Patemo-BUchi photocycloaddition reaction

The Patemo-BUchi photocycloaddition reaction of various carbonyl compounds to furans was initially investigated by Sakurai in 1965 and was found to afford only the head-to-head photoproducts with high exo relative face selectivity. An NMR study by Whipple and Evanega ° later confirmed the exo mode of cycloaddition. Since the time of the origin report the photoreaction has been systematically studied by several groups and the 2,7-dioxabicyclo[3.2.0]hept-3-ene ring system has been exploited in several facets of synthesis. 

Reviews of the Patemo-Buchi photocycloaddition reaction for the synthesis of mono- and bi-cyclic oxetanes <00SL1699> and the stereoselective synthesis of oxetanones are available... 

Oxygen atoms in the tetrahydrofuran rings embedded in inositol-based tris(spirotetrahydrofuranyl)ionophores were useful as chelation sites towards selective alkali metal ion binding and as a result rodlike supramolecular ionic polymer frameworks were prepared and analyzed in this study <01JA4974>. The temperature and viscosity dependence of the spin-directed stereoselectivity of the Patemo-Buchi photocycloaddition reactions between 2,3-dihydrofuran and benzaldehyde were studied <01 AG(E)4684>. 

XVIII in a yield of 88 to 92%. It is here that the relationship between the Patemo-Buchi photocycloaddition process and the aldol reaction is most readily apparent. 

The chemo-, regio- and stereo-selectivity observed in the Patemo-Buchi 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 synthetic advances. Knowledge of these precedents will likely initiate further scrutiny and applications of the reaction to the synthesis of architecturally complex targets. 

The intramolecular attack of an excited carbonyl on an alkene can occur to provide oxetane products, even in cases when the corresponding intermolecular reaction is imsuccessful. Thus the intramolecular reaction surely benefits from favorable entropic considerations. Jones and Carless have summarized the scope and utility of intramolecular Patemo-BUchi photocycloadditions. There is general agreement that successful implementation of an intramolecular reaction requires that the Norrish type II photoreactionsand other hydrogen abstraction processes be overcome. In addition, the intramolecular reaction provides access to polyoxygenated ring systems that can exhibit remarkable properties because of their strain. 

The synthesis of oxetanes from alkenic precursors has been demonstrated and the range and scope of Patemo-BUchi 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 understood. Synthetic applications are numerous owing to the rapid stereocontrolled assembly of multifunctional targets. 

The enol ether double bond contained within the ds-fused dioxa-bicyclo[3.2.0]heptene photoadducts can also be oxidized, in a completely diastereoselective fashion, with mCPBA. Treatment of intermediate XXII, derived in one step from a Patemo-Buchi reaction between 3,4-dimethylfuran and benzaldehyde, with mCPBA results in the formation of intermediate XXIII. Once again, consecutive photocycloaddition and oxidation reactions furnish a highly oxygenated system that possesses five contiguous stereocenters, one of which is quaternary. Intermediate XXIII is particularly interesting because its constitution and its relative stereochemical relationships bear close homology to a portion of a natural product known as asteltoxin. 

The Patemo-Buchi reaction is the photocycloaddition of an alkene with an aldehyde or ketone to form oxetanes. This transformation has been shown to proceed through a biradical intermediate, and up to three new stereocenters can be formed as a result of this reaction. A general mechanism for the reaction between an aldehyde and a chiral enol silyl ether is shown in Eq. (13.7) [18]. Allylic 1,3-strain is cited as the control element in reactions of this type, and diastereomeric ratios of >95 5 are reported for products 30 containing four contiguous stereocenters. Examples of photocyclizations of amino acid derivatives proceeding through biradical intermediates have been repotted [19]. 

The Patemo-Buchi reaction is one of the more predictable photocycloaddition reactions. Regiocontrol of the photoproduced oxetane is a function of the stepwise addition of the carbonyl chromophore to the alkene [30]. In the case of electron-rich alkenes, excitation of the carbonyl group produces a triplet species that adds to the alkene. The product regioselectivity is a result of addition that generates the most stable biradical, and the triplet lifetime of the intermediate biradical allows for substantial stereoselectivity prior to closing (see Scheme 2). Electron poor alkenes are more likely to undergo cycloaddition with carbonyl groups directly from an exciplex [31]. 

Fleming and Gao reported that the photocycloaddition of the trimethylsilyl ether of phenylpropenol with benzaldehyde proceeds with high stereoselectivity to give the fra/is-oxetane 60 in 20% yield. In competition to the Patemo-Buchi reaction, cis/trans isomerization leads to cis and trans isomers of the... 

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