Carbonyl compounds photoenolization

Case Study 6.23 Organic synthesis - Diels-Alder trapping of photoenols 

The total synthesis of cytotoxic agents hamigerans was achieved via the photoenoliza-tion of substituted benzaldehydes and the subsequent Diels Alder (dark) reaction.983 Two species, E- and Z-isomers of 305, while being interconverted by E Z 

Case Study 6.24 Photoremovable protecting groups - 2,5-dimethyl-phenacyl chromophore 

For example,3n,7t excited 1,4-benzoquinone affords spiro-oxetanes in the presence of alkenes exclusively, whereas 1,4-naphthoquinone, in which the energies of the 3n,Jt and 37t,7t states are close, produces both spiro-oxetanes and cyclobutanes.990 Introduction of electron-donor substituents on quinones further destabilizes the 3n,Jt state. 

Case Study 6.25 Green photochemistry - photochemical Friedel-Crafts acylation 

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There are four distinct processes initiated by y-hydrogen abstraction in excited carbonyl compounds Norrish type II photoelimination, Yang photo-cyclization (cyclobutanol formation), Yang photoenolization (o-xylylenol formation), and (3-cleavage of radicals from carbons adjacent to the radical sites of the 1,4-biradicals. Some of these require unique structures and generate distinct products. 

Many photoreactions of aromatic compounds proceed by way of nonaromatic intermediates which are then either trapped inter- or intra-molecularly, or revert to starting materials. Such processes involving aromatic groups include the di-rr-methane rearrangement and photoenolization of aromatic carbonyl compounds. These particular reactions are considered fully elsewhere in this Volume, but examples of this type of photoisomerization from the current literature are given here for illustrative purposes. 

Photoenolization is a frequently encountered process for aromatic and a,j6-imsaturated carbonyl compounds. Typically, an ally lie or benzylic y-hydrogen is abstracted by the... 

The E enols such as (367) obtained by photoenolisation of ortho-alkyl aromatic carbonyl compounds can be trapped by Diels Alder dienophiles and this has frequently been applied for synthetic purposes. A new example of this application has been published in which the ortho-methylbenzaldehydes (373) were irradiated in the presence of diallyl glutaconate (374) to give (375) as the products of regioselective Diels Alder trapping of the intermediate photoenol. ... 

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