Ene reaction, of singlet oxygen

Following the discovery of the ene reaction of singlet molecular oxygen ( Ap (Scheme 15) in 1953 by Schenck [88], this fascinating reaction continues to receive considerable mechanistic attention today. The importance of a path via the perepoxide intermediate or a perepoxide-Iike transition state [13] or the perepoxide quasi-intermediate [70] was proposed for the ene reactions of singlet oxygen with alkenes affording allylic hydroperoxides. 

As is outlined for ene reactions of singlet oxygen in Scheme 15, the prototypical ene reaction starts with the electron delocalization from the HOMO of propene to the LUMO of X=Y. The delocalization from the HOMO, a combined n and orbital with larger amplitude on n, leads to a bond formation between the C=C and X=Y bonds. Concurrent elongation of the bond enables a six-membered ring transition stracture, where partial electron density is back-donated from the LUMO of X=Y having accepted the density, to an unoccupied orbital of propene localized on the bond. As a result, the partial electron density is promoted (pseudoex-cited) from the HOMO (it) to an unoccupied orbital (ct n ) of alkenes. This is a reaction in the pseudoexcitation band. 

TABLE 6. Results of the regio- and diastereoselective ene reaction of singlet oxygen with chiral aUyUc alcohols, acetates, amines (and acyl derivatives), silyl ethers and ethers... 

Highly regioselective ene reactions of singlet oxygen with alkyl-substituted alkenes can also be used for synthetic purposes. For example, in the photooxygenation ofl-t-butylcycloheptene only one allylic hydroperoxide, by the exclusive geminal hydrogen... 

Bald and coworkers synthesized four out of the ten possible diastereomers of querci-tols (Scheme 62), some of which are natural products, by using the ene reaction of singlet oxygen as the key step and the commercially available 1,4-cyclohexadiene as starting material. In this fashion, they managed to avoid the use of natural products, or compounds which require many steps to synthesize, employed as starting materials in all previously reported syntheses. 

Singh and coworkers have carried ont a large amonnt of work on the synthesis of sprro 1,2,4-trioxanes by employing the ene reaction of singlet oxygen with substituted ally lie alcohols The resulting hydroperoxy alcohols 86a can be condensed with a variety of different carbonyl compounds inclnding cyclic ketones (Scheme 33). Some of these trioxanes 86b have excellent in vivo activity (see Table 1). 

Replacement of the Ti in these catalysts with Zr, Hf, or Ta does not improve the chiral efficiency (5S). Dibenzyl tartramide in place of tartrates is also effective as chiral auxiliary of the Ti complex and results in high ee (59). The catalyst systems formed from Ti(0-/-C3H7)4 and the tartramide in 2 2 and 2 1 mole ratio leads to opposite enantioselec-tion as shown in Scheme 24. The ene reaction of singlet oxygen and alkenes in the presence of a Ti-DET complex has been used to prepare chiral epoxy alcohols in up to 72% ee (Scheme 25) (60). 

The ene reaction of singlet oxygen with alkenes was originally discovered [22,23] by Schenck in 1943. It is of synthetic [21,24], biochemical [25-27], and environmental significance [25,28] (Scheme 1). Over the years, the mechanism of this re-... 

In conclusion, the ene reaction of singlet oxygen with simple alkenes is a highly stereospecific suprafacial process. It proceeds via an irreversible formation... 

Stratakis, M. and Orfanopoulos, M. (2000) Regioselectivity in the ene reaction of singlet oxygen with alkenes. Tetrahedron, 56 (12), 1595-1615. 

Meyer, M.P., Leach, A.G., Kuwata, K.T., Chen, J.S., Greer, A., Foote, C.S. and Houk, K.N. (2003) Mechanism of ene reactions of singlet oxygen. A Two-Step No-intermediate mechanism. Journal of the American Chemical Society, 125, 1319-1328. 

Salamci, E., Secen, H., Siitbeyaz, Y. and Bald, M. (1997) A condse and convenient synthesis of dl-proto-quercitol and dl-golo-quercitol via ene reaction of singlet oxygen combined with [2 + 4] cycloaddition to cyclohexadiene. Journal of Organic Chemistry, 62 (8), 2453-2457. 

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. 

Figure 4. The ene reaction of singlet oxygen with unsaturated fatty acid.

Leach, A. G., Houk, K. N. Diels-Alder and ene reactions of singlet oxygen, nitroso compounds and triazolinediones transition states and mechanisms from contemporary theory. Chem. Common. 2002,1243-1255. 

Diels-Alder and ene reactions of singlet oxygen, nitroso compounds, and triazo-linediones 02CC1243. 

Salamci, E., Secen, H., Sutbeyaz, Y., Balci, M., A Concise and Convenient Synthesis of dl proto Quercitol and dl gala Quercitol via Ene Reaction of Singlet Oxygen Combined with [2 + 4] Cycloaddition to Cyclohexadiene, J. Org. Chem. 1997, 62, 2453 2457. 

A. Maras, H. Secen, Y. Sutbeyaz, and M. Balci, Convenient synthesis of (+)-ta/o-quercitol (1-deoxy-neo-inositol) and (+) vibo-quercitol (1-deoxy-myo-inositol via ene reaction of singlet oxygen, J. Org. Chem., 63 (1998) 2039-2041. 

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