Paterno-Biichi reactions

Photo-induced oxetane formation from a ketone and an olefin. 

Schore, N. E. In Comprehensive Organic Synthesis, Paquette, L. A. Fleming, I. Trost, B. M. Eds, Pergamon Oxford, 1991, Vol. 5, p.l037. 

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One of the best-known and highly useful photochemical synthetic procedures is the Paterno-Biichi reaction [17]. This transformation has also been adapted as basic principle for domino processes by different research groups. Agosta and coworkers published a procedure by which tetrasubstituted furans such as 5-65 can be built up from 5-61 and 5-62 (Scheme 5.13) [18],... 

Cycloadditions of olefins and carbonyl compounds (Paterno-Biichi reaction)... 

The photoinduced [2 + 2] cycloaddition of carbonyl acceptors with electron-rich olefins leads to oxetanes (Paterno-Biichi reaction) with high regio- and stereoselectivities (equation 25). 

The photocycloaddition of an aldehyde or ketone with an olefin to yield an oxetane was reported by Paterno and Chieffi in 1909. 58> Contemporary studies on the synthetic utility and mechanistic features were initiated nearly 50 years later by Biichi et al. 59) Two review articles summarizing synthetic aspects of Paterno-Biichi reactions have been published 6.12)) and mechanistic studies have been reviewed several times. 6,38,60-62) The reaction involves the addition to olefin of a photo-excited carbonyl moiety. This circumstance makes it advantageous to review this reaction before a discussion of olefin-olefin additions, because the solution photochemistry of carbonyl compounds is probably better understood than any other aspect of organic photochemistry. Many of the reactions of carbonyl compounds have been elucidated during studies of the important phenomena of energy transfer and photosensitization. 63-65)... 

A clear division of Paterno-Biichi reactions into several distinct categories is possible on the basis of the type of reacting carbonyl compound (alkyl or aromatic), the excited state responsible for reaction (n—71 or Ti—n, singlet or triplet), and the type of olefin (electron deficient or electron-rich). Some examples of these reactions are given in Eqs. 7—11, where only the oxetane products are shown. 

For instance, Kochi and co-workers [89,90] reported the photochemical coupling of various stilbenes and chloranil by specific charge-transfer activation of the precursor donor-acceptor complex (EDA) to form rrans-oxetanes selectively. The primary reaction intermediate is the singlet radical ion pair as revealed by time-resolved spectroscopy and thus establishing the electron-transfer pathway for this typical Paterno-Biichi reaction. This radical ion pair either collapses to a 1,4-biradical species or yields the original EDA complex after back-electron transfer. Because the alternative cycloaddition via specific activation of the carbonyl compound yields the same oxetane regioisomers in identical molar ratios, it can be concluded that a common electron-transfer mechanism is applicable (Scheme 53) [89,90]. 

The photocycloaddition of a carbonyl compound to an alkene was discovered as early as 1909 by Paterno and Chiefifi [78] who employed sunlight as the irradiation source. In the 1950s the reaction was more intensively investigated by Biichi et al. [79] using artificial light sources. The Paterno-Biichi reaction has been studied mechanistically [80] and some important aspects are summarized in Scheme 37. Upon n r -excitation (1=280-350 nm), aldehydes... 

With many other ketones and aldehydes, reaction between the photoexcited carbonyl chromophore and alkene can result in formation of four-membered cyclic ethers (oxetanes). This reaction is often referred to as the Paterno-Biichi reaction.127 128 129... 

Sauers and coworkers have applied the Paterno-Biichi reaction to engeneral formula 427 (Scheme XXXIV) Reductive cleavage of these products with lithium aluminium hydride is also regioselective and leads, following oxidation, to ketones... 

Trimethyloxazole 257 undergoes photochemically induced [2 + 2] cycloaddition with aromatic and aliphatic aldehydes to provide bicyclic oxazolines 258 with excellent regiochemical and stereochemical control. Diastereoselec-tivities from 75-99% can be achieved, which is the first reported example of a Paterno-Biichi reaction involving an oxazole. The oxetane cycloadducts can be hydrolyzed to a-amino-(3-hydroxy ketones. Other oxazoles have not been evaluated to determine if they undergo the photochemical cycloaddition (Scheme 8.71). 

The Paterno-Biichi reaction has been employed in the synthesis, often in high yield, of a large variety of substituted oxetanes. In addition to simple aliphatic and aromatic alkenes, cycloaddition of ketones to, for example, fumaronitrile,284 l,3-diacetylimidazolin-2-one288 [Eq. (73)], and allenes286 has been reported. Allenes yield both 1,5- and l,6-dioxaspiro[3.3]heptanes as well as the 2-alkylidene-oxetane this is illustrated for benzophenone and tetramethylallene in Eq. (74). Cycloaddition of ketones to ketenimines to form 2- and... 

The intramolecular equivalent of the Paterno-Biichi reaction has been observed in a number of unsaturated ketones. A series of y,S-unsaturated ketones (278) are converted in this way into the oxabicyclo[2.2.0]hexanes (279) and the oxabicyclo[2.1.1]hexanes (280), both photoproducts being the result of 1,2-cycloaddition.303... 

Tandem intermolecular Paterno-Biichi reaction of benzophenone with trimethylsilyl vinylcyclopropyl ether 72 (Scheme 31) leads in its first step to the formation of seven-membered 73 rather than to four-membered heterocycle while its second step proceeds in a normal fashion. With aromatic aldehydes, only tetrahydrooxepine derivatives 74 are formed <1998J(P1)2363>. 

III. The Paterno-Biichi Reaction on Pentaatomic Heterocycles Different from Furan... 

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