minimizing redox operations and the manipulation of functional groups in the correct order.