Patemo-Biichi photocycloaddition reaction

Griesbeck and co-workers" described the first examples of oxazoles reacting as the alkene component in a Patemo-Biichi photocycloaddition reaction with aldehydes and a-ketoesters. The authors envisioned that such a process would generate... 

The synthesis of oxetanes from alkenic precursors has been demonstrated and the range and scope of Patemo-Biichi photocycloadditions are broad. In general, both the reaction regioselectivity and stereoselectivity can often be predicted by considering the reaction mechanism (Section 2.4.2) for example, the directing effects of alkene substituents are readily imderstood. Synthetic applications are numerous owing to the rtq>id stereocontrolled assembly of multifimctional targets. 

The chemo-, regio- and stereo-selectivity observed in the Patemo-Biichi photocycloaddition make the reaction a valuable synthetic method as its numerous applications to targets in synthesis attest. This review will summarize the important mechanistic and syndetic advances. Knowledge of these precedents will likely initiate further scrutiny and applications of the reaction to the synthesis of architecti ly complex targets. 

The [2+2] photocycloaddition of an aldehyde or a ketone to an alkene to form an oxetane (e. g. Ill + IV —> V, Scheme 1) is a process that is known as the Patemo-Biichi reaction 8 it is a reaction that was first reported by Patemo in 19094 and was confirmed approximately forty-five years later by Biichi.Sa A particularly interesting variant of the Patemo-Biichi reaction is illustrated in... 

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]. 

The Patemo-Biichi reaction is a photocycloaddition reaction of a n,ji carbonyl compound to an alkene in the ground state from either the Si or the rl i state. The reaction can occur through the initial C O bond formation or through a previous formation of the C—C bond. A frontier orbitals approach can be used to explain the formation of oxetanes. We can observe the HSOMO-LUMO interaction in which the half-occupied ji carbonyl orbital interacts with the unoccupied ji molecular orbital of an electron-deficient alkene, and a C,0-biradical is formed. The LSOMO-HOMO interaction in which the half-occupied n orbital of the carbonyl O atom interacts with the ji orbital of an electron-rich alkene, and a C,C-biradical is formed [13, 14]. 

One well-known class of photocycloadditions is that of aldehydes and ketones with olefins to give oxetanes375 (e.g. the reaction of 462 with 463 to give 464 and 465).376 This kind of reaction is known as the Patemo-Biichi reaction. The excited state of the ketone is the n-rc one, and it is the orbitals of this state which interact with the ground-state orbitals of the olefin. Often it is the triplet state which is involved, but occasionally the singlet state is important. The orientation usually observed is shown in the following examples, where C- and X-substituted olefins are involved.377,376... 

Many other methods of asymmetric induction in photochemistry, such as template-induced enantioselective photoreactions,591 asymmetric photochemistry in zeolites,592 or simple photoreactions of a chiral starting material, parallel their ground-state chemistry counterparts. Diastereoselective photocycloaddition in the Patemo Biichi reaction of chiral reactants (Case Study 6.17) may serve as an additional example. 

The use of cheap and readily available chiral auxiliaries such as menthol, 8-phenylmen-thol, and franj-2-rerr-butylcyclohexanol in Patemo-Biichi reactions is described by H.-D. Scharf. High diastereoselectivities are obtained in the photocycloaddition of the corresponding pyruvic esters to electron rich cycloalkenes. Both the synthesis of the chiral auxiliary as well as of the cycloalkene are interesting non-photochemical steps. 

The [2 + 2] photocycloaddition of two alkenes is widely used to form cyclobutanes. The reaction proceeds in the forward direction because the product cannot absorb light of the wavelengths that the starting material can absorb. In the Patemo-Biichi reaction, one of the two-atom components is a ketone or an aldehyde instead of an alkene. Why these cycloadditions require light to proceed will be discussed later. 

Closure of the biradical leads to a four-membered ring containing oxygen, which is known as an oxetane. The overall 1,2-photocycloaddition reaction is often called the Patemo-Biichi reaction (equation 12.60). The Patemo-Biichi reaction provides synthetic entry into highly strained ring systems containing oxygen atoms, as exemplified by equations 12.61 and 12.62 (where Ar is 2-naphthyl). ... 

Another photoreaction between ketones and olefins or dienes, which has often been connected to the involvement of exciplexes, is the Patemo-Biichi reaction [4,5], i.e., the photocycloaddition of C=C double bonds to carbonyl C=0 bonds under formation of oxetanes [17,78,207-217], Especially for electron-rich olefins such as ethyl vinyl ether or 1,2-diethoxyethylene, intermediary exciplexes have often been postulated [212], with the consequence of a diminished legioselectivity and stereospecificity for oxetane formation. On the other hand, electron-deficient olefins such as a,P-unsaturated nitriles react with a high legioselectivity and stereospedlicity due to a well defined transition state, which is based on the electronic leqniranents of n,7t -excited ketones [17]. 

Photochemical Cycloadditions. Isobutene has been widely used in intermolecular [2 + 2] photocycloaddition reactions with enones. The weakly polarized isobutene is often used to study the regioselectivity of the photocycloaddition. Cyclohexenones (eq 9), cyclopentenones (eq 10), and functionalized enones (eq 11 ) undergo cycloaddition with isobutene. Patemo-Biichi... 

The Patemo-Biichi reaction of 5-substituted adamantan-2-ones 55 with fumaro-nitrile to form oxetanes 56 and 57 (Scheme 20) has been studied by Turro and coworkers [93,94]. In the CD-free photocycloaddition performed in acetonitrile or water, the antf-oxetane 56 formed through the 5jn-face attack of fumaronitrile is slightly favored, with the 57/56 ratio varying from 47/53 to 36/64. When the photocycloaddition reaction is carried out in the presence of -CD in aqueous solution, the stereochemical preference is switched to 5jn-oxetane 57 and the... 

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