Paterno-Biichi photocycloaddition

The Paterno-Biichi photocycloaddition to glycals occurs with a total regioselectivity according to the stability of the supposed intermediate radicals. Acetone in presence... 

Abe, M., Fujimoto, K., Nojima, M. Notable Sulfur Atom Effects on the Regio- and Stereoselective Formation of Oxetanes in Paterno-Biichi Photocycloaddition of Aromatic Aldehydes with Silyl 0,S-Ketene Acetals. J. Am. Chem. Soc. 2000, 122, 4005-4010. 

The Paterno-Biichi photocycloaddition of silyl 0,X-ketene acetals (with X = O, S, Se) and aromatic aldehydes was intensively investigated by Abe and co-workers in the last decade. The regioselectivity of the reaction (69 vs. 70) is highly affected by the heteroatom. The regioselectivity is rationalized by (1) the relative stability of the 1,4-biradicals, and (2) the relative nucleophiUcity of sp -carbons in the respective 0,X-ketene acetal. 

Abe, M., Fujimoto, K., and Nojima, M., Notable sulfur atom effects on the regio- and stereoselective formation of oxetanes in Paterno-Biichi photocycloaddition of aromatic aldehydes with silyl O, S-... 

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)... 

On irradiation with alkenes, carbonyl compounds undergo photocycloaddition to give oxetanes in the Paterno-Biichi reaction ... 

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... 

The regioselectivity of the Paterno-Biichi reaction with acyclic enol ethers is substantially higher than with the corresponding unsymmetrically alkyl-substituted olefins. This effect was used for the synthesis of a variety of 3-alkoxyoxetanes and a series of derivatives [55]. The diastereoisomeric cis-and tnms-l-methoxy-l-butenes were used as substrates for the investigation of the spin state influence on reactivity, regio- and stereoselectivity [56]. The use of trimethylsilyloxyethene 62 as electron rich alkene is advantageous and several 1,3-anhydroapiitol derivatives such as 63 could be synthesized via photocycloaddition with l,3-diacetoxy-2-propanone 61 (Sch. 17) [57]. 

The difference between the simple diastereoselectivities in photocycloaddition reactions following the singlet vs. the triplet route were studied by determination of the concentration dependence of the Paterno-Biichi reaction [155]. Carbonyl substrates which have both reactive singlet and triplet states, exhibit one characteristic substrate concentration where a 1 1 ratio of single and triplet reactivity, i.e., spinselectivity, could be detected. The shape of these concentration/diastereoselectivity corrrelations reflects the different kinetic contributions to this complex reaction scenario. 

The concentration dependence of the photocycloaddition of cis- and trans-cyclo-cocetene 136 with aliphatic aldehydes was studied in order to test the spin-selectivity with respect to the cis/trans oxetane ratio and/or the eftdo/exo-selectivity relevant for the cis-photoadduct [156]. The results indicate a moderate but still significant spin correlation effect in the Paterno-Biichi reaction of cyclo-octene with aliphatic aldehydes. The exo-diastereoisomers were formed with similar probability as the e do-diastereo-isomers in the singlet carbonyl manifold, whereas the triplet excited aldehydes preferred the formation of the e Jo-diastereoisomer and trans fused products. 

The reactivity and selectivity trends described for the intermolecular version of the Paterno-Biichi reaction are also valid for the intramolecular version. Special aspects of intramolecular photocycloadditions have been reviewed [157]. A crucial role for the reactivity mode of unsaturated carbonyl compounds is the tether length, i.e., the distance between reactive carbonyl and C-C double bond. Conjugated systems, i.e., oc,(3-unsaturated carbonyls, prefer photodeconjugation or photocycloaddition invovling the C-C double... 

Buhr, S., Griesbeck, A. G., Lex, J., Mattay, J., Schroeer, J. Electronic control of stereoselectivity in photocycloaddition reactions. 7. Stereoselectivity in the Paterno-Biichi reaction of 2,2-diisopropyl-1,3-dioxole with methyl trimethylpyruvate. Tetrahedron Lett. 1996, 37, 1195-1196. 

Similarly to carbonyl compounds (Section 6.3.1), thiocarbonyl compounds abstract hydrogen upon irradiation however, both n,7t and n,n excited states are reactive and the hydrogen atom can be added to either the sulfur (Table 6.17, entry 1) or carbon (entry 2) atoms of the C=S bond. Aliphatic and aromatic thiocarbonyl compounds can also undergo photocycloaddition to unsaturated compounds from both singlet or triplet excited states to form thietanes (analogously to the Paterno Biichi reaction see Section 6.3.2) (entry 3) or 1,4-dithianes. On the other hand, fragmentation of the S C bond is a typical primary process observed in excited sulfones and sulfonates (entry 4), followed by efficient SO2 extrusion from the radical intermediate. 

The Paterno-Biichi Reaction. One well-known class of photocycloadditions is the Paterno-Buchi reaction in which aldehydes or ketones combine with alkenes to give oxetanes.1108,1109 The excited state of the ketone is n-n, and it is the orbitals of this state which interact with the ground-state orbitals of the alkene. Often it is the triplet state which is involved,1110 but occasionally the singlet state is important. The orientation usually observed for C- and X-substituted alkenes is usually counter-thermodynamic, with the more-substituted atoms bonded to each other.1111,1112... 

Recent reports emphasizing the role of 1,4-diradicals in triplet Paterno-Biichi reactions should also be mentioned. The first to be discussed concerns the chiral induction of photocycloadditions of various olefins to chiral phenylglyoxalates11 28,29, The high diastereoselectivity of these reactions often reaches de values of >96% and shows a characteristic temperature dependence with specific points of inversion. This behavior is a result of competition between enthalpy- and entropy-controlled partial selection steps. 

This reaction can be done in the presence of a rather Wide range of functionality. It has been used with aromatic compounds such as 2-furylmethanol derivatives, as in the reaction of 382 with benzophenone to give a 65% yield of 383 along with a 20% yield of 384.322 -ghg photocycloaddition product of the Paterno-Biichi reaction with these substrates is an acetal or a ketal, and it can be hydrolyzed to produce acyl derivatives or other functionality. This synthetic route is attractive since achiral fragments generate cycloadducts with several contiguous chiral centers whose relative stereochemistry can usually be predicted with great accuracy. 

High selectivity has been obtained in the Paterno-Biichi reaction of aldehydes with furans and this reaction has formed the basis of a number of total syntheses. Thus, photocycloaddition of furan with nonanal gave the exo product 170, which was converted to the antifungal metabolite avenaciolide (3.115). 

An extensive review of 2 + 2-photocycloadditions of dienones and quinones has been published. The photocycloaddition of heterocyclic 2,3-diones (18) with electron-rich alkenes in the presence of the photosensitizer benzophenone yields 2-1-2-cycloadducts (19) and (20) with high regio- and stereo-selectivity (Scheme 1) Time-resolved spectroscopy has shown that in the photochemical cycloaddition between stilbenes and quinones, the (singlet) ion-radical pair [S ", Q+ ] is the primary reaction intermediate and therefore establishes the electron-transfer pathway for this typical Paterno-Biichi transformation. A kinetic study of the Paterno-Biichi cycloaddition of stilbene to chloranil shows that solvent polarity and donicity control the formation as well as the reaction path of the ion-radical. The photoirradiation of chloranil with... 

Another photocycloaddition reaction that has been known for a long time is the Paterno-Biichi reaction, which involves the formation of oxetanes through the addition of an excited carbonyl compound to olefins ... 

Ketones with homotopic faces have been widely used to determine the stereoselectivity in Paterno-Biichi reactions with E- and Z-isomers of electron-rich and electron-poor olefins. The photocycloaddition of... 

The induced diastereoselectivity in a Paterno-Biichi reaction resulting from a stereogenic center in the alkene part was recently described by Bach and co-workers in the photocycloaddition of chiral silylenol ethers 67 with benzaldehyde 18. The substituents, R, at the stereogenic center were varied in order to evaluate the influence of steric bulk and possible electronic effects. In accord with the 1,3-aUyhc strain model, the facial diastereoselectivity was at a maximum with large (R = t-Bu, SiMejPh) and polar (R = OMe) substituents at the y-position of silyl enol ether (diastereomeric ratio of oxetanes 68 > 95 5). 

Adam, W., Stegmann, V. R., and Weinkoetz, S., Unusual temperature-dependent diastereoselectivity in the [2-1-2]-photocycloaddition (Paterno-Biichi reaction) of benzophenone to ds- and trans-cyclooctene through conformational control, /. Am. Chem. Soc., 123, 2452, 2001. 

The photocycloaddition of triplet benzophenone to norbornene was originally reported by Scharf and Korte. The photoproduct 101 that is formed in high exo-selectivity could be thermally cleaved to the 5,e-unsaturated ketone 102, an appHcation of the carbonyl-olefin metathesis (COM) concept. The 1,4-biradical formed in the interaction of norbornene with o-dibenzoyl-benzene was trapped in an intramolecular fashion by the second carbonyl moiety. A highly regioselective reaction of triplet benzophenone was reported with 5-methylenenorborn-2-ene, with preferential attack toward the exo CC double bond. A number of publications have discussed the photocycloaddition reactions of triplet carbonyl compounds to norbornadiene and quadricyclane, as weU as the competition between the Paterno-Biichi reaction and the sensitized norbornadiene/quadricyclane interconversion. Oxetane formation has also been reported for the photoreaction of biacetyl and para-quinones with benzvalene. ... 

The Patemo-Biichi reaction is a milestone in organic photochemistry (Scheme 3.1). Paterno and Chieffi [1] reported obtaining oxetanes from the photocycloaddition of ketones to olefins in 1909, but this reaction was recognized as an important synthetic reaction only after the work of Biichi et al. [2]. 

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