Norrish-Yang Photocyclization Research Articles

Stereoselective Construction of the ABCDE Pentacyclic Motif of Phainanoids via Norrish-Yang Photocyclization Reaction.

A Norrish-Yang photocyclization reaction has been applied to regio- and stereoselective construction of the ABCDE pentacyclic motif of natural product phainanoids. The observed substrate conformation control implicates this powerful reaction could be applied to the construction of structurally diverse natural product scaffolds.

Visible‐Light‐Mediated Energy Transfer Enables the Synthesis of β‐Lactams via Intramolecular Hydrogen Atom Transfer

AbstractThe synthesis of 2‐azetidinones (β‐lactams) from simple acrylamide starting materials by visible‐light‐mediated energy transfer catalysis is reported. The reaction features a C(sp3)−H functionalization via a variation of the Norrish–Yang photocyclization involving a carbon‐to‐carbon 1,5‐hydrogen atom transfer (supported by deuterium labelling and DFT calculations) and can be used for the construction of a diverse range of β‐lactam products.

Visible-Light-Mediated Energy Transfer Enables the Synthesis of β-Lactams via Intramolecular Hydrogen Atom Transfer.

The synthesis of 2-azetidinones (β-lactams) from simple acrylamide starting materials by visible-light-mediated energy transfer catalysis is reported. The reaction features a C(sp3 )-H functionalization via a variation of the Norrish-Yang photocyclization involving a carbon-to-carbon 1,5-hydrogen atom transfer (supported by deuterium labelling and DFT calculations) and can be used for the construction of a diverse range of β-lactam products.

Regio- and Stereoselectivity of the Norrish-Yang Photocyclization of Dialkyl 1,2-Diketones: Solution versus Solid State Photochemistry of Two Polymorphs.

As shown by X-ray crystallography, crystals of 3β-acetoxy-16,17-seco-17,20-dioxopregn-5-ene-16-nitrile are dimorphic. The regioselectivity of the Norrish-Yang type II photocyclization under visible light of this steroidal 1,2-diketone, which bears primary, secondary, and tertiary nonequivalent abstractable γ-hydrogens, dramatically increases in the crystalline state of both polymorphs. X-ray crystallography and molecular mechanics calculations reveal crystal structure-solid state photochemistry relationships.

Stereoselective Synthesis of trans-Decalin-Based Spirocarbocycles via Photocyclization of 1,2-Diketones.

Diastereoselective synthesis of the trans-decalin-based α-hydroxyl butanone spirocarbocycles bearing all-carbon quaternary stereogenic centers has been achieved via Norrish–Yang photocyclization of trans-decalin-substituted-2,3-butanediones using daylight. Density functional theory (DFT) calculations suggest that this diastereoselective reaction is affected by both substrate conformation and intramolecular hydrogen bonds. The developed chemistry could be applied to synthesizing the derivatives of the trans-decalin-based biologically important natural products.

Regulation of an Ambient‐Light‐Induced Photocyclization Pathway (Norrish–Yang Versus 6π) by Substituent Choice

A series of donor/acceptor chromophores 1d-3d containing two types of photoresponsive motifs, namely an electron-deficient BF2-chelated ketone fused with an electron-rich thiophene have been synthesized and their photochromic performance are investigated. The results reveal that simple variation of R substituents on the diaryl moiety allows one to control the intramolecular photocyclization mechanism with high photochemical selectivity, e.g., under ambient light, methyl-substituted 1d and 2d undergo 6π photocyclization, whereas ethyl-substituted 3d exclusively undergoes Norrish-Yang photocyclization. Single-crystal X-ray analysis of Norrish-Yang cyclization products reveals the formation of four pairs of conformational enantiomers differing in the dihedral angle between benzothiophene and the BF2 core. The UV-vis absorption spectra of 1d-3d cover a broad visible light region (380-572 nm), while theoretical calculations reveal that absorption in this region is dominated by the charge transfer (CT) transition from the thiophene-centered HOMO to the LUMO of the electron-deficient π-conjugated BF2-chelated unit. The spatial separation of the HOMO and LUMO of these dyes promotestriplet state generation and self-photosensitizes intramolecular photocyclization in the visible light region. Three-dimensional time-resolved and steady-state emission spectra of 3d show that the Norrish-Yang photocyclization takes place within milliseconds with excellent conversion efficiency (96%).

Visible light promoted continuous flow photocyclization of 1,2-diketones.

The continuous flow Norrish-Yang photocyclization of 1,2-diketones has been developed and used for the synthesis of a large number of functionalized 2-hydroxycyclobutanones, under blue light irradiation and employing acetone as a solvent. This eco-friendly procedure represents a valid alternative to the reactions carried out in batches thus reducing the reaction times, the formation of secondary products and simplifying the purification steps. The use of differently substituted diketone compounds has allowed us to obtain a wide range of 2 and 3-functionalized cyclobutanones, thus allowing the evaluation of the scope and limitations of this procedure.

The tetraalkylammonium-accelerated Norrish-Yang photocyclization of 2-substituted acetophenones

The tetrabutylammonium-accelerated Norrish-Yang cyclization of 2-ethoxycarbonylmethyloxy- and 2-cyanomethyloxyacetophenones afforded trans-dihydrobenzofuranols in good stereoselectivities through cation-π interactions between the ammonium and the carbonyl and cyano groups. Furthermore, a new type of organocatalyst possessing both conformation-controlling and triplet-sensitizing units was developed, and was found to be effective in accelerating the Norrish-Yang photocyclization of 2-cyanomethyloxyacetophenone.

ChemInform Abstract: Photochemistry of α‐Diketones in Carbohydrates: Anomalous Norrish Type II Photoelimination and Norrish—Yang Photocyclization Promoted by the Internal Carbonyl Group.

A series of four α-diketones placed as 1α-pyruvoyl tethers on D-glucopyranose and D-glucopyranosiduronic acid skeletons was prepared in order to determine the influence of captodative and stereoelectronic effects on the regioselectivity of the hydrogen atom transfer (HAT) in Norrish typeII photochemical processes. We observed that the 1,5-HAT regioselectivity can be switched between the two potentially abstractable syn-1,3-diaxial hydrogens at H6 and H8. Highly unusual photoproducts from Norrish typeII photoelimination and Norrish-Yang photocyclization initiated by the excited internal carbonyl group were obtained, in some cases in excellent synthetic yield. The 1,5-HAT transition state in the Norrish typeII photoelimination was investigated by photochemical experiments in the crystalline state.

Photochemistry of α‐Diketones in Carbohydrates: Anomalous Norrish Type II Photoelimination and Norrish–Yang Photocyclization Promoted by the Internal Carbonyl Group

A series of four α-diketones placed as 1α-pyruvoyl tethers on D-glucopyranose and D-glucopyranosiduronic acid skeletons was prepared in order to determine the influence of captodative and stereoelectronic effects on the regioselectivity of the hydrogen atom transfer (HAT) in Norrish type II photochemical processes. We observed that the 1,5-HAT regioselectivity can be switched between the two potentially abstractable syn-1,3-diaxial hydrogens at H6 and H8. Highly unusual photoproducts from Norrish type II photoelimination and Norrish-Yang photocyclization initiated by the excited internal carbonyl group were obtained, in some cases in excellent synthetic yield. The 1,5-HAT transition state in the Norrish type II photoelimination was investigated by photochemical experiments in the crystalline state.

Two-Photon Chemistry from Upper Triplet States of Thymine

Photolysis of the benzophenone chromophore by means of high energy laser pulses has been used as a tool to populate upper thymine-like triplet states via intramolecular sensitization. These species undergo characteristic nπ* triplet photoreactivity, as revealed by the Norrish-Yang photocyclization of 5-tert-butyluracil.

Approach for expanding triterpenoid complexity via divergent Norrish-Yang photocyclization.

Triterpenoids comprise a very diverse family of polycyclic molecules that is well-known to possess a myriad of medicinal properties. Therefore, triterpenoids constitute an attractive target for medicinal chemistry and diversity-oriented synthesis. Photochemical transformations provide a promising tool for the rapid, green, and inexpensive generation of skeletal diversity in the construction of natural product-like libraries. With this in mind, we have developed a diversity-oriented strategy, whereby the parent triterpenoids bryonolic acid and lanosterol are converted to the pseudosymmetrical polyketones by sequential allylic oxidation and oxidative cleavage of the bridging double bond at the B/C ring fusion. The resultant polyketones were hypothesized to undergo divergent Norrish-Yang cyclization to produce unique 6/4/8-fused triterpenoid analogues. The subtle differences between parent triterpenoids led to dramatically different spatial arrangements of reactive functionalities. This finding was rationalized through conformational analysis to explain unanticipated photoinduced pinacolization, as well as the regio- and stereochemical outcome of the desired Norrish-Yang cyclization.

ChemInform Abstract: Regio‐ and Stereoselective Acylation of Saturated Carbocycles via Norrish—Yang Photocyclization.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Regio- and stereoselective acylation of saturated carbocycles via Norrish–Yang photocyclization

A regio- and stereoselective acylation of saturated carbocycles has been achieved through two-step reactions involving the Norrish–Yang photocyclization of 1,2-diketones and subsequent ring opening of the α-hydroxy-cyclobutanones. The C–H activation of the carbocycle proceeds regioselectively at vicinal to the diketone moiety resulting in stereoselective formation of cis-fused bicycles. The following C–C cleavage affords vicinally cis-acylated carbocycles. Predictability, generality, and practicality of the present strategy have been demonstrated using variously modified saturated carbocycles.

Mechanism of the Norrish–Yang Photocyclization Reaction of an Alanine Derivative in the Singlet State: Origin of the Chiral‐Memory Effect

Remember how it was …︁? A mechanistic explanation for the chiral-memory effect in Norrish–Yang photoreactions has been obtained by computing the reaction pathway (see figure) at the ab initio level. The results show that both the ultrafast excited-state decay, which occurs at a conical intersection (CI), and the specific hydrogen-bonded structure of the primary diradical intermediate determine the high stereoselectivity of the reaction.

Mechanism of the Norrish–Yang Photocyclization Reaction of an Alanine Derivative in the Singlet State: Origin of the Chiral‐Memory Effect

Noch in Erinnerung? Eine mechanistische Erklärung des chiralen Gedächtnisses bei Norrish-Yang-Photoreaktionen wurde mithilfe von Ab-initio-Rechnungen zum Reaktionsweg abgeleitet (siehe Bild). Danach sind der ultraschnelle Zerfall des angeregten Zustands, der an einer konischen Durchdringung (CI) erfolgt, und die spezifische wasserstoffverbrückte Struktur der primären Diradikalzwischenstufe für die hohe Stereoselektivität der Reaktion entscheidend. Supporting information for this article (coordinates and energies of all optimized structures) is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2005/z461898_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Photocyclization of an isopentafulvene–benzoquinone adduct: a vinylogous Norrish–Yang reaction

The Diels–Alder reaction of N-methyl-4H-1,2,4-triazoline-3,5-dione (MTAD) with a mixture of isopentafulvenes 2a, b resulted in two 1:1 adducts 3a, b in a 65:35 ratio. The less reactive dienophile benzoquinone gave only one diastereomerically pure pentacyclic 1:2 adduct 4. The photolysis of 4 initiated a highly regioselective allylic hydrogen transfer reaction and resulted in the formation of the cyclobutanol 5 via an unusual vinylogous Norrish–Yang photocyclization. Prolonged photolysis of 5 gave the supercage molecule 6 via intramolecular photocycloaddition.