Paterno-Btichi reaction

M425>. Here the selectivity was only low with respect to the site of addition, which could be either the benzoyl or 4-substituted benzoyl group. Phenyl glyoxylates can also be successfully utilized as reactive carbonyls in the Paterno-Btichi reaction as demonstrated by Hu and Neckers <1997JOC564>. Oxetanes were formed in very high yields with electron-rich (e.g., polyalkylated) alkenes, but with monosubstituted alkenes there was no oxetane formation due to the prevalence of Norrish II type hydrogen abstraction (Scheme 22). 

The photo-Diels-Alder reaction of a-acetonaphthone 145 with the chiral a-enaminonitrile 146 yielded the cycloadduct 147 with almost complete diastere-oselectivity (Scheme 37) [91,92]. The intermediately formed biradical R is particularly stable owing to delocalization of the radical on the aromatic moiety and to a captodative effect on the enamine moiety. In analogy to the Paterno-Btichi reaction (see Scheme 7), the chiral induction occurred in two steps. In the first step, a chiral center is created at the a-position of the acyl group. In the second step of the diastereoselection, one of the two diasteromeric intermediates undergoes preferential cyclization to yield the final product 147, while the other one is decomposed to form the starting material [92]. 

In contrast to the preferred meta mode of intramolecular photoaddition of 5-phenyl-l-pentenes, where the alkene and benzene groups are separated by three atoms, irradiation of the styrene (64) yields a single stereoisomer of the ortho adduct (65). In (64), not only are the reacting units separated by 4 atoms, but also it is the styrene rather than the benzene which is excited. Comparable photoreactivity is seen for phenanthrene-styrene systems such as (66) which yield 2+2 adducts (67) along with products derived from competing Paterno-Btichi reaction of the ester carbonyl with the alkene side chain. The photochemical cycloaddition also proceeds in an intermolecular fashion between the ester of 9-phenanthrene carboxylic acid and para-methoxy-0-methylstyrene. The mechanism of this reaction is shown to involve addition of the styrene to the singlet excited state of the phenanthrene derivative. °... 

The lowest-lying excited state of ketones most often corresponds to a o 7t c=o transition. The maximum of this band is around 280 nm with simple aldehydes or ketones and is shifted to the red for conjugated or aryl derivatives. As hinted above, the unpaired electron on the hq orbital gives to these states electrophilic properties similar to those of alkoxy radicals, and indeed the observed chemistry is similar in the two cases. Typical reactions are a-fragmentation, inter- or intramolecular (from the easily accessible y position) hydrogen abstraction and attack of alkenes (finally resulting in a formal 2h-2 cycloaddition to give an oxetane, the Paterno-Btichi reaction). 

Hoye and Richardson have published an ingeneous synthesis of the tricyclic iridoid sarracenin (170) which relied on the Paterno-Btichi cycloaddition between acetaldehyde and cyclopentadiene as the intial step (Scheme 38). This reaction provided a 5 1 mixture of adducts 166a and 166b. The major adduct was opened with camphor-10-sulfonic acid (CSA) in methanol and the alcohol was tosylated to give 167. Displacement with malonate 168 and decarboalkoxylation/demethylation steps gave 169. Ozonolysis, reductive workup and acid-catalyzed acetalization then furnished 170. 

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