The RuO4-Catalyzed Ketohydroxylation. Part 1. Development, Scope, and Limitation

A Concise Synthesis of (2R,6R)-Hydroxynorketamine

A concise synthesis of (2R,6R)-hydroxynorketamine was accomplished in eight steps, starting from commercially available materials. This synthesis features a cerium chloride-enhanced Stork-Danheiser reaction, an asymmetric reduction of ketone by the Corey-Bakshi-Shibata reaction, a signature Overman rearrangement, and a facial selective dihydroxylation of an electronically deficient olefin by RuCl3/NaIO4. The overall yield is 7.3% with 94.5% ee.

Total Synthesis of the Phenylnaphthacenoid Type II Polyketide Antibiotic Formicamycin H via Regioselective Ruthenium-Catalyzed Hydrogen Auto-Transfer [4 + 2] Cycloaddition

The first total synthesis of the pentacyclic phenylnaphthacenoid type II polyketide antibiotic formicamycin H is described. A key feature of the synthesis involves the convergent, regioselective assembly of the tetracyclic core via ruthenium-catalyzed α-ketol-benzocyclobutenone [4 + 2] cycloaddition. Double dehydration of the diol-containing cycloadduct provides an achiral enone, which upon asymmetric nucleophilic epoxidation and further manipulations delivers the penultimate tetracyclic trichloride in enantiomerically enriched form. Subsequent chemo- and atroposelective Suzuki cross-coupling of the tetracyclic trichloride introduces the E-ring to complete the total synthesis. Single-crystal X-ray diffraction analyses of two model compounds suggest that the initially assigned stereochemistry of the axially chiral C6-C7 linkage may require revision.

Concise Total Syntheses of the 6-7-5 Hamigeran Natural Products

Herein, we report the total syntheses of four hamigeran natural products featuring a 6-7-5 tricyclic carbon skeleton. We utilized a palladium-catalyzed intramolecular cyclopropanol ring opening cross-coupling to build the central seven-membered ring and a series of oxidations including a challenging aromatic C-H oxidation to introduce the peripheral functionalities. This approach enabled us to achieve the first total syntheses of hamigeran C (14 steps), debromohamigeran I (12 steps), and hamigeran I (13 steps). Our synthesis also resulted in hamigeran G in 13 steps, which is significantly shorter than the previously reported one (24 steps, longest linear sequence).

Accessing Diverse Cross-Benzoin and α-Siloxy Ketone Products via Acyl Substitution Chemistry

An approach to diverse cross-benzoin and α-siloxy ketone products which leverages a simple yet underutilized C-C bond disconnection strategy is reported. Acyl substitution of readily accessible α-siloxy Weinreb amides with organolithium compounds enables access to a broad scope of aryl, heteroaryl, alkyl, alkenyl, and alkynyl derivatives. Enantiopure benzoins can be accessed via a chiral pool approach, and the utility of accessible cross-benzoins and α-siloxy ketones is highlighted in a suite of downstream synthetic applications.

Tandem Olefin Metathesis/α-Ketohydroxylation Revisited

EWG-activated and polar quaternary ammonium salt-tagged ruthenium metathesis catalysts have been applied in a two-step one-pot metathesis-oxidation process leading to functionalized α-hydroxyketones (acyloins). In this assisted tandem process, the metathesis catalyst is used first to promote ring-closing metathesis (RCM) and cross-metathesis (CM) steps, then upon the action of Oxone™ converts into an oxidation catalyst able to transform the newly formed olefinic product into acyloin under mild conditions.

Passerini-type reaction of boronic acids enables α-hydroxyketones synthesis

Multicomponent reactions (MCRs) facilitate the rapid and diverse construction of molecular scaffolds with modularity and step economy. In this work, engagement of boronic acids as carbon nucleophiles culminates in a Passerini-type three-component coupling reaction towards the synthesis of an expanded inventory of α-hydroxyketones with skeletal diversity. In addition to the appealing features of MCRs, this protocol portrays good functional group tolerance, broad substrate scope under mild conditions and operational simplicity. The utility of this chemistry is further demonstrated by amenable modifications of bioactive products and pharmaceuticals as well as in the functionalization of products to useful compounds.

Multicomponent reactions enable the rapid construction of diverse molecular scaffolds with modularity and step economy. In this work, the authors report the use of boronic acids as carbon nucleophiles in a Passerini-type three-component coupling reaction towards an expanded inventory of α-hydroxyketones.

Mechanistic Insights into Selective Oxidation of Polyaromatic Compounds using RICO Chemistry

Ruthenium-ion-catalyzed oxidation (RICO) of polyaromatic hydrocarbons (PAHs) has been studied in detail using experimental and computational approaches to explore the reaction mechanism. DFT calculations show that regioselectivity in these reactions can be understood in terms of the preservation of aromaticity in the initial formation of a [3+2] metallocycle intermediate at the most-isolated double bond. We identify two competing pathways: C-C bond cleavage leading to a dialdehyde and C-H activation followed by H migration to the RuO_x complex to give diketones. Experimentally, the oxidation of pyrene and phenanthrene has been carried out in monophasic and biphasic solvent systems. Our results show that diketones are the major product for both phenanthrene and pyrene substrates. These diketone products are shown to be stable under our reaction conditions so that higher oxidation products (acids and their derivatives) are assigned to the competing pathway through the dialdehyde. Experiments using ¹⁸ O-labelled water do show incorporation of oxygen from the solvents into products, but this may take place during the formation of the reactive RuO₄ species rather than directly during PAH oxidation. When the oxidation of pyrene is carried out using D₂ O, a kinetic isotope effect (KIE) is observed implying that water is involved in the rate-determining step leading to the diketone products.

Pd-Catalyzed Aerobic Oxidation Reactions: Strategies To Increase Catalyst Lifetimes

The palladium complex [(neocuproine)Pd(μ-OAc)]₂[OTf]₂ (1, neocuproine = 2,9-dimethyl-1,10-phenanthroline) is an effective catalyst precursor for the selective oxidation of primary and secondary alcohols, vicinal diols, polyols, and carbohydrates. Both air and benzoquinone can be used as terminal oxidants, but aerobic oxidations are accompanied by oxidative degradation of the neocuproine ligand, thus necessitating high Pd loadings. Several strategies to improve aerobic catalyst lifetimes were devised, guided by mechanistic studies of catalyst deactivation. These studies implicate a radical autoxidation mechanism initiated by H atom abstraction from the neocuproine ligand. Ligand modifications designed to retard H atom abstractions as well as the addition of sacrificial H atom donors increase catalyst lifetimes and lead to higher turnover numbers (TON) under aerobic conditions. Additional investigations revealed that the addition of benzylic hydroperoxides or styrene leads to significant increases in TON as well. Mechanistic studies suggest that benzylic hydroperoxides function as H atom donors and that styrene is effective at intercepting Pd hydrides. These strategies enabled the selective aerobic oxidation of polyols on preparative scales using as little as 0.25 mol % of Pd, a major improvement over previous work.

Oxidation of Vicinal Diols to α-Hydroxy Ketones with H2O2 and a Simple Manganese Catalyst

α-Hydroxy ketones are valuable synthons in organic chemistry. Here we show that oxidation of vic-diols to α-hydroxy ketones with H₂O₂ can be achieved with an in situ prepared catalyst based on manganese salts and pyridine-2-carboxylic acid. Furthermore the same catalyst is effective in alkene epoxidation, and it is shown that alkene oxidation with the Mn^II catalyst and H₂O₂ followed by Lewis acid ring opening of the epoxide and subsequent oxidation of the alkene to α-hydroxy ketones can be achieved under mild (ambient) conditions.

Oxidation of Polynuclear Aromatic Hydrocarbons using Ruthenium-Ion-Catalyzed Oxidation: The Role of Aromatic Ring Number in Reaction Kinetics and Product Distribution

Oxidation of aromatic hydrocarbons with differing numbers of fused aromatic rings (2-5), have been studied in two solvent environments (monophasic and biphasic) using ruthenium-ion-catalyzed oxidation (RICO). RICO reduces the aromaticity of the polyaromatic core of the molecule in a controlled manner by selective oxidative ring opening. Moreover, the nature of the solvent system determines the product type and distribution, for molecules with more than two aromatic rings. Competitive oxidation between substrates with different numbers of aromatic rings has been studied in detail. It was found that the rate of polyaromatic hydrocarbon oxidation increases with the number of fused aromatic rings. A similar trend was also identified for alkylated aromatic hydrocarbons. The proof-of-concept investigation provides new insight into selective oxidation chemistry for upgrading of polyaromatic molecules.

A Sequential Cycloaddition Strategy for the Synthesis of Alsmaphorazine B Traces a Path Through a Family of Alstonia Alkaloids

Driven by a new biogenetic hypothesis, the first total synthesis of alsmaphorazine B and several related indole alkaloids has been achieved. Numerous early approaches proved unsuccessful owing to unproductive side reactivity; nevertheless, they provided important clues that guided the evolution of our strategy. Critical to our success was a major improvement in our Zincke aldehyde cycloaddition strategy, which permitted the efficient gram-scale synthesis of akuammicine. The sequential chemoselective oxidations of akuammicine leading up to the key oxidative rearrangement also yielded several biogenetically related indole alkaloids en route to alsmaphorazine B.

Dual Role of H2O2 in Palladium-Catalyzed Dioxygenation of Terminal Alkenes

A palladium-catalyzed, environmentally friendly dioxygenation reaction of simple alkenes has been developed that enabled rapid assembly of valuable α-hydroxy ketones with high atom economy. Notably, control experiments and ¹⁸O isotope-labeling experiments established that H₂O₂ played a dominant dual role in this transformation.

Green organocatalytic α-hydroxylation of ketones

An efficient and green method for the α-hydroxylation of substituted ketones has been developed.

Selectfluor-mediated highly selective radical dioxygenation of alkenes

Selectfluor mediated highly selective radical dioxygenation of alkenes using hydroxylamines and O₂ was achieved under mild conditions.

Ruthenium tetroxide and perruthenate chemistry. Recent advances and related transformations mediated by other transition metal oxo-species

In the last years ruthenium tetroxide is increasingly being used in organic synthesis. Thanks to the fine tuning of the reaction conditions, including pH control of the medium and the use of a wider range of co-oxidants, this species has proven to be a reagent able to catalyse useful synthetic transformations which are either a valuable alternative to established methods or even, in some cases, the method of choice. Protocols for oxidation of hydrocarbons, oxidative cleavage of C-C double bonds, even stopping the process at the aldehyde stage, oxidative cleavage of terminal and internal alkynes, oxidation of alcohols to carboxylic acids, dihydroxylation of alkenes, oxidative degradation of phenyl and other heteroaromatic nuclei, oxidative cyclization of dienes, have now reached a good level of improvement and are more and more included into complex synthetic sequences. The perruthenate ion is a ruthenium (VII) oxo-species. Since its introduction in the mid-eighties, tetrapropylammonium perruthenate (TPAP) has reached a great popularity among organic chemists and it is mostly employed in catalytic amounts in conjunction with N-methylmorpholine N-oxide (NMO) for the mild oxidation of primary and secondary alcohols to carbonyl compounds. Its use in the oxidation of other functionalities is known and recently, its utility in new synthetic transformations has been demonstrated. New processes, synthetic applications, theoretical studies and unusual transformations, published in the last eight years (2006-2013), in the chemistry of these two oxo-species, will be covered in this review with the aim of offering a clear picture of their reactivity. When appropriate, related oxidative transformations mediated by other metal oxo-species will be presented to highlight similarities and differences. An historical overview of some aspects of the ruthenium tetroxide chemistry will be presented as well.

Synthetic studies directed toward the AB decalin common to HMP-Y1 and hibarimicinone

Efforts toward the synthesis of the decalin ring system common to the hibarimicin shunt metabolite HMP-Y1 and parent aglycone hibarimicinone are reported herein. An intramolecular Diels-Alder cyclization rapidly generated the decalin framework. Two approaches toward completion of the AB decalin were vetted. Incorporation of a phenylsulfonyl leaving group β- to both a ketone and a γ-lactone followed by base-induced elimination of sulfinate led to the undesired α,β-unsaturated lactone. Methanolysis of the γ-lactone followed by elimination produced the unexpected bridged cyclic ether by way of an intramolecular oxy-Michael addition of the endo oriented C13 alcohol.

Aminodiols via stereocontrolled oxidation of methyleneaziridines

A highly diastereoselective Ru-catalyzed oxidation/reduction sequence of bicyclic methyleneaziridines provides a facile route to complex 1-amino-2,3-diol motifs. The relative anti stereochemistry between the amine and the vicinal alcohol are proposed to result from 1,3-bischelation in the transition state by the C1 and C3 heteroatoms.

Chemoselective Pd-catalyzed oxidation of polyols: synthetic scope and mechanistic studies

The regio- and chemoselective oxidation of unprotected vicinal polyols with [(neocuproine)Pd(OAc)]2(OTf)2 (1) (neocuproine = 2,9-dimethyl-1,10-phenanthroline) occurs readily under mild reaction conditions to generate α-hydroxy ketones. The oxidation of vicinal diols is both faster and more selective than the oxidation of primary and secondary alcohols; vicinal 1,2-diols are oxidized selectively to hydroxy ketones, whereas primary alcohols are oxidized in preference to secondary alcohols. Oxidative lactonization of 1,5-diols yields cyclic lactones. Catalyst loadings as low as 0.12 mol % in oxidation reactions on a 10 g scale can be used. The exquisite selectivity of this catalyst system is evident in the chemoselective and stereospecific oxidation of the polyol (S,S)-1,2,3,4-tetrahydroxybutane [(S,S)-threitol] to (S)-erythrulose. Mechanistic, kinetic, and theoretical studies revealed that the rate laws for the oxidation of primary and secondary alcohols differ from those of diols. Density functional theory calculations support the conclusion that β-hydride elimination to give hydroxy ketones is product-determining for the oxidation of vicinal diols, whereas for primary and secondary alcohols, pre-equilibria favoring primary alkoxides are product-determining. In situ desorption electrospray ionization mass spectrometry (DESI-MS) revealed several key intermediates in the proposed catalytic cycle.