Asymmetric Synthesis of Axially Chiral 2‐Aminobiaryls by Rhodium‐Catalyzed Benzannulation of 1‐Arylalkynes with 2‐(Cyanomethyl)phenylboronates

Asymmetric benzannulation of 1‐arylalkynes, where the aryl group is an ortho‐substituted aromatic group, with 2‐(cyanomethyl)phenylboronate was catalyzed by a rhodium complex coordinated with a chiral diene ligand to give high yields of axially chiral 2‐aminobiaryls with greater than 90 % ee.     

A divergent and stereoselective approach to phenolic 1,7-dihydroxy-bisabolane sesquiterpenes: asymmetric total synthesis of (+)-curcutetraol, (+)-sydonol, (+)-sydonic acid,and (+)-7-O-methylsydonic acid

The combined use of the Sharpless asymmetric epoxidation, a number of stereospecific chemical transformations, and the 3,5-hexadienoic acid benzannulation protocol allowed us to devise a new, divergent, and stereoselective approach to terpenes with a chiral tertiary hydroxyl group at the ortho-position of a phenol functional group. Accordingly, the natural occurring enantiomeric forms of the bisabolane sesquiterpenes (+)-curcutetraol, (+)-sydonol, (+)-sydonic acid, and (+)-7-O-methylsydonic acid were synthesized with high enantiomeric purity starting from geraniol. The latter two acids were prepared in enantioenriched form for the first time.     

A benzannulation strategy for the synthesis of phenols and heteroaromatic compounds based on the reaction of (trialkylsilyl)vinylketenes with lithium ynolates

(Trialkylsilyl)vinylketenes react with lithium ynolates to generate 3-(oxido)dienylketenes, which undergo rapid 6π-electrocyclization. The ultimate products of this benzannulation are highly substituted resorcinol monosilyl ethers, which are formed via a [1,3] carbon to oxygen silyl group shift. Further transformations of the benzannulation products are described providing efficient access to ortho-benzoquinones and benzofuran, benzoxepine, and benzoxocine ring systems.     

The stereodivergent asymmetric synthesis of a range of 2-(1′-hydroxyalkyl)phenols

The use of the (S)-α-methylbenzyl group as a chiral auxiliary has allowed the diastereoselective ortho-deprotonation of a chromium tricarbonyl complexed phenoxy ring. When the resultant ortho-anion is treated with an aldehyde two diastereoisomeric complexes are formed, in relatively poor dr, which differ in the configuration of the newly formed benzylic stereogenic centre. However, both ortho-formylation followed by treatment with Grignard reagents and ortho-acylation followed by reduction with Super-Hydride^® were found to be completely diastereoselective, giving access to either epimer of the corresponding benzylic alcohol complexes in >99:1 dr. Subsequent oxidative removal of the chromium tricarbonyl unit, followed by cleavage of the O-α-methylbenzyl chiral auxiliary gives enantiopure 2-(1′-hydroxyalkyl)phenols. Following this stereodivergent procedure, either enantiomer of the product may be accessed from a single antipode of [(α-methylbenzyloxy)benzene]Cr(CO)₃.     

AuCl-catalyzed reaction of ortho-alkynyl(oxo)benzene with benzenediazonium 2-carboxylate as a synthetic method towards anthracene, triptycene, and phthalazine derivatives

The AuCl-catalyzed benzannulation of ortho-alkynylphenyl ketones with benzenediazonium 2-carboxylate proceeded efficiently at 40 °C in (CH₂Cl)₂ and a variety of anthracene derivatives, having a ketone group at 9-position, were produced in good to high yields. On the other hand, the reaction of ortho-alkynylbenzaldehydes with benzenediazonium 2-carboxylate afforded triptycyl ketones. The reactions most probably proceed through the formation of a zwitterionic intermediate by the gold-induced electrophilic cyclization of ortho-alkynyl(oxo)benzenes, followed by the cycloaddition of benzyne. In contrast, when the above reaction was carried out at rt in 1,4-dioxane, phthalazine derivative was produced without the generation of benzyne.     

Asymmetric synthesis of 1,2,3-trisubstituted indanes via an enantioselective copper(II)-catalyzed asymmetric nitroaldol reaction followed by an intramolecular Michael cyclization

Herein we describe a novel approach for the synthesis of a chiral 1,2,3-trisubstituted indane, a privileged substructure in medicinal chemistry, via an enantioselective nitroaldol reaction and subsequent intramolecular Michael addition. The asymmetric copper(II)-catalyzed reactions of ortho-formyl cinnamates, ortho-formyl cinnamonitrile, or ortho-formyl α-benzalketones with nitromethane were carried out using the C1-symmetric chiral secondary diamine L1 as a ligand, which afforded the nitroaldol products in high yields (up to 92%) and with good to excellent enantioselectivities (up to 97%) under mild conditions. The notable effect of the base and the dosage of nitromethane on the reaction are also discussed.     

Determination of energy barriers to rotation and absolute conformations of thermally interconvertible 5,5-dimethyl-3-(o-aryl)-2,4-oxazolidinedione enantiomers

The activation barriers for the interconversion between the enantiomers of 5,5-dimethyl-3-(o-aryl)-2,4-oxazolidinediones (M⇌P) have been determined by temperature dependent NMR and by enantioresolution on chiral sorbents via HPLC. The activation barriers were found to increase linearly with the size of the van der Waals radii of the ortho-halogen substituents. The enantiomers of the o-iodo derivative were micropreparatively enriched on a Chiralpak AD column, leading to the determination of its barrier to rotation via thermal racemization and resulting in the assignments of conformations in the presence of the optically active chiral auxiliary (S)-(+)-1-(9-anthryl)-2,2,2-trifluoro ethanol [(S)-TFAE].     

Highly diastereoselective ortho-lithiation of chiral ferrocenecarboxamides

Planar chirality has been introduced into chiral ferrocenecarboxamides via an amide-directed ortho-lithiation with diastereoselectivity of up to 99%. The use of an easily accessible, camphane-based chiral auxiliary as an ortho-directing group led to single diastereoisomers with a variety of electrophiles. The approach allows the preparation of widely applicable ferrocene derivatives.     

TARTROL-derived chiral phosphine–phosphite ligands and their performance in enantioselective Cu-catalyzed 1,4-addition reactions

By using (R,R,R,R)-2,3-dimethoxy-2,3-dimethyl-1,4-dioxane-5,6-bis-diphenylmethanol (TARTROL) as a chiral building block, a set of six modular phosphine–phosphite ligands (with a 1,2-phenylene backbone) were synthesized and evaluated in the Cu-catalyzed asymmetric 1,4-addition of Grignard reagents to cyclohexenone. Ligands with bulky substituents at the ortho- and para-positions to the chiral phosphite moiety were found to be the most selective affording the 1,4-addition products with enantioselectivities of up to 84% ee.     

Chiral P,O-ligands derived from N,O-phenylene prolinols for palladium-catalyzed asymmetric allylic alkylation

CuI-catalyzed N,O-arylation of (S)-prolinols and 1-bromo-2-iodobenzene afforded cyclic N,O-(1,2-phenylene)prolinols, and subsequent ortho-lithiation and phosphination provided a new type of chiral P,O-ligands. Their palladium-complex-catalyzed asymmetric allylic alkylation of dimethyl malonate with 1,3-diphenyl 2-propenyl acetate gave high yields and good enantioselectivities.     

α-(3,7-Dioxa-r-1-azabicyclo[3.3.0]oct-c-5-ylmethoxy)-diazines. Part 2: Functionalisation via directed ortho-metallation and cross-coupling reactions

The functionalisation of the title compounds via regioselective Directed ortho-Metallation (DoM) and cross-coupling reactions is studied. The compatibility of the 3,7-DiOxa-r-1-AzaBicyclo[3.3.0]Oct-c-5-ylmethoxy system (DOABO-CH₂O) to typical reaction conditions is established. Its role as Directed ortho-Metallation Group (DoMG) is examined, including competition with classical DoMGs, chlorine and methoxy. The chelating ability of some functionalised terms such as DOABO-CH₂O substituting chiral diarylmethanols and polyaza analogues of 2,6-terpyridine is discussed as intramolecular steric relationships determining configuration and aptitude to bind selectively transition metals, respectively.     

Rapid access to benzo-annelated heterocycles, naphthalenes, and polysubstituted benzenes through a novel benzannulation reaction

A novel, efficient, and powerful methyl mercaptoacetate triggered benzannulation reaction is described. The precursors are heterocyclic, aromatic or acyclic compounds bearing a carbonyl group at ortho position to an internal alkyne. The methodology does not require transition-metal catalysts and moreover it is general for the preparation of wide range of benzo-annelated heterocycles, naphthalenes and benzenes.     

Asymmetric synthesis of a chiral arsinophosphine via a metal template promoted asymmetric Diels-Alder reaction between diphenylvinylphosphine and 2-furyldiphenylarsine

The asymmetric Diels-Alder reaction between 2-furyldiphenylarsine and diphenylvinylphosphine was achieved stereospecifically by utilizing an organoplatinum reaction promoter containing the ortho-metalated (R)-(1-(dimethylamino) ethyl)-naphthalene as the chiral auxiliary. The optically pure (+)-As-P heterobidentate cycloadduct could be liberated from the template product by successive treatment with concentrated hydrochloric acid and aqueous potassium cyanide.     

Mercury(II) mediated cyclisation of R-1-(1′-hydroxyethyl)-2-(1″-propenyl)-3-alkoxy-4-methoxybenzenes to chiral isochromanes

A protocol has been established for the transformation of chiral ortho 1-hydroxyethyl propenyl benzenes under both anaerobic and oxidative mercury(II) mediated conditions to produce chiral isochromanes. Further transformations of the former products yielded chiral isochromanquinones, while the latter afforded the corresponding chiral 4-hydroxyisochromanquinones.     

Template effects on the asymmetric cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylphosphine and their arsenic elimination reaction

The organopalladium complex containing ortho-metalated (S)-[1-(dimethylamino)ethyl]naphthalene as the chiral template was employed to promote the asymmetric cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylphosphine. It has been proven that the chiral template incorporating napthylamine is more efficient than the benzylamine based analogue as evidenced by the drastic improvement in stereoselectivity and reaction rate. However, when no chiral template was employed, trans-[PdI₂(3,4-dimethyl-1-phenylarsole)₂] reacted with trans-[PdI₂(diphenylvinylphosphine)₂] producing a structurally novel diiodo complex, as a result of an interesting selective cleavage of one As-C bond in the norbornene skeleton and subsequent rearrangements within the skeletal framework. The molecular structure of the diiodo product has been confirmed by X-ray crystallography. Structural analysis showed that in addition to the normal As-P five-membered ring, there is one new five-membered ring containing As-O bond being formed during the course of the reaction along with another seven-membered ring incorporating a hydroxy group.     

Novel chiral cyclic polysulfides with a biphenyl backbone: investigation of atropisomerism and pentathiepin ring inversion

Novel axially chiral benzopolysulfides were synthesized on biaryls by sulfurization of dithiastannoles. Pentathiepin, trithiole, and trithiole 2-oxide rings were observed as single isomer on 1,1′-biaryls. The rotational energy barrier of chiral axis was increased by incorporation of a methyl group at ortho-position. In that case, both trithiole oxide and pentathiepin rings appeared as diastereomer. ortho-Tolyl functionality was also replaced by naphthyl moiety to create more rotational hindrance. Chiral axis was incorporated at the neighborhood of polysulfide functionality by Suzuki-Miyaura cross-coupling reaction. Calculated rotational energy barriers were very much consistent with experimental observations to show atropisomerism. Energy barrier for the inversion of pentathiepin ring was experimentally determined by variable temperature ¹H NMR. The kinetic data suggested that pentathiepin ring inversion was prompt in solution. Insufficient rotational energy barriers of chiral axis and pentathiepin ring inversion make substantially impossible to separate optically pure diastereomer even by chiral chromatography [Preliminary report: Sato, R.; Ohta, H.; Yamamoto T.; Nakajo, S.; Ogawa, S.; Alam, A. Tetrahedron Lett.2007, 48, 4991-4994.].     

Convenient synthesis of selected meta- and ortho-substituted pentaarylpyridines via the Suzuki-Miyaura cross-coupling reaction

The first synthesis of sterically demanding, stable at room temperature atropisomeric derivatives of penta-(ortho-substituted phenyl)pyridines is described. The Suzuki-Miyaura cross-coupling reaction of pentabromopyridine and selected meta- and ortho-tolylboronic acids afforded a series of pentaarylpyridine derivatives. The structures of two room temperature stable atropisomeric derivatives of penta-(o-tolyl)pyridines were confirmed by single-crystal X-ray analysis. Racemic atropisomers were examined by ¹H NMR spectroscopy with a chiral solvating agent in order to visualize the presence of individual enantiomers.     

Asymmetric synthesis of pharmaceutically relevant 1-aryl-2-heteroaryl- and 1,2-diheteroarylcyclopropane-1-carboxylates

This study describes general methods for the enantioselective syntheses of pharmaceutically relevant 1-aryl-2-heteroaryl- and 1,2-diheteroarylcyclopropane-1-carboxylates through dirhodium tetracarboxylate-catalysed asymmetric cyclopropanation of vinyl heterocycles with aryl- or heteroaryldiazoacetates. The reactions are highly diastereoselective and high asymmetric induction could be achieved using either (R)-pantolactone as a chiral auxiliary or chiral dirhodium tetracarboxylate catalysts. For meta- or para-substituted aryl- or heteroaryldiazoacetates the optimum catalyst was Rh₂(R-p-Ph-TPCP)₄. In the case of ortho-substituted aryl- or heteroaryldiazoacetates, the optimum catalyst was Rh₂(R-TPPTTL)₄. For a highly enantioselective reaction with the ortho-substituted substrates, 2-chloropyridine was required as an additive in the presence of either 4 Å molecular sieves or 1,1,1,3,3,3-hexafluoroisopropanol (HFIP). Under the optimized conditions, the cyclopropanation could be conducted in the presence of a variety of heterocycles, such as pyridines, pyrazines, quinolines, indoles, oxadiazoles, thiophenes and pyrazoles.

The dirhodium tetracarboxylate-catalysed asymmetric cyclopropanation has been applied to the enantioselective syntheses of pharmaceutically relevant 1-aryl-2-heteroaryl- and 1,2-diheteroarylcyclopropane-1-carboxylates.     

Asymmetric synthesis of a chiral diarsine ligand via a cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine

The organopalladium complex containing ortho-metallated (S)-[1-(dimethylamino)ethyl]naphthalene as a chiral auxiliary has been successfully employed to promote the asymmetric cycloaddition reaction between 3,4-dimethyl-1-phenylarsole and diphenylvinylarsine. In the intramolecular cycloaddition reaction, a pair of separable diastereomeric palladium complexes was obtained in the ratio of 6:1. The chiral naphthylamine auxiliary could be removed chemoselectively from the template by treatment with HCl and subsequently NaI to generate the neutral diiodo complex [(As–As)PdI₂]. Treatment of the diiodo complex with KCN gave the enantiomerically pure As–As bidentate ligand in quantitative yield. In contrast to the reported similar P–P and As–P analogues, both arsenic donors in the diiodo complex could be readily eliminated to produce a structurally novel dimetallic complex.     

A novel class of bidentate ligands with a conformationally flexible biphenyl unit built into a planar chiral [2.2]paracyclophane backbone

We report the synthesis of a novel class of planar chiral bidentate aryl[2.2]paracyclophane ligands. For the first time in the [2.2]paracyclophanyl series the Pd-catalyzed Suzuki cross-coupling was employed for the formation of the arylparacyclophanyl skeleton. From the two possible approaches: (a) cross-coupling of [2.2]paracyalophanylboronic acids with aryl halides; (b) cross-coupling of [2.2]paracyclophanyl halides with arylboronic acids, the latter was found to be more efficient. This approach was successfully used for the synthesis of a wide range of aryl[2.2]paracyclophanes with different types of substitution patterns (ortho-, pseudo-ortho- or pseudo-gem-arrangement of the functionally-substituted aryl fragment with respect to the substituent in the paracyclophane ring).