Photooxygenation diastereoselectivity

Synthetic applications involving regioselective or diastereoselective photooxygenation ene reactions, as key steps, are very common in the literature and some examples have already been reported in the previous sections. Additionally, the number of natural products... 

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. 

Adam, W., Peters, E. M., Peters, K., Prein, M., and von Schnering, H. G., Diastereoselective photooxygenation of chiral naphthyl alcohols the hydroxy group directing effect in singlet oxygen [4+2] cycloaddition to arenes, /. Am. Chem. Soc., 117, 6686,1995. 

Stratakis, M., Sofikiti, N., Baskakis, C. and Raptis, C. (2004). Dye-sensitized intrazeolite photooxygenation of 4-substituted cyclohexenes. Remote substituent effects in regioselectivity and diastereoselectivity. Tetrahedron Lett. 45, 5433-5436... 

A sequence of transformations involving diastereoselective singlet oxygen photooxygenation of TBS-protected 3,5-cycloheptadienol 525 to predominantly 5yw-endoperoxide syn-526 and some anti-526, followed by reduction into the all-ci5 triol 527 as described in Scheme 146, was used by Johnson for synthesis of enantiopure methyl 2,4-dideoxyhexa-pyranosides D-528 and L-5284°5.406... 

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... 

This problem was solved by Adam and coworkers in 1994-1998. They presented a high-yielding and diastereoselective method for the preparation of epoxydiols starting from enantiomerically pure allyhc alcohols 39 (Scheme 69). Photooxygenation of the latter produces unsaturated a-hydroxyhydroperoxides 146 via Schenck ene reaction. In this reaction the (Z)-allylic alcohols afford the (5, 5 )-hydroperoxy alcohols 146 as the main diastereomer in a high threo selectivity (dr >92 8) as racemic mixmre. The ( )-allylic alcohols react totally unselectively threolerythro 1/1). Subsequent enzymatic kinetic resolution of rac-146 threolerythro mixture) with horseradish peroxidase (HRP) led to optically active hydroperoxy alcohols S,S) and (//,5 )-146 ee >99%) and the... 

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 33. Threo/erythro diastereoselectivity in the photooxygenation of aUyhc alcohols 115-117... 

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... 

SCHEME 53. Regioselectivity/diastereoselectivity in the photooxygenation of 2-methyl-5-phenyl-2-hexene (values in parentheses relative reactivity in solution)... 

SCHEME 56. Na -directing regioselectivity and diastereoselection in the intrazeolite photooxygenation of a-terpinyl acetate... 

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... 

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