Specific and Strong Acceleration of Nitrosation by Nitrosyl Sulfonates Using the Bis-Coordinating Lewis Acid TiCl4 or SnCl4

Nitrosyl triflate is an excellent donor of NO+ to C═C and very unreactive benzenoids, in contrast to the much less reactive nitrosyl mesylate. However, both of these reagents are strongly activated by the bis-coordinating Lewis acids TiCl4 and SnCl4 and become super reactive NO+ donors. Strong catalysis and activation are even observed with the inherently less electrophilic nitrosyl trifluoroacetate. Monocoordinating strong Lewis acids, e.g., MeAlCl2, are relatively ineffective.

Synthetically Useful Transformations of Olefins via Cationic 1,2-Oxazetium Intermediates

Nitrosyl triflate serves as a NO+ donor in reactions with many olefinic substrates to form reactive, cationic 1,2-oxazetium cycloadducts that can be converted selectively into a wide range of useful products depending on reagents and conditions.

Synthesis of Tetracyclic C2-Symmetric Ketiminium Salts and Evaluation as Catalysts for Epoxidation

Short synthetic routes to the nonplanar tetracyclic ketiminium salt A and two close analogs are described. These are helical nonplanar structures as shown by X-ray crystallography.

Two Unprecedented Reactions of Nitrosyl Triflate: NO+-Induced Cationic Cascade Cyclization and C═C Oxidative Cleavage

Nitrosyl triflate is shown to react with olefins in a CH₂Cl₂ solution by three different pathways depending on structure and reaction conditions: (1) cationic cyclization, (2) C═C cleavage, and (3) allylic nitrosation. Possible reaction pathways are described.

Mechanism of the Reaction of Olefins with Nitrous Anhydride (O═N-O-N═O) to Form 1,2-Oxazetes

The mechanistic pathway for the formation of 1,2-oxazetes by reaction of olefins with nitrous anhydride has been clarified. The initial reaction intermediate, a β-nitroso nitrite ester that is sensitive to light, undergoes O-NO fission to form a β-nitroso alkoxy radical, even with ambient fluorescent lighting but much faster with blue light irradiation. The oxygen of the alkoxy radical subsequently adds to the adjacent nitroso group to generate a cyclic four-membered nitrosyl radical. The 1,2-oxazete is then produced by elimination to generate the C═N bond. No 1,2-oxazete formation occurs in the dark.

Nitrosyl Triflate and Nitrous Anhydride, Same Mode of Generation, but Very Different Reaction Pathways. Direct Synthesis of 1,2-Oxazetes, Nitroso or Bisoxazo Compounds from Olefins

Nitrosyl triflate (TfONO) can be generated in situ from tetra-n-butylammonium nitrite and triflic anhydride (1:1) in CH₂Cl₂ solution at ca. -30 °C. It acts as a powerful and soluble nitrosating agent with a wide range of olefinic or aromatic substrates. Nitrous anhydride (O═N-O-N═O) can be generated in the same way using tetra-n-butylammonium nitrite and triflic anhydride (2:1) in CH₂Cl₂ solution at ca. -30 °C. Each reagent has been isolated and characterized. They react with olefins to give different products. Nitrosyl triflate is an excellent reagent for generating either vinylic or allylic nitroso compounds or their dimeric bisoxazo derivates. Nitrous anhydride efficiently converts many olefins to 1,2-oxazetes in a single step, making this class of compounds readily available from olefins for the first time. We have also discovered another route to 1,2-oxazetes involving a novel rearrangement/isomerization of allylic nitroso compounds which is catalyzed by Pd-C/H₂.

Contrasting Diastereoselectivity between Cyclic Nitrones and Azomethine Ylides. Stereocontrolled Pathways to cis-anti-anti-cis-Oxazatetraquinanes from a Bicyclic Nitrone

A study of [3 + 2] cycloaddition reactions of a bicyclic nitrone with various cyclopentenes has clarified the diastereomeric preferences as a function of the olefinic structure. It has also identified an important stereochemical difference between nitrones and the analogous azomethine ylides in [3 + 2] cycloaddition reactions.

A Useful Method for the Conversion of Olefins to Nitro Olefins

Triflyl nitrate is easily generated from tetra-n-butylammonium nitrate in CH₂Cl₂ solution and serves as an effective nitrating agent for a wide range of unsaturated substrates to form nitro olefins.

Unraveling the C2-Symmetric Azatetraquinane System. Simple, Enantioselective Syntheses

Concise stereocontrolled synthetic routes to the C₂-symmetric azatetraquinane 1 (or, also, the enantiomer) are described. The successful execution of the synthesis involved innovation in the methodology for [3+2] cycloaddition and stereochemical control.

A Method for the Catalytic Enantioselective Synthesis of Chiral α-Azido and α-Amino Ketones from Racemic α-Bromo Ketones, and Its Generalization to the Formation of Bonds to C, O, and S

A new and practical method has been developed for the transformation of racemic α-bromo ketones to chiral α-azido and α-amino ketones with high enantioselectivity using phase transfer, ion-pair mediated reactions with a recoverable chiral quaternary salt (10 mol %) as catalyst in fluorobenzene-water. The process has been generalized to a variety of other attachments including of C, O, S, and NHR.

Enantioselective Conversion of Oligoprenol Derivatives to Macrocycles in the Germacrene, Cembrene, and 18-Membered Cyclic Sesterterpene Series

A new enantio-and diastereoselective process has been developed for the efficient conversion of farnesol and other oligoprenyl alcohols to chiral 10-, 14-, and 18-membered cyclization products, including germacrenol, (+)-costunolide, 3-β-elemol, and epi-mukulol. The key cyclization reaction utilizes ω-bromo aldehyde substrates, a chiral ligand, and indium powder as the reagent at -78 °C and generates 10-, 14-, and 18-membered cyclic products in 70-74% yield and 94-95% ee.

Highly Diastereoselective Cationic Cyclization Reactions Convert a Common Monocyclic Enone to Bicyclic Precursors for the Synthesis of Retigeranic Acids A and B

A novel method is demonstrated for the diastereoselective conversion of a monocyclic enone to a pair of bicyclic intermediates which enable the stereocontrolled enantioselective syntheses of retigeranic acids A and B.

A systematic study of functionalized oxiranes as initiating groups for cationic polycyclization reactions

Three different methods have been developed that effectively utilize chiral oxiranes derived from Katsuki-Sharpless epoxidation of allylic alcohols as initiating groups for cationic cyclization of unsaturated substrates to form chiral polycycles. This type of transformation has previously been problematic. These employ either epoxy-methoximes, vinyl-substituted oxiranes, or hydroxymethyl oxiranes. All three approaches are described in detail. In addition, this research has led to possible explanations for previously encountered difficulties in this area and provided two new insights into the Lewis acid activation of oxiranes. The methodology described herein constitutes a valuable link between two powerful synthetic constructions, enantioselective Katsuki-Sharpless epoxidation and cationic polycyclization reactions.

Reversal of the importance of steric and electronic effects in the base-promoted α-silylation of sulfides

Lithiation of α-C-H groups in organic substrates by RLi or R2NLi followed by silylation with R'3SiCl generally provides analogous products regardless of the R' group of R'3SiCl. A striking exception using 3,4-benzothiophane as substrate depending on whether R' is methyl, phenyl, or isopropyl is demonstrated. With R' = Me or Ph, the geminal α,α-bis-silylated products result whereas with i-Pr3SiCl the trans-α,α'-bis-silylated sulfide is formed. The latter pathway provides ready access to the C2-symmetric enantiomers of trans-2,5-bis(triisopropylsilyl)-3,4-benzothiophane.

Preferential geminal bis-silylation of 3,4-benzothiophane is caused by the dominance of electron withdrawal by R3Si over steric shielding effects

Benzylic C–H lithiation of 3,4-benzothiophane and subsequent treatment with triphenyl- or trimethylchlorosilane under a variety of conditions leads to α,α- rather than α,α′-bis-silylation products as a consequence of anion stabilization by R₃Si and very fast deprotonation of the intermediate monosilylated product, even with a sterically bulky base such as lithium diisopropylamide.

Selective formation of six-membered oxa- and carbocycles by the In(III)-activated ring closure of acetylenic substrates

Fifteen examples are disclosed of efficient In(III)-catalyzed six-membered ring closure leading to bi-, tri-, and tetracyclic products.

Mechanism of the enantioselective oxidation of racemic secondary alcohols catalyzed by chiral Mn(III)-salen complexes

The experiments described here clarify the mechanism and origin of the enantioselectivity of the oxidation of racemic secondary alcohols catalyzed by chiral Mn(III)-salen complexes using HOBr, Br(2)/H(2)O/KOAc or PhI(OAc)(2)/H(2)O/KBr as a stoichiometric oxidant. Key points of the proposed pathway include (1) the formation of a Mn(V)-salen dibromide, (2) its subsequent reaction with the alcohol to give an alkoxy-Mn(V) species, and (3) carbonyl-forming elimination to produce the ketone via a highly organized transition state with intramolecular transfer of hydrogen from carbon to an oxygen of the salen ligand.

Synthetic access to bent polycycles by cation-pi cyclization

The presence of an ether oxygen within a chain undergoing cation-polyene cyclization has a profound influence on the stereochemistry of this important construction, apparently due to nucleophilic participation of oxygen in the cyclization process and formation of an oxonium intermediate, leading to bent fused ring systems.

Enantioselective synthesis of a chiral C(3)-symmetric bridgehead amine

An efficient enantioselective synthesis of the above C(3)-symmetric chiral quinuclidine starting with N-tert-butoxycarbonyl-4-pyridone has been developed.

Enantioselective pathway for the synthesis of laurenditerpenol

Simple enantioselective routes to the two key intermediates shown above (at center) for the synthesis of laurenditerpenol have been developed using a Diels-Alder step and the same catalyst system for each.

A short total synthesis of (+)-omaezakianol via an epoxide-initiated cationic cascade reaction

(+)-Omaezakianol was synthesized in only three steps from racemic chlorohydrin 4 by Shi epoxidation followed by cascade cyclization and reduction.

[4 + 2] Cycloaddition reactions catalyzed by a chiral oxazaborolidinium cation. Reaction rates and diastereo-, regio-, and enantioselectivity depend on whether both bonds are formed simultaneously

The reaction rates and products in enantioselective Diels-Alder reactions with a range of dienophiles correlate with the expected degree of concertedness of bond formation in the transition state.

Catalytic enantioselective formation of chiral-bridged dienes which are themselves ligands for enantioselective catalysis

This paper reports a method for highly enantioselective Diels-Alder reaction with an acetylene equivalent to produce chiral-bridged dienes. These dienes, by coordination to Rh(I), can serve as catalysts for the enantioselective addition of vinyl or aryl groups to alpha,beta-unsaturated ketones.

Catalytic enantioselective Diels--Alder reactions: methods, mechanistic fundamentals, pathways, and applications

One hundred years after the birth of Kurt Alder and seventy-five years after the discovery of his famous reaction, one of the most important and fascinating transformations in chemistry, research on that process continues to surprise, excite, delight, and inform the chemical community. This article is based on presentations given first at the University of Cologne, Germany (Kurt Alder lecture, 1992), then at the Roger Adams Award Symposium (1993), and later at the Bürgenstock Conference of 2001, and describes research by our group on the development and understanding of enantioselective versions of the Diels-Alder reactions. The elements of this review include (1) development of new chiral Lewis acid catalysts for highly enantioselective (>25:1) [4+2] cycloadditions; (2) the fine mechanistic details and pre-transition-state assemblies of these reactions; (3) the fundamental understanding of catalytic activity and enantioselectivity for highly enantioselective Diels-Alder processes; and (4) applications to the synthesis of complex molecules. The range and power of the Diels-Alder reaction have steadily increased over seven decades. The end of this remarkable development is not in sight, a high compliment to this field of Science and to its great inventor.

A short and efficient synthesis of (-)-7-methylomuralide, a potent proteasome inhibitor

Short, practical, and scalable syntheses of (+/-)-7-methylomuralide and (-)-7-methylomuralide have been developed. Three consecutive tandem reaction pairs establish all of the carbons and the stereochemistry of the target molecule, vastly simplifying the synthetic scheme from N-trichloroethoxycarbonyl glycine. The chiral directing group controls the absolute stereochemistry of the key aldol reaction.