14-membered Lactone Research Articles

Cespitulactones A and B, New Diterpenoids from Cespitularia taeniata.

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Enantioselective synthesis of aurisides A and B, cytotoxic macrolide glycosides of marine origin

The enantioselective synthesis of aurisides A and B, macrolide glycosides of marine origin, was achieved by a convergent approach. The C1–C9 segment 4 was prepared from ( R)-pantolactone, and the C10–C17 segment 14 was synthesized from ( R)-glycidyl trityl ether. The Nozaki–Hiyama–Kishi reaction between 4 and 14 and subsequent reactions gave seco acid 10, which was converted into the aglycon ( 3) of aurisides by construction of the 14-membered lactone and bromine-substituted conjugated diene. The glycosylation reaction of the aglycon provided aurisides A and B.

Determination of the Preferred Conformation of the Bicyclic Galerucella Pheromone Using Density Functional Theory Optimization and Calculations of Chemical Shifts

A pheromone from the beetle, Galerucella calmariensis, was recently isolated and identified (Bartelt, R. J. et al. J. Chem. Ecol. 2006, 32, 693-712) as a 14-carbon, bicyclic dimethylfuran lactone, with the systematic name 12,13-dimethyl-5,14-dioxabicyclo[9.2.1]tetradeca-1(13),11-dien-4-one. The main 12-membered lactone ring is very flexible; as a result, there exist multiple possible conformations. The preferred conformation cannot be deduced solely from room-temperature NMR measurements. Using density functional (DFT) studies, 26 unique conformers with energies within 10.0 kcal/mol of the global minimum-energy structure were found. A mirror-image plane exists so that each conformer has an "inverse" structure with the same energy, for which the dihedral angles around the flexible ring have opposite sign. The isotropic 1H and 13C NMR chemical shifts of the DFT-optimized structures were calculated using the gauge-including atomic orbital (GIAO) method. By considering the relative energies of the conformers and the calculated and observed NMR spectra, we concluded that the molecule exists primarily as a mixture of two distinct conformers at room temperature, each being present with its mirror-image inverse. Structural interconversions among these likely occur on a time scale that is fast compared to the NMR experiments. Using mode-following and dihedral-driving techniques, several potential pathways were found for the conversion of the lowest-energy conformer to its mirror-image structure. Ab initio molecular dynamics (AIMD) using the 4-31G basis set was carried out for 50 ps to test the availability of various low-energy minima and the transition states found from the searches noted above.

Cespitulactones A and B, new diterpenoids from Cespitularia taeniata

Two new diterpenoids, designated cespitulactones A ( 1) and B ( 2), were isolated from a sample of the soft coral Cespitularia taeniata collected in Taiwan. Compound 1 possesses a novel structure with a bond cleavage between C-10 and C-11, and having a 14-membered lactone ring junction between C-10 and C-12. Their structures were elucidated on the basis of extensive spectroscopic analysis and chemical derivatization. The isolated compounds were also evaluated for cytotoxicity toward human cancer cell lines.

New Silicon‐Mediated, Sequential Ring Expansions of n‐Sized 2‐Cycloalkenones into Hydroxyolefinic n + m + p Medium‐Sized Lactones: Short Synthesis of (‐)‐Phoracantholide‐J.

[reaction: see text] In only four steps from 2-cyclopentenone and 2-cyclohexenone, sequential three- or four-atom and then one- to three-atom ring enlargements produce nine- to 12-membered hydroxyolefinic lactones on a gram scale. 2-Cyclopentenone undergoes this serial 5 + 3 + 2 process to form 10-membered ring natural (-)-phoracantholide-J in six linear steps and 26% overall yield.

Reductive Aldol Cyclizations: Synthesis of β-Hydroxylactones

This paper reports the construction of synthetically useful β-hydroxylactones via reductive aldol cyclizations. Various copper salts, hydride sources and phosphine ligands were screened and Cu(OAc)2, 1,1,3,3-tetramethylhydrosiloxane (TMDS), and (R)-3,5-xyl-MeO-BIPHEP proved to be the optimal reagents, but only modest enantioselectivity was obtained (up to 83% ee). Reasonable yields (60-70%) were attained for the formation of 5- and 6-membered lactones as well as high diastereoselectivity.

Enantioselective Radical Methods for Lactone Synthesis: Use of Unprotected Haloalcohols as Radical Precursors.

AbstractFor Abstract see ChemInform Abstract in Full Text.

New Silicon-Mediated, Sequential Ring Expansions of n-Sized 2-Cycloalkenones into Hydroxyolefinic n + m + p Medium-Sized Lactones: Short Synthesis of (−)-Phoracantholide-J

[reaction: see text] In only four steps from 2-cyclopentenone and 2-cyclohexenone, sequential three- or four-atom and then one- to three-atom ring enlargements produce nine- to 12-membered hydroxyolefinic lactones on a gram scale. 2-Cyclopentenone undergoes this serial 5 + 3 + 2 process to form 10-membered ring natural (-)-phoracantholide-J in six linear steps and 26% overall yield.

Enantioselective Total Synthesis of (‐)‐Microcarpalide.

Abstract The enantioselective total synthesis of the actin-targeting metabolite (−)-microcarpalide is described. Key steps include ring-closing metathesis (RCM) for the final construction of the 10-membered lactone framework and stereoselective homologation of boronic esters for the insertion of all stereocentres with the desired absolute configuration. In particular, the acidic fragment was prepared in seven steps from a suitable chiral bromomethane boronate by means of two sequential stereoselective homologations to install the two stereocentres with the correct final R stereochemistry, employing (−)-pinanediol as the chiral director. Subsequent elaboration to the required C 7 backbone entailed nucleophilic displacement with a vinyl Grignard reagent, oxidative cleavage of the boronic scaffold and protection–deprotection manipulations. Interestingly, when the tribenzyloxy diene ester resulting from DCC-mediated coupling of the two key synthons was subjected to RCM in the presence of Grubbs' catalyst, the reaction proceeded stereoselectively to yield the desired trans oxecin-2-one, albeit with poor conversion.

Design and Synthesis of 16‐Membered Hybrid Macrolide Having a Thiazole Side Chain on the Carbonolide Skeleton.

Design and synthesis of a 16-membered hybrid macrolide, which contained a 16-membered carbonolide-type lactone framework with a thiazole sidechain, were described.

Synthetic Studies toward Ecteinascidin 743

An efficient synthesis of a fully functionalized tetracycle (A-B-C-H) 7 containing a 1,4-bridged 10-membered lactone was developed. Phenolic aldol condensation between 2-methylsesamol (15) and Garner's aldehyde provided the protected amino diol 16, which was converted to free amine 11 in excellent yield. A Pictet-Spengler reaction between 11 and ethyl glyoxylate under carefully controlled conditions (LiCl, toluene, 1,1,1,3,3,3-hexafluoro-2-propanol, room temperature) provided the acid-sensitive tetrahydroisoquinoline (18) in high yield, which was converted to the amino alcohol 9. Enantioselective alkylation of a glycine template in the presence of a catalytic amount of chiral cinchonidium salt was the key step for the access of enantiomerically pure amino aldehyde 10. Union of the two fragments 9 and 10 via oxazolidine intermediate afforded amino nitrile 39, which upon esterification of the primary alcohol with (R)-N-(S-4,4',4' '-trimethoxyltrityl) Cys (42) afforded 43. Cyclization of 43 (1% trifluoroacetic acid in trifluoroethanol) provided compound 44 by a domino process involving (a) unmasking of the S-trimethoxytrityl group, (b) fragmentation of dioxane assisted by an electron-rich aromatic ring, and (c) formation of a 1,4-bridged 10-membered lactone via formation of a sulfide linkage. Treatment of 7, obtained in two steps from 44b, under acidic conditions (0.5% methyl sulfonic acid in acetonitrile) afforded the pentacyclic compound 51 via fragmentation of the 10-membered cyclic sulfide followed by an intramolecular Pictet-Spengler reaction.

Enantioselective total synthesis of (−)-microcarpalide

The enantioselective total synthesis of the actin-targeting metabolite (−)-microcarpalide is described. Key steps include ring-closing metathesis (RCM) for the final construction of the 10-membered lactone framework and stereoselective homologation of boronic esters for the insertion of all stereocentres with the desired absolute configuration. In particular, the acidic fragment was prepared in seven steps from a suitable chiral bromomethane boronate by means of two sequential stereoselective homologations to install the two stereocentres with the correct final R stereochemistry, employing (−)-pinanediol as the chiral director. Subsequent elaboration to the required C 7 backbone entailed nucleophilic displacement with a vinyl Grignard reagent, oxidative cleavage of the boronic scaffold and protection–deprotection manipulations. Interestingly, when the tribenzyloxy diene ester resulting from DCC-mediated coupling of the two key synthons was subjected to RCM in the presence of Grubbs' catalyst, the reaction proceeded stereoselectively to yield the desired trans oxecin-2-one, albeit with poor conversion.

Enantioselective sulfoxide-directed preparation of 1,2-diols from oxalic compounds: formal synthesis of the 10-membered lactone core of ascidiatrienolides and didemnilactones

The synthesis of diene 17, which is available in both possible absolute configurations is described. This diene constitutes the key intermediate of a previous synthesis of the 10-membered lactone core of the marine natural products ascidiatrienolides and didemnilactones. This intermediate is available via two successive highly diastereoselective sulfoxide-directed reductions of oxalic diamide 18.

Stereoselective synthesis of (−)-microcarpalide

A highly convergent and efficient synthesis of (−)-microcarpalide, a 10-membered lactone displaying remarkable microfilament disrupting activity is described. Ring-closing metathesis and Sharpless asymmetric dihydroxylations are the key steps. Our strategy highlights the application of novel hydroxy lactone precursors for the stereoselective synthesis of (−)-microcarpalide.

Enantioselective Radical Methods for Lactone Synthesis: Use of Unprotected Haloalcohols as Radical Precursors

We have developed an efficient free radical method for the synthesis ofenantioenriched 6- and 7-membered lactones in one synthetic operation.

Identification of Diverse Microbial Metabolites as Potent Inhibitors of HIV-1 Tat Transactivation

HIV-1 Tat is one of six regulatory proteins that are required for viral replication and is an attractive target for the development of new anti-HIV agents. Screening of microbial extracts using a whole cell Tat-dependent transactivation assay, which guided the separation of the active broths, led to the identification of five structurally diverse classes (M(R) range 232-1126) of natural products. These include i) three sesquiterpenoids, namely, sporogen-AO1, petasol, and 6-dehydropetasol, ii) two resorcylic 14-membered lactones, namely monorden and monocillin IV, iii) a ten-membered lactone, iv) a quinoline and quinoxiline bicyclic octadepsipeptides, namely echinomycin and UK-63598, and v) a cyclic heptapeptide, ternatin. These compounds displayed varying degrees of potencies with IC50 values ranging from 0.0002 to 100 microM. The most active compound was the quinoxiline bicyclic octadepsipeptides, UK-63598, which inhibited Tat-dependent transactivation with an IC50 value of 0.2 nM and exhibited a 100-fold therapeutic window with respect to toxicity. In a single-cycle antiviral assay, UK-6358 inhibited viral replication with an IC50 value of 0.5 nM; however, it appeared to be equally toxic at that concentration. Monocillin IV was significantly less active (Tat transactivation inhibitory IC50 of 5 microM) but was not toxic at 100 microM in an equivalent cytotoxicity assay. The compound exhibited antiviral activity with an IC50 value of 6.2 microM in the single-cycle antiviral assay and a sixfold therapeutic window. Details of the isolation, fermentation, and biological activities of these structurally diverse natural products are described.

Kinetics of Macrolide Action: THE JOSAMYCIN AND ERYTHROMYCIN CASES

Members of the macrolide class of antibiotics inhibit peptide elongation on the ribosome by binding close to the peptidyltransferase center and blocking the peptide exit tunnel in the large ribosomal subunit. We have studied the modes of action of the macrolides josamycin, with a 16-membered lactone ring, and erythromycin, with a 14-membered lactone ring, in a cell-free mRNA translation system with pure components from Escherichia coli. We have found that the average lifetime on the ribosome is 3 h for josamycin and less than 2 min for erythromycin and that the dissociation constants for josamycin and erythromycin binding to the ribosome are 5.5 and 11 nM, respectively. Josamycin slows down formation of the first peptide bond of a nascent peptide in an amino acid-dependent way and completely inhibits formation of the second or third peptide bond, depending on peptide sequence. Erythromycin allows formation of longer peptide chains before the onset of inhibition. Both drugs stimulate the rate constants for drop-off of peptidyl-tRNA from the ribosome. In the josamycin case, drop-off is much faster than drug dissociation, whereas these rate constants are comparable in the erythromycin case. Therefore, at a saturating drug concentration, synthesis of full-length proteins is completely shut down by josamycin but not by erythromycin. It is likely that the bacterio-toxic effects of the drugs are caused by a combination of inhibition of protein elongation, on the one hand, and depletion of the intracellular pools of aminoacyl-tRNAs available for protein synthesis by drop-off and incomplete peptidyl-tRNA hydrolase activity, on the other hand.

Low-temperature EPR and quantum chemical study of lactone radical cations and their transformations

Radical cations of a number of lactones ( β-butyro-, γ-butyro-, γ-valero-, δ-hexano-, δ-valero- and ε-capro-) were radiolytically generated in CF 3CCl 3 matrix and investigated by EPR spectroscopy. The primary radical cation of the 4-membered ring β-butyrolactone is unstable even at 77 K and undergoes spontaneous ring opening and fragmentation, leading to the deprotonated neutral (CH 2CHCH 2) radical. The stability of the primary carbonyl-centred radical cations of the 5-, 6- and 7-membered lactone rings towards intramolecular H-shift from the C1 in α-position to carbonyl oxygen depends primarily on the ring size, which determines the activation energy of the transformation and distance L(H–O) of the carbonyl oxygen to the nearest H-atom on the ring. The larger the ring, the smaller the L(H–O) and also activation energy of the H-shift, making the transformation of the primary radical cation more feasible. The quantum chemical calculations facilitated the interpretation of the EPR spectra of the secondary radical cations.

Antineoplastic Agents. 510. Isolation and structure of dolastatin 19 from the Gulf of California sea hare Dolabella auricularia.

The Gulf of California shell-less mollusc Dolabella auricularia has been found to contain a new 14-membered macrocyclic lactone linked to a 2,4-di-O-methyl-l-alpha-rhamnopyranoside, designated dolastatin 19 (1). The new cancer cell growth inhibitor (1, 8.33 x 10(-8)% yield) was obtained by bioassay (P388 lymphocytic leukemia and human cancer cell lines) directed isolation, accompanied by debromoaplysiatoxin (9.17 x 10(-7)% yield) and anhydrodebromoaplysiatoxin (2.0 x 10(-7)% yield). The structures were determined on the basis of analyses of high-resolution mass spectra and high-field NMR data. All the relative stereochemistry for the chiral centers was designated by utilizing NMR techniques.

New antibacterial agents derived from the DNA gyrase inhibitor cyclothialidine.

Cyclothialidine (1, Ro 09-1437) is a potent DNA gyrase inhibitor that was isolated from Streptomyces filipinensis NR0484 and is a member of a new family of natural products. It acts by competitively inhibiting the ATPase activity exerted by the B subunit of DNA gyrase but barely exhibits any growth inhibitory activity against intact bacterial cells, presumably due to insufficient permeation of the cytoplasmic membrane. To explore the antibacterial potential of 1, we developed a flexible synthetic route allowing for the systematic modification of its structure. From a first set of analogues, structure-activity relationships (SAR) were established for different substitution patterns, and the 14-hydroxylated, bicyclic core (X) of 1 seemed to be the structural prerequisite for DNA gyrase inhibitory activity. The variation of the lactone ring size, however, revealed that activity can be found among 11- to 16-membered lactones, and even seco-analogues were shown to maintain some enzyme inhibitory properties, thereby reducing the minimal structural requirements to a rather simple, hydroxylated benzyl sulfide (XI). On the basis of these "minimal structures" a modification program afforded a number of inhibitors that showed in vitro activity against Gram-positive bacteria. The best activities were displayed by 14-membered lactones, and representatives of this subclass exhibit excellent and broad in vitro antibacterial activity against Gram-positive pathogens, including Staphylococcus aureus, Streptococcus pyogenes, and Enterococcus faecalis, and overcome resistance against clinically used drugs. By improving the pharmacokinetic properties of the most active compounds (94, 97), in particular by lowering their lipophilic properties, we were able to identify congeners of cyclothialidine (1) that showed efficacy in vivo.