Total synthesis and stereochemical reassignment of (+)-dolastatin 19, a cytotoxic marine macrolide isolated from Dolabella auricularia

Cobalt-Catalyzed Enantioselective Hydroboration of α-Substituted Acrylates

Even though metal-catalyzed enantioselective hydroborations of alkenes have attracted enormous attention, few preparatively useful reactions of α-alkyl acrylic acid derivatives are known, and most use rhodium catalysts. No examples of asymmetric hydroboration of the corresponding α-arylacrylic acid esters are known. In our continuing efforts to search for new applications of earth-abundant cobalt catalysts for broadly applicable organic transformations, we have identified 2-(2-diarylphosphinophenyl)oxazoline ligands and mild reaction conditions for efficient and highly regio- and enantioselective hydroboration of α-alkyl- and α-aryl- acrylates, giving β-borylated propionates. Since the C-B bonds in these compounds can be readily replaced by C-O, C-N, and C-C bonds, these intermediates could serve as valuable chiral synthons, some from feedstock carbon sources, for the synthesis of propionate-bearing motifs including polyketides and related molecules. Two-step syntheses of "Roche" ester from methyl methacrylate (79%; er 99:1), arguably the most widely used chiral fragment in polyketide synthesis, and tropic acid esters (∼80% yield; er ∼93:7), which are potential intermediates for several medicinally important classes of compounds, illustrate the power of the new methods. Mechanistic studies confirm the requirement of a cationic Co(I) species [(L)Co]+as the viable catalyst in these reactions and rule out the possibility of a [L]Co-H-initiated route, which has been well-established in related hydroborations of other classes of alkenes. A mechanism involving an oxidative migration of a boryl group to the β-carbon of an η4-coordinated acrylate-cobalt complex is proposed as a plausible route.

Lewis Base-Brønsted Acid-Enzyme Catalysis in Enantioselective Multistep One-Pot Syntheses

Establishing one-pot, multi-step protocols combining different types of catalysts is one important goal for increasing efficiency in modern organic synthesis. In particular, the high potential of biocatalysts still needs to be harvested. Based on an in-depth mechanistic investigation of a new organocatalytic protocol employing two catalysts {1,4-diazabicyclo[2.2.2]octane (DABCO); benzoic acid (BzOH)}, a sequence was established providing starting materials for enzymatic refinement (ene reductase; alcohol dehydrogenase): A gram-scale access to a variety of enantiopure key building blocks for natural product syntheses was enabled utilizing up to six catalytic steps within the same reaction vessel.

Conformation-guided analogue design identifies potential antimalarial compounds through inhibition of mitochondrial respiration

The synthesis of a 2-methyl-substituted analogue of the natural product, neopeltolide, is reported in an effort to analyze the importance of molecular conformation and ligand-target interactions in relation to biological activity. The methyl substitution was incorporated via highly diastereoselective ester enolate alkylation of a late-stage intermediate. Coupling of the oxazole sidechain provided 2-methyl-neopeltolide and synthetic neopeltolide via total synthesis. The substitution was shown to maintain the conformational preferences of its biologically active parent compound through computer modeling and NMR studies. Both compounds were shown to be potential antimalarial compounds through the inhibition of mitochondrial respiration in P. falciparum parasites.

An Expedient Total Synthesis of Chivosazole F: an Actin-Binding Antimitotic Macrolide from the Myxobacterium Sorangium Cellulosum

A unified strategy for the chemical synthesis of the chivosazoles is described. This strategy is based on two closely related approaches involving the late‐stage installation of the isomerization‐prone (2Z,4E,6Z,8E)‐tetraenoate motif, and an expedient fragment‐assembly procedure. The result is a highly convergent total synthesis of chivosazole F through the orchestration of three mild Pd/Cu‐mediated Stille cross‐coupling reactions, including the use of a one‐pot, site‐selective, three‐component process, in combination with controlled installation of the requisite alkene geometry.

The vinylogous Mukaiyama aldol reaction (VMAR) in natural product synthesis

This review will provide an overview on the recent developments of polyketide synthesis using the vinylogous Mukaiyama aldol reaction for the construction of advanced intermediates. In general, four different motifs can be constructed efficiently using the recent developments of asymmetric variants of this strategy.

Synthesis of the C15-C35 northern hemisphere subunit of the chivosazoles

An advanced C15-C35 subunit of the chivosazole polyene macrolides was prepared in a convergent manner, exploiting boron-mediated aldol reactions for the stereocontrolled construction of the C15-C26 and C27-C35 segments, followed by their Pd/Cu-promoted Stille coupling to configure the signature (23E,25E,27Z)-triene motif. Correlation with a known C28-C35 degradation fragment of chivosazole A was also achieved.

Marine natural products: synthetic aspects

An overview of marine natural products synthesis during 2007 is provided. As with earlier installments in this series, the emphasis is on total syntheses of molecules of contemporary interest, new total syntheses, and syntheses that have resulted in structure confirmation or stereochemical assignments.1 Introduction, 2 Review articles, 3 Azaspiracid, 4 Polyethers, 5 Guanidinium alkaloids, 6 Amphidinolides, 7 Total syntheses of other compounds, 8 Acknowledgements, 9 References.

Marine natural products

This review covers the literature published in 2007 for marine natural products, with 948 citations(627 for the period January to December 2007) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidarians,bryozoans, molluscs, tunicates, echinoderms and true mangrove plants. The emphasis is on new compounds (961 for 2007), together with the relevant biological activities, source organisms and country of origin. Biosynthetic studies, first syntheses, and syntheses that lead to the revision of structures or stereochemistries, have been included.1 Introduction, 2 Reviews, 3 Marine microorganisms and phytoplankton, 4 Green algae, 5 Brown algae, 6 Red algae, 7 Sponges, 8 Cnidarians, 9 Bryozoans, 10 Molluscs, 11 Tunicates (ascidians),12 Echinoderms, 13 Miscellaneous, 14 Conclusion, 15 References.

Total synthesis of cytotoxic marine macrolides: Callipeltoside A, aurisides A and B, and dolastatin 19

Synthetic studies pertaining to a novel class of structurally related glycosidic 14-membered macrolides of marine origin are reported. The evolution of a versatile aldol-based strategy that culminated in the total syntheses of callipeltoside A and aurisides A and B is detailed. Using a combination of biogenetic considerations and conformational analysis, a revised stereochemical assignment for the related polyketide dolastatin 19 was proposed and validated by total synthesis.