Reactions of ketene acetals-14 The use of simple mixed vinylketene acetals in the annulation of quinones

Total Synthesis of the Proposed Structure of the Anthrapyran Metabolite δ-Indomycinone

The first total synthesis of the proposed structure of delta-indomycinone has been accomplished. The key steps involve the Diels-Alder reaction of a bromonaphthoquinone (6) and 1-methoxy-3-methyl-1-trimethylsiloxy-1,3-butadiene (7) to access the anthraquinone skeleton, representing a common building block of other naturally occurring anthraquinone antibiotics, regioselective bromination of anthraquinone (14) and a highly diastereoselective alkylation of enantiomerically pure dioxolanone 8. The reported synthetic approach has the advantage of high yields, excellent selectivity and a remarkable general applicability for the total synthesis of other anthrapyran natural products. The spectroscopic data obtained for the synthetic compounds 2 and 36 are not in agreement with those reported for the natural product, and therefore revision of the assigned structure is required.

Synthesis of tetra-ortho-substituted, phosphorus-containing and carbonyl-containing biaryls utilizing a Diels-Alder approach

The application of the Diels-Alder approach to biaryls (DAB) is described for the synthesis of tetra-ortho-substituted biaryl compounds containing orthogonally functionalized substituents. The syntheses of phosphorus-containing, disubstituted alkynes and carbonyl-containing, disubstituted alkynes were accomplished in two to three steps from commercially available reagents. Subsequent Diels-Alder cycloadditions with a range of oxygenated dienes yielded the target biaryls. Further functionalization through palladium-couplings is demonstrated on the phosphorus-containing biaryls. In addition, selective manipulation of each of the remaining ortho substituents on the phosphorus-containing biaryls is demonstrated. One of these phosphorus-containing derivatives is utilized as a highly active catalyst for Suzuki coupling. For the carbonyl-containing series, a wide range of dienophile substituents were screened including esters, ketones, and amides. The key Diels-Alder cycloadditions proceeded smoothly with the commercially available 1-methoxy-1,3-cyclohexadiene to yield the resultant tetra-ortho-substituted biaryls with excellent regioselectivity. The scope of the cycloaddition process was also explored on the carbonyl-containing dienophiles with a series of cyclic dienes. Acyclic dienes were also screened; however, they did not prove effective in the Diels-Alder process with the carbonyl-containing acetylenes. The ability to isolate enantiomerically pure biaryl atropisomers using a benzyl oxazolidinone is disclosed. Finally, the subsequent conversion to an axially chiral anilino alcohol is also reported.

Diels−Alder Approach to Tetra-ortho-Substituted Biaryls Employing Propargylic Tertiary Alcohols as Dienophiles

The efficient synthesis of a series of tetra-ortho-substituted biaryls is described utilizing a Diels-Alder reaction between propargylic tertiary alcohols and cyclic oxygenated dienes. The successful resolution of one of the biaryls is achieved through derivatization with menthyl chloroformate followed by crystallization. The menthyl carbamate is cleaved under basic conditions to reveal enantiomerically pure biaryl compounds.

Diels-Alder approach for the construction of halogenated, o-nitro biaryl templates and application to the total synthesis of the anti-HIV agent siamenol

A rapid Diels-Alder approach to halogenated biaryl templates is described. These biaryl templates are available in two steps from the corresponding aromatic aldehydes. The scope of subsequent Suzuki couplings on the biaryl chlorides is explored. Good tolerance for both electron-donating and electron-withdrawing groups in the coupling process can be achieved. Further functionalization of the biaryl templates is described. Hydrogenation of the nitro moiety with concomitant removal of the benzyl ether yields the o-anilino, o-phenolic polyaryls. Selective reduction of the nitro group can be accomplished. Alternatively, the benzyl ether can be selectively removed under Lewis acidic conditions. The utilization of the Diels-Alder adducts for the synthesis of a series of chlorinated carbazoles via the Cadogan cyclization is also demonstrated. Finally, application of this technology to the total synthesis of siamenol, an anti-HIV agent, is reported.

Novel strategies for the synthesis of anthrapyran antibiotics: discovery of a new antitumor agent and total synthesis of (S)-espicufolin

Two high-yielding strategies for the synthesis of 4H-anthra[1,2-b]pyran antibiotics have been developed giving access to novel antitumor agent (ED(50) 1.5 microm) and to (S)-espicufolin (3). A key step for the assembly of the tetracyclic 4H-anthra[1,2-b]pyran-4,7,12-trione skeleton is the nucleophilic addition of an aryl lithium species onto an aldehyde which allows the introduction of either an ynone or 1,3-diketo side chain, serving as precursors for an acid-catalysed cyclisation.

Total synthesis and stereochemical reassignment of (+)-dolastatin 19

[reaction: see text] A revised configurational assignment for the cytotoxic marine macrolide dolastatin 19 is proposed and validated by total synthesis. Key features of the route include an asymmetric vinylogous aldol reaction to install the isolated C13 stereocenter and (E)-trisubstituted alkene, two sequential 1,4-syn boron-mediated aldol reactions, and a Mukaiyama glycosylation to append the l-rhamnose-derived pyranoside.

Synthesis of Hypericin via Emodin Anthrone Derived from a Two-fold Diels-Alder Reaction of 1,4-Benzoquinone

The six-step synthesis of hypericin by the regioselective two-fold Diels-Alder reaction of 1,4-benzoquinone first with (1-methoxy-3-methylbuta-1,3-dienyloxy)trimethylsilane leading to 7-methyljuglone, and next with (1,3-dimethoxybuta-1,3-dienyloxy)trimethylsilane, to give emodin and its O-methylated derivative. The reduction of both compounds with tin(II) chloride in acidic media was accompanied by acid hydrolysis that produced emodin anthrone, whose oxidative dimerization with iron (III) chloride hydrate gave the bianthrone in high yield. The oxidation of the bianthrone in the presence of N-ethyldiisopropylamine gave protohypericin, which was converted into hypericin upon irradiation.

Bisoxazoline-Lewis Acid-Catalyzed Direct-Electron Demand oxo-Hetero-Diels−Alder Reactions of N-Oxy-pyridine Aldehyde and Ketone Derivatives

A general catalytic oxo-hetero-Diels-Alder reaction for pro-chiral aldehyde and ketone N-oxy-pyridines is presented. The catalytic and asymmetric oxo-hetero-Diels-Alder reaction of electron-rich dienes with N-oxy-pyridine-2-carbaldehyde and ketone derivatives, catalyzed by chiral copper(II)-bisoxazoline complexes, gives optically active six-membered oxygen heterocycles in moderate to good yields and with excellent enantioselectivities.

First Enantioselective Total Synthesis and Structure Determination of the Anthrapyran Metabolite γ-Indomycinone

The first total synthesis of (R)-gamma-indomycinone has been achieved which allowed the determination of the configuration of the stereogenic center of natural gamma-indomycinone as (S). The approach stands out for its generality and efficiency. [reaction: see text]

Isolation and identification of anthraquinones of Caloplaca cerina and Cassia tora

Five hydroxyanthraquinone derivatives physcion, emodin, fallacinal, teloschistin, and 1,3-dimethoxy-8-hydroxy-6-methyl-9,10-anthraquinone were isolated from the lichen Caloplaca cerina growing in Serbia. Three anthraquinone derivatives, namely physcion, emodin, and rhein were isolated and identified from the Thai medicinal plant Cassia tora.

Concise Formal Total Synthesis of Hybocarpone and Related Naturally Occurring Naphthazarins

A concise formal total synthesis of the cytotoxic bisnaphthazarin derivative hybocarpone has been completed through the development of routes to the synthetic precursor, 3-ethyl-2-hydroxy-5,7,8-trimethoxy-6-methyl-1,4-naphthoquinone. The oxidation of 3-ethyl-1,2,4,5,7,8-hexamethoxy-6-methylnaphthalene under Rapoport conditions gave 3-ethyl-2-hydroxy-5,7,8-trimethoxy-6-methyl-1,4-naphthoquinone in modest yields after basic hydrolysis. In addition, treatment of 3-ethyl-1,2,4,5,7,8-hexamethoxy-6-methylnaphthalene with boron tribromide provided access to the naturally occurring naphthazarin, boryquinone. The analogous oxidative demethylation of 3,6-dimethyl-1,2,4,5,7,8-hexamethoxynaphthalene and 3-ethyl-1,2,4,5,7,8-hexamethoxynaphthalene resulted in the synthesis of 2,5,7,8-tetrahydroxy-3,6-dimethyl-1,4-naphthoquinone (aureoquinone) and 3-ethyl-2,5,7,8-tetrahydroxy-1,4-naphthoquinone, respectively. An alternative selective synthetic route to 3-ethyl-2-hydroxy-5,7,8-trimethoxy-6-methyl-1,4-naphthoquinone was also developed utilizing an intramolecular Claisen condensation of methyl 2-butyryl-3,5,6-trimethoxy-4-methylphenylacetate with concomitant in situ aerial oxidation.

A Formal Total Synthesis of (−)-FR901483, Using a Tandem Cationic Aza-Cope Rearrangement/Mannich Cyclization Approach

A formal total synthesis of the immunosuppressant FR901483 has been accomplished. The key step in the synthesis utilizes a tandem cationic aza-Cope rearrangement/Mannich cyclization reaction for accessing the unprecedented bridging tricyclic azaspirane substructure of this compound. The tandem reaction proceeds through a bridgehead iminium ion, a functionality that has rarely been explored in the context of natural product syntheses. Improved stereoselectivity was observed in an aldol reaction when using a Boc-protected amino aldehyde and zinc chloride as an additive. A stereoselective epimerization of the aldehyde-containing stereocenter was achieved with l-phenylalanine upon completion of the Mannich cyclization. Finally, this synthesis is the only one to date that controls the stereochemistry of the oxygen-bearing stereocenters. All other synthetic routes required late stage adjustments to at least one of these stereocenters.

Stereocontrolled total synthesis of (-)-callipeltoside A

[structure: see text] A highly stereocontrolled total synthesis of the cytotoxic macrolide (-)-callipeltoside A has been achieved in 23 steps (4.8% overall). Notable features include a novel asymmetric vinylogous aldol reaction to install the C13 stereocenter and (E)-trisubstituted alkene, an anti-selective aldol addition, a Sonogashira coupling, and, last, a Schmidt-type glycosylation to attach the sugar unit.

Synthesis of dihydronaphthofurandiones and dihydrofuroquinolinediones with trypanocidal activity and analysis of their stereoelectronic properties

The synthesis of dihydronaphthofurandione and dihydrofuroquinolinedione derivatives 4-11 was performed through Diels-Alder reactions of dihydrobenzofurandione 1 with several carbodienes and acrolein N,N-dimethylhydrazone. Then, the use of 5-bromobenzofurandione 2 toward 1,3-pentadiene and the 1-azadiene afforded quinones 6 and 11 with a total regioselectivity. All the prepared quinones were tested for trypanocidal activity in vitro against Trypanosoma epimastigotes, Tulahuen strain. Among the tested compounds, the furoquinolinediones 10 and 11 have shown potent trypanocidal activities but, only the 1,5-regioisomer (11) was found active as a redox cycling agent. Calculation of their stereoelectronic properties by the density-functional theory method provided a new insight for the trypanocidal activity of these heterocyclic quinones.

Cytotoxic anthraquinones from the stems of Rubia wallichiana Decne

From the stems of Rubia wallichiana DECNE, thirty-four structurally related compounds were isolated and identified. Three of them, namely rubiawallin-A (1), -B (2), and -C (3), constitute the first report of their occurrence from the natural source. Their structures were determined by comprehensive analyses of their 1D and 2D NMR, and electron impact (EI) mass spectral data. Furthermore, an in vitro screening of cytotoxicity of the isolated compounds was also evaluated. Among the testing compounds, 1-hydroxy-2-hydroxymethyl-3-methoxyanthraquinone (4) demonstrated most effective cytotoxicity towards Hepa-3B and Colo-205 cells.

Atropo-enantioselective total synthesis of knipholone and related antiplasmodial phenylanthraquinones

The "lactone concept" has been efficiently employed for the first atropo-enantioselective synthesis of knipholone and related natural phenylanthraquinones. Besides the regio- and stereoselective construction of the biaryl axis, another important step was the "synthetically late" introduction of the C-acetyl group, either by a Friedel-Crafts type acetylation or by an ortho-selective Fries rearrangement first tested on simplified model systems and subsequently applied to the highly atroposelective preparation of the natural products and of simplified analogs thereof for biotesting. The synthetic availability of these natural biaryls, their precursors, and their unnatural analogs permitted a broader investigation of the antiplasmodial activities of these interesting biaryls.

Four new compounds from the seeds of Cassia fistula

Four new compounds, 5-(2-hydroxyphenoxymethyl)furfural (1), (2'S)-7-hydroxy-5- hydroxymethyl-2-(2'-hydroxypropyl)chromone (2), benzyl 2-hydroxy-3,6-dimethoxybenzoate (3), and benzyl 2beta-O-D-glucopyranosyl-3,6-dimethoxybenzoate (4), together with four known compounds, 5-hydroxymethylfurfural, (2'S)-7-hydroxy-2-(2'-hydroxypropyl)-5-methylchromone, and two oxyanthraquinones, chrysophanol and chrysophanein, were isolated and identified from the seeds of Cassia fistula. The structures of 1-4 were determined on the basis of spectral data explanation, and the synthesis of compound 1 was carried out.

Synthesis of an orange anthrathiophene pigment isolated from a Japanese bryozoan

A short, regiospecific synthesis of the naturally occurring anthrathiophene 1 from naphthazarin (7) is described.

Synthesis of two naphthoquinone antibiotics, dehydroherbarin and 6-deoxybostrycoidin

The synthesis of two naphthoquinone antibiotics, dehydroherbarin (7) and 6-deoxybostrycoidin (5), was accomplished by reaction of 3-acetonyl-2-bromomethyl-6,8-dimethoxy-1,4-naphthoquinone (23) with either triethylamine or ammonia, respectively. This is the first report on their synthesis.

Direct Synthesis of G-2N

The synthesis of angularly fused quinone natural products has been achieved using a photoenolization reaction and a Diels-Alder reaction in the key carbon-carbon bond forming steps.