Ni(ii)-Catalyzed vinylic C-H functionalization of 2-acetamido-3-arylacrylates to access isotetronic acids.

A ligand-free Ni(ii)-catalyzed cascade annulation reaction for the synthesis of 4-aryl-substituted isotetronic acids from 2-acetamido-3-arylacrylates via vinylic C-H functionalization is reported. The reaction proceeds through heteroatom guided electrophilic insertion of nickel to the vinylic double bond followed by annulation with dibromomethane. This unconventional route features cascade steps, sole product formation, multiple functional group tolerance, low cost of catalysts and reagents, and readily available starting materials. Using this method, various aryl-substituted isotetronic acids have been synthesized which are biologically relevant. The annulation of 2-acetamido-3-arylacrylates has also been assessed with 1,2-dichloroethane, which resulted in the rearranged annulated products of 5-methyl substituted isotetronic acids.

Anionic Annulation of 3-Cyanophthalides with Allene Carboxylates: A Carbon-Conserved Synthesis of Naphtho[ b]furanones.

The reaction of 3-cyanophthalides with allene carboxylates in the presence of tBuOLi results in a tandem annulation furnishing naphtho[ b]furanones in good yields with no loss of carbon. The carbon economy is explained by a tandem process, in which the initially expelled cyanide induces the second annulation.

A Representative Synthetic Route for C5 Angucycline Glycosides: Studies Directed toward the Total Synthesis of Mayamycin.

This study discloses an efficient synthetic route for the regiospecific construction of a C5 glycoside angucycline representative of mayamycin. The key steps are intramolecular aldol condensation and Hauser annulation, and the key precursor for the aldol reaction is accessible through utilization of α-lithiation of a vinyl ether.

Applications of [4+2] Anionic Annulation and Carbonyl-Ene Reaction in the Synthesis of Anthraquinones, Tetrahydroanthraquinones, and Pyranonaphthoquinones.

Hexa-2,5-dienoates, susceptible to isomerization by acids and bases, are suitable for the [4+2] anionic annulation to give 3-(2-alkenyl)naphthoates in regiospecific manner. When combined with intramolecular carbonyl-ene reaction (ICE), the accessibility of the naphthoates culminates in a new synthesis of anthraquinones and diastereoselective synthesis of tetrahydroanthraquinones. This strategy has also resulted in a 3-step synthesis of dehydroherbarin from a 3-methallylnaphthoate.

Regioselective synthesis of naphthoquinone/naphthoquinol-carbohydrate hybrids by [4 + 2] anionic annulations and studies on their cytotoxicity.

An efficient and regioselective synthetic route to naphthoquinone/naphthoquinol-carbohydrate hybrids has been developed. It is based upon anionic annulation of 3-nucleofugalphthalides with an acrylate appended sugar moiety. In each of the annulations studied, the arene-carbohydrate hybrids were obtained in good to excellent yields. The in vitro cytotoxic activity of the synthetic naphthoquinone/naphthonol-carbohydrate hybrids were evaluated against the human cervical cancer cell line (HeLa), and a few of them were found to exhibit potent anticancer activity against the cell line.

Transpositive Tandem Annulation of Phthalides with Allene Carboxylates: Regioselective Synthesis of Arylnaphthalene Lignans.

Allene carboxylates, scarcely used as Michael acceptors, serve as acceptors in the annulation with phthalides in the presence of LDA and provide a one-pot synthesis of naphtho[c]furanones in very good yields. This tandem annulation is proposed to proceed via transposition of the hydroxy group resulting from the initial annulation.

Regiodefined synthesis of brominated hydroxyanthraquinones related to proisocrinins.

Dibromobenzoisofuranone 12, synthesized in six steps, was regiospecifically annulated with 5-substituted cyclohexenones 13/36 in the presence of LiOt-Bu to give brominated anthraquinones 14/38 in good yields. Darzens condensation of 30 was shown to give chain-elongated anthraquinone 32. Alkaline hydrolysis of 38 furnished 39 representing desulfoproisocrinin F.

Synthesis of Vitamin K and Related Naphthoquinones via Demethoxycarbonylative Annulations and a Retro-Wittig Rearrangement.

Anionic annulations of 3-nucleofugal phthalides with α-alkyl(aryl)acrylates involving a demethoxycarbonylation provide a succinct synthesis of vitamin K and related naphthoquinones. Also reported is a new cascade reaction stemming from a Cope-retro-Wittig rearrangement. This cascade leads to direct formation of 1-hydroxy-4-prenyloxynaphthalene-2-carboxylates from the corresponding α-prenyl acrylate acceptors.

A regioselective facile synthesis of furo[3,4-b]carbazolones: application to the total synthesis of mafaicheenamine E and claulansine D.

1-Hydroxycarbazole-2,3-dicarboxylates have been shown to undergo chemoselective reductive cyclization to furo[3,4-b]carbazolones on reaction with LiAlH4. One of the furocarbazolones is utilized to accomplish the first total synthesis of claulansine D and mafaicheenamine E in 9 and 6 steps respectively. The other key steps of the syntheses are addition of an allylic indium reagent and CC double bond isomerization.

Regiodivergent and short total synthesis of calothrixins.

The anionic annulation of MOM-protected furoindolone with 4-bromoquinoline followed by deprotection of the N-MOM group provides calothrixin B, whereas that with 3-bromoquinoline yields isocalothrixin B. The outcomes are explained by the divergence of the reaction mechanism from commonly perceived quinolyne intermediate. A sequence of addition-cyclization-elimination is proposed to account for the formation of calothrixin from 4-bromoquinoline.

Angucycline C5 glycosides: regio- and stereocontrolled synthesis and cytotoxicity.

This study discloses a general and convergent route for the regio- and stereospecific construction of the C5 glycosyl angucycline framework of mayamycin. C-Glycosidation, dearomatization, and Hauser annulation are the key steps. The synthetic analogues show cytotoxicity against different human cancer cell lines with IC50 values between 16.4 and 1.2 µM.

Total synthesis of chlorocyclinone A, a PPAR-γ antagonist.

The first total synthesis of chlorocyclinone A (1) is regioselectively completed in 28 steps. The key steps are Pd-catalyzed methoxycarbonylation, unprecedented Hauser annulation, Krohn photo-oxidation, and regioselective gem-dichlorination.

First example of a heterobimetallic 'Pd-Sn' catalyst for direct activation of alcohol: efficient allylation, benzylation and propargylation of arenes, heteroarenes, active methylenes and allyl-Si nucleophiles.

Arenes, heteroarenes, 1,3-dicarbonyls and organosilicon nucleophiles undergo highly efficient alkylation with allylic, propargylic and benzylic alcohols in the presence of a new 'Pd-Sn' bimetallic catalyst in nitromethane; water being the sole byproduct. The plausible mechanism of alkylation and the intermediacy of ether has been enumerated.

Anionic [4+3] heteroannulation of 2-azidoacrylates: a modular synthesis of 2-benzazepin-1-ones.

2-Azidoacrylates undergo [4+3] annulation with phthalides under anionic conditions at low temperatures to furnish 5-hydroxy-2-benzazepinones, the formation of which represents a new concept for the construction of azepines as well as a new reactivity of 2-azidoacrylates.

Stereoselective synthesis of hydroxy stilbenoids and styrenes by atom-efficient olefination with thiophthalides.

The synthesis of stilbenoids and styryl carboxylic acids is accomplished with high E-stereoselectivity by olefination of aldehydes with thiophthalides under basic conditions. The olefination is highly atom-efficient as it only loses elemental sulfur during the reaction. This olefination, in conjunction with retro Kolbe-Schmitt reaction, allows facile synthesis of E-hydroxystilbenoids with minimal employment of protecting groups. This study also discloses two important findings: formation of i) 4-methylsulfanyl isocoumarins and ii) an 2-arylindenone.

Benzannulation for the regiodefined synthesis of 2-alkyl/aryl-1-naphthols: total synthesis of arnottin I.

The annulation of phthalides with α-alkyl/arylacrylates in the presence of LDA/LHMDS is shown to directly give alkyl/aryl-1-naphthols. The method involving a novel dealkoxycarbonylation obviates the regiochemical issues in the synthesis of polysubstituted naphthalenes, and it forms the key step in a three-step total synthesis of arnottin I, a naphthobenzopyranone natural product.

A rapid entry to C-prenylcarbazoles: total synthesis of clausamine C-D, clausevatine D and clausine F.

The key prenylcarbazole precursor 33 was readily assembled from diester 30 by an ester-driven para-Claisen rearrangement followed by selective removal of the ester function. Unusual oxidative cyclization of 33 by m-CPBA resulted in the total synthesis of tetracyclic carbazole natural products 3 and 11.

Total synthesis of euplectin, a natural product with a chromone fused indenone.

The first total synthesis of euplectin, a rare metabolite with a chromone annulated indenone motif, has been accomplished in 17 steps. This has been possible through interplay among three key reactions: a Hauser sulfoxide annulation, a new chromone formation and a late-stage retro-Diels-Alder reaction. The entire regiochemical integrity of the successful route is established by an iodine-catalyzed aromatization of a cyclohexane-1,3-dione and the Hauser annulation.

Synthesis and rearrangement of quinone-embedded epoxycyclopentenones: a new avenue to pyranonaphthoquinones and indenopyranones.

The epoxyquinones (e.g., 24), readily assembled in one step from the quinols (e.g., 27) by a simplified version of the Dowd oxidation, are shown to undergo rearrangement to pyranonaphthoquinones (e.g., 28) and their ring contracted homologues (e.g., 29) on flash vacuum pyrolysis at 450 degrees C and 0.01 Torr. The rearrangement has been demonstrated to be useful for a regiospecific synthesis of lambertellin (3). Similarly, the masked aziridinocyclopentanone 9 rearranges to 2-pyridone (37).