Total synthesis of (+/-)-spiroxin C

Elucidation of Palmarumycin Spirobisnaphthalene Biosynthesis Reveals a Set of Previously Unrecognized Oxidases and Reductases

Spirobisnaphthalenes (SBNs) are a class of highly oxygenated, fungal bisnaphthalenes containing a unique spiroketal bridge, that displayed diverse bioactivities. Among the reported SBNs, palmarumycins are the major type, which are precursors for the other type of SBNs structurally. However, the biosynthesis of SBNs is unclear. In this study, we elucidated the biosynthesis of palmarumycins, using gene disruption, heterologous expression, and substrate feeding experiments. The biosynthetic gene cluster for palmarumycins was identified to be distant from the polyketide synthase gene cluster, and included two cytochrome P450s (PalA and PalB), and one short chain dehydrogenase/reductase (PalC) encoding genes as key structural genes. PalA is an unusual, multifunctional P450 that catalyzes the oxidative dimerization of 1,8-dihydroxynaphthalene to generate the spiroketal linkage and 2,3-epoxy group. Chemical synthesis of key intermediate and in vitro biochemical assays proved that the oxidative dimerization proceeded via a binaphthyl ether. PalB installs the C-5 hydroxy group, widely found in SBNs. PalC catalyzes 1-keto reduction, the reverse 1-dehydrogenation, and 2,3-epoxide reduction. Moreover, an FAD-dependent oxidoreductase, encoded by palD, which locates outside the cluster, functions as a 1-dehydrogenase. These results provided the first genetic and biochemical evidence for the biosynthesis of palmarumycin SBNs.

De Novo Diastereoselective Synthesis of 1-Hydroxyl Allogibberic Methyl Ester en Route to Diverse Bioactive Molecules

The first de novo synthesis of 1-hydroxyl allogibberic methyl ester, en route to pharbinilic acid and other bioactive molecules, is accomplished in diastereoselective manner. Key reactions of the synthesis include a Pd-catalyzed Suzuki-Miyaura cross-coupling reaction, a Lewis acid-catalyzed reductive Prins cyclization reaction, and a SmI₂-mediated transannular pinacol coupling reaction. The synthesis provides a new avenue to access diverse relevant bioactive molecules.

Total synthesis, structure revision and cytotoxic activity of Sch 53825 and its derivatives

The first total synthesis of Sch 53825 (14) was achieved in 12 steps from 5-hydroxy-1-tetralone in 16% overall yield through N-benzyl cinchoninium chloride-catalyzed asymmetric epoxidation and a Mitsunobu reaction as the key steps. On this basis, the synthesis of palmarumycin B₆ was improved using the same raw material with 6 steps and 32% overall yield. Also, three new analogues with two chlorine atoms were synthesized. Their structures were characterized by ¹H, ¹³C NMR, HR-ESI-MS and X-ray diffraction data. The structure of natural Sch 53825 was revised as an epimer of compound 1 with the anti-hydroxy epoxide at C-4. Their cytotoxic activities against several tumor cell lines (HCT116, U251, BGC823, Huh-7 and PC9) showed that compound 11 exhibited excellent cytotoxicity against above mentioned cancer cell lines with IC₅₀ < 0.5 μM.

The first total synthesis of Sch 53825 (14) was achieved in 12 steps from 5-hydroxy-1-tetralone in 16% overall yield through N-benzyl cinchoninium chloride-catalyzed asymmetric epoxidation and a Mitsunobu reaction as the key steps.

Applications of Norrish type I and II reactions in the total synthesis of natural products: a review

Natural products and their analogue have played a key role in the drug discovery and development process. In the laboratory, the total synthesis of secondary metabolites is very useful in ascertaining the hypothetical complex structure of molecules of natural origin. Total synthesis of natural products using Norrish type I and II reactions as a crucial step has been explored in this overview. Norrish reactions are important photo-induced transformations of carbonyl compounds in organic synthetic chemistry and are connected in numerous industrially and biologically relevant procedures and the processing of carbonyl compounds in the atmosphere. The present review tries to focus on the brilliant applications of Norrish type I and II photochemical reactions as a key step in the total synthesis of natural products and highlights on natural sources, structures, and biological activities of the promising natural products for the first time elegantly.

Enantioselective Total Synthesis of (-)-Spiroxins A, C, and D

Spiroxins A, C, and D are metabolites that have been identified in the marine fungal strain LL-37H248. Their unique polycyclic structures and intriguing biological activities make them attractive targets for the synthetic community. Based on a scalable enantioselective epoxidation of 5-substituted naphthoquinone, an oxidation/spiroketalization cascade, ortho-selective chlorination of the phenol unit, and oxime-ester-directed acetoxylation, an enantioselective total synthesis of (-)-spiroxins A and C and the first total synthesis of (-)-spiroxin D have been achieved.

Total Synthesis of (±)-Spiroaxillarone A

Spiroaxillarone A, a novel and unique spirocyclic dinaphthalene natural product with significant antimalarial activity, was regioselectively synthesized from tetrahydrocurcumin in five steps with an overall 10% yield. Key features of the synthesis involved an oxidized free radical cycloaddition to build the spiro ring central skeleton and an oxidized dehydrogenation to introduce two double bonds via enol silicon ether from diketones.

Development of Facile and Simple Processes for the Heterogeneous Pd-Catalyzed Ligand-Free Continuous-Flow Suzuki–Miyaura Coupling

The Suzuki–Miyaura coupling reaction is one of the most widely utilized C–C bond forming methods to create (hetero)biaryl scaffolds. The continuous-flow reaction using heterogeneous catalyst-packed cartridges is a practical and efficient synthetic method to replace batch-type reactions. A continuous-flow ligand-free Suzuki–Miyaura coupling reaction of (hetero)aryl iodides, bromides, and chlorides with (hetero)aryl boronic acids was developed using cartridges packed with spherical resin (tertiary amine-based chelate resin: WA30)-supported palladium catalysts (7% Pd/WA30). The void space in the cartridge caused by the spherical catalyst structures enables the smooth flow of a homogeneously dissolved reaction solution that consists of a mixture of organic and aqueous solvents and is delivered by the use of a single syringe pump. Clogging or serious backpressure was not observed.

Modular synthesis of new C-aryl-nucleosides and their anti-CML activity

The C-aryl-ribosyles are of utmost interest for the development of antiviral and anticancer agents. Even if several synthetic pathways have been disclosed for the preparation of these nucleosides, a direct, few steps and modular approaches are still lacking. In line with our previous efforts, we report herein a one step - eco-friendly β-ribosylation of aryles and heteroaryles through a direct Friedel-Craft ribosylation mediated by bismuth triflate, Bi(OTf)₃. The resulting carbohydrates have been functionalized by cross-coupling reactions, leading to a series of new C-aryl-nucleosides (32 compounds). Among them, we observed that 5d exerts promising anti-proliferative effects against two human Chronic Myeloid Leukemia (CML) cell lines, both sensitive (K562-S) or resistant (K562-R) to imatinib, the "gold standard of care" used in this pathology. Moreover, we demonstrated that 5d kills CML cells by a non-conventional mechanism of cell death.

[Overview of 40 years' chemical study]

1,6-Dihydro-3(2H)-pyridinone, designed as a common synthon for synthesis of various natural products, was found to be easily prepared in large scale and successfully used to synthesize a variety of alkaloids such as ibogamine, quinine and tecomanine. A tricyclo[3.3.0.0(2.8)]octane was also served as a common synthon for several sesquiterpenes such as pentalenene and quadrone. Synthetic studies by using sulfinyl chirality via an intramolecular Michael addition gave the novel route to construct spiro-ketal moiety in enantiomerically pure form. By applying this method, many natural spiro-ketal compounds were asymmetrically synthesized effectively. 3-Sulfinylated 1,4-dihydropyridine, a chiral NADH model compound, reduced activated ketones such as methyl benzoylformate to give the corresponding alcohols in excellent optical yields. A kind of 3-O-substituted pyridoxal chiral model compound was useful for preparation of alpha,alpha-dialkylated alpha-amino acids by asymmetric alpha-alkylation of alpha-amino acids. 2'-O,4'-C-Bridged nucleic acid analogs, BNAs, developed as novel type of artificial nucleic acids, showed an extraordinarily high binding affinity toward single stranded RNA and double stranded DNA complements along with excellent nuclease-resistant ability. Oligonucleotides containing BNA monomer units were proved to be very useful for various biotechnologies, such as antisense and antigene methodologies.

Suzuki−Miyaura, α-Ketone Arylation and Dehalogenation Reactions Catalyzed by a Versatile N-Heterocyclic Carbene−Palladacycle Complex

[reaction: see text] The activity of the complex (IPr)PdCl(eta2-N,C-C12H7NMe2), 1 [IPr = (N,N'-bis(2,6-diisopropylphenyl)imidazol)-2-ylidene], in the Suzuki-Miyaura cross-coupling reaction involving unactivated aryl chlorides and triflates with arylboronic acids at room temperature in technical grade 2-propanol is described. These conditions allow for the synthesis of di- and tri-ortho-substituted biaryls in very short reaction times. This complex also displays very high activity for alpha-ketone arylation and dehalogenation reactions of activated and unactivated aryl chlorides.

Marine natural products

This review covers the literature published in 2003 for marine natural products, with 619 citations (413 for the period January to December 2003) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, coelenterates, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (656 for 2003), together with their relevant biological activities, source organisms and country or origin. Biosynthetic studies or syntheses that lead to the revision of structures or stereochemistries have been included (78), including any first total syntheses of a marine natural product.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2003 is provided. The emphasis on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure conformation or stereochemical assignments.