Thermal electrocyclic ring-opening of cyclobutenes: stereoselective routes to functionalised conjugated (Z,E)- and (E,E)-2,4-dienals

New insights of QTAIM and stress tensor to finding non-competitive/competitive torquoselectivity of cyclobutene

This study aims to investigate the phenomenon of torquoselectivity through three thermal cyclobutene ring-opening reactions (N1-N3). This research focuses on the nature of the chemical bond, electronic reorganization, predicting non-competitive or competitive reactions, and torquoselectivity preference within Quantum Theory of Atoms in Molecules (QTAIM) and stress tensor frameworks. Various theoretical analyses for these reactions, such as metallicity ξ(r_b), ellipticity ε, total local energy density H(r_b), stress tensor polarizability ℙ_σ, stress tensor eigenvalue λ_3σ, and bond-path length, display differently for non-competitive and competitive reactions as well as for the conrotatory preferences either it is the transition state outward conrotatory (TS_out) or transition state inward conrotatory (TS_in) directions by presenting degeneracy or non-degeneracy in their results. The ellipticity profile provides the motion of the bond critical point locations due to the different substituents of cyclobutene. In agreement with experimental results, examinations demonstrated that N1 is a competitive reaction and N2-N3 are non-competitive reactions with TS_out and TS_in preference directions, respectively. The concordant results of QTAIM and stress tensor scalar and vectors with experimental results provide a better understanding of reaction mechanisms.

Photochemically Produced Aminocyclobutanes as Masked Dienes in Thermal Electrocyclic Cascade Reactions

Cyclobutane products of a triplet sensitized enamide-alkene intramolecular [2 + 2] photocycloaddition have been shown to undergo fragmentation under acidic conditions. This lability has been exploited by inducing a complexity-generating thermal electrocyclic cascade sequence involving the in situ formation of a cyclobutene, followed by electrocyclic ring opening, Diels-Alder cycloaddition, and subsequent lactamization. This combination of excited state photochemistry and thermal electrocyclic cascade reactions allows simple planar molecules to be rapidly transformed into sp³-rich scaffolds.

Advances in Quantum Mechanochemistry: Electronic Structure Methods and Force Analysis

In quantum mechanochemistry, quantum chemical methods are used to describe molecules under the influence of an external force. The calculation of geometries, energies, transition states, reaction rates, and spectroscopic properties of molecules on the force-modified potential energy surfaces is the key to gain an in-depth understanding of mechanochemical processes at the molecular level. In this review, we present recent advances in the field of quantum mechanochemistry and introduce the quantum chemical methods used to calculate the properties of molecules under an external force. We place special emphasis on quantum chemical force analysis tools, which can be used to identify the mechanochemically relevant degrees of freedom in a deformed molecule, and spotlight selected applications of quantum mechanochemical methods to point out their synergistic relationship with experiments.

Response to Comment on T. Stauch, A. Dreuw, “Stiff-stilbene photoswitch ruptures bonds not by pulling but by local heating”, Phys. Chem. Chem. Phys., 2016, 18, 15848

Computational analysis of mechanical stress energy revealed that photoisomerization of stiff-stilbene cannot initiate the ring opening of cyclobutene via a purely mechanical pathway.

Stiff-stilbene photoswitch ruptures bonds not by pulling but by local heating

The forces generated by stiff-stilbene during photoswitching are too low to cause bond rupture, which is instead initiated by heating.

An atom-economical and stereoselective domino synthesis of functionalised dienes

Open sesame: A direct synthesis of functionalised and stereodefined dienes, relying on a domino allylic alkylation/electrocyclic ring-opening sequence, is reported. This method allows concise access to doubly vinylogous esters. A further systematic study of ring-opening rates of carbon-substituted cyclobutenes allowed the design of substrates amenable to sequential pericyclic reactions (see scheme).

Functionalized Cyclobutenes via Multicomponent Thermal [2 + 2] Cycloaddition Reactions

[reaction: see text] Enamine [2 + 2] cycloadditions can be achieved in useful yields simply by stirring a mixture of an aldehyde, diethylamine, a dialkyl fumarate, and potassium carbonate in acetonitrile at 25 degrees C, conditions that are compatible with the presence of a potential leaving group on the beta-position of the intermediate enamine. Methylation and elimination of the product cyclobutanes completes a mild nonphotochemical route to functionalized cyclobutenes.

A new synthesis of 2-azabicyclo[2.1.1]hexanes

An efficient synthesis of the 2-azabicyclo[2.1.1]hexane ring system has been accomplished starting from cis-cyclobut-3-ene-1,2-dicarboxylic anhydride 7, which was prepared using a photochemical method. The key step of this new strategy involved a stereoselective electrophilic addition of phenylselenyl bromide to the double bond of cyclobutene dicarbamate 16 derived from 7. The subsequent ring closure of 17a in the presence of sodium hydride afforded the 2-azabicyclohexane compound 18 with a satisfying overall yield. Reductive removal of the phenylselenyl group and subsequent deprotection led rapidly to the amino derivative 4a functionalized on the carbon ring. Syntheses of the hydroxy and carboxylic derivatives 4b,c were then achieved from the intermediary disulfonamide 23. Displacement of the activated amino group by potassium acetate yielded hydroxy derivative 4b after three additional steps. Finally, oxidation of the alcohol function of 4b under Jones conditions followed by hydrogenolysis afforded the carboxylic derivative 4c, which is the first reported beta-isomer of 2,4-methanoproline 1.

A short stereoselective preparation of dienamides from cyclobutene compounds. Application in the synthesis of a new cyclohexene nucleoside

A short stereoselective synthesis of N-acylamino-1,3-dienes was developed starting from the cyclobutene lactam 8, which was obtained from 2-hydroxypyridine by a photochemical electrocyclic reaction. The tert-butoxycarbonyl derivative 17 was prepared to facilitate nucleophilic attacks to the carbonyl group, and the subsequent thermal ring opening provided dienes 18-21. One of these (20) was used in the synthesis of the cyclohexene nucleoside 30. A Diels-Alder reaction between diene 20 and maleic anhydride provided the endo-cycloadduct 22a. Three additional steps yielded amine 26. Construction of the uracil moiety afforded intermediate 29. Cyclization and removal of the protecting groups occurred in one step in the presence of ammonia, giving the target molecule 30. Diene 20 also underwent [4 + 2] cycloaddition with methyl acrylate to provide predominantly the endo-product 23a, regioselectively.

Synthesis, resolution, and absolute stereochemistry of (1R, 2S)-(+)-cis-1-methoxycarbonyl-2-methylcyclobutane

Racemic cis-1-methoxycarbonyl-2-methylcyclobutane uncontaminated with the trans isomer was prepared efficiently in five steps; the corresponding amides from (R)-(-)-phenylglycinol were separated. An X-ray crystallographic structure determination of the amide from (+)-cis-1-methoxycarbonyl-2-methylcyclobutane showed that it was of (1R,2S) absolute stereochemistry, a revision of configurational assignment.

Synthesis of cis-disubstituted cyclobutenyl nucleoside analogues

cis-Disubstituted cyclobutene nucleosides analogues were prepared by a linear synthesis starting from cis-cyclobutene dicarboxylic anhydride. This strategy involved mild reaction conditions with intent to restrict the thermal electrocyclic ring opening into (Z,E)-dienes.

Synthesis of Norcarbovir Analogues, the First Examples of Cyclobutene Nucleosides Unsubstituted at the Vinylic Position

Two cyclobutene nucleosides, 27 and 29, analogous to the yet unknown norcarbovir, and with adenine and hypoxanthine as the base moieties, respectively, were synthesized starting from cis-3-cyclobutene-1,2-dicarboxylic anhydride (6). Its reduction to lactone 9 followed by reaction with ammonia and then Hofmann rearrangement led to cyclic carbamate 15 which was the key intermediate of these syntheses. Its tert-butoxycarbonyl derivative 17 led to the ring opening of the heterocyclic moiety at low temperature. Compound 18 was thus obtained, and the successive benzylation and then treatment with hydrochloric acid yielded hydrochloride 21. Construction of bases was achieved in satisfying overall yields provided that mild experimental conditions from 21 to 27 or 29 were used to restrict the unwanted electrocyclic ring opening. Nitropyrimidine 31 was also prepared from 21 via the intermediate 23.

Interplay of Theory and Experiment: Reversal of the Torquoselectivity of the Electrocyclic Ring Opening of 3-Acetylcyclobutene by a Lewis Acid

Theoretical predictions, accompanied by firm experimental verification, are presented on the "torquoselectivity" of the thermal ring opening of 3-acetylcyclobutene (1). 3-Acetylcyclobutene was synthesized from commercially available 1,1-cyclobutanedicarboxylic acid. Thermolysis of 3-acetylcyclobutene resulted in a mixture of E- and Z-dienes with a slight preference for the E-diene. This preference was reversed by the Lewis acid, ZnI(2), as predicted from theoretical calculations.