Stereoselective photo-oxygenation

Recent developments in the stereoselective singlet oxygen allylic photo-oxygenations of alkenes have been reviewed. A number of factors, such as solvent, electronic effects, and non-bonded interactions that dictate the ene product selectivity, and also the various mechanisms of this reaction, have been highlighted.239... 

The phenyl ring of styrene substrates directs a syn selectivity in their ene reactions with singlet oxygen [71], This effect was demonstrated by the photo-oxygenation of (3,(3-dimethyl styrene. This substrate, a part of the ene product, produces the 1,2-dioxetane and two diastereomeric diendoperoxides [72,73], as shown in Scheme 12. The stereospecific labeling of the anti methyl by deuterium in compound 24 to produce substrate 25 was required in order to study the syn/anti stereoselectivity of the ene products. 

Many mechanisms had been proposed in the past to rationalize this selectivity (trioxanes, perepoxide, exciplex, dipolar, or biradical intermediates) however, it is now generally accepted [100] that the mechanism proceeds through an intermediate exciplex which has the structural requirements of a perepoxide. This assumption is supported (a) by the lack of stereoselectivity in the reactions with chiral oxazolines [101] and tiglic acid esters [102], (b) by the comparison of the diastereoselectivity of dialkyl-substituted acrylic esters [103] with structurally similar nonfunctionalized alkenes, (c) by intermolecular isotope effects [104] in the photo-oxygenation of methyl tiglate, and (d) by solvent effects on regioselec-tivity [105],... 

The hydroxyl group at the allylic position has a significant effect on the syn/anti methyl stereoselectivity [67,68] and the diastereoselectivity [63,64] of the photo-oxygenation ene reaction (see Sec. II.B). To assess the effect of the hydroxyl at the more remote homoallylic position, the reaction of O with the geminal dimethyl trisubstituted homoallylic alcohols (85, 86, 89) and the cis dis-ubstituted 90 was examined in nonpolar solvents [116], The regioselectivity trend was compared with that of the structurally similar trisubstituted alkenes (87, 88, 91) [105], The results are summarized in Table 12. 

Photo-oxygenation of a-pinene in MeCN-HaO differed from that in aprotic media the main hydroperoxide was the same but it was accompanied by (218).546 Similar reaction of the p-isomer in the presence of metal oxides gave myrtenol, myrtenal, nopinone, and pinocarvone.547 a-Pinene was reduced to m-pinane (96% stereoselective) over Ru,648 and electrochemical reduction of 2,4-dibromo-pinocamphone gave a variety of products devoid of Br.549 Hydroboration of amino-and azido-pinanes550 and the conversion of a-pinene into 2-hydroxy-3-amino- and 3-hydroxy-5-amino-pinanes have been reported.551-553 syn-Addition of 2HC1 to the... 

The Type II photo-oxygenation reactions with which we are concerned occur by a completely different mechanism. The main feature of these reactions is that an activated oxygen is formed during the reaction, which can react stereoselectively with certain substrates to give the addition products. Today, we have reason to believe that the activated oxygen is the excited singlet oxygen, 02. 

In preparative organic chemistry, much use has been made of the stereoselective Type II photo-oxygenation reaction since the primarily produced allylic hydroperoxides can be reduced under retention of configuration. However, depending on the nature of the allylic hydroper-... 

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