Stereoselectivity, 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). 

Buhr, S., Griesbeck, A.G., Lex, J., Mattay, J., and Schroer, J. (1996) Stereoselectivity in the Paterno-Biichi reaction of 2,2-diiso-propyl-l,3-dioxol with methyl trimethyl-pyruvate. Tetrahedron Letters, 37, 1195-1196. 

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 second report concerns the factors controlling the stereoselectivity of Paterno-Biichi reactions between aromatic aldehydes and 2,3-dihydrofuran32-34, Whereas furan yields exclusively 6-phenyl-2,7-dioxabicyclo[3.2.0]hept-3-ene (9) with the aryl substituent in the ecto-posi-tion 10,32-36, 2,3-dihydrofuran forms the eWo-compound 11, interestingly this occurs even with sterically demanding substituents. 

In the previous sections the multiplicity of the excited state has been discussed. In general, electron-deficient alkenes react preferentially with alkanones in the nit singlet state preserving the stereochemistry of the alkene (see Section I.6.I.4.3.2.)6,, 3 27,47-49. Note that ZjE isomerization of the alkenes can precede and as a consequence change the stereoselectivity of oxetane formation61 J 27. Nevertheless, the majority of Paterno- Biichi reactions are concerned with electron-rich olefins and here triplet states of the carbonyl compounds are most likely involved. As briefly discussed in Section 1.6.1.4.3.1., 1,4-diradicals play a decisive mechanistic role and consequently also control the stereochemical outcome some examples have already been given11 28 29 32-J6. The controlling factors can be seen in the mechanistic scheme shown in... 

Asymmetric induction in the Paterno Biichi reaction has been increasingly studied since the first successful example, reported in 1979, which gave up to 53% de51. Since then high dia-stereoselectivities with either optically active carbonyl compounds or alkenes have been reported. The most extensive studies have been performed by Scharfs group1 ,28 29, who found that the degree of stereoselectivity depends not only on the structure of the chiral auxiliary (mainly chiral alcohols bound to a-oxo acids) but also on the olefinic partner, the reaction conditions and, in particular, on the temperature. The following items will be discussed here. 

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... 

Effect of Concentration on the Stereoselectivity of the Paterno-Biichi Reaction... 

Morris, T. H., Smith, E. H., and Walsh, R., Oxetane synthesis methyl vinyl sulphides as new traps of excited benzophenone in a stereoselective and regiospecific Paterno-Biichi reaction, J. Chem. Soc., Chem. Commun., 964, 1987. 

Buschmann, H., Scharf, H.-D., Hoffmann, N., Plath, M., and Runsink, J., Chiral induction in photochemical reactions. 10. The principle of isoinversion a model of stereoselection developed from the diastereoselectivity of the Paterno-Biichi reaction, /. Am. Chem. Soc., Ill, 5367, 1989. 

Bach, T, The Paterno-Biichi reaction of 3-heteroatom-substituted alkenes as a stereoselective entry to polyfunctional cyclic and acycHc molecules, Liebigs Ann./Recueil, 1627, 1997. 

Developments in the field of stereoselective 1,3-dipolar cycloadditions and their synthetic applications are outlined in Sections 18.2 to 18.4 [22, 23, 28-34]. Subsequently, related [2-i-3]-cycloadditions with 1,3-diradicals and trimethylenemethane equivalents that furnish the corresponding five-membered carbocycles are covered (Section 18.5) [38—41]. The chapter closes with a collection of [2-i-2]-cycloadditions involving ketenes (Section 18.6) [19, 35, 36], photochemical olefin cycloadditions, and Paterno-Biichi reactions (Section 18.7) [36, 37],... 

Abe, M. and Nojima, M., The Paterno-Biichi photocydoaddition reactions of silyl ketene acetals controlling factors on the regio- and stereoselectivity in the oxetane formation,/. Synth. Org. Chem. Jpn., 59, 855, 2001. 

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