Total Synthesis of (−)-7-Epicylindrospermopsin, a Toxic Metabolite of the Freshwater Cyanobacterium Aphanizomenon ovalisporum, and Assignment of Its Absolute Configuration

Absolute Configuration of 4-Chloroisoleucine in Cyclolithistide A: An Antifungal Peptide Lactone Discovered in Marine Lithistid Sponges

Cyclolithistide A is a peptide lactone isolated from marine lithistid sponges. Its entire structure, including absolute configurations, has been reported except the relative and absolute configurations of its characteristic residue, 4-chloroisoleucine (4-CIle). We synthesized four isomers of 4-CIle from furfural-derived N-Boc imine and propionaldehyde. Analysis of the acid hydrolysate of cyclolithistide A and the synthetic samples of 4-CIle after derivatization with l- and d-FDAA permitted us to propose the absolute configuration of the 4-chloroisoleucine residue in cyclolithistide A as 2S,3R,4R.

Synthesis of pyrimidine-containing alkaloids

This review deals with the synthesis of naturally occurring alkaloids containing partially or completely saturated pyrimidine nuclei. The interest in these compounds is associated with their structural diversity, high biological activity and toxicity. The review is divided into four parts, each of which describes a number of synthetic methodologies toward structurally different naturally occurring alkaloids containing saturated cyclic six-membered amidine, guanidine, aminal and urea (thiourea) moieties, respectively. The development of various synthetic strategies for the preparation of these compounds has remarkably increased during the past few decades. This is primarily due to the fact that some of these compounds are isolated only in limited quantities, which makes it practically impossible to study their full structural characteristics and biological activity.

Formal total synthesis of (±)-7-deoxycylindrospermopsin and its 8-epi isomer

A concise formal synthesis of (±)-7-deoxycylindrospermopsin in 14 steps was reported.

Short Total Synthesis of [15N5]-Cylindrospermopsins from 15NH4Cl Enables Precise Quantification of Freshwater Cyanobacterial Contamination

Fresh water cyanobacterial algal blooms represent a major health risk because these organisms produce cylindrospermopsin, a toxic, structurally complex, zwitterionic uracil-guanidine alkaloid recognized by the EPA as a dangerous drinking water contaminant. At present, the ability to detect and quantify the presence of cylindrospermospin in water samples is severely hampered by the lack of an isotopically labeled standard for analytical mass spectrometry. Herein, we present a concise, scaled total synthesis of 15N cylindrospermosin from 15N ammonium chloride, which leverages a unique stereoselective intramolecular double conjugate addition step to assemble the tricyclic guanidine core. In addition to providing the first pure isotopically labeled probe for precise quantification of this potent biotoxin in fresh water sources, our results demonstrate how unique constraints associated with isotope incorporation compel novel solutions to synthesis design.

Asymmetric Petasis Borono-Mannich Allylation Reactions Catalyzed by Chiral Biphenols

Chiral biphenols catalyze the asymmetric Petasis borono-Mannich allylation of aldehydes and amines through the use of a bench-stable allyldioxaborolane. The reaction proceeds via a two-step, one-pot process and requires 2-8 mole % of 3,3'-Ph2 -BINOL as the optimal catalyst. Under microwave heating the reaction affords chiral homoallylic amines in excellent yields (up to 99 %) and high enantioselectivies (er up to 99:1). The catalytic reaction is a true multicomponent condensation reaction whereas both the aldehyde and the amine can possess a wide range of structural and electronic properties. Use of crotyldioxaborolane in the reaction results in stereodivergent products with anti- and syn-diastereomers both in good diastereoselectivities and enantioselectivities from the corresponding E- and Z-borolane stereoisomers.

Interpretation of the longitudinal (13)C nuclear spin relaxation and chemical shift data for five bromoazaheterocycles supported by nonrelativistic and relativistic DFT calculations

The longitudinal relaxation times of (13)C nuclei and NOE enhancement factors for 2-bromopyridine (1), 6-bromo-9-methylpurine (2), 3,5-dibromopyridine (3), 2,4-dibromopyrimidine (4), and 2,4,6-tribromopyrimidine (5) have been measured at 25 °C and B0 = 11.7 T. The most important contributions to the overall relaxation rates of nonbrominated carbons, i.e., the relaxation rates due to the (13)C-(1)H dipolar interactions and the shielding anisotropy mechanism, have been separated out. For 3 and 5, additionally, the T2,Q((14)N) values have been established from (14)N NMR line widths. All of these data have been used to determine rotational diffusion tensors for the investigated molecules. The measured saturation recovery curves of brominated carbons have been decomposed into two components to yield relaxation times, which after proper corrections provided parameters characterizing the scalar relaxation of the second kind for (13)C nuclei of (79)Br- and (81)Br-bonded carbons. These parameters and theoretically calculated quadrupole coupling constants for bromine nuclei have allowed the values of one-bond (13)C-(79)Br spin-spin coupling constants to be calculated. Independently, the coupling constants and magnetic shielding constants of the carbon nuclei have been calculated theoretically using the nonrelativistic and relativistic DFT methods F/6-311++G(2d,p)/PCM and so-ZORA/F/TZ2P/COSMO (F = BHandH or B3LYP), respectively. The agreement between the experimental and theoretical values of these parameters is remarkably dependent on the theoretical method used.

Direct stereoselective synthesis of enantiomerically pure anti-β-amino alcohols

Enantiomerically pure anti-β-amino alcohols were synthesized from optically pure α-(N,N-dibenzylamino)benzyl esters, derived from α-amino acids, by the sequential reduction to aldehyde with DIBAL-H at -78 °C and subsequent in situ addition of Grignard reagents. Besides anti-β-amino alcohols, anti-2-amino-1,3-diols and anti-3-amino-1,4-diols were obtained in good yields (60-95%) and excellent stereoselectivity (de > 95%). Our technique is compatible with free hydroxyl groups present in the substrate. To demonstrate the versatility of the method, spisulosine and sphinganine were synthesized in two steps from the appropriate N,N-dibenzyl-l-aminobenzyl ester in 42% and 45% yield, respectively.

Preparation of an ABC tricyclic model of the cylindrospermopsin alkaloids via a biomimetically inspired pathway

The protected guanidine18was convertedviaa 4-step sequence (Boc-deprotection, tethered Biginelli condensation, deallylation/decarboxylation and reduction) to give the tricyclic guanidines26and27, models of the tricyclic core of cylindrospermopsin1.

A review on cylindrospermopsin: the global occurrence, detection, toxicity and degradation of a potent cyanotoxin

Cylindrospermopsin is an important cyanobacterial toxin found in water bodies worldwide. The ever-increasing and global occurrence of massive and prolonged blooms of cylindrospermopsin-producing cyanobacteria poses a potential threat to both human and ecosystem health. Its toxicity is associated with metabolic activation and may involve mechanisms that adversely affect a wide variety of targets in an organism. Cylindrospermopsin has been shown to be cytotoxic, dermatotoxic, genotoxic, hepatotoxic in vivo, developmentally toxic, and may be carcinogenic. Human exposure may occur through drinking water, during recreational activities and by consuming foods in which the toxin may have bioaccumulated. Drinking water shortages of sufficient quality coupled with growing human pressures and climate variability and change necessitate an integrated and sustainable water management program. This review presents an overview of the importance of cylindrospermopsin, its detection, toxicity, worldwide distribution, and lastly, its chemical and biological degradation and removal by natural processes and drinking water treatment processes.

Use of sulfur fragments for the synthesis of nitrogen heterocycles

This contribution contains a representative sampling from the Padwa laboratory of the conjugate addition of oximes with 2,3-bis(phenylsulfonyl)-1,3-butadiene followed by a subsequent dipolar cycloaddition cascade to produce a variety of alkaloids. The resulting cycloadducts are cleaved reductively to provide azapolycyclic scaffolds with strategically placed functionality for further manipulation of the target compounds.

Synthesis of alkaloid (-)-205B via stereoselective reductive cross-coupling and intramolecular [3+2] cycloaddition

An asymmetric synthesis of alkaloid (-)-205B, a tricyclic member of the architecturally diverse family of natural products isolated from the skin of neotropical poison frogs, is described that proceeds through two recently developed stereoselective synthetic methods: (1) Ti-mediated allylic alcohol-imine reductive cross-coupling and (2) intramolecular [3+2] cycloaddition of a glyoxylate-based homoallylic nitrone. The utility of this latter cycloaddition process for the assembly of the stereochemically dense piperidine core of 205B is noteworthy, as this method enables direct [3+2] cycloaddition of an intermediate homoallylic (E)-nitrone via a pathway that is stereochemically unscathed by competitive [3,3]-sigmatropic rearrangement processes. Overall, the synthesis is asymmetric, concise, and highly stereoselective-features which point to the potential future utility of these chemical methods in natural product synthesis and medicinal chemistry.

Synthetic Routes to a Series of Proximal and Distal 2'-Deoxy Fleximers

Two series of innovative 2'-deoxy nucleoside analogues have been designed where the nucleobase has been split into its imidazole and pyrimidine subunits. This structural modification serves to introduce flexibility into the nucleobase scaffold while still retaining the elements required for recognition. The synthetic efforts to realize these analogues are described within.

Diastereo- and enantioselective ruthenium-catalyzed hydrohydroxyalkylation of 2-silyl-butadienes: carbonyl syn-crotylation from the alcohol oxidation level

Exposure of alcohols 2a-2j to 2-silyl-butadienes in the presence of ruthenium complexes modified by (R)-SEGPHOS or (R)-DM-SEGPHOS results in redox-triggered generation of allylruthenium-aldehyde pairs, which combine to form products of carbonyl crotylation 4a-4j in the absence of stoichiometric byproducts and with high levels of syn-diastereo- and enantioselectivity. In the presence of isopropanol under otherwise identical conditions, aldehydes 3a-3j are converted to an equivalent set of adducts 4a-4j. Whereas reactions conducted using conventional heating require 48 h, microwave irradiation enables full conversion in only 4 h. Finally, as illustrated in the conversion of adduct 4a to compounds 6a and 6b, diastereoselective hydroboration-Suzuki cross-coupling with aryl and vinyl halides followed by Fleming-Tamao oxidation enables generation of anti,syn-stereotriads found in numerous polyketide natural products.

Iridium Catalyzed Hydro-hydroxyalkylation of Butadiene: Carbonyl Crotylation

Exposure of alcohols 1a-1i to butadiene in the presence of a cyclometallated iridium catalyzed derived from allyl acetate, 4-methoxy-3-nitrobenzoic acid and BIPHEP (2,2'-bis(diphenylphosphino)biphenyl) results in hydrogen transfer to generate aldehyde-allyliridium pairs, which engage in C-C coupling to form products of carbonyl crotylation. Under related conditions using 1,4-butanediol as hydrogen donor, butadiene reductively couples to aldehydes 2e-2g and 2i to furnish carbonyl crotylation products 3e-3g and 3i. Thus, butadiene mediated carbonyl crotylation occurs with equal facility from the alcohol or aldehyde oxidation level with complete levels of branched regioselectivity.

Organozinc generation via the titanium-catalyzed activation of alkyl halides

A protocol for the generation of organozinc reagents via catalytic activation of alkyl halides is described herein. Subsequent nucleophilic addition to carbonyl derivatives provided the desired products in good to excellent yields (76-99%). Evidence suggests that titanocene dichloride catalyzes the formation of an organozinc species. This discovery will have wide ranging applicability in the generation of highly reactive organometallic reagents.

Enantioselective iridium-catalyzed carbonyl allylation from the alcohol oxidation level via transfer hydrogenation: minimizing pre-activation for synthetic efficiency

Existing methods for enantioselective carbonyl allylation, crotylation and tert-prenylation require stoichiometric generation of pre-metallated nucleophiles, and often employ stoichiometric chiral modifiers. Under the conditions of transfer hydrogenation employing an ortho-cyclometallated iridium C,O-benzoate catalyst, enantioselective carbonyl allylations, crotylations and tert-prenylations are achieved in the absence of stoichiometric metallic reagents or stoichiometric chiral modifiers. Moreover, under transfer hydrogenation conditions, primary alcohols function dually as hydrogen donors and aldehyde precursors, enabling enantioselective carbonyl addition directly from the alcohol oxidation level.

1,n-glycols as dialdehyde equivalents in iridium-catalyzed enantioselective carbonyl allylation and iterative two-directional assembly of 1,3-polyols

Unstable? We're able! 1,n-Glycols serve as synthetic equivalents to unstable dialdehydes in two-directional carbonyl allylation from the alcohol oxidation level under iridium-catalyzed transfer hydrogenation conditions. Iterative asymmetric allylation employing 1,3-propanediol enables the rapid assembly of protected 1,3-polyol substructures with exceptional levels of stereocontrol.

Rearrangement of beta-amino alcohols via aziridiniums: a review

This tutorial review focuses on the rearrangement of beta-amino alcohols via aziridinium intermediates. It covers the literature from 1947 to January 2009 (55 references). The rearrangement of beta-amino alcohols can be performed by activation of the hydroxy group followed by the addition of nucleophiles (Nu). In most examples, an aziridinium intermediate is involved in the rearrangement. The ratio of amines resulting from the attack of nucleophiles at either the C-1 or C-2 position of the aziridinium intermediate, depends on the nature of the nucleophiles and the R(2) substituent. In some cases, solvent as well as temperature can influence the ratio of amines.

Enantioselective iridium-catalyzed carbonyl allylation from the alcohol or aldehyde oxidation level via transfer hydrogenative coupling of allyl acetate: departure from chirally modified allyl metal reagents in carbonyl addition

Under the conditions of transfer hydrogenation employing an iridium catalyst generated in situ from [Ir(cod)Cl]2, chiral phosphine ligand (R)-BINAP or (R)-Cl,MeO-BIPHEP, and m-nitrobenzoic acid, allyl acetate couples to allylic alcohols 1a-c, aliphatic alcohols 1d-l, and benzylic alcohols 1m-u to furnish products of carbonyl allylation 3a-u with exceptional levels of asymmetric induction. The very same set of optically enriched carbonyl allylation products 3a-u are accessible from enals 2a-c, aliphatic aldehydes 2d-l, and aryl aldehydes 2m-u, using iridium catalysts ligated by (-)-TMBTP or (R)-Cl,MeO-BIPHEP under identical conditions, but employing isopropanol as a hydrogen donor. A catalytically active cyclometallated complex V, which arises upon ortho-C-H insertion of iridium onto m-nitrobenzoic acid, was characterized by single-crystal X-ray diffraction. The results of isotopic labeling are consistent with intervention of symmetric iridium pi-allyl intermediates or rapid interconversion of sigma-allyl haptomers through the agency of a symmetric pi-allyl. Competition experiments demonstrate rapid and reversible hydrogenation-dehydrogenation of the carbonyl partner in advance of C-C coupling. However, the coupling products, which are homoallylic alcohols, experience very little erosion of optical purity by way of redox equilibration under the coupling conditions, although isopropanol, a secondary alcohol, may serve as terminal reductant. A plausible catalytic mechanism accounting for these observations is proposed, along with a stereochemical model that accounts for the observed sense of absolute stereoinduction. This protocol for asymmetric carbonyl allylation transcends the barriers imposed by oxidation level and the use of preformed allyl metal reagents.

Asymmetric allylboration of acyl imines catalyzed by chiral diols

Chiral BINOL-derived diols catalyze the enantioselective asymmetric allylboration of acyl imines. The reaction requires 15 mol % (S)-3,3'-Ph2-BINOL as the catalyst and allyldiisopropoxyborane as the nucleophile. The reaction products are obtained in good yields (75-94%) and high enantiomeric ratios (95:5-99.5:0.5) for aromatic and aliphatic imines. High diastereoselectivities (diastereomeric ratio > 98:2) and enantioselectivities (enantiomeric ratio > 98:2) are obtained in the reactions of acyl imines with crotyldiisopropoxyboranes. This asymmetric transformation is directly applied to the synthesis of Maraviroc, the selective CCR5 antagonist with potent activity against HIV-1 infection. Mechanistic investigations of the allylboration reaction including IR, NMR, and mass spectrometry studies indicate that acyclic boronates are activated by chiral diols via exchange of one of the boronate alkoxy groups with activation of the acyl imine via hydrogen bonding.

Chemo- and Regioselective Functionalization of Uracil Derivatives. Applications to the Synthesis of Oxypurinol and Emivirine

[reaction: see text] A novel route for the synthesis of 4,5-difunctionalized uracils using a chemo- and regioselective bromine/magnesium exchange reaction on 5-bromo-4-halogeno-2,6-dimethoxypyrimidines has been developed. Applications to the synthesis of pharmaceuticals such as oxypurinol and emivirine are reported.

Cyanobacterial (blue-green algal) toxins in water supplies: Cylindrospermopsins

The toxic alkaloid cylindrospermopsin is produced by a range of cyanobacterial species worldwide. It was first identified in the species Cylindrospermopsis raciborskii from tropical waters, and has since been isolated from four other genera in locations ranging from Israel to Japan. High concentrations of the organisms and toxin have been identified in reservoirs, natural lakes, and rivers in summer in the USA and in Australia. The toxin is a particular problem in drinking water sources as concentrations in the free water are appreciable, so that removal of the filaments during water treatment does not remove the toxin. The toxicity resulting from oral ingestion is seen in the liver, kidneys, stomach, intestine, and white blood cells, with some vascular damage in mice. Gastrointestinal as well as liver injury has been observed in human poisoning. Studies of toxicity in vitro have shown inhibition of protein synthesis. Genotoxicity has also been demonstrated, and there is preliminary evidence for carcinogenicity. A Guideline Value for safe water supply of 1 microg/L has been proposed. Research into toxin measurement techniques and water treatment methods has indicated that effective control measures may be practicable for this toxin in drinking water. Considerably more research is needed to fully define the health risks from this toxin.

Metabolites produced by cyanobacteria belonging to several species of the familyNostocaceae

This paper provides a comprehensive overview of metabolites, including lipids and lipid-like compounds, nitrogen metabolites, oligopeptides and amino acid derivatives, produced by cyanobacteria of the genera Anabaenopsis, Aphanizomenon, Aulosira, Cylindrospermopsis, Cylindrospermum, Nodularia, and Richelia of the family Nostocaceae.