Spontaneous Lactonization Research Articles

Total synthesis of Comfreyn A and structural analogues via two photochemical key steps.

In this work the influence of o-fluorine substituents on the photo-dehydro-Diels-Alder (PDDA) reaction was investigated and the findings of this study were applied to the total synthesis of natural products. The reactant molecules consisted of two alkyl arylpropiolates, connected by a suberic acid tether and bearing fluorine substituents in each of the o-positions. While quantum chemical calculations suggested that a fluorine substituent prevents an attack of the adjacent carbon atom in the second C-C bond forming step of the PDDA reaction, this attack took place nevertheless. The subsequent fluoride elimination, assisted by protic solvents or trialkylsilanes, resulted in an "Umpolung" of the 4-position of the cycloallene intermediate enabling the introduction of nucleophiles at this position. The nucleophilic replacement of the second fluorine substituent could also be triggered photochemically. After removal of the tether, the two arene moieties stand nearly perpendicular to each other and a selective excitation of the naphthalene moiety was possible. This led to an intramolecular photoinduced electron transfer (PET) followed by a nucleophilic replacement of the fluoride according to a SR+N1Ar* mechanism. The formed phenolic hydroxyl group underwent spontaneous lactonization with the adjacent ester group. Based on these results, the first total synthesis of the lignan Comfreyn A and some structural analogues were developed.

Asymmetric Aldol Reaction of Pyruvate Promoted by Chiral Tertiary Amines

AbstractThe direct asymmetric aldol reaction of α‐ketoesters catalyzed by chiral tertiary amines is reported. The described methodology is characterized by mild reaction conditions and distinct product selectivity determined by the starting materials. In the developed transformation pyruvates undergo highly selective self‐condensation reaction, whereas cross‐aldol reaction take place predominantly for their carbon chain homologues in presence of aldehyde acceptors. Notably, addition of bulky phenol moiety into pyruvate inhibit the spontaneous lactonization of products and enhance the enantioselectivity.

Synthesis of chiral 1,4-oxazepane-5-carboxylic acids from polymer-supported homoserine.

The preparation of novel 1,4-oxazepane-5-carboxylic acids bearing two stereocenters is reported in this article. Fmoc-HSe(TBDMS)-OH immobilized on Wang resin was reacted with different nitrobenzenesulfonyl chlorides and alkylated with 2-bromoacetophenones to yield N-phenacyl nitrobenzenesulfonamides. Their cleavage from the polymer support using trifluoroacetic acid (TFA) led to the removal of the silyl protective group followed by spontaneous lactonization. In contrast, TFA/triethylsilane (Et3SiH)-mediated cleavage yielded 1,4-oxazepane derivatives as a mixture of inseparable diastereomers. The regioselectivity/stereoselectivity depended on the substitution of the starting 2-bromoacetophenones and was studied in detail. Catalytic hydrogenation of the nitro group improved the separability of the resulting diastereomeric anilines, which allowed us to isolate and fully characterize the major isomers.

Identification and functional characterization of a p-coumaroyl CoA 2'-hydroxylase involved in the biosynthesis of coumarin skeleton from Peucedanum praeruptorum Dunn.

A p-coumaroyl CoA 2'-hydroxylase responsible for the formation of coumarin lactone ring was identified from Peucedanum praeruptorum Dunn and functionally characterized in vitro. Coumarins are important plant secondary metabolites with a variety of biological activities. Ortho-hydroxylation of cinnamates leads to the formation of coumarin lactone ring and is generally thought to be a key step in coumarin biosynthesis. However, ortho-hydroxylases, especially p-coumaroyl CoA 2'-hydroxylase (C2'H) responsible for the biosynthesis of the most common coumarin skeleton, have received insufficient attention. Here, a putative ortho-hydroxylase PpC2'H was isolated from P. praeruptorum Dunn, a traditional Chinese medicinal herb rich in coumarins. Expression profile indicated that PpC2'H exhibited the highest transcript level in roots and could be up-regulated by MeJA elicitation. Subcellular localization of PpC2'H was demonstrated to be cytosol in planta. In order to functionally characterize PpC2'H, the purified recombinant protein was incubated with various potential substrates. HPLC-ESI-MS analysis indicated that PpC2'H catalyzed the conversion of p-coumaroyl CoA into hydroxylated intermediate, which then underwent spontaneous lactonization to generate umbelliferone. Our data also showed that light would promote the spontaneous process. In addition, based on homology modeling and site-directed mutagenesis, amino acid residues Phe-130, Lys-141, Asn-207, His-224, Asp-226, His-282 and Phe-298 were verified essential for enzymatic activity. These findings provide insight into structure-function relationship of this pivotal ortho-hydroxylase and also contribute to elucidating the biosynthetic mechanism of coumarin skeleton.

Photoenolization of o-Methylvalerophenone Ester Derivative.

Photolysis of ester 1 in argon-saturated methanol and acetonitrile does not produce any product, whereas irradiation of 1 in oxygen-saturated methanol yields peroxide 2. Laser flash photolysis studies demonstrate that 1 undergoes intramolecular H atom abstraction to form biradical 3 (λmax ~340nm), which intersystem crosses to form photoenols Z-4 and E-4 (λmax ~380nm). Photoenols 4 decay by regenerating ester 1. With the aid of density functional theory calculations, it was concluded the photoenol E-4 does not undergo spontaneous lactonization or electrocyclic ring closure because the transition state barriers for these reactions are too large to compete with reketonization of E-4 to form 1.

A 2‐oxoglutarate‐dependent dioxygenase from Ruta graveolens L. exhibits p‐coumaroyl CoA 2′‐hydroxylase activity (C2′H): a missing step in the synthesis of umbelliferone in plants

Coumarins are important compounds that contribute to the adaptation of plants to biotic or abiotic stresses. Among coumarins, umbelliferone occupies a pivotal position in the plant phenylpropanoid network. Previous studies indicated that umbelliferone is derived from the ortho-hydroxylation of p-coumaric acid by an unknown biochemical step to yield 2,4-dihydroxycinnamic acid, which then undergoes spontaneous lactonization. Based on a recent report of a gene encoding a 2-oxoglutarate-dependent dioxygenase from Arabidopsis thaliana that exhibited feruloyl CoA 6'-hydroxylase activity (Bourgaud et al., 2006), we combined a bioinformatic approach and a cDNA library screen to identify an orthologous ORF (Genbank accession number JF799117) from Ruta graveolens L. This ORF shares 59% amino acid identity with feruloyl CoA 6'-hydroxylase, was functionally expressed in Escherichia coli, and converted feruloyl CoA into scopoletin and p-coumaroyl CoA into umbelliferone with equal activity. Its bi-functionality was further confirmed in planta: transient expression of JF799117 in Nicotiana benthamiana yielded plants with leaves containing high levels of umbelliferone and scopoletin when compared to control plants, which contained barely detectable traces of these compounds. The expression of JF799117 was also tightly correlated to the amount of umbelliferone that was found in UV-elicited R.graveolens leaves. Therefore, JF799117 encodes a p-coumaroyl CoA 2'-hydroxylase in R.graveolens, which represents a previously uncharacterized step in the synthesis of umbelliferone in plants. Psoralen, which is an important furanocoumarin in R.graveolens, was found to be a competitive inhibitor of the enzyme, and it may exert this effect through negative feedback on the enzyme at an upstream position in the pathway.

Lettuce Costunolide Synthase (CYP71BL2) and Its Homolog (CYP71BL1) from Sunflower Catalyze Distinct Regio- and Stereoselective Hydroxylations in Sesquiterpene Lactone Metabolism

Sesquiterpene lactones (STLs) are terpenoid natural products possessing the γ-lactone, well known for their diverse biological and medicinal activities. The occurrence of STLs is sporadic in nature, but most STLs have been isolated from plants in the Asteraceae family. Despite the implication of the γ-lactone group in many reported bioactivities of STLs, the biosynthetic origins of the γ-lactone ring remains elusive. Germacrene A acid (GAA) has been suggested as a central precursor of diverse STLs. The regioselective (C6 or C8) and stereoselective (α or β) hydroxylation on a carbon of GAA adjacent to its carboxylic acid at C12 is responsible for the γ-lactone formation. Here, we report two cytochrome P450 monooxygenases (P450s) capable of catalyzing 6α- and 8β-hydroxylation of GAA from lettuce and sunflower, respectively. To identify these P450s, sunflower trichomes were isolated to generate a trichome-specific transcript library, from which 10 P450 clones were retrieved. Expression of these clones in a yeast strain metabolically engineered to synthesize substrate GAA identified a P450 catalyzing 8β-hydroxylation of GAA, but the STL was not formed by spontaneous lactonization. Subsequently, we identified the closest homolog of the GAA 8β-hydroxylase from lettuce and discovered 6α-hydroxylation of GAA by the recombinant enzyme. The resulting 6α-hydroxy-GAA spontaneously undergoes a lactonization to yield the simplest form of STL, costunolide. Furthermore, we demonstrate the milligram per liter scale de novo synthesis of costunolide using the lettuce P450 in an engineered yeast strain, an important advance that will enable exploitation of STLs. Evolution and homology models of these two P450s are discussed.

Novel and Efficient One-Step Parallel Synthesis of Dibenzopyranones via Suzuki−Miyaura Cross Coupling

Microwave-promoted novel and efficient one-step parallel synthesis of dibenzopyranones and heterocyclic analogues from bromo arylcarboxylates and o-hydroxyarylboronic acids via Suzuki-Miyaura cross coupling reaction is described. Spontaneous lactonization gave dibenzopyranones and heterocyclic analogues bearing electron-donating and -withdrawing groups on both aromatic rings in good to excellent yields.

Glycomic analysis of sialic acid linkages in glycans derived from blood serum glycoproteins.

A number of alterations to the normal glycomic profile have been previously described for a number of diseases and disorders, thus underscoring the medical importance of studying the glycans associated with proteins present in biological samples. An important alteration in cancer progression is an increased level of alpha2,6-sialylation, which aids in increasing the metastatic potential of tumor cells. Here we report a glycomic method that selectively amidates alpha2,6-linked sialic acids, while those that are alpha2,3-linked undergo spontaneous lactonization. Following subsequent permethylation, MALDI-TOF MS analysis revealed that many sialylated glycans present on glycoproteins found in blood serum featured increased levels of alpha2,6-sialylation in breast cancer samples. On the basis of the altered ratios of alpha2,3-linked to alpha2,6-linked sialic acids, many of these glycans became diagnostically relevant when they did not act as such indicators when based on traditional glycomic profiling alone.

Easy and Stereoselective Approach to α,β-Unsaturated γ-Lactones Fused to Pyranoses from Furanose Scaffolds

The first facile and efficient route to pyranose-fused butenolides from furanose scaffolds, convenient for scaling up production, is described. Wittig olefination of 1,2-O-isopropylidene pentofuranos- or hexofuranos-3-uloses with a resonance-stabilized ylide led to the stereoselective formation of the (Z)-alpha,beta-unsaturated ester. In the presence of acid labile 5-O- or 5,6-di-O-protecting groups, acid hydrolysis of the Wittig product resulted in isomerization to the pyranose form and spontaneous lactonization to give the target molecules in good overall yield.

Hydrolyzable Tannins with the Hexahydroxydiphenoyl Unit and the m-Depsidic Link: HPLC-DAD-MS Identification and Model Synthesis

This study was designed to develop efficient analytical tools for the difficult HPLC-DAD-MS identification of hydrolyzable tannins in natural tissue extracts. Throughout the study of the spectroscopic characteristics of properly synthesized stereodefined standards, it was observed that the UV-vis spectra of compounds with the m-depsidic link showed a characteristic shoulder at 300 nm, consistent with the simple glucogalloyl esters, whereas compounds with the hexahydroxydiphenoyl (HHDP) unit gave a diagnostic fragmentation pattern, caused by a spontaneous lactonization in the mass spectrometer. These observations were confirmed by HPLC-DAD-MS analyses of tannic acid and raspberry extracts, which are rich in hydrolyzable tannins with the m-depsidic link and the HHDP unit, respectively.

Organic nitrates. II. Synthesis and biological activities of 4-nitrooxymethylphenyl-1,4-dihydropyridines.

Both 2-nitrooxymethyl-4-phenyl- (2) and 4-nitrooxymethylphenyl-1,4-dihydropyridines (3) represent new combinations of two different vasodilating structures. 2 could not be isolated due to its spontaneous lactonization. Derivatives of 3 were obtained via Hantzsch synthesis using nitrooxymethylated benzaldehydes. The inotropic potency in isolated porcine trabecular muscles and the vasodilator activity in isolated porcine coronary arteries of four nitrooxyphenyl-dihydropyridines were determined. Nitrendipine (NTD) and glyceryl trinitrate (GTN) were used for reference. 3 were negative inotropic, however, less than NTD and--except for the dicyano derivative 3d--more than GTN. Vasodilator properties were less pronounced than that of both nitrendipine and GTN. Vascular selectivity was low.

Ring‐chain tautomerism provides a route to 7a‐hydroxy‐3a‐methyl‐2,7‐dioxoperhydrobenzofuran. Synthesis of the hydroxy γ‐lactone substructure of myrocin and other bioactive natural products

Abstract(2‐Oxocyclohexyl)acetic acid 10 was converted into bicyclic alkoxy γ‐lactones 12a–d via intermediate chloro lactone 11 and alkanolysis. Similarly, lactones 15a, b and 18 were prepared. In contrast, simple ring‐chain tautomerism directly afforded the title heterocycle 28, a crystalline, stable and well‐defined compound. The angular methyl group seems essential for the spontaneous lactonization. 5,5‐Dialkoxy‐4‐oxocarboxylic acids 26a, b without an angular methyl group did not cyclize to bicyclic hydroxy γ‐lactones.

Purification and characterization of a novel extracellular Streptomyces lividans 66 enzyme inactivating fusidic acid

The wild-type strain Streptomyces lividans 66 is resistant against the steroid-like antibiotic fusidic acid. Comparative studies of the wild-type strain and a fusidic acid-sensitive mutant allowed the identification of an extracellular enzyme which inactivates fusidic acid. With the help of a combination of ultrafiltration and chromatographies with Phenyl-Sepharose and an anion exchanger, the enzyme was highly purified. Its apparent molecular mass is 48 kDa, its optimal activity ranges between 45 and 55 degrees C, and its optimal pH is 6.0 to 9.0. It is stimulated by neither monovalent nor divalent ions. The enzyme acts as a specific esterase which removes the acetyl group at C-16 from fusidic acid. The resulting intermediate is unstable, and spontaneous lactonization between C-21 and C-16 occurs rapidly.

Synthetic methods. 36. Utilization of ethyl 2-((phenylsulfonyl)methyl)acrylate for the synthesis of .alpha.-methylenevalerolactones

A two-step sequence is described for conversion of cyclic and acyclic ketone enolates into α-methylenevalerolactones. The first step involves Michael addition to ethyl α-((phenylsulfony)methyl)acrylate 1 with concomitant elimination of PhSO 2 and formation of unsaturated keto esters 2-7. In the next sequence chemoselective ketone reduction is usually followed by spontaneous lactonization of acidification. Contrary to the five- and six-membered ring systems, the cis-fused isomer predominates in the seven- and eight- membered ring compounds 12 and 13. Spiro α-methylenevalerolactones 17a,b are as well obtainable by a short sequence of steps from 2-oxocyclohexanecarboxylate and 1

Pilocarpine Prodrugs II. Synthesis, Stability, Bioconversion, and Physicochemical Properties of Sequentially Labile Pilocarpine Acid Diesters

Various novel diesters of pilocarpic acid were synthesized and evaluated as prodrug forms for pilocarpine with the aim of improving the ocular delivery characteristics of the drug. The pilocarpic acid monoesters previously studied cyclized spontaneously to pilocarpine in aqueous solution and although they showed enhanced corneal permeability when compared with pilocarpine these monoesters suffered from poor solution stability. The present study shows that this problem can be totally overcome by blocking the free hydroxyl group in the monoesters. Diesters of pilocarpic acid were obtained by esterification of this group. Such compounds were found to possess a high stability in aqueous solution (shelf lives of more than 5years at 20°C were estimated) but at the same time were readily converted to pilocarpine under conditions simulating those occurring in vivo through a sequential process involving enzymatic hydrolysis of the O-acyl bond followed by spontaneous lactonization of the intermediate pilocarpic acid monoester. Rate data are given for the conversion of the diesters in human plasma and in various rabbit eye homogenates. The pH–solubility profile was derived for a diester and lipophilicity parameters were determined for the compounds. All diesters were markedly more lipophilic than pilocarpine and the corresponding pilocarpic acid monoesters. The results suggest that pilocarpic acid diesters may be potentially useful pilocarpine prodrugs as they combine a high solution stability with an adequate rate of conversion to pilocarpine under in vivo conditions.

Synthesis of N-Trityl-L-homoserine

Abstract L-Homoserine was tritylated via its silylated product with Me2SiCl2 or Ph2SiCl2 in 65-68% yield. Also N-Trt-L-methionine upon treatment with Mel and degradation of the resulting sulfonium salt with KOH yielded N-Trt-L-homoserine in 75% yield. Both routes provided N-Trityl-L-homoserine with identical physical data. In contrast to N-alkyloxycarbonyl-L-homoserine derivatives, N-Trt-L-homoserine shows no tendency to spontaneous lactonization.

Synthesis of four stereoisomers of 4-amino-2-(hydroxymethyl)tetrahydrofuran-4-carboxylic acid

The 5-benzyl ether, 15, of a 1,2,4,5-pentanetetrol of known 2 S configuration was made by a multistep synthesis from d-ribose. Ring-closure of the 1- O-tosyl derivative, 17, with retention of configuration, followed by oxidation, gave the 2 S enantiomer, 22, of 2-benzyloxymethyl-4-oxotetrahydrofuran. The latter was converted by a hydantion synthesis into the 4-amino-4-carboxylic acid (mixture of 2 S,4 R and 2 S,4 S isomers, 28 and 29). Spontaneous lactonization of the 2 S,4 R diastereomer proved it to have the “cis” configuration. The remaining, 2 S,4 S diastereomer then must be “trans” it is identical with a natural compound recently isolated from an acid hydrolyzate of diabetic urine. In a parallel synthesis, the 4- O-mesyl derivative (de- O-isopropylidenated 19) was cyclized, with inversion at ring-position 2, leading after oxidation to the 2 R enantiomer, 25, of the 4-oxotetrahydrofuran. The hydantoin synthesis this time yielded a mixture of the 2 R,4 R and 2 R,4 S amino-acids. Spontaneous lactonization of the latter showed it to have the “cis” configuration. Absolute configurations were assigned to the four optically active products, based on the known absolute configuration of d-ribose and the known mechanisms of the synthetic reactions.

Cyclic unsaturated compounds Communication 51. direction of lactonization of bicyclo[2.2.1]-2-heptene-5,6-dicarboxylic acids

1. A study was made of the NMR spectra of some methyl-substituted lacto esters that are derivatives of the acids that are formed in the spontaneous lactonization of the correspondingbicyclo[2.2.1]-2-heptene-5.6-dicarboxylic acids. 2. In the mentioned acids with a monosubstituted double bond the closure of the γ-lactone ring is accomplished at the C2-center (containing the substituent). In the acids with a disubstituted double bond the substituent at C3 in the formed lactones assumes an exo-configuration. 3. Additional proof was obtained for the endo-configuration of the anhydride ring in the adducts of substituted cyclopentadienes with maleic anhydride, and in the case of the 5-substituted cyclopentadienes, for the syn-configuration of the substituent at the C7-center (at the bridge) in the adducts that predominate in the corresponding mixtures of the stereoisomers.