Rapid and Stereoselective Synthesis of Spirocyclic Ethers via the Intramolecular Piancatelli Rearrangement

The first example of a Piancatelli rearrangement of alcohols is demonstrated utilizing dysprosium(III) triflate as a catalyst to access oxaspirocycles in a highly diastereoselective manner. The cascade reaction constructs the spirocyclic ether ring system and the tertiary stereocenter in a single operation and is experimentally easy to perform

Aza-Piancatelli Rearrangement Initiated by Ring Opening of Donor–Acceptor Cyclopropanes

The development of a new platform to initiate the cascade rearrangement of furans for the formation of functionalized cyclopentenone building blocks is reported. This methodology allows the creation of congested vicinal stereogenic centers with high diastereoselectivity through a 4π-electrocyclization process

Asymmetric Electrophilic α‑Amination of Silyl Enol Ether Derivatives via the Nitrosocarbonyl Hetero-ene Reaction

The first example of a general asymmetric nitrosocarbonyl hetero-ene reaction is described. The procedure uses a copper-catalyzed aerobic oxidation of a commercially available chiral nitrosocarbonyl precursor (EleNOr) and is operationally simple. The transformation is both high yielding and highly diastereoselective for a range of silyl enol ether derivatives. A variety of synthetically useful postfunctionalization reactions are presented along with a mechanistic rationale that can be used as a predictive model for future asymmetric reactions with nitrosocarbonyl intermediates

Nitrosocarbonyl Hetero-Diels–Alder Cycloaddition: A New Tool for Conjugation

It is demonstrated that nitrosocarbonyl hetero-Diels–Alder chemistry is an efficient and versatile reaction that can be applied in macromolecular synthesis. Polyethylene glycol functionalized with a hydroxamic acid moiety undergoes facile coupling with cyclopentadiene-terminated polystyrene, through a copper-catalyzed as well as thermal hetero-Diels–Alder reaction. The mild and orthogonal methods used to carry out this reaction make it an attractive method for the synthesis of block copolymers. The resulting block copolymers were analyzed and characterized using GPC and NMR. The product materials could be subjected to thermal retro [4 + 2] cycloaddition, allowing for the liberation of the individual polymer chains and subsequent recycling of the diene-terminated polymers

Electrophilic α‑Amination Reaction of β‑Ketoesters Using <i>N</i>‑Hydroxycarbamates: Merging Aerobic Oxidation and Lewis Acid Catalysis

The copper-catalyzed α-amination of carbonyl compounds using nitrosoformate intermediates as the electrophilic source of nitrogen is reported. The reaction merges aerobic oxidation and Lewis acid catalysis. The scope of the reaction is broad in terms of both the N-substituted hydroxylamines and the β-ketoesters. The new methodology harnesses the power of nitrosoformate intermediates and demonstrates their potential as a viable electrophilic source of nitrogen in α-functionalization reactions

Copper-Catalyzed Aerobic Oxidation of <i>N</i>-Substituted Hydroxylamines: Efficient and Practical Access to Nitroso Compounds

A general and efficient aerobic oxidation of <i>N</i>-substituted hydroxylamines is described. The mild reaction conditions employed provide a catalytic and sustainable alternative to stoichiometric oxidation methods to gain access to a range of nitroso compounds, such as acylnitroso, nitrosoformate, nitrosoformamide, iminonitroso, arylnitroso, and <i>P</i>-nitrosophosphine oxide derivatives in excellent yield

Synthesis of Hindered α‑Amino Carbonyls: Copper-Catalyzed Radical Addition with Nitroso Compounds

The synthesis of sterically hindered anilines has been a significant challenge in organic chemistry. Here we report a Cu-catalyzed radical addition with in situ-generated nitroso compounds to prepare sterically hindered amines directly from readily available materials. The transformation is conducted at room temperature, uses abundant copper salts, and is tolerant of a range of functional groups

Design and Synthesis of Donor–Acceptor Stenhouse Adducts: A Visible Light Photoswitch Derived from Furfural

The development of an easily synthesized, modular, and tunable organic photoswitch that responds to visible light has been a long-standing pursuit. Herein we provide a detailed account of the design and synthesis of a new class of photochromes based on furfural, termed donor–acceptor Stenhouse adducts (DASAs). A wide variety of these derivatives are easily prepared from commercially available starting materials, and their photophysical properties are shown to be dependent on the substituents of the push–pull system. Analysis of the switching behavior provides conditions to access the two structural isomers of the DASAs, reversibly switch between them, and use their unique solubility behavior to provide dynamic phase-transfer materials. Overall, these negative photochromes respond to visible light and heat and display an unprecedented level of structural modularity and tunabilty

Design and Synthesis of Donor–Acceptor Stenhouse Adducts: A Visible Light Photoswitch Derived from Furfural

The development of an easily synthesized, modular, and tunable organic photoswitch that responds to visible light has been a long-standing pursuit. Herein we provide a detailed account of the design and synthesis of a new class of photochromes based on furfural, termed donor–acceptor Stenhouse adducts (DASAs). A wide variety of these derivatives are easily prepared from commercially available starting materials, and their photophysical properties are shown to be dependent on the substituents of the push–pull system. Analysis of the switching behavior provides conditions to access the two structural isomers of the DASAs, reversibly switch between them, and use their unique solubility behavior to provide dynamic phase-transfer materials. Overall, these negative photochromes respond to visible light and heat and display an unprecedented level of structural modularity and tunabilty

Brønsted-Acid-Catalyzed Exchange in Polyester Dynamic Covalent Networks

The effect of catalyst strength on polyester–alcohol dynamic covalent exchange was systematically studied using Brønsted acids and a low-<i>T</i>_g poly­(4-methylcaprolactone) vitrimer formulation. Relaxation times, activation energies, and Arrhenius prefactors are correlated with p<i>K</i>_a. Strong protic acids induce facile network relaxation at 25 °C on the order of 10⁴–10⁵ s, significantly faster than Lewis acid alternatives that function only above 100 °C. Activation energies span 49–67 kJ/mol and increase as p<i>K</i>_a decreases. The opposite trend is observed with the Arrhenius prefactor. We anticipate that the quantitative understanding of Brønsted acid effects disclosed herein will be of utility in future studies that exploit acid-catalyzed dynamic covalent bond exchange