Stereoselectivity Paterno-Buchi reaction

Attempts to obtain stereoselective Paterno-Buchi reactions were performed carrying out the reaction between 3,4-dimethylfuran and R-isopropylidene dia-stereoisomer. The coupling products were obtained with an overall yield of 35% as a 1.2 1 mixture of diastereoisomers. Furthermore, the compound 37 was obtained with 54% ee (Scheme 3.39) [76, 77]. This behavior suggests the operation of a mechanism that is insensitive to the substitution pattern of chiral aldehydes. Reaction between an excited aldehyde (singlet or triplet state) and furan... 

The intramolecular Paterno-Buchi reaction is capable of installing new oxetane units with significant regio- and stereoselectivity, as is evident in the construction of a tetracycle related to merrilactone (Equation (26) see also Section 2.06.12.2) <2005OL3969>. The chirality of an atropoisomer recrystallized from an interconverting (4/z = 468 s at — 20 °C) mixture of enantiomers was preserved by photocycloaddition (Equation 27) the atropoiso-meric oxetane products do not interconvert unless heated <2003JOC942>. 

Bach, T. (1996) N-acyl enamines in the Paterno-Buchi reaction stereoselective preparation of 1,2-amino alcohols by C-C bond formation. Angewandte Chemie, International Edition in English, 35 (8), 884-886. 

A clear evidence for a long-lived intermediate came from investigations of the stereoselectivity of the Paterno-Buchi reaction with cis and trans 2-butene as substrates, when acetone [52] or benzaldehyde [53] was used as carbonyl addends, complete stereo-randomization was observed. Acetelde-hyde and 2-naphthaldehyde showed stereoselective addition reactions which accounts for the singlet mi as the reactive state [43]. Fleming and Gao... 

This peculiar stereoselectivity might be attributed to a memory effect from the approach geometry between the triplet excited benzaldehyde and the alkene. Abe and coworkers have also observed a comparable stereochemical effect in the Paterno-Buchi reaction of 4-cycanobenzalde-hyde with O-silylated thioketene acetals 129 (Sch. 43) resulting in the highly functionalized oxetanes 130 [64]. 

The first total synthesis of the cytotoxic agent (+)-euplotin A was completed by the research team of R.L. Funk. The key step of the synthetic effort was the intramolecular hetero DIels-Alder cycloadditlon of a 3-acyl oxadiene (generated from 5-acyl-4H-1,3-dioxins via thermal retrocycloaddition) with a substituted dihydrofuran to afford the tricyclic skeleton of the natural product. The correct relative stereochemistry of the required dihydrofuran substrate was established using the Paterno-Buchi reaction between ethyl glyoxylate and furan. Subsequently, the oxetane ring was opened stereoselectively under Lewis acid catalysis. 

Bach, T. The Paterno-Buchi reaction of 3-heteroatom-substituted alkenes as a stereoselective entry to polyfunctional cyclic and acyclic molecules. Liebigs Ann. Chem. 1997,1627-1634. 

D Auria, M., Emanuele, L., Poggi, G., Racioppi, R., Romaniello, G. On the stereoselectivity of the Paterno-Buchi reaction between carbonyl compounds and 2-furylmethanol derivatives. The case of aliphatic aldehydes and ketones. Tetrahedron 2002, 58, 5045-5051. 

Morris, T. H., Smith, E. H., Walsh, R. Oxetane synthesis. Methyl vinyl sulfides as new traps of excited benzophenone in a stereoselective and regiospecific Paterno-Buchi reaction. J. Chem. Soc., Chem. Common. 1987, 964-965. 

The regio- and stereochemistry of the Paterno-Buchi reaction depend on the structures of the reactants, on the electronic energy of the excited state carbonyl compound, and on the multiplicity of the excited state. With unsymmetrical alkenes, the products suggest preferential formation of the more substituted radical center in the biradical intermediate, but steric and electronic factors are also important. Stereoselectivities depend on the multiplicity of the excited state. For example, the reaction of excited singlet and triplet states of propanal (84) with 2,3-dihydrofuran (85) gave the diastereomers of 7-ethyl-2,6-dioxabicyclo[3.2.0]heptane (86) in different ratios (equation 12.63). ... 

Buschmann, H., Scharf, H.-D., Hoffmann, N., Plath, M. W., and Runsink, J., Chiral induction in photochemical reactions. The principle of isoinversion a model of stereoselection developed from the diastereoselectivity of the Paterno-Buchi reaction,/. Am. Chem. Soc., Ill, 5367,1989. 

The photochemical reaction of carbonyl compounds and alkenes, which is referred to as the Paterno-Buchi (PB) reaction, was developed in 1909 [13], and is currently one of the most widely used methods for oxetane synthesis (Scheme 7.4). As exemplified in the PB reaction of benzophenone with 2-methylpropene [14], a selective formation of the oxetane is possible even when the photochemical reaction involves highly unstable molecules that is, the excited state of carbonyls. Due to its synthetic importance and mechanistic interest, the PB reaction is the most extensively studied synthetic method for oxetanes. Thus, several extensive reviews describing the PB reaction have been published since 1968, and the reader is directed towards these for further information [15]. In this chapter, methods that allow for the control of the regioselective and stereoselective formation of synthetically important oxetanes will be described. 

The stereoselectivity of the Buchi-Paterno reaction between 3-hydroxy-2,3-dihydrofuran and benzophenone was found to be influenced by solvent, temperature and steric effect <06TL2527>. A Dotz benzannulation involving a dihydrofuran chromium carbene complex and an alkyne was employed to form the aflatoxin skeleton, providing the annulated product as the only regioisomer <06TL2299>. Cycloaddition involving 2,3-dihydrofuran, 1-aminoanthraquinone and salicy aldehyde was catalyzed by... 

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