Diels-Alder Reaction of Photochemically Generated (E)-Cyclohept-2-enones: Diene Scope, Reaction Pathway, and Synthetic Application

Stereospecific Conversion of Boronic Esters into Enones using Methoxyallene: Application in the Total Synthesis of 10-Deoxymethynolide

Enones are widely utilized linchpin functional groups in chemical synthesis and molecular biology. We herein report the direct conversion of boronic esters into enones using commercially available methoxyallene as a three-carbon building block. Following boronate complex formation by reaction of the boronic ester with lithiated-methoxyallene, protonation triggers a stereospecific 1,2-migration before oxidation generates the enone. The protocol shows broad substrate scope and complete enantiospecificity is observed with chiral migrating groups. In addition, various electrophiles could be used to induce 1,2-migration and give a much broader range of α-functionalized enones. Finally, the methodology was applied to a 14-step synthesis of the enone-containing polyketide 10-deoxymethynolide.

Construction of Unique Spiro[dibenzo[a,f]azulene-6,2'-indenes] via Unprecedented Annulation of ortho-C-H Bond of Benzylidene Group

The domino reaction of alkyl and aryl isocyanides with two molecules of 2-arylidene-1,3-indanediones in acetonitrile at 80 °C resulted in unique functionalized spiro[dibenzo[a,f]azulene-6,2'-indenes] in good yields, in which the two 2-arylidene-1,3-indanediones acted as different building blocks to construct the polycyclic system. More importantly, the unprecedented anticipation of the ortho-position of benzylidene group to form a novel dibenzo[a,f]azulene ring through a formal [5 + 2] cycloaddition process was first observed. On the other hand, DABCO-promoted reaction of the isocyanides with two molecules of 2-arylidene-1,3-indanediones in acetonitrile at 80 °C afforded functionalized spiro[cyclopenta[a]-indene-2,2'-indene] derivatives.

Photochemical Isomerization of Cyclohept-1-ene-1-carbaldehyde: Strain-Release Cycloadditions and Ene Reactions

Cyclohept-1-ene-1-carbaldehyde undergoes photoinduced E → Z isomerization at λ = 350 nm. The ring strain facilitates Diels-Alder cycloaddiions with 1,3-dienes, [3 + 2] cycloadditions with 1,3-dipoles, and ene reactions with olefins. Products are trans-fused at the cycloheptane core and were obtained in yields of up to 82%. Single crystal X-ray analyses corroborated the constitution and relative configuration of key products. With BF3 as a Lewis acid and 2,3-dimethylbuta-1,3-diene, cyclohept-1-ene-1-carbaldehyde reacted in the dark and rearranged stereoselectively to a tricyclic ketone (87%).

Efforts toward Synthetic Photosynthesis: Visible Light-Driven CO2 Valorization

Current methods of urethane preparation from amines invariably involve high-energy and often toxic or cumbersome molecules to make the process exergonic. CO2 aminoalkylation using olefins and amines represents an attractive albeit endergonic alternative. We report a moisture-tolerant method that uses visible light energy to drive this endergonic process (+25 kcal/mol at STP) using sensitized arylcyclohexenes. They convert much of the photon's energy to strain upon olefin isomerization. This strain energy greatly enhances alkene basicity, allowing for sequential protonation by and interception of ammonium carbamates. Following optimization steps and amine scope evaluation, an example product arylcyclohexyl urethane underwent transcarbamoylation with some demonstrative alcohols to form more general urethanes with concomitant regeneration of the arylcyclohexene. This represents a closure of the energetic cycle, producing H2O as the stoichiometric byproduct.

Time-resolved X-ray and XUV based spectroscopic methods for nonadiabatic processes in photochemistry

The photochemistry of numerous molecular systems is influenced by conical intersections (CIs). These omnipresent nonadiabatic phenomena provide ultra-fast radiationless relaxation channels by creating degeneracies between electronic states and decide over the final photoproducts. In their presence, the Born-Oppenheimer approximation breaks down, and the timescales of the electron and nuclear dynamics become comparable. Due to the ultra-fast dynamics and the complex interplay between nuclear and electronic degrees of freedom, the direct experimental observation of nonadiabatic processes close to CIs remains challenging. In this article, we give a theoretical perspective on novel spectroscopic techniques capable of observing clear signatures of CIs. We discuss methods that are based on ultra-short laser pulses in the extreme ultraviolet and X-ray regime, as their spectral and temporal resolution allow for resolving the ultra-fast dynamics near CIs.

Photochemical Ring Contraction of 5,5-Dialkylcyclopent-2-enones and in situ Trapping by Primary Amines

If substituted in the 5,5-position, cyclopent-2-enones undergo a smooth photochemical rearrangement to ketenes. A concomitant cyclopropane formation occurs due to a 1,3-shift of the C5 carbon atom from the carbonyl carbon atom (C1) to carbon atom C3. In this study, the cyclopropyl-substituted ketene intermediates were trapped in situ by primary amines providing an efficient entry into 2,2-disubstituted cyclopropaneacetic amides (24 examples, 49-95% yield). A remarkable feature of the reaction is the fact that the photochemical rearrangement can occur from either the first excited singlet (S₁) or the respective triplet state (T₁). In line with experimental results (triplet quenching, sensitization), XMS-CASPT2 calculations support the existence of efficient reaction pathways to the intermediate ketene both on the singlet and on the triplet hypersurface.