Photochemical Transformations of Proteinogenic and Non-Proteinogenic Amino Acids

The photochemistry of N-activated enantiomerically pure α-amino acids is described with emphasis on chemo-, regio-, stereo-, and spin selectivity. An especially valuable chromophore is the phthalimido group. The first excited singlet states are short-lived and deactivated (chemically) via homolytic CH cleavage or (physically) via electron-transfer steps. The first excited triplet states are chemically deactivated via electron-transfer reactions and subsequent deprotonation/coupling steps. A wide variety of product types were synthesized, and potential target molecules were available by tuning the reaction conditions. Also remote groups can be activated by means of electron-transfer steps, which represents an attractive new synthetic protocol for macrocyclization

Spin-dependent diastereoselectivity in the photocycloaddition of aldehydes to 2,2-dimethyl-2,3-dihydrofuran

The simple (non-induced) diastereoselectivity of the photocycloaddition of aliphatic as well as aromatic aldehydes to 2,2-dimethyl-2,3-dihydrofuran (1) was analyzed as a function of the substrate concentration (spin mapping) and rationalized in terms of optimal spin-orbit coupling controlled triplet biradical geometries

Photosensitized [2+2]-Cycloaddition of Complex Acceptor-Donor Combinations: A Regio/Diastereoselectivity Study

The photosensitized [2 + 2]-cycloaddition of chalcones, conjugated cyclopentenones, and cyclohexenones with electron-rich alkenes such as cyclic enolethers and polymethylated alkenes was investigated. While cyclic enones showed high regioand stereoselectivity, acyclic enones resulted in a more complex product mixture containing dimers as well as four dominant regioand diastereoisomers. This complex product mixture can be controlled by adjusting the reaction conditions such as sensitizer, solvents, or additives

Photoinduced electron transfer chemistry of phthalimides: an efficient tool for C–C-bond formation

A collection of intra- and intermolecular photoinduced electron transfer (PET) reactions is presented which all are based on the phthalimide chromophore as the oxidizing species. Electron-donating groups versatile for PET processes are ethers, thioethers, amines, alkenes, arenes, and carboxylates as well as α-trialkylsilyl activated heteroatom-substituents. These reactions can be efficiently applied for the synthesis of five- and six-membered ring heterocycles, medium-sized and macrocyclic products such as macrolides, cyclopeptides, crown ethers or thioethers as well as (from intermolecular processes) Grignard-alike products

Spin Photochemistry: Electron Spin Multiplicity as a Tool for Reactivity and Selectivity Control

Spin chemistry involving small organic molecules without heavy atoms is highly sensitive to spin-orbit-coupling (SOC) modulating biradical conformation as well as hyperfine coupling (HFC) modulating magnetic isotope interactions. Several easily available reaction properties such as chemo-, regio-, and diastereoselectivity as well as quantum yields serve as analytical tools to follow intersystem crossing (ISC) dynamics and allow titrating spin selectivities

IUPAC-project SYNPHO - a collection of experimental standard procedures in synthetic photochemistry

The IUPAC project intends to collect topical experimental procedures in the field of preparative photochemistry with emphases on essential experimental and mechanistic details. This collection may become a standard for every new report on synthetic photochemistry, thus guaranteeing a maximum of reproducibility and mechanistic understanding. To achieve this, SynPho will gather a large assortment of photochemical reactions and useful synthetic methods that utilize light-initiated and/or light-driven (i.e. photon catalytic or stoichiometric) processes. These will include descriptions of reactor setups (geometries, optics, materials, lamps, filters, wavelengths) and photon-specific information (quantum yields, quantum efficiencies, absorption and emission properties of substrates, intermediates and products)

Photoinduced electron-transfer processes of phthalimides

In terms of synthetic applications, the phthalimide system has attracted much attention over the past three decades, as noticeable by a number of summarizing reviews. Although the photochemistry of phthalimide derivatives is similar to that of carbonyl compounds, it covers additional reactivity features due to the remarkably high oxidizing power of the excited singlet and triplet states. Thus, the presence\ud of energetically feasible electron donor groups leads to the generation of radical ions that can undergo\ud nonproductive (back) electron transfer, direct radical ion combination, or mesolytic extrusion of a suitable leaving group (e.g., a proton, silyl cation, or carbon dioxide), respectively. The competition between these processes can be controlled by varying the redox potentials, the stability of the radical cations, and the leaving group ability. The photophysical and electrochemical properties of phthalimides are well documented. In acetonitrile, -alkylphthalimides show relatively unstructured UV absorption spectra with absorption maxima around 235 nm (π,π*) and 290 nm (π,π*), respectively. In ethanol or acetonitrile at room temperature, they exhibit weak fluorescence with low quantum yields (Φf < 1 × 10−3).\ud In the absence of oxygen in alcohol, N-alkylphthalimides show broad structureless phosphorescence centered around 450 nm with quantum yields between Φp = 0.4–0.7 and triplet lifetimes of τp = 0.7–1.04 s (at −196°C). N-Methylphthalimide is reversibly reduced to the corresponding radical anion at ca. −1.35 V in DMF,\ud and at ca. −1.5 V in acetonitrile (vs. SCE),respectively, but the presence of a hydrogen donor site in the side chain has a dramatic effect on the redox properties. Based on the available photophysical and electrochemical data, it is possible to estimate the feasibility of a photoinduced electron transfer (PET)for various phthalimide/donor pairs. The limiting maximum oxidation potential of the electron donorthus depends on the nature of the electronically excited state of the phthalimide electron acceptor and\ud can be estimated from the Rehm-Weller equation. Thus, if the first excited singlet state is involved (E00= 3.8 eV), the limiting oxidizing power for an isoenergetic electron transfer is ca. 2.4 V (vs. SCE). If the first excited triplet state is involved (E00 = 3.1 eV), the limiting oxidizing power decreases to ca. 1.7 V(vs. SCE). In cases where the spectroscopically nondetectable second triplet state is populated (E00 = 3.6eV), the oxidizing power increases by about 500 mV

5-Adamantylated 1,2,4-Trioxanes: Adamantane Position is Crucial for Antiparasitic Activity

Allylic alcohols with 1'- and 2'-adamantanyl functionalization, available by reaction of 3-methylcrotonaldehyde with 1- and 2-bromoadamantane, respectively, were reacted with singlet oxygen under solution conditions with high diastereoselectivities to give the beta-hydroperoxy alcohols. Lewis acid catalyzed peroxyacetalization of the beta-hydroperoxy alcohols resulted in two sets of 1,2,4-trioxanes. The allylic alcohol 11 delivered the rearrangement product - the less strained primary hydroperoxide - upon singlet oxygenation and Lewis acid treatment

Decarboxylative photoadditions of heteroatom-substituted carboxylates to phthalimides

a-Thioalkyl and a-oxoalkyl-substituted potassium carboxylates efficiently decarboxylate in the presence of electronically excited phthalimides. Radical coupling and reduction products result from secondary steps. In contrast, b-thioalkyl-substituted carboxylates are unreactive under the reaction conditions whereas b-oxoalkyl-substituted carboxylates react quantitatively. This behaviour is rationalized by assuming primary photoinduced electron transfer with sulfur oxidation, respectively, and no oxidation of the ether oxygen. In b-thioalkyl-substituted carboxylates, the thioether radical cation serves as hole trap and prevents the carboxylate of oxidative cleavage