Paterno-Buchi reactions, cycloadditions

Typical chemical reactions of photoexcited aldehydes and ketones are cleavage reactions, usually designated as Norrish Type I [equation (54)], II [equation (55)] and III [equation (56)], hydrogen abstraction [equation (57)] and cycloadditions, such as the Paterno-Buchi reaction [equation (58)]. Of these, Norrish Type II cleavage and the related... 

In general however the various possible reaction pathways give rise to formation of a mixture of products. The type I-cleavage reaction is only of limited synthetic importance, but rather an interfering side-reaction—e.g. with an attempted Paterno-Buchi reaction, or when an aldehyde or ketone is used as sensitizer in a [2 + 2]-cycloaddition reaction. 

The cycloaddition reactions to give oxetanes readily occurs whenTj state is ( , 7T ) in character and an electrophilic centre is created on carbonyl oxygen atom on photoexcitation (Section 8.1). Paterno-Buchi reaction. 

PATERNO-BUCHI REACTION. Formation of oxetanes by photochemical cycloaddition of carbonyl compounds to olefins. 

Photoinduced [2+2] cycloaddition (the Paterno-Buchi reaction) of 1-acetylisatin with acyclic enol ethers afford the spiro(3f/-indole-3,2 -oxetane)s 43 with moderate regio- and diastereoselectivity via the mi triplet state of the isatin derivative without involvement of single electron transfer <02JCS(P1)345>. 

There have only been a few new reports of reactivity of this type. The Paterno-Buchi reaction of 92 with substituted pyruvates gives the endo- and < ti-products shown (Equation 2) <1996TL1195>. In a later study, the corresponding cycloadditions of l,3-dioxol-2-one with a variety of aldehydes and ketones were also examined <2007ARK(viii)58>. 

The cycloaddition of carbonyl-containing compounds to alkenes to yield oxetans (the Paterno-Buchi reaction) was one of the first photoreactions to be described in the literature. The study of the mechanism and synthetic applications of this process continues to attract widespread attention, and numerous papers have been published during the period covered by this report. Although competing photoreactions and energy transfer processes are often observed in such investigations, the reaction provides a valuable route to oxetan-containing systems. 

Ketones and aldehydes can undergo photochemical [2-1-2] cycloaddition reactions with alkenes to give oxetanes. This is called the Paterno-Buchi reaction. For alkyl carbonyl compounds both singlet and triplet excited states seem to be involved, but for aromatic compounds the reaction occurs through the triplet state.The regiochemistry can usually be accounted for on the basis of formation of the most stable 2-oxa-1,4-diradical. For example, styrene and benzaldehyde give 2,3- not 2,4-diphenyloxetane. ... 

Among the [2+2] cycloadditions, the Paterno-Buchi reaction (see p 39) with pyrroles has been investigated. The oxetanes isomerize to give 3-(hydroxyalkyl)pyrroles under the reaction conditions ... 

By far the most important method of preparation of fluorinated oxetanes is [2 + 2] cycloaddition that can either be photoinitiated (Paterno-Buchi reaction) or result from electrophilic condensation. 

The photochemical [2 + 2]-cycloaddition reactions of carbonyl compounds with alkenes are known as the Paterno-Buchi reactions This reaction was first reported by the group of E. Patemo and G. Buchi on the reaction of benzaldehyde with 2-... 

The photochemical (2 + 2)-cycloaddition of carbonyl compounds (aldehydes/ketones) and alkenes, known as Paterno-Buchi reaction [3], likewise leads to oxetanes ... 

A comparable approach was used by some of the same authors in the study of the regioselectivity of the photochemical [2 -I- 2] cycloaddition of triplet carbonyl compounds with a series of ground state electron-rich and electron-poor alkenes, the Paterno-Buchi reaction,yielding oxetanes. Very recently, the regioselectivity of the [2 -I- 2] photocycloaddition reaction between triplet (n-n ) acrolein and substituted olens in their... 

A striking difference exists between the photochemical reactivity of a, 3-unsaturated enones and the corresponding ynones. Whereas many cyclic enones undergo (2-t2]-cycloaddition to alkenes at the CC double bond of the enone (probably fi om the triplet n,n state) to yield cyclobutanes, acyclic enones easily deactivate without radiation by rotation about the central CC single bond. Ynones, on the other hand, behave much more like alkyl-substituted carbonyl compounds and add to (stericaUy less encumbered) alkenes to yield oxetanes. - The regioselectivity of the Paterno-Buchi reaction is similar to that... 

Mattay, J., Conrads, R, and Hoffmann, R., Oxetanes via [2-1-2] cycloadditions to carbonyl compounds (Paterno-Buchi reaction), in Houben-Weyl, 4th ed., VoL E21c, Helmchen, G., Hofiftnan, R. W., Mulzer, J., and Schaumann, E., Eds., Thieme, Stuttgart, 1995, 3133. 

Hoye and Richardson have published an ingeneous synthesis of the tricyclic iridoid sarracenin (170) which relied on the Paterno-Buchi cycloaddition between acetaldehyde and cyclopentadiene as the intial step (Scheme 38)79. 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. 

These cycloadducts, at their most elementary level, are excellent intermediates for the synthesis of 3-substituted furan derivatives. For example, Kawanisi and coworkers reported a synthesis of perillaketone 174 in which the critical step was a Paterno-BUchi photocycloaddition between furan and 4-methylpentanal in the presence of methanesul-fonic acid (Scheme 39)82. This reaction furnished two initial photoadducts, 172 and 173. The unexpected product 173 presumably arises from a Norrish Type II cleavage of 4-methylpentanal to give acetaldehyde, and subsequent cycloaddition with furan. The desired cycloadduct 172 was then converted uneventfully to 174 via acid-catalyzed aromatization and oxidation. 

Important studies by Sakurai have highlighted the use of Paterno-Buchi cycloaddition reactions of carbonyl compounds and furans to give fused ox-etanes with high regio- and diastereocontrol [143]. An elegant synthetic application was reported by Schreiber, who used the conversion of furan into cycloadduct 228 as the only observed isomer (>99%, Scheme 18.37) [144, 145]. This cycloaddition could readily be conducted on a 50 g scale and formed the cornerstone in the total synthesis of the antifungal metabolite avenaciolide (229) [145]. 

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