Photochemistry of acyloximes: synthesis of heterocycles and natural products

Catalytic Oxidative Cyclization of 2′-Arylbenzaldehyde Oxime Ethers under Photoinduced Electron Transfer Conditions

“…5,6 Previously, the photochemical reactions of oxime esters studied by Rodriguez 14 and Walton 16 were proposed to proceed via iminyl radical intermediates. A plot of the data points from Walton’s study against σ + shows a line with a slope that is close to zero, consistent with a neutral radical intermediate such as the iminyl radical.…”

Interplay of ortho- with spiro-cyclisation during iminyl radical closures onto arenes and heteroarenes

“…This research initiated spiralling interest by synthetic chemists in iminyl radical-mediated preparations. Recently iminyls have been generated from quite a variety of precursors [10–15], and their cyclisations onto arenes [16–21] and heteroarenes [22–24] have attracted attention. Iminyl cyclisations have also been utilised in natural-product syntheses [10,16,25–26].…”

Light-Induced Iminyl Radicals: Generation and Synthetic Applications

“…Table 1. Free energies (G, kcal/mol) for the cyclization step to five-or six-membered rings, relative to the corresponding iminyl radical, at the B3PW91/6-31+G* level DFT calculated structures and energies have also been used to understand the influence of some substituents on the reactivity of iminyl radicals (Alonso et al, 2010). The most significant is the reduction of the energy barrier, from 14.6 to 13.2 kcal/mol at the B3PW91/6-31G* level, when a methyl group is located on the iminic carbon (Figure 2).…”

“…As shown in Scheme 21, the use of a combination of reagents with five-and six-membered rings and cyclization of the intermediate iminyl radicals onto phenyl, thiophenyl or pyridinyl rings led to a variety of fused rings with different heteroatoms on the structure in good to excellent yields (48 to 90%) (Alonso et al, 2010 The versatility of this methodology has been exploited in the preparation of some interesting natural products. On using the appropriate structure (Scheme 22), direct irradiation of Oacyloximes allowed the preparation of several phenanthridine derivatives (Alonso et al, 2010), such as the alkaloid trisphaeridine (R 1 ,R 2 = OCH 2 O; R 3 = H ) o r t h e v a s c o n i ne precursor (R 1 = R 2 = OCH 3 ; R 3 = CH 2 CH 2 OH), which can also be used to obtain assoanine, oxoassoanine and pratosine (Rosa et al, 1997). Trisphaeridine was also prepared in 59% yield by irradiation of the corresponding dioxime oxalate in the presence of 4-methoxyacetophenone as a photosensitizer (Portela-Cubillo et al, 2008).…”