Epoxidation of a,3-Unsaturated Carbonyl Compounds

The epoxidation reaction of a,P-unsaturated aldehydes has only been successfully accompUshed using secondary amine catalysts, with diarylprohnols, such as 

which are especially useful for the synthesis of chiral heteroaromatic compounds [100]. 

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Many examples of the phase-transfer catalysed epoxidation of a,(3-unsaturated carbonyl compounds using sodium hypochlorite have been reported [e.g. 7-10]. The addition of transition metal complexes also aids the reaction [11], but advantages in reaction time or yields are relatively insignificant, whereas the use of hexaethyl-guanidinium chloride, instead of a tetra-alkylammonium salt, enhances the rate of epoxidation while retaining the high yields (>95%) [10]. Intermediate (3-haloalkanols are readily converted into the oxiranes under basic conditions in the presence of benzyltriethylammonium chloride [12]. 

Several methods for the epoxidation of a,(3-unsaturated carbonyl compounds have been reported. The use of amino acid derivatives or peptides as chiral ligands for epoxidation continues to be an active area of investigation. The use of silica bound poly-L-leucine, 21, with sodium percarbonate appears to be an excellent route to enantiomerically pure keto... 

A biphasic system consisting of the ionic liquid [BMIM]PF6 and water was used for the epoxidation reactions of a, 3-unsaturated carbonyl compounds with hydrogen peroxide as an oxidant at room temperature 202). This biphasic catalytic system compared favorably with the traditional phase transfer catalysts. For example, under similar conditions (15°C and a substrate/NaOH ratio of five), the [BMIM]PF6/H20 biphasic system showed a mesityl oxide conversion of 100% with 98% selectivity to oc, 3-epoxyketone, whereas the phase-transfer catalyst with tet-rabutylammonium bromide in a CH2CI2/H2O biphasic system gave a conversion of only 5% with 85% selectivity. 

A rather new concept in the context of domino radical cydizations has been developed by Gansauer and coworkers utilizing titanocene-complexes for the radical opening of unsaturated epoxides. The titanocene-catalyzed reactions [61] of 3-145 primarily led to radical 3-146, which underwent a subsequent intermolecular addition to a present a,(3-unsaturated carbonyl compound to form bicyclic carbocy-cles of type 3-148 via the intermediate 3-147 after aqueous work-up (Scheme 3.38) [62]. From a kinetic point of view, the reaction is remarkable since the intermolecular addition of simple radicals to a,(3-unsaturated carbonyl compounds is not an easy task, as highlighted above. 

With an operating catalytic cycle for the reductive opening of epoxides at hand, we decided to investigate the preparatively more important formation of carbon—carbon bonds. Inter-molecular addition reactions to a,(3-unsaturated carbonyl compounds have been described for the stoichiometric process of Nugent and RajanBabu [5], The general concept behind the catalytic conditions is outlined in Scheme 12.16. 

In the benzoin condensation, one molecule of aldehyde serves as an electrophile. If a carbanion is generated from protected cyanohydrins, a-aminonitriles or dithioacetals, it can react with electrophiles such as alkyl halides, strongly activated aryl halides or alkyl tosylates to form ketones. Amongst other electrophiles which are attacked by the above carbanions are heterocyclic A -oxides, carbonyl compounds, a,3-unsaturated carbonyl compounds, a,3-unsaturated nitriles, acyl halides, Mannich bases, epoxides and chlorotrimethyl derivatives of silicon, germanium and tin. 

Hydroxybenziodoxole 104 can be readily converted into its acetoxy derivative, l-acetoxy-1,2-benziodoxole-3(l//)-one (88, 2X = O, Y = OAc), by heating 104 with acetic anhydride and the acetoxy derivative can be further converted into the alkoxy derivatives by treatment with an appropriate alcohol [280], The tetrabutylanmionium salt of hydroxybenziodoxole 104 has been prepared by the reaction of hydroxybenziodoxole with tetrabutylammonium fluoride in THF it is a mild oxidant that is useful for the preparation of epoxides from a,p-unsaturated carbonyl compounds [281], l-Hydroxy-l,2-benziodoxole-3(l//)-one and l-acetoxy-l,2-benziodoxole-3(l//)-one have found wide application as starting compounds for the synthesis of various benziodoxole-based hypervalent iodine reagents by ligand exchange on iodine [239]. 

The epoxidation of electron-deficient alkenes, particularly a,P-unsaturated carbonyl compounds, continues to generate much activity in the literature, and this has been the subject of a recent concise review <00CC1215>. Additional current contributions in this area include a novel epoxidation of enones via direct oxygen atom transfer from hypervalent oxido-).3-iodanes (38), a process which proceeds in fair to good yields and with complete retention of... 

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