Acid Rearrangement Research Articles

B-norsteroids from Hymenoscyphus pseudoalbidus

Two viridin-related B-norsteroids, B-norviridiol lactone (1) and B-norviridin enol (2), both possessing distinct unprecedented carbon skeletons, were isolated from a liquid culture of the ash dieback-causing fungus Hymenoscyphus pseudoalbidus. Compound 2 was found to degrade to a third B-norsteroidal compound, 1β-hydroxy-2α-hydro-asterogynin A (3), which was later detected in the original culture. The proposed structure of 1 is, regarding connectivity, identical to the original erroneous structure for TAEMC161, which was later reassigned as viridiol. Compound 2 showed an unprecedented 1H–13C HMBC correlation through an intramolecular hydrogen bond. The five-membered B-ring of compounds 1-3 was proposed to be formed by a benzilic acid rearrangement. The known compound asterogynin A was found to be formed from 3 by a β-elimination of water. All compounds were characterized by NMR spectroscopy, LC-HRMS and polarimetry.

Access to new sequiterpenoids by catalytic acid rearrangement of 9α-hydroxyparthenolide

The catalytic acid rearrangement of 9α-hydroxyparthenolide (1), the main constituent of Anvillea radiata, has been studied. Several new sesquiterpene compounds were obtained and their structures were fully elucidated. A mechanism to explain the formation of these compounds is proposed.

Domino Reactions Consisting of Heterocyclization and 1,2‐Migration—Redox‐Neutral and Oxidative Transition‐Metal Catalysis

Two cats, two paths: two novel domino reactions starting from 6-hydroxy-2-alkyl-2-alkynylcyclohexanones have been discovered. While redox-neutral platinum catalysis gives rise to furans through a sequence of cyclization, 1,2-shift, and Grob fragmentation, oxidative copper catalysis provides an entry to bicyclic 2,3-dihydrofurans. Upon cyclization and oxidation, an unusual benzilic acid rearrangement can take place in this case.

First insights into the mode of action of a “lachrymatory factor synthase” – Implications for the mechanism of lachrymator formation in Petiveria alliacea, Allium cepa and Nectaroscordum species

A study of an enzyme that reacts with the sulfenic acid produced by the alliinase in Petiveria alliacea L. (Phytolaccaceae) to yield the P. alliacea lachrymator (phenylmethanethial S-oxide) showed the protein to be a dehydrogenase. It functions by abstracting hydride from sulfenic acids of appropriate structure to form their corresponding sulfines. Successful hydride abstraction is dependent upon the presence of a benzyl group on the sulfur to stabilize the intermediate formed on abstraction of hydride. This dehydrogenase activity contrasts with that of the lachrymatory factor synthase (LFS) found in onion, which catalyzes the rearrangement of 1-propenesulfenic acid to ( Z)-propanethial S-oxide, the onion lachrymator. Based on the type of reaction it catalyzes, the onion LFS should be classified as an isomerase and would be called a “sulfenic acid isomerase”, whereas the P. alliacea LFS would be termed a “sulfenic acid dehydrogenase”.

A Practical Asymmetric Synthesis of Isopropyl (1R,2S)-Dehydrocoronamate

A novel asymmetric synthesis of isopropyl (1R,2S)-dehydrocoronamate is described from (S)-1,2,4-butanetriol as the starting material in 28% overall yield. Highlights of this synthetic route include selective cyclopropanation between chiral cyclic sulfate 5 and diisopropyl malonate (8c), formation of vinylcyclopropane 3c via elimination of halide 4c, selective monohydrolysis of diisopropyl ester 3c, and Curtius rearrangement of acid 10 to form isopropyl (1R,2S)-dehydrocoronamate TsOH salt 13 in >99% ee. With the involvement of only three isolations, this chromatography-free process provides a rapid and practical access to (1R,2S)-1-amino-2-vinylcyclopropane-1-carboxylic acid derivatives.

Specific Basic and Aromatic Residues Optimize the Nucleic Acid Chaperone Activity of HIV-1 Nucleocapsid Protein

As an efficient nucleic acid chaperone protein, the human immunodeficiency virus type 1 (HIV-1) nucleocapsid (NC) protein facilitates the rearrangement of nucleic acid secondary structure during reverse transcription and recombination. The biophysical basis for NC's chaperone activity includes the ability to strongly aggregate nucleic acids, destabilize nucleic acid secondary structure, and facilitate rapid protein-nucleic acid interaction kinetics. HIV-1 NC contains only 55 amino acids, with 11 basic residues throughout its structure as well as two zinc fingers, each having a single aromatic residue (F16 and W37), which is highly conserved among retroviral NC proteins. Despite its simple structure, HIV-1 NC facilitates nucleic acid rearrangement efficiently. To determine the mechanism behind this high efficiency, we performed single-molecule DNA stretching experiments in the presence of wild-type and mutant HIV-NC. We measured the ability of the proteins to aggregate and destabilize dsDNA and determined their ability to facilitate nucleic acid annealing. We show that mutant F16W and F16W-W37F exhibit similar properties as wild-type NC. In contrast, the single aromatic substitutions W37A and F16A significantly reduce NC's rapid DNA interaction kinetics, while retaining a portion of its dsDNA destabilization capability. Furthermore, removing both aromatics leads to a complete loss of chaperone activity. Thus, we can conclude that both aromatic residues significantly contribute to HIV-1 NC's optimal chaperone activity. Moreover, experiments on basic residue mutants show that positively charged residues play a very important role in protein binding. While mutating basic residues in any region of the protein significantly reduces reannealing kinetics, residues in the zinc fingers and in the N-terminus are particularly important for rapid protein-nucleic acid interaction kinetics. This work was funded in part by Federal Funds from NCI, NIH under contract N01-CO-12400 (RJG).

Rapid and highly selective conversion of biomass into value-added products in hydrothermal conditions: chemistry of acid/base-catalysed and oxidation reactions

An overview of recent advances in research on selective conversion of carbohydrates into products with hydrothermal chemistry is given. Conversion methods tend to use acid- or base-catalysed reactions and oxidation reactions. Conversion products that can be selectively produced include acetic acid, formic acid, lactic acid, levulinic acid, 5-hydroxymethyl-2-furaldehyde (HMF) and 2-furaldehyde (2-FA). Future conversion paths and possible new products that can be formed with hydrothermal chemistry include aldol/reverse condensation, dehydration, benzilic acid rearrangement, keto-enol tautomerization, Lobry de Bruyn–Alberda van Ekenstein transformation and oxidation reactions.

Non-truncating hMLH1 variants identified in Slovenian gastric cancer patients are not associated with Lynch Syndrome: a functional analysis report

Hereditary non-polyposis colorectal cancer is the most common known genetic syndrome that predisposes to various types of cancer including gastric cancer and occures mainly due to pathogenic germline mutations in DNA mismatch repair (MMR) genes, such as MLH1, MSH2 and MSH6. Impaired MMR activity can lead to microsatellite instability (MSI) in tumor tissues. Interpreting the pathogenic significance of identified mutations in MMR genes, especially missense alterations and short in-frame deletions and insertions is challenging and functional analysis is often needed to accurately assess their pathogenicities. The purpose of this study was to evaluate functional significance of MLH1 missense mutations, previously identified in unrelated Slovenian patients with MSI-positive gastric carinomas. A novel in vivo yeast-based approach and in silico predictions were used. Variant E433Q was characterized for the first time and was shown to have no effect on MLH1 protein function. Functional analysis of amino acid rearrangement K618A, with previously reported contradictory results of its pathogenicity, suggests that the variant is a neutral polymorphism. Results of our study imply that there is either germline mutation or an epigenetic inactivation of another MMR gene, which causes MSI phenotype in the referred gastric cancer cases.

First Stereoselective Synthesis of Penicillenol A1 via Novel O- to C-Acyl Rearrangement of O-Acyltetramic Acid

The first stereoselective synthesis of penicillenol A1 has been accomplished in nine steps from l-threonine. The 3-acyltetramic acid core of penicillenol A1 was constructed by successful O- to C-acyl rearrangement.

A Synthetic Entry into the Taiwaniaquinoids Based on a Biogenetic Hypothesis: Total Synthesis of (−)‐Taiwaniaquinone H

(Chemical Equation Presented) Less carbon: Following a biogenetic proposal, the unusual 6-5-6 carbon skeleton of C19 taiwaniaquinoids was obtained by an intramolecular benzilic acid rearrangement from a C20 precur-sor. A protecting-group-free total synthesis then allowed for the preparation of (-)-taiwaniaquinone H (see scheme). © 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Connecting C19 Norditerpenoids to C20 Diterpenes: Total Syntheses of 6-Hydroxy-5,6-dehydrosugiol, 6-Hydroxysugiol, and Taiwaniaquinone H, and Formal Synthesis of Dichroanone

Oxidation of the B-ring of abietane derivatives by Sharpless­ dihydroxylation gave the natural products 6-hydroxy-5,6-dehydrosugiol and 6-hydroxysugiol. Moreover, further oxidation gave a hydroxy dione derivative that provides a synthetic entry into the C19 taiwaniaquinoid family of natural products. This route is based on biosynthetic considerations and involves a benzilic acid rearrangement followed by decarboxylation. On the basis of this approach, a total synthesis of (-)-taiwaniaquinone H and a formal synthesis of (-)-dichroanone have been achieved.

Novel Thione-Thiol Rearrangement of Dithiocarbonic Acid O-(2,2,6,6-Tetramethylpiperidin-1-yl) Ester in the Context of Controlled Radical Polymerization

Addition of the lithium salt of N-hydroxy-2,2,6,6-tetramethylpiperidine 10 to carbon disulfide and subsequent methylation affords the rearranged dithiocarbonic acid S-methyl-S'-(2,2,6,6-tetramethylpiperidin-1-yl)-ester 9 rather than the expected xanthate ester S-methyl-O'-(2,2,6,6-tetramethylpiperidin-1 -yl)-ester 8. n-Butylacrylate could be polymerized with the novel compound 9 even though the polymerization was not controlled.

Ultrafast time-resolved transient infrared and resonance Raman spectroscopic study of the photo-deprotection and rearrangement reactions of p-hydroxyphenacyl caged phosphates

This is a picosecond and nanosecond time-resolved infrared absorption spectroscopic study of p-hydroxphenacyl diethylphosphate (HPDP) in aqueous solution examining the deprotection reaction and subsequent evolution to produce the p-hydroxyphenylacetic acid (HPAA) rearrangement product. The data reported here support previous ultrafast fluorescence, Raman and transient UV/visible absorption studies and provide new insight into the role of water in the deprotection reaction. It also provides insight into the nature of the intermediate steps that take place following initial formation of the triplet state and its reaction with water to eventually produce the solvent-separated ions and solvolytic reaction product HPAA. In addition, density functional theory (DFT) calculations were performed to investigate the reactions of the triplet state of HPDP with water molecules to induce deprotection, the formation of a contact ion pair and the subsequent intermediates that lead to the HPAA rearrangement product. A more detailed reaction mechanism is proposed based on the time-resolved spectroscopy experiments and DFT computational results.

Chemical synthesis of 3β-sulfooxy-7β-hydroxy-24-nor-5-cholenoic acid: An internal standard for mass spectrometric analysis of the abnormal Δ 5-bile acids occurring in Niemann-Pick disease

In Niemann-Pick disease, type C1, increased amounts of 3β,7β-dihydroxy-5-cholenoic acid are reported to be present in urinary bile acids. The compound occurs as a tri-conjugate, sulfated at C-3, N-acetylglucosamidated at C-7, and N-acylamidated with taurine or glycine at C-24. For sensitive LC–MS/MS analysis of this bile acid, a suitable internal standard is needed. We report here the synthesis of a satisfactory internal standard, 3β-sulfooxy-7β-hydroxy-24-nor-5-cholenoic acid (as the disodium salt). The key reactions involved were (1) the so-called “second order” Beckmann rearrangement (one-carbon degradation at C-24) of hyodeoxycholic acid (HDCA) 3,6-diformate with sodium nitrite in a mixture of trifluoroacetic anhydride and trifluoroacetic acid, (2) simultaneous inversion at C-3 and elimination at C-6 of the ditosylate derivatives of the resulting 3α,6α-dihydroxy-24-nor-5β-cholanoic acid with potassium acetate in aqueous N, N-dimethylformamide, and (3) regioselective sulfation at C-3 of an intermediary 3β,7β-dihydroxy-24-nor-Δ 5 derivative using sulfur trioxide–trimethylamine complex. Overall yield of the desired compound was 1.8% in 12 steps from HDCA.

Synthesis and pharmacological evaluation of 5-carboxamide-substituted tetrahydrochromeno[7,8- d]imidazoles

A fast access to novel 5-carboxamide-substituted tetrahydrochromeno[7,8- d]imidazoles 4 was developed using the readily available preclinical candidate BYK 405879 ( 1) as starting material. The 5-amino function was installed by the Curtius rearrangement of carboxylic acid 2 or by the Hofmann rearrangement of carboxamide 8 furnishing benzimidazole 3 as key intermediate. In the Ghosh Schild rat, some of the target compounds 10– 14 showed noteworthy activity as potassium-competitive acid blockers.

Total Synthesis of Meloscine by a [2+2]‐Photocycloaddition/Ring‐Expansion Route

The unusual monoterpenoid indole alkaloid meloscine was synthesized starting from a protected aminoethylquinolone in 15 steps and an overall yield of 9%, employing a [2+2]-photocycloaddition as the stereochemistry defining key step. After the initial plan of a Wagner-Meerwein type rearrangement of a [4.2.0]- into a [3.3.0]-bicyclic substructure could not be realized, the required ring enlargement of a cyclobutane was eventually achieved by a retro-benzilic acid rearrangement. Generation of the central pyrrolidine ring was possible by a three-step reductive amination domino sequence. The final ring was built up by a ring-closing metathesis after the last quaternary stereocenter had been constructed by a Johnson-Claisen rearrangement. The synthesis was concluded by a selenylation-elimination sequence to build up the exocyclic vinyl group of meloscine. Using our methodology for enantioselective [2+2]-photocycloaddition mediated by a chiral complexation agent, the experimentally very simple synthesis could be performed in an enantioselective fashion (7 % overall yield). The enantioselective synthesis of (+)-meloscine represents the first example of a natural product synthesis employing an enantioselective [2+2]-photocycloaddition as its key step, and illustrates nicely the synthetic potential of photochemical transformations for the construction of complex heterocyclic structures.

Regioselective glycosylation of 7-azapteridines and conversion of the 7-azapteridine nucleosides into 6-azapurine nucleosides

The regioselective glycosylation of reumycins (3) reacted with 1-O-acetyl-2,3,5-tri-O-benzoyl-beta-D-ribofuranose (4) and BSTFA in acetonitrile at 90 degrees C followed by reaction of SnCl(4) in dioxane at room temperature afforded the 1-(2',3',5'-tri-O-benzoyl-beta-D-ribofuranosyl)- 6-methylpyrimido[5,4-e][1,2,4]triazine-5,7(1H,6H)-diones (5) (toxoflavin type nucleosides), while the similar alkylations with 1-bromo-2,3,5-tri-O-benzoyl-beta-D-ribofuranose (6) and KHCO(3) in DMF at 100 degrees C gave predominantly the 8-(2',3',5'-tri-O-benzoyl-beta-D-ribofuranosyl)-6-methylpyrimido[5,4-e]-[1,2,4]triazine-5,7(6H,8H)-diones (7) (fervenulin type nucleosides). On the other hand, treatment of the 7-azapteridine nucleosides (5 and 7) in alkali solution at room temperature yielded the corresponding 1-(beta-D-ribofuranosyl)-5-methyl-1H-imidazo[4,5-e][1,2,4]- triazin-6(5H)-ones (8) and 7-(beta-D-ribofuranosyl)-5-methyl-5H-imidazo[4,5-e][1,2,4]triazin-6(7H)-ones (9) [6-azapurine nucleosides] by benzilic acid rearrangement. Some 7-azapteridine nucleosides (5 and 7-9) showed antitumor activities and anti-coccidiosis activities.

Cyclobutane Ring Opening Reactions of 1,2,2a,8b-Tetrahydrocyclobuta[c]-quinolin-3(4H)-ones

Two pathways are presented, which allow for the selective cleavage of the C1/C8b bond in the title compounds. The first reaction was found by serendipity when reductive debenzylation was attempted on a heteroanellated azabicyclo[4.2.0]octane. It yielded an eight-membered lactam ring (azocane). The second transformation relies on a rationally designed 1,2-rearrangement reaction, which was introduced because attempts to induce classical Wagner-Meerwein-type 1,2-shifts did not meet with success. The reaction, a retro-benzilic acid rearrangement, allowed for the conversion of a heteroanellated hydroxycyclobutanecarboxylate into the respective cyclopentadione (76% yield).

Enantioselective Total Synthesis of the Melodinus Alkaloid (+)‐Meloscine

Enantioselective synthesis in a new light: The template-controlled [2+2] photocycloaddition leading to product 1 is the first example of this type of reaction in natural product synthesis. In addition, a retro-benzilic acid rearrangement (→2), a Claisen rearrangement (→3), and a ring-closing metathesis played decisive roles in the synthesis of the alkaloid (+)-meloscine (4).

Nonvolatile Oxidation Products of Glucose in Maillard Model Systems: Formation of Saccharinic and Aldonic Acids and Their Corresponding Lactones

By using pyrolysis-gas chromatography-mass spectrometry-based methodologies, nonvolatile oxidation products of isotopically labeled glucose/glycine model systems were studied through a postpyrolytic in situ derivatization technique by using trimethylsilyldiethylamine. Analysis of the data indicated that the known reactive sugar intermediates such as glucosone and its deoxy derivatives can undergo in Maillard model systems three types of transformations: oxidation of the aldehydic groups into carboxylic acids, oxidative cleavage of alpha-dicarbonyl moieties into aldonic acids, and benzylic acid rearrangement of 1-deoxy-glucosone into saccharinic acids. The aldonic and saccharinic acids were identified through silylation of their lactone derivatives, and their origin was verified through (13)C-labeling studies. The following lactones were identified in glucose and glucose/glycine model systems: trans-dihydro-3,4-bis[(trimethylsilyl)oxy]-2(3 H)-furanone, cis-dihydro-3,4-bis[(trimethylsilyl)oxy]-2(3H)-furanone, 2-C-methyl-2,3,5-tris-O-(trimethylsilyl)-D-ribonic acid gamma-lactone, 3-deoxy-2,5,6-tris-O-(trimethylsilyl)-D-ribo-hexonic acid gamma-lactone, 2-deoxy-3,5-bis-O-(trimethylsilyl)-pentonic acid gamma-lactone, and 2,3,5-tris-O-(trimethylsilyl)-D-arabinonic acid gamma-lactone. The observed reduction in color and aroma in Maillard reactions performed under oxidative conditions may be attributed to the oxidation of reactive dicarbonyls into the corresponding carboxylic acids or their corresponding lactones.