1,2-Dioxetanes reduction

As already hinted at above, chiral dioxetanes, obtained through the highly stereoselective [2 + 2] cycloaddition of singlet oxygen to the chiral enecarbamate, provide a convenient preparation of optically active 1,2 diols as building blocks for asymmetric synthesis (Scheme 5) . Reduction of the dioxetane 2c by L-methionine, followed by release of the oxazolidinone auxiliary by NaBH4/DBU reduction, affords the enantiomerically pure like-5 diol (for additional cases, see Table 4 in Reference 19e). 

CIEEL mechanism was originally proposed by Schuster for intense luminescence (or bioluminescence) in the decomposition of endoperoxides and dioxetanes. The relevance and importance of ET in the latter has recently been discussed. While the mechanism of reduction of peroxides in these early studies was described essentially as a concerted dissociative process, little insight into the fine detail of the mechanism could be provided. 

Dioxetanes are readily reduced to 1,2-diols (Scheme 30) by lithium aluminum hydride (75CJC1103), thiols (88AG(E)429), and biologically important reductants (89MI 133-02) such as ascorbic acid, tocopherol, dihydronicotinamide adenine dinucleotide (NADH), and riboflavin adenosine diphosphate (FADH2). 

Dioxetanes are readily reduced to 1,2-diols by lithium aluminium hydride and thiols. However, no notable examples have been reported since 1996. An intriguing and very mild method for the reduction of 1,2-dioxetanes, which utilizes L-methionine as the reductant, has been reported recently. For example, treatment of dioxetanes of type 38 with L-methionine at 0°C produces the desired 1,2-diols 39 (Scheme 7) <2004JOC1704>. Yields ranged from 25% to 41%. 

Dioxetanes are converted to vzc-diols with reductants as lithium aluminum hydride [132a] or thiols [132b] (Sch. 77). 

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. 

In the case of alkyl enol ethers the normal oie process competes with solvent incorporated and 1,2-di-oxetane products. Here however the ene process seems to be less inevitaUe when allylic protons are available and the product distribution may be effectively contndled by manipulation of solvent and temperature combinations. Best results are nonetheless achieved where the competitive processes are restricted. Thus enol ether (79) produces hydroxylated dimethoxy acetal (W) via direct incorporation of methanol or through reduction of the 1,2-dioxetane (81). 

Catalytic reductions over platinum or palladium, which are usually quantitative methods for the identification of organic peroxides, are problematic. Little of the expected 1,2-diol is obtained because the dioxetane fragments into its carbonyl products due to metal catalysis." However, lithium aluminium hydride reduction under subambient conditions affords the expected 1,2-diol quantitatively. Again, the sterically hindered dioxetane (9) is an exception. Here zinc in acetic acid proved successful. ... 

Superoxide ion reacts with vicinal dibromoalkanes to form aldehydes (Table 7-1), The mechanism proposed for these reactions (Scheme 7-5) is a nucleophilic attack on carbon, followed by a one-electron reduction of the peroxy radical and nucleophilic displacement on the adjacent carbon to form a dioxetane that subsequently cleaves to form two moles of aldehyde.21... 

Little has been reported in the period covered by this review on the photoreactions of oxetans. 2,2,4,4-Tetraphenyloxetan-3-one undergoes decarbony-lation on irradiation in benzene to give tetraphenyloxiran and reduction on irradiation in propan-2-ol to give the corresponding oxetan-3-ol. The photodecomposition of 1,2-dioxetans is of particular interest tetramethyl-... 

Dioxetanes can also be transformed into 1,2-diols by reduction with LiAlH4 or into epoxides by treatment with phosphines in the dark.1421... 

Iodometric titration is one of the most direct methods for identifying 1,2-dioxetanes, since we are specifically analyzing for peroxide content. When the necessary precautions are taken to avoid thermal decomposition, many 1,2-dioxetanes titrate quantitatively.12 However, the adamantylidene (17) does not react with iodide ion since it is too sterically hindered.28 The product of I- reduction of la is the corresponding diol.12... 

Reduction with lithium aluminum hydride is a useful method of identification since the respective diols are formed.12,34 Catalytic hydrogenation usually leads to decomposition of the dioxetanes into the corresponding carbonyl products.12,31 This is not surprising since transition metals promote fragmentation of these labile materials.49 In the case of dioxetane lz, the use of zinc in acetic acid was the only successful method for reducing it to its diol.31... 

A novel reduction was discovered by Baumstark,126 involving the reaction of dioxetane lb with stannous chloride, leading to pinacol and stannic ion on hydrolysis [Eq. (52)]. Formally it could be considered as... 

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