Photoinduced Electron Transfer Reactions of α-Cyclopropyl- and α-Epoxy Ketones. Tandem Fragmentation−Cyclization to Bi-, Tri-, and Spirocyclic Ketones

Abstract

Reductive photoinduced electron transfer (PET) reactions have been performed with various bicyclic alpha-cyclopropyl-substituted ketones and tertiary amines. The reaction resulted in a regioselective cleavage of one cyclopropyl bond under formation of an exocyclic radical with an endocyclic enolate unit. In the case of bicyclic ketones with an unsaturated side chain, various bicyclic, spirocyclic, and tricyclic products are accessible via radical cyclization, depending on the position of the alkenyl substituent. In addition to triethylamine, N-silylated amines have also been used as electron donors, leading to a variety of compounds, among them are silylated fragmentation products, indicating that a proton is transferred from not only the amine radical cation but also the cationic silyl group. The intramolecular Paternó-Büchi reaction has also been studied for cyclopropane derivatives of the jasmone type leading to tetracyclic oxetanes. Finally, alpha-epoxy-substituted ketones have been investigated under PET conditions, yielding ring-opened products.

Studies on azaspiracid biotoxins. I. Ultrafast high‐resolution liquid chromatography/mass spectrometry separations using monolithic columns

“…The products were isolated by HPLC and unambiguously identified by spectroscopic analysis (mostly one and two dimensional NMR as reported in the Experimental Section). The stereochemistry of the cyclisation products 28 and 29 was assigned by comparison with reported NMR data 25. The structure of the tricyclic compound 31 was established by comparison with the 1 H NMR data of the cyclisation products of the complex cascade reactions, which were identified by qualitative and partially quantitative NOESY spectroscopy (see Table 2).…”

Synthetic Applications in Radical/Radical Cationic Cascade Reactions

“…The stereochemistry of the cyclisation products 28 and 29 was assigned by comparison with reported NMR data. [25] The structure of the tricyclic compound 31 was established by comparison with the 1 H NMR data of the cyclisation products of the complex cascade reactions, which were identified by qualitative and partially quantitative NOESY spectroscopy (see Table 2). …”

Photoreactions of Tricyclic α‐Cyclopropyl Ketones and Unsaturated Enones – Synthesis of Polyquinanes and Analogous Ring Systems

“…We also studied alternative methods such as the use of samarium( II ) iodide (SmI 2 ) 57. However, as already shown for other unsaturated cyclopropyl ketones,25 this procedure gave lower yields than the PET method: 30 was formed from 15 in only 35 % yield with SmI 2 . The proposed mechanism of the PET cyclization of 15 is shown in Scheme .…”

“…The mechanistic details of the solvent and salt effects with various type of ion pairs have been described by us previously 56. In accordance with the described advantages,24,25 we used a variant with one equivalent of LiClO 4 as an additive in the acetonitrile solution of the α‐cyclopropyl ketone and five equivalents of TEA under standard PET conditions in the synthesis of polycyclic compounds of both angular triquinane and propellane type. The results of the PET reductive cyclization reactions of the α‐cyclopropyl indenones 15 , 18 and 21a are shown in Scheme and Table 2.…”

“…The proposed mechanism of the PET cyclization of 15 is shown in Scheme . For saturation of the radical 36 we assume a hydrogen atom transfer from a suitable donor (e.g., TEA and/or solvent) 25…”