Asymmetric total syntheses of marine cyclic depsipeptide halipeptins A-D

A Comparative Synthetic Strategy Perspective on α-Amino Acid- and Non-Amino Acid-Derived Synthons towards Total Syntheses of Selected Natural Macrolides

Macrocyclic alkaloids (macrolides) and cyclopeptides have an immense range of applications in drug discovery research because of their natural abundance and potential biological and physicochemical properties. Presently, more than 100 approved drugs or clinical drug candidates contain macrocyclic scaffolds as the biologically active component. This review provides an interesting perspective about the use of amino acid-derived chiral pools versus other methods derived from miscellaneous synthons towards the total synthesis of non-peptidic macrolides. The synthetic routes and the key strategies involved in the total syntheses of ten natural macrolides have been discussed. Both the amino acid-derived and non-amino acid-derived synthetic routes have been illustrated to present a comparative study between the two approaches.

Marine natural product peptides with therapeutic potential: Chemistry, biosynthesis, and pharmacology

The oceans are a uniquely rich source of bioactive metabolites, of which sponges have been shown to be among the most prolific producers of diverse bioactive secondary metabolites with valuable therapeutic potential. Much attention has been focused on marine bioactive peptides due to their novel chemistry and diverse biological properties. As summarized in this review, marine peptides are known to exhibit various biological activities such as antiviral, anti-proliferative, antioxidant, anti-coagulant, anti-hypertensive, anti-cancer, antidiabetic, antiobesity, and calcium-binding activities. This review focuses on the chemistry and biology of peptides isolated from sponges, bacteria, cyanobacteria, fungi, ascidians, and other marine sources. The role of marine invertebrate microbiomes in natural products biosynthesis is discussed in this review along with the biosynthesis of modified peptides from different marine sources. The status of peptides in various phases of clinical trials is presented, as well as the development of modified peptides including optimization of PK and bioavailability.

Ester coupling reactions – an enduring challenge in the chemical synthesis of bioactive natural products

In this review we investigate the use of complex ester fragment couplings within natural product total syntheses. Using examples from the literature up to 2014 we illustrate the state-of-the-art as well as the challenges within this area of organic synthesis.

Total synthesis of (+)-18-epi-latrunculol A: development of a synthetic route

The evolution of an enantioselective total synthesis of (+)-18-epi-latrunculol A, a congener of the marine-sponge-derived latrunculins A and B, is reported. Key steps include a late-stage Mitsunobu macrolactonization to construct the 16-membered macrolactone, a mild Carreira alkynylation to unite the northern and southern hemispheres, a diastereoselective, acid-mediated δ-hydroxy enone cyclization/equilibration sequence, and a functional-group-tolerant cross-metathesis to access the enone cyclization precursor.

Total synthesis of sandramycin and its analogues via a multicomponent assemblage

The total synthesis of sandramycin has been accomplished by using a Staudinger/aza-Wittig/diastereoselective Ugi three-component reaction sequence as a key step to obtain a linear pentadepsipeptide. Subsequent [5 + 5] coupling of the penptapeptide, macrolactamization, and introduction of the quinaldin chromophores afforded sandramycin. Dihydroxy and diacetoxy analogues were also prepared, and the cytotoxic activity of these analogues against a range of human cancer cell lines was evaluated.

Diverse synthesis of marine cyclic depsipeptide lagunamide A and its analogues

The asymmetric total synthesis of lagunamide A (3.0%, 20 steps longest linear sequence) and its five analogues, including the structure dehydrated at the C37 position, are detailed in this report. The key feature in this diverse synthesis includes the elaboration of four consecutive chiral centers at C37-40 and the final macrocyclization. Starting from chiral aldehyde 10, we synthesized both 1,3-anti and 1,3-syn homoallylic alcohols 20a and 20b through asymmetric aldol condensation and stereoselective allylation. The following esterification to introduce the L-N-Me-Ala unit resulted in significant epimerization. This problem was finally overcome by coupling the alcohols with the corresponding acid chloride of the L-alanine derivative. The key α,β-unsaturated carboxylic acid unit was produced by cross-metathesis (CM) of methacrylaldehyde and related olefins. Interestingly, we found that the C7 configuration dramatically affected the ring closure. Natural lagunamide A (1a), its 39-epimer (1c), and its 2-epimer (1d) were obtained through macrolactamization between alanine and isoleucine moieties.

Total synthesis of the marine cyclic depsipeptide viequeamide a

The first total synthesis of viequeamide A, a natural cyclic depsipeptide isolated from a marine button cyanobacterium, was achieved with the N-Me-Val-Thr peptide bond as the final macrocyclization site. The synthetic product gave nearly identical spectroscopic data to that reported for the natural product.

Second-generation DBFOX ligands for the synthesis of beta-substituted alpha-amino acids via enantioselective radical conjugate additions

A set of second-generation DBFOX ligands possessing extended aryl or benzyl-type groups was synthesized. The requisite amino alcohols were either commercially available (DBFOX/Bn) or constructed via Sharpless asymmetric aminohydroxylation (DBFOX/Nap, DBFOX/ t-BuPh, DBFOX/Pip) or phase-transfer-catalyzed asymmetric alkylation (DBFOX/MeNap). Complexes of the ligands with Mg(NTf2)2 were evaluated as promoters of enantioselective radical conjugate additions to alpha,beta-unsaturated alpha-nitro amides and esters. Reactions employing the DBFOX/Nap ligand exhibited improved enantioselectivity relative to previously published additions mediated by DBFOX/Ph. However, the relatively modest increase in diastereomeric ratio suggests that our substrate-Lewis acid binding model, which was formulated based on results from DBFOX/Ph-promoted radical conjugate additions, is in need of revision.

Marine natural products

This review covers the literature published in 2006 for marine natural products, with 758 citations (534 for the period January to December 2006) referring to compounds isolated from marine microorganisms and phytoplankton, green algae, brown algae, red algae, sponges, cnidaria, bryozoans, molluscs, tunicates and echinoderms. The emphasis is on new compounds (779 for 2006), together with their 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.