Dienes reaction with singlet oxygen

Acetylenic ethers 7 can be hydrozirconated, and subsequent iododezirconation leads to (fc)-iodo enol ethers 8 (Scheme 4.4) [18], These species undergo efficient Sonogashira couplings to give (E)-enynes, which are ultimately converted to stereodefined dienol ethers. These dienes have proven useful in studies of diastereoselective cycloaddition reactions with singlet oxygen, where R in 8 is a nonracemic auxiliary (e. g., menthyl) (Procedure 3, p. 140). 

In the case of chiral base catalysis of the E2 elimination, enantioenriched 7-hydroxyenones from the corresponding endoperoxides were obtained <2006JA12658> in fact, a one-pot asymmetric 1,4-dioxygenation of 1,3-cyclohepta-diene by sequential reaction with singlet oxygen and 5 mol% chiral catalyst provided the 7-hydroxyenones 80 in 90% yield and 92% ee (Scheme 18). 

The photosensitized oxygenation of 12(Z)-abienol has been studied in a biomimetic synthesis of tobacco labdanoids. In the reaction with singlet oxygen the major process is an ene-reaction involving attack at C-13 and yielding after reduction the 11(H),13(/ and 5)-labda-ll,14-diene-8,13-diols. Some minor products arose from attack at C-12 and include the 12(i )- and 12(5)-labda-13(16)-14-diene-8,12-diols. Reaction of c -abienol with m-chloroperbenzoic acid gave some 8,12- and 8,13-epoxides. The X-ray crystal structure of 12(5),13(S)-8,12 12,15-diepoxy-13-bromolabdane has been determined. The synthesis has been described of some 13,8 13,17-ethers of the enantio-14(15)-dinorlabdane series from eperuic acid. 

Katsumura, Kitaura and their coworkers [74] found and discussed the high reactivity of vinylic vs allylic hydrogen in the photosensitized reactions of twisted 1,3-dienes in terms of the interaction in the perepoxide structure. Yoshioka and coworkers [75] investigated the effects of solvent polarity on the product distribution in the reaction of singlet oxygen with enolic tautomers of 1,3-diketones and discussed the role of the perepoxide intermediate or the perepoxide-Uke transition state to explain their results. A recent review of the ene reactions of was based on the significant intervention of the perepoxide structure [76], which can be taken as a quasi-intermediate. 

Danishefsky dienes [98] cycloadd to Cjq in refluxing toluene or benzene [5, 38, 99-101]. The diene 103 adds in 60% yield to Cjq to give the desilylated ketone 104 [5,101]. Acid-catalyzed methanol elimination then furnishes the enone 105 in 82% yield (Scheme 4.17). As already described, this enone can be reduced by DIBAL-H to the corresponding alcohol for further functionalization. The same a,(3-un-saturated alcohol can also be obtained in better yield by Diels-Alder reaction of Cg0 with butadiene, followed by oxidation with singlet oxygen to the allylic hydroperoxide and PPhj reduction to the desired alcohol [101]. This sequence yields the allylic alcohol in 53%, starting from Cjq without the need of isolating intermediates. 

Tetradehydrodianthracene undergoes 4 + 2-cycloaddition with electron-deficient dienes such as 1,2,4,5-tetrazines.263 The Diels-Alder reactions of [3.3 ortho-anthracenophanes witli A-(p-nitro, chloro, or methoxy-substituted phenyl)malehnides yield approximately equal quantities of inside and outside adducts.264 The photooxidation of bulky water-soluble 1,4-disubstituted naphthalenes with singlet oxygen yields both the expected 1,4- and the unexpected 5,8-endoperoxides.265... 

The thermal rearrangement of unsaturated bicyclic 1,4-peroxides, readily available from the reaction of conjugated dienes with singlet oxygen, is a convenient route fw the preparation of bisepoxides. Epoxidation of cholesteryl acetate (176) with air in the presence of a catalytic amount of dioxo(tetra-mesitylporphyrinato)ruthenium(VI) fimiishes in 8S% yield a 99 1 mixture the epoxides (177a) aitd (177b)." >... 

Isomerization of 1,3-diene epoxides. Noyori etalf have reported three types of isomerization of allylic oxides in the presence of a catalytic amount of this palladium(O) complex. One class, exemplified by the isomerization formulated in equation (I), involves transfer of a hydrogen atom from the Ca-alkyl group and results in a dienol of the type obtained in the ene reaction of singlet oxygen with 1,3-dienes. 

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