New insights into the disulfide bond formation enzymes in epidithiodiketopiperazine alkaloids

Epidithiodiketopiperazines (ETPs) are a group of bioactive fungal natural products and structurally feature unique transannular disulfide bridges between α, α or α, β carbons. However, no enzyme has yet been demonstrated to catalyse α, β-disulfide bond formation in these molecules. Through genome mining and gene deletion approaches in Trichoderma hypoxylon, we identified a putative biosynthetic gene cluster of pretrichodermamide A (1), which requires a FAD-dependent oxidoreductase, TdaR, for the irregular α, β-disulfide formation in 1 biosynthesis. In vitro assays of TdaR, together with AclT involved in aspirochlorine and GliT involved in gliotoxin biosynthesis, proved that all three enzymes catalyse not only the conversion of red-pretrichodermamide A (4) to α, β-disulfide-containing 1 but also that of red-gliotoxin (5) to α, α-disulfide-containing gliotoxin (6). These results provide new insights into the thiol-disulfide oxidases responsible for the disulfide bond formation in natural products with significant substrate and catalytic promiscuities.

A FAD-dependent oxidoreductase TdaR was responsible for α, β-disulfide formation in the biosynthesis of pretrichodermamide A. TdaR, together with its homologs AclT and GliT, catalysed not only α, α- but also α, β-disulfide formation in fungi.     

Concise Biosynthesis of Phenylfuropyridones in Fungi

Phenylfuropyridone natural products from fungi exhibit a range of antibacterial and cytotoxicity activities, and can potentiate azole antifungal compounds. We elucidated the biosynthetic pathway of compounds in the citridone family through heterologous reconstitution of the pfp pathway. We demonstrate that multiple members of this family can be accessed from a reactive ortho-quinone methide (o-QM) intermediate through electrocyclization, cycloisomerization, or conjugate addition. Formation of the quaternary carbon center in citridone B is catalyzed by an epoxide-forming P450 enzyme, followed by carbon skeletal rearrangement. Our results showcase how nature harvests the reactivities of an o-QM intermediate to biosynthesize complex natural products.     

Ortho-Quinone Methide Driven Synthesis of New O,N- or N,N-Heterocycles

To synthesize functionalized Mannich bases that can serve two different types of ortho-quinone methide (o-QM) intermediates, 2-naphthol and 6-hydroxyquinoline were reacted with salicylic aldehyde in the presence of morpholine. The Mannich bases that can form o-QM and aza-o-QM were also synthesized by mixing 2-naphthol, 2-nitrobenzaldehyde, and morpholine followed by reduction of the nitro group. The highly functionalized aminonaphthol derivatives were then tested in [4+2] cycloaddition with different cyclic imines. The reaction proved to be both regio- and diastereoselective. In all cases, only one reaction product was obtained. Detailed structural analyses of the new polyheterocycles as well as conformational studies including DFT modelling were performed. The relative stability of o-QMs/aza-o-QM were also calculated, and the regioselectivity of the reactions could be explained only when the cycloaddition started from aminodiol 4 . It was summarized that starting from diaminonaphthol 25 , the regioselectivity of the reaction is driven by the higher nucleophilicity of the amino group compared with the hydroxy group. 12H-benzo[a]xanthen-12-one ( 11 ), formed via o-QM formation, was isolated as a side product. The proton NMR spectrum of 11 proved to be very unique from NMR point of view. The reason for the extreme low-field position of proton H-1 could be accounted for by theoretical calculation of structure and spatial magnetic properties of the compound in combination of ring current effects of the aromatic moieties and steric compression within the heavily hindered H(1)-C(1)-C(12b)-C(12a)-C(12)=O structural fragment.     

Preparation of poly-p-xylylenes by electrolysis

Poly-p-xylylenes were prepared by electrolytic reduction of α,α′-dihalo-p-xylenes at controlled cathode potentials (c.p.). Polymers and halides are formed at the cathode; at the anode the halide is oxidized to halogen. Poly-p-xylylene was prepared from α,α′-dichloro-p-xylene (c.p. ?1.2 v.) and α,α′-dibromo-p-xylene (c.p. ?1.2 v.); poly-p-2-chloroxylylene from α,α′,2-trichloro-p-xylene (c.p. ?1.4 v.) and α,α′-dibromo-2-chloro-p-xylene (c.p. ?1.2 v.); poly-α,α,α′,α′-tetrachloro-p-xylylene from α,α,α,α′,α′,α′-hexachloro-p-xylene (c.p. ?0.7 v.), and poly-α,α,α′,α′-tetrafluoro-p-xylylene from α,α′-dibromo-α,α,α′,α′-tetrafluoro-p-xylene (c.p. ?1.1 v.). The cathode potentials were measured and controlled with respect to a saturated calomel electrode. Current efficiencies up to 96% were observed. α,α,α′,α′-Tetrachloro-p-xylylene was identified as an intermediate in the reduction of α,α,α,α′,α′,α′-hexachloro-p-xylene. A general mechanism for these reactions is suggested and discussed. It involves elimination of halide by a two-electron charge transfer with formation of a xylyl anion, followed by an elimination of halide in α′-position yielding xylylenes which then polymerize.     

Reaction of substituted benzylideneacetylacetones with hydroxylamine hydrochloride

The reaction of α,β-unsaturated β-diketones, such as 3-(o-chloro, m-nitro, and o-nitrobenzylidene)acetylacetones (I, II, and III) with hydroxylamine hydrochloride was carried out. Among them, compound I and II in acetonitrile, methanol, and acetic acid afforded 4-(α-hydroxy, methoxy, acetoxy, and chlorobenzyl)-3,5-dimethylisoxazoles (IV-XI) in fairly good yield. On the other hand, III yielded 3-(3′,5′-dimethylpyrazolo)-5-chloroanthranil (XV) under the almost same conditions by the participation of o-nitro group.     

Discovery and Biomimetic Synthesis of a Phloroglucinol-Terpene Adduct Collection from Baeckea frutescens and Its Biogenetic Origin Insight

A phloroglucinol-terpene adduct (PTA) collection consisting of twenty-four molecules featuring three skeletons was discovered from Baeckea frutescens. Inspired by its biosynthetic hypothesis, we synthesized this PTA collection by reductive activation of stable phloroglucinol precursors into highly reactive ortho-quinone methide (o-QM) intermediates and subsequently Diels–Alder cycloaddition. We also demonstrated, for the first time, the generation process of the active o-QM by performing dynamic NMR and HPLC-MS monitoring experiments. Moreover, the PTA collection showed significant antifeedant effect toward the Plutella xylostella larvae.     

The synthesis,configuration, and conformation of cis- and trans-3-amino-3,4-dihydro-1-hydroxy-4-methylcarbostyrils and other configurationally related compounds

The cis- and trans-3-amino-3,4-dihydro-1-hydroxy-4-methylcarbostyrils (Ia and Ib) were synthesized by catalytic hydrogenation of erythro- and threo-α-amino-β-(o-nitrophenyl)butyric acid hydrochlorides, IIIa and IIIb, respectively, under acidic conditions. The free bases of IIIa and IIIb were catalytically hydrogenated under neutral conditions to yield the erythro- and threo-α-amino-β-(o-aminophenyl)butyric acids (VIa and VIb), which were converted by acidification to their corresponding lactams, cis- and trans-3-amino-3,4-dihydro-4-methylcarbostyrils, IIa and IIb. The erythro and threo isomers of α-amino-β-(o-nitrophenyl)-butyric acid were prepared and separated by liquid chromatography via a diastereomeric mixture of (V) of methyl α-acetamido-β-(o-nitrophenyl)butyrates. The configurations and conformational assignments of the cyclic hydroxamic acids Ia and Ib were first established by analysis of the proton nmr spectra. In turn, the configurations of the o-nitroaromatic amino acids IIIa and IIIb were assigned as well as the other structurally related compounds (VIa, VIb, IIa and IIb) derived therefrom.     

Cyclic α, β-epoxysilanes: Acid-catalysed reactions of 1-trimethylsilylcyclooctene oxide

The acid-catalysed reactions of medium ring α,β-epoxysilane 1 are described. The epoxide 1 gives exclusively the bicyclic alcohol 2 with boron trifluoride etherate. With aqueous sulphuric acid the products are due to both transannular ring closure, 2, and transannular hydride migration, 3 and 4. trans-Cyclooctene derivatives 3 and 4 are formed by the trans-elimination of trimethylsilanol from the likely intermediate β-hydroxysilanes 10 and 11. The results with methanolic sulphuric acid are similar. It is also noticed that the hydride migration is facilitated by the nucleophilic strength of the medium.     

Synthesis of the 3-methyl and 4-methyl derivatives of 3-amino-3,4-dihydro-1-hydroxycarbostyril and related compounds

The 3-methyl and 4-methyl derivatives of 3-amino-3,4-dihydro-1-hydroxycarbostyril were synthesized by the reductive cyclization of α-methyl-β-(o-nitrophenyl)alanine and α-amino-β-(o-nitrophenyl)butyric acid hydrohalides, respectively, under conditions of catalytic hydrogenation in acidic solution. The free bases of the latter two o-nitroaromatic amino acids were also catalytically hydrogenated under neutral conditions to yield the respective α-methyl-β-(o-aminophenyl)alanine and α-amino-β-(o-aminophenyl)butyric acid which were converted to the corresponding lactams, 3-methyl- and 4-methyl-3-amino-3,4-dihydrocarbostyrils. α-Methyl-β-(o-nitrophenyl)alanine was obtained by acid hydrolysis of 5-methy)-5-(o-nitrobenzyl)hydantoin which was prepared by treatment of o-nitrophenylacetone with potassium cyanide and ammonium carbonate. α-Amino-β-(o-nitrophenyl)butyric acid was synthesized by condensation of α-bromo-o-nitroethylbenzene with diethyl acetamidomalonate, followed by acid hydrolysis of the condensation product. The 4-methylated compounds were obtained as synthetic mixtures of two diasteromeric racemates in nearly the same amounts as shown by nmr spectral analysis. Unlike the demethylated parent compound, 3-amino-3,4-dihydro-1-hydroxycarbostyril, neither the 3-methyl nor 4-methyl analog was found to possess any antibacterial activity.     

Synthesis and characterization of some heterocyclic derivatives from 1,2:3,4-Di-o-isopropylidene-α-d-galacto-1,6-hexadialdo-1,5-pyranose oxime

We report the synthesis of 5-[5′-(1′,2′:3′,4′-di-O-isopropylidene-β-L-arabinopyranosyl)]tetrazole, from 1,2:3,4-di-O-isopropylidene-α-D-galacto-1,6-hexodialdo-1,5-pyranose oxime via 1,2:3,4-di-O-isopropylidene-α-D-galcturononitrile as intermediate by 1,3-dipolar cycloaddition. We also report the synthesis of 5-methyl- and 5-phenyl-2-[5′-(1′,2′:3′,4′-di-O-isopropylidene-β-L-arabinopyranosyl)]-1,3,4-oxadiazole from the tetrazole derivative. The physical and spectroscopic characterizations of the heterocyclic derivatives as well as the intermedi ate nitrile and the principal by product are described and we discuss its possible formation pathway. We present the preferential conformation in solution using computational calculation and spectroscopic data.     

Melanotropin Receptors I. Synthesis and Biological Activity of Nα-(5-Bromovaleryl)-Nα-deacetyl-α-melanotropin

Chemical synthesis and biological activities of a new α-melanotropin derivative are described. N^α-(5-Bromovaleryl)-N^α-deacetyl-α-melanotropin contains the 5-bromopentanoyl group as a chemical ‘handle’ in place of the acetyl group of the natural hormone. The synthesis involved a new protected intermediate which allowed the selective deprotection of either the N^α or N^α amino group. The title compound reacted with sodium thiosulfate to give N^α-deacetyl-N^α-(5-(sulfothio)valeryl)-α-melanotropin, a key intermediate for the preparation of tobaccomosaic virus/α-melanotropin disulfide conjugates. As a basis for the study of the conjugates, biological activities of the title compound on Cloudman S-91 mouse melanoma cell cultures (tyrosinase stimulation, binding, and cyclic AMP accumulation) were determined. They proved to be quite similar to the corresponding α-melanotropin activities. Differences in bindings may be explained by stronger hydrophobic interaction of the new derivative with the lipid phase of the target cell membranes.     

Aza-Ortho-Quinone Methides as Reactive Intermediates: Generation and Utility in Contemporary Asymmetric Synthesis

The aza-ortho-quinone methide (aza-o-QM) chemistry has overwhelmingly progressed in the past few decades. This review aims to integrate various transition metal-catalyzed and organocatalytic strategies in taming aza-o-QM intermediates, including the aza-ortho-vinylidene quinone methide (aza-o-VQM), aza-ortho-alkynyl quinone methide (aza-o-AQM), aza-para-quinone methide (aza-p-QM), and indole-based aza-o-QM analog. These transient species are often utilized for the direct and enantioselective synthesis of complex (hetero)polycyclic or fused-ring molecular scaffolds such as tetrahydroquinoline and indoline, among others, which are abundant in many natural products, bioactive compounds, and pharmaceuticals.     

Steroide und Sexualhormone. 261. Mitteilung. Quassinoide Bitterstoffe II. Partialsynthetischer Zugang zu Quassin: Überführung von Testosteron in eine Schlüsselverbindung mit angulärer 8β-Methylgruppe

Partial Synthesis of Quassin: Synthesis of a Key Intermediate with an Angular 8β-Methyl Group from Testosterone A key intermediate in the partial synthesis of quassin ( 1 ) was synthesized in 28 steps starting from testosterone ( 9 ) (Scheme 3). The key features are: (i) The conversion of testosterone ( 9 ) into the 1α, 2β, 3β-O-substituted 4α-methylandrostane 19 (Scheme 3) and its transformation into an intermediate 26 with the ring A partial structure of quassin (Scheme 4). (ii) The conversion of 19 to the vinylogous α-hydroxyketone 5 (Scheme 6 and 7). (iii) The photochemically induced [2+2]-cycloaddition of allene to hydroxyenone 5 , affording the 8β, 14β-cyclobutano-derivative 6 (Scheme 2 and 8). (iv) The conversion of 6 into the key compound 7 . In connection with this last transformation a new method for the degradation of phenylselenoesters of carboxylic acids to the corresponding nor-alkanes was developed (see Scheme 8). Details of this reaction will be published elsewhere [18].     

Steroidal quinoxalines

The treatment of 3β-hydroxy-16α-bromo-5α-androstan-17-one, 3β-acetoxy-16α-bromo-5-androsten-17-one and 21-bromo-5-pregnen-3β-ol-20-one with 4,5-dimethyl-o-phenylenediamine gave substituted quinoxalines. Hydrolysis of 3β-acetoxy-5-androsteno[16,17-b]-6′,7′-dimethylquinoxaline produced the corresponding 3β-hydroxy compound. 3-Oxo-4-androsteno[16,17-b]-6′,7′-dimethylquinoxaline was obtained by Oppenauer oxidation of the corresponding alcohol.     

o-Quinone methide with overcrowded olefin component as a dehydridation catalyst under aerobic photoirradiation conditions

An o-quinone methide (o-QM) featuring an overcrowded olefinic framework is introduced, which exhibits dehydridation activity owing to its enhanced zwitterionic character, particularly through photoexcitation. The characteristics of this o-QM enable the operation of dehydridative catalysis in the oxidation of benzylic secondary alcohols under aerobic photoirradiation conditions. An experimental analysis and density functional theory calculations provide mechanistic insights; the ground-state zwitterionic intermediate abstracts a hydride and proton simultaneously, and the active oxygen species facilitate catalyst regeneration.

An o-quinone methide (o-QM) featuring an overcrowded olefinic framework is introduced, which exhibits dehydridation activity owing to its enhanced zwitterionic character, particularly through photoexcitation.     

Photochemical Reactions. 153th communication. Photochemistry of acylsilanes: Photolyses and thermolyses of α,β-epoxy silyl ketones

The photolyses and thermolyses of the α,β-epoxy silyl ketones 5 and 6 are described. On n,π*-excitation, the silyl ketones 5 and 6 were transformed to the ketone 7 and the ketene 8 in quantitative yield. The formation of 8 may be explained by initial cleavage of the C(α)? O bond and subsequent C(1)→C(2) migration of the (t-Bu)Me₂Si group. In contrast to the acylsilanes 5 and 6 , the photolyses of the analogous methyl ketones 11 and 12 gave a very complex mixture of products. On thermolysis, 5 and 6 yielded the ketone 7 and the acetylenic compound 9 , which were probably formed via a siloxycarbene intermediate. In addition, the 1,3-dioxle 10 was formed via an initial C(α)? C(β) bond cleavage leading to the ylide g and subsequent intramolecular addition of the carbonyl group. The analogous 1,3-dioxole 13 was obtained on pyrolysis of the methyl ketones 11 and 12 .     

New Insights into the Hexaphenylethane Riddle: Formation of an α,o‐Dimer

Upon reduction of a 1H‐cyclobuta[de]naphthalene‐4,5‐diylbis(diarylmethylium) species, a new C? C bond is formed between the C_α and C_ortho atoms of the two chromophores, which presents an unprecedented coupling pattern for the dimerization of two trityl units. By attaching an annulated cyclobutane ring at the opposite peri position of the naphthalene core, the distance between the C_α carbon atoms was elongated beyond the limit of σ‐bond formation through “scissor effects”. The suppression of C_α? C_α bond formation, which would lead to hexaphenylethane‐type compounds, is key to the first successful isolation of the α,o‐adducts. The 5‐diarylmethylene‐6‐triarylmethyl‐1,3‐cyclohexadiene unit in the α,o‐adducts is stable, and isomerization of the cyclohexadiene unit into an aromatic system was not observed. The newly formed C_α? C_ortho bond was cleaved upon two‐electron oxidation to regenerate the dicationic dye.     

Synthesis of Guanosine 5′-(β-L-Fucopyranosyl)-Diphosphate Revisited

ABSTRACT

It will be demonstrated that a successful synthesis of β-L-fucopyranose-1-phosphate (2), a key intermediate in the preparation of guanosine 5′-(β-L-fucopyranose)-diphosphate (1), strongly depends on the nature of the acyl protecting groups for the non-anomeric hydroxyl functions. Thus, the perbenzoylated, instead of peracetylated, α-L-fucopyranosyl trichloroacetimidate (11) or the corresponding ethyl β-thiofucopyranoside proved to be a convenient starting compound for the preparation of 2. Further, condensation of N,N′-dicyclohexyl-4-morpholinecarboxamidinium guanosine 5′-morpholidophosphate with excess 2 gave the title compound without concomitant formation of bisguanosine-5′-diphosphate (16).     

Isotope effects and mechanism in the bromination of alpha- and beta-carbon-14 labeled 4-nitro-4′-methylstilbenes

In the bromination of 4-nitro-4′-methylstilbene, the carbon-14 αC isotope effect is larger, k/^α k = 1.013, than the ^αC one, k/^βk = 1.0035. These results support an open carbocation intermediate mechanism with the positive charge on the carbon adjacent to the methyl-containing ring.     

A transannular reaction during lithium ammonia reduction of an α,β-unsaturated ketone

The lithium-ammonia reduction of the α,β-unsaturated bicyclic dione 2-methyl-Δ^1,6-bicyclo[6,4,0]cyclododecene-5,9-dione 1 has been found to give two isomeric products 2α-methyl-5-oxo-6β-tricyclo[6,4,0,0^1,9] dodecan-9α-ol 2 and 2-β-methyl-5-oxo-6β-tricyclo[6,4,0,0^1,9]dodecan-9α-ol 3 involving a transannular reaction. Chemical and spectral evidence are presented to support the assigned structures. The stereochemistry of 2 arid 3 is also discussed.