Photooxygenation chiral diastereoselectivity

In a search for more effective approaches to the problem of stereoselective dioxygenation, alternative methods have been developed. While the inclusion of alkenes and O2 within a chiral cyclodextrin cavity furnished hydroperoxides with modest ee values, neighboring stereocenters and chiral auxiliaries could induce highly stereoselective dioxygenation. In 1987 Kropf and Reichwaldt, and three years later Adam and coworkers reported on the photooxygenation of phenyl-substituted alkenes 36 producing allyUc hydroperoxides 37 and 38 with high diastereoselectivity (dr 80/20). In the best example... 

Many mechanisms had been proposed in the past to rationalize this selectivity (tri-oxanes, perepoxide, exciplex, dipolar or biradical intermediates) however, it is now generally accepted that the reaction proceeds through an intermediate exciplex which has the structural requirements of a perepoxide. This assumption is supported by (a) the lack of stereoselectivity in the reactions with chiral oxazolines and tiglic acid esters (b) the comparison of the diastereoselectivity of dialkyl substituted acrylic esters with structurally similar non-functionalized aUtenes (c) the intermolecular isotope effects in the photooxygenation of methyl tiglate and (d) the solvent effects on regioselectivity. ... 

SCHEME 37. Threo/erythro diastereoselectivity in the photooxygenation of chiral olefins without heteroatoms at the stereogenic centre... 

It has already been mentioned (Section III) that the study of the diastereoselection in the electrophilic addition of singlet oxygen to the n face of chiral alkenes is of primary interest for the achievement of a selective oxyfunctionalization reaction. Zeolite confinement and cation- 7r interactions might be expected to affect significantly the diastereoselectivity in the photooxygenation of chiral alkenes. 

TABLE 24. Regioselectivity and diastereoselectivity in the photooxygenation of chiral alkenes within zeoUte Na-Y and in solution (values in parentheses)... 

For the photooxygenation of chiral alkenes in solution bearing a stereogenic centre at the or more remote position with respect to the double bond, low or negligible diastereoselection is expected. The photooxygenation of 2-methyl-5-phenyl-2-hexene 156, a chiral alkene that bears a stereogenic centre at the /3-position with respect to the double bond, gave in solution low diastereoselectivity ca 10% By Na-Y... 

Singlet oxygen affords a variety of regio and diastereoselective reactions with chiral allylic alcohols amines - (e.g. substrate 161, Scheme 58) and chiral cyclohexadienes - that are useful for synthetic transformations. For example, the photooxygenation of a chiral allylic alcohol was used recently as the key step in the total syntheses of plakorin 162 and ewawfio-chondrilin (Scheme 58). If the photooxidation... 

The oxazolidinone-substituted olefin Ic (Scheme 3) constitutes another fortunate substrate for the diastereoselective synthesis of a chiral dioxetane , which is of preparative value for the enantiomeric synthesis of 1,2 diols . For example, the photooxygenation of the enecarbamate Ic produces the asymmetric dioxetane 2c in >95% jt-facial diastereoselectivity. The attack of the O2 occurs from the jt face anti to the isopropyl... 

The first example of a chiral-auxiliary-induced [2+2] cycloaddition between 02 and oxazolidinone-functionalized enecarbamates, which proceeds with complete diastereoselectivity as a result of steric repulsions, has been reported to afford 57 <02JACS8814>. The optically active enecarbamates bearing Evans chiral auxiliary were photooxygenated at -35 °C with 5,10,15,20-tetrakis(pentafluorophenyl)porphine (TPFPP) as sensitizer and an 800 W sodium lamp as light source. The dioxetanes 57 were obtained exclusively, but they readily decomposed at room temperature to the expected carbonyl products because of their thermally labile nature. The absolute configuration of the dioxetanes 57 was established by reduction to the corresponding diols with L-methionine. 

Adam W, Bosio SG, Turro NJ (2002) Flighly diastereoselective dioxetane formation in the photooxygenation of enecarbamates with an oxazolidinone chiral auxiliary steric control in the [2+2] cycloaddition of singlet oxygen through conformational alignment. J Am Chem Soc 124 8814-8815... 

Adam, W. and Nestler, B., Photooxygenation of chiral aUyHc alcohols hydroxy-directed regio- and diastereoselective ene reaction of singlet oxygen, /. Am. Chem. Soc., 114, 6549,1992. 

Linker, T. and Frohlich, L., Regioselective and diastereoselective photooxygenation of chiral 2,5-cyclohexadiene-l-carboxyHc acids, Angew. Chem. Int. Ed. Engl., 33, 1971, 1994. 

Linker, T., Rebien, E, and Toth, G., Highly diastereoselective photooxygenation of chiral 1,2-dihydronaphthalenes evidence for a common intermediate in the ene reaction and the [4 -i- 2] cycloaddition, J. Chem. Soc., Chem. Commun., 2585, 1996. 

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