Production of desmethyl-gregatin A, a possible causative toxin of brown stem rot in adzuki bean, by Phialophora gregata f. sp. adzukicola.

To elucidate the cause of brown stem rot in the adzuki bean, we re-evaluated the phytotoxins produced in cultures of the causative agent, Phialophora gregata f. sp. adzukicola. The ethyl acetate-soluble acidic fraction of the culture, as well as the neutral fraction, inhibited the growth of alfalfa seedlings. In the neutral fraction, known phytotoxins gregatin A, B, and C or D and penicilliol A were present. Although the phytotoxins in the acidic fraction were unstable, liquid chromatography-mass spectrometry analysis of the partially purified material suggested that one phytotoxin present was the non-methylated gregatin desmethyl-gregatin A (gregatinic acid A).

Regioselective Anti-Silyllithiation of Propargylic Alcohols.

Among the known hydrosilylation or carbosilylation conditions of alkynes, anti-addition of the two units across the triple bond is considered rare compared to the syn counterpart. For anti-silylative vicinal difunctionalizations, transition-metal catalysts, such as ruthenium or palladium complexes, are generally required. Accordingly, silyl alkali metals have not been employed for those anti-addition transformations. Here we demonstrate that silyllithiums can add across the triple bond of a series of propargylic alkoxides regioselectively in an anti-fashion. Upon treatment with a variety of electrophiles, the trisubstituted alkenyl lithium intermediates were transformed into highly functionalized ß-silyl allylic alcohols with high regiocontrol, eventually providing tri- or tetrasubstituted alkenylsilanes stereoselectively. A classic trick for anti-addition with propargylic alkoxides has transformed anti-silylative functionalizations into a robust and reliable strategy.

Zincation of Styrylsulfonium Salts.

We report the formation of zinc reagents by the reaction of styrylsulfonium salts with zinc powder. Transition metals and other additives are not required for promoting zincation. Zincation tolerates a variety of sensitive functional groups, including esters, bromides, and boronic esters, and proceeds with complete retention of stereochemistry. This method presents a practical approach to the formation of zinc reagents that can be used in a variety of functionalizations, such as halogenation, carboxylation, and Negishi cross-couplings.

Synthesis of meso-Free Decaphyrin(1.1.1.1.1.1.1.1.1.1) by a Hydrodebromination Protocol; Aromaticity and Solvent-Polarity Dependent Conformational Change.

A meso-free ß-bromodecaphyrin has been constructed by [2+3+2+3] type acid-catalyzed cross-condensation and subsequent oxidation. Hydrodebromination of this tetrabromodecaphyrin with NaBH4 in the presence of TMEDA and palladium catalyst afforded ß-unsubstituted doubly meso-free [46]decaphyrin(1.1.1.1.1.1.1.1.1.1) as the first example of ß-unsubstituted meso-free regular expanded porphyrins with the number of pyrrole units larger than eight. It exhibits distinct aromaticity originating from its 46π-conjugated electronic circuit and flexible conformational change between non-twisted and doubly twisted forms depending on the solvent-polarity. Their distinct conformations have been analyzed by combined experimental and theoretical investigations.

Five-Fold Symmetric Pentaindolo- and Pentakis(benzoindolo)Corannulenes: Unique Structural Dynamics Derived from the Combination of Helical and Bowl Inversions.

Peripherally π-extended corannulenes bearing quintuple azahelicene units, 10 and 11, were prepared and their dynamic behaviors were studied experimentally and theoretically. The fused corannulenes were synthesized from sym-pentabromocorannulene in three steps. X-Ray diffraction analysis for 10 displayed a conformer possessing a P(M) bowl chirality and a PPMPM (PMPMM) helical chirality, which was found to be the most stable conformer(s). Variable-temperature NMR measurements of 10 and 11 revealed that their structural isomers can be interconvertible in solution, depending on the steric congestion around the helical scaffolds. Automated search for conformers in the equilibrium and transition states by Artificial Force Induced Reaction (AFIR) method revealed their interconversion networks, including bowl-inversion and helical-inversion. This analysis indicated that the co-existing corannulene and azahelicene moieties influence the conformational dynamics, which leads to mitigation of the activation energy barriers for isomerization.

Reductive Cleavage of Propargylic Ethers with Alkali Metal: Application to the Synthesis of Allenylboronates.

Treatment of propargylic ethers with sodium dispersion in the presence of lithium iodide results in the generation of the corresponding carbanion species via cleavage of the propargylic C-O bond. The anionic species react with trimethoxyborane to yield the allenylboronates including highly substituted ones that are difficult to synthesize.

The dioxasilepanyl group as a versatile organometallic unit: studies on stability, reactivity, and utility.

Organic synthesis is performed based on precise choices of functional groups and reactions employed. In a multistep synthesis, an ideal functional group should be compatible with various reaction conditions and unaltered until it is subjected to a selective conversion. The current study was set out to search for a silicon functionality that meets these criteria. Here we have established a new silicon-based synthetic methodology centred on a bulky 7-membered dialkoxysilyl group (2,4,4,7,7-pentamethyl-1,3,2-dioxasilepan-2-yl) that uniquely has both stability and on-demand reactivity. The exceptional stability of this functional group was corroborated by both experimental and computational studies which demonstrated that key factors for its stability were a 7-membered structure and steric hindrance. In turn, the dioxasilepanyl group was found to become reactive and to be easily transformed in the presence of appropriate activators. Combined with the development of easy and robust methods to introduce the dioxasilepanyl group onto aryl rings, these findings have allowed a shorter and more efficient synthesis of a bioactive molecule, thus demonstrating the potential utility of the easily accessible dioxasilepanyl group in organic synthesis.

Aromatic Metamorphosis of Thiophenes by Means of Desulfurative Dilithiation.

A new mode of aromatic metamorphosis has been developed, which allows thiophenes and their benzo-fused derivatives to be converted to a variety of exotic heteroles. This transformation involves 1) the efficient generation of key 1,4-dianions by means of desulfurative dilithiation with lithium powder and 2) the subsequent trapping of the dianions with heteroatom electrophiles in a one-pot manner. Via the desulfurative dilithiation, the sulfur atoms of thiophenes are replaced also with a carbon-carbon double bond or a 1,2-phenylene for the construction of benzene rings.

Palladium-Catalyzed C-H Iodination of Arenes by Means of Sulfinyl Directing Groups.

C-H iodination of aromatic compounds has been accomplished with the aid of sulfinyl directing groups under palladium catalysis. The reaction proceeds selectively at the peri-position of polycyclic aryl sulfoxides or at the ortho-position of phenyl sulfoxides. The iodination products can be further converted via iterative catalytic cross-coupling at the expense of the C-I and C-S bonds. Computational studies suggest that peri-C-H palladation would proceed via a non-directed pathway, wherein neither of the sulfur nor oxygen atom of the sulfinyl group coordinates to the palladium before and at the transition state.

Aromatic Metamorphosis of Indoles into 1,2-Benzazaborins.

Among the plethora of aromatic compounds, indoles represent a privileged class of substructures that is ubiquitous in natural products and pharmaceuticals. While numerous exocyclic functionalizations of indoles have provided access to a variety of useful derivatives, endocyclic transformations involving the cleavage of the C2-N bond remain challenging due to the high aromaticity and strength of this bond in indoles. Herein, we report the "aromatic metamorphosis" of indoles into 1,2-benzazaborins via the insertion of boron into the C2-N bond. This endocyclic insertion consists of a reductive ring-opening using lithium metal and a subsequent trapping of the resulting dianionic species with organoboronic esters. Considering that 1,2-azaborins have attracted increasing academic and industrial attention as BN isosteres of benzene, the counterintuitive aromatic metamorphosis presented herein can feasibly be expected to substantially advance the promising chemistry of 1,2-azaborins.

Copper-Catalyzed Ring-Opening Silylation of Benzofurans with Disilane.

The catalytic ring-opening silylation of benzofurans has been achieved by employing a copper catalyst and 1,2-di-tert-butoxy-1,1,2,2-tetramethyldisilane, which could be easily prepared and handled without special care. The reaction afforded (E)-o-(ß-silylvinyl)phenols with complete stereoselectivity. The scope of benzofurans was well explored, and functional groups such as chloro, fluoro, and acetal were compatible with the reaction conditions. DFT calculations were used to determine the energy profile of the silylation and the origin of the stereoselectivity. The silylated product was proven to be useful as a synthetic intermediate and subsequently underwent transformations such as Pd-catalyzed cross-coupling with iodoarenes.

Manganese-Catalyzed Ring Opening of Benzofurans and Its Application to Insertion of Heteroatoms into the C2-O Bond.

A new class of aromatic metamorphosis in which benzofurans are converted into diverse six-membered oxaheterocycles has been developed. This transformation is composed of two reactions in one pot: manganese-catalyzed arylative or alkylative ring-opening of benzofurans affording dianionic intermediates and subsequent trapping with multivalent heteroatom electrophiles. Various electrophiles containing silicon, boron, phosphorus, germanium, and titanium could be applied to this heteroatom insertion.

Nickel-Catalyzed Boron Insertion into the C2-O Bond of Benzofurans.

Treatment of benzofurans with bis(pinacolato)diboron and Cs2CO3 under nickel-NHC catalysis resulted in the insertion of a boron atom into the C2-O bond of benzofurans to afford the corresponding oxaborins. The scope of benzofuran substrates is wide, and the reactions proceeded without loss of functional groups such as fluoro, methoxy, and ester that are potentially reactive under nickel catalysis. The boron-inserted products proved to be useful building blocks and subsequently underwent a series of transformations, one of which led to the synthesis of fluorescent π-expanded oxaborins.