Quantum Mechanical Calculation of Coupling Constants in the Configurational Analysis of Flexible Systems: Determination of the Configuration of Callipeltin A

Stereoselective Synthesis of a Protected Side Chain of Callipeltin A

Matteson homologations of chiral boronic esters proved to be an excellent tool for the synthesis of highly functionalized amino and hydroxy acid residues. This method provides straightforward stereoselective access to the side chain of callipeltin A, a natural marine product with interesting biological activities. Furthermore, this protocol should allow for variations in the substitution pattern in future SAR studies, simply by choosing suitable nucleophiles during the homologation steps.

Enigmazole C: A Cytotoxic Macrocyclic Lactone and Its Ring-Opened Derivatives from a New Species of Homophymia Sponge

A new macrolide, enigmazole C (1), and two additional analogues, enigmazoles E (2) and D (3), were obtained from a new species of the Homophymia genus as part of an ongoing discovery program at PharmaMar to study cytotoxic substances from marine sources. The structures were fully characterized by cumulative analyses of NMR, IR, and MS spectra, along with density functional theory computational calculations. All three of the new compounds feature an unusual 2,3-dihydro-4H-pyran-4-one moiety, but only enigmazoles C (1) and D (3) showed cytotoxic activity in the micromolar range against A-549 (lung), HT-29 (colon), MDA-MB-231 (breast), and PSN-1 (pancreas) tumor cells.

Synthesis of the cyclic heptapeptide core of callipeltin A

Macrolactonisation of a suitable heptapeptide precursor provides the cyclic depsipeptide core of callipeltin A in excellent yield.

Elucidating heteroatom influence on homonuclear 4 J(H,H) coupling constants by DFT/NMR approach

We report the structural dependency of long range scalar J-coupling constant across four bonds as function of the dihedral angles Φ1 and Φ3. The calculated homonuclear coupling constants ⁴ J(_H,H ), obtained at a density functional theory level, were measured between C(1)─X(2) and X(2)─C(3) bonds in three-term models, where C, N, O, and S were systematically used as the second atom of the alkyl structures (1-4). The ⁴ J_(H,H) calculated values, tabulated for variation of 30° for both Φ1 and Φ3, have disclosed an unexpected detectable coupling constant (⁴ J(_H,H ) ≥ 1 Hz) across heteroatoms, useful to provide valuable structural information. A 2-methyl-1,3-dithiane sulfide (5) was used as a case study to prove the applicability and reliability of the calculated values to real issues. The ⁴ J(_H,H ) values obtained at density functional theory for the system 4 have reproduced with good accuracy an unexpected experimental ⁴ J(_H2ax-H4ax ) = 1.01 Hz of sulfide molecule (5), suggesting these calculated coupling constant values as a new powerful tool for the organic synthesis and stereochemical analysis.

Long-range proton-carbon coupling constants: NMR methods and applications

A general review of novel NMR methods to measure heteronuclear long-range proton-carbon coupling constants ((n)JCH; n>1) in small molecules is made. NMR experiments are classified in terms of NMR pulse scheme and cross-peak nature. A discussion about simplicity, general applicability and accuracy for each particular NMR experiment is presented and exemplified. Important aspects such as the sign determination and measurement of very small coupling values involving protonated and non-protonated carbons as well as the complementarity between different experiments are also discussed. Finally, a compilation of applications in structural and conformational analysis of different types of molecules since 2000 is surveyed.

Neamphamide B, new cyclic depsipeptide, as an anti-dormant mycobacterial substance from a Japanese marine sponge of Neamphius sp

A new cyclic depsipeptide, designated neamphamide B (1), was isolated from a marine sponge of Neamphius sp. collected at Okinawa, Japan in 1993 as an anti-mycobacterial substance against active and dormant bacilli. The planar structure of neamphamide B (1) was determined on the basis of spectroscopic analysis, and stereostructure of amino acid was deduced by chromatographic comparison of the acid hydrolysate of 1 with appropriate amino acid standards after derivatizing with FDAA or GITC. Neamphamide B (1) showed potent anti-mycobacterial activity against Mycobacterium smegmatis under standard aerobic growth conditions as well as dormancy-inducing hypoxic conditions with MIC of 1.56 μg/mL. Neamphamide B (1) was also effective to Mycobacterium bovis BCG with MIC in the ranging of 6.25-12.5 μg/mL.

Natural products from the Lithistida: a review of the literature since 2000

Lithistid sponges are known to produce a diverse array of compounds ranging from polyketides, cyclic and linear peptides, alkaloids, pigments, lipids, and sterols. A majority of these structurally complex compounds have very potent and interesting biological activities. It has been a decade since a thorough review has been published that summarizes the literature on the natural products reported from this amazing sponge order. This review provides an update on the current taxonomic classification of the Lithistida, describes structures and biological activities of 131 new natural products, and discusses highlights from the total syntheses of 16 compounds from marine sponges of the Order Lithistida providing a compilation of the literature since the last review published in 2002.

Theoretical prediction and assignment of vicinal 1H-1H coupling constants of diastereomeric 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes

Spin-spin coupling constants between nuclei in NMR spectroscopy reflect their spatial arrangement. A number of calculation methods, applying different levels of theory, have been developed to support the stereochemical assignment of novel compounds. Nevertheless, revisions of the assignment of structures in the literature are not rare. In the present work, the reliability of the calculation methods amenable for a theoretical prediction of spin-spin coupling constants of vicinal protons to support correct stereochemical assignment of substitution at five-membered rings of 3-alkoxy-6,7-epoxy-2-oxabicyclo[3.3.0]octanes was studied. Experimental (3)J(H,H) coupling constants were compared with the coupling constants calculated for all possible diastereomers. The fully quantum chemical approach provided theoretical (3)J(H,H) coupling constants with an absolute deviation of no more than 1.1 Hz for 91% of the experimentally studied coupled spins, whereas the methods without quantum chemical geometry optimization resulted in completely unreliable predictions. Consequently, for a reliable stereochemical assignment of small and medium size molecules, the protocol for calculating the coupling constants based on the results of the quantum chemical geometry optimization is recommended.

Microbial Transformation of Marine Halogenated Sesquiterpenes

The sesquiterpene pacifenol is one of the main constituents of the red alga Laurencia claviformis. Earlier work on the semisynthetic derivatives of pacifenol afforded a series of halogenated sesquiterpenes. The aim of the present work was to obtain new hydroxylated derivatives of halogenated sesquiterpenes by means of microbial transformation using Aspergillus níger, Gibberella fujikuroi and Mucor plumbeus. The best results were obtained with M. plumbeus. The microbiological transformation by M. plumbeus of pacifenol, and two semisynthetic derivatives, is described. The structures of the new compounds obtained were determined by spectroscopic means.

Solid-phase total synthesis and structure proof of callipeltin B

The cytotoxic, cyclic heptadepsipeptide, natural product callipeltin B was synthesized on a solid-phase support in 15% overall yield. Comparison of the 1H NMR spectra of three synthetic isomers with those of callipeltin B confirmed the configurational reassignment of its threonine residues as d-allothreonine and the assignment of the configuration of its beta-methoxytyrosine residue as (2R,3R).