A novel simplified procedure affords consistently 0.09-0.10 M solutions of distilled dimethyldioxirane in acetone: other than control of the reaction temperature below 15°C and vigorous mechanical stirring, no other precautions are mandatory.The importance of isolated dimethyldioxirane (Eq. 1) for preparative oxidation chemistry, as witnessed by the surge of activity2) since its isolation3) in 1985, demands a convenient method of isolation of this efficient oxidant. Presently, we provide a much simplified version of a recently reported procedure4), which should be of general interest and utility. Me (Eq. 1) H20. NaHC03 CH,COGH, + KHSO, pH Z 7 4 . 5 -1 O o C MeKey features of the new version are that even normal water (not distilled from EDTA) and commercial-grade acetone (directly from the metal canister) can be employed, that the high-efficiency

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The synthesis of epoxides 3a -r is achieved in excellent yields by reaction of the a,P-unsaturated ketones l a -c , 4,4'-disubstituted (E)-chalcones 1 d -0 , and 2'-hydroxy-4-substituted (E)-chalcones l p -r with isolated dimethyldioxirane (2a) (as acetone solution) and/or in situ generated ethyl(methy1)dioxirane (2b). This method constitutes a useful alternative to the Weitz-Scheffer epoxidation (alkaline H202) of such electronpoor substrates.Chalcone epoxides are natural products which are commonly formed in plants ') and exhibit important biological activity, e.g. they act as potent inhibitors of the cytosolic epoxide hydrolase'). The synthetic utility of such functionalized epoxides has been unquestionably demonstrated3), a fact that underscores the need of new methods for the convenient and efficient preparation of these valuable "building blocks" in organic synthesis.Two evident methodologies are either Sharpless epoxidation4) of allylic alcohols and subsequent oxidation of the hydroxy functionality, or alternatively direct epoxidation of a$-unsaturated ketones by alkaline hydrogen peroxide and its alkyl and silyl derivatives (Weitz-Scheffer reaction5)). For base-sensitive functional groups, e. g. phenolic substituents, undesirable oxidations of the aromatic moiety may become prominent. The use of electrophilic oxidants such as peracids are frequently complicated by the facile Baeyer-Villiger rearrangement 6, of such electron-poor substrates. The basecatalyzed dehydrohalogenation of halohydrin may provide a useful entry into chalcone epoxides7), but the preparation of the halohydrin precursors from the chalcones is not compatible with a number of functionalities.Dimethyldioxirane (2a) is a selective and powerful oxygen transfer agent *I, that besides its high propensity to epoxidize electron-rich double bonds, has recently been shown to work well in its isolated form as acetone solution9) with disubstituted a$-unsaturated acids and esters"), p-0x0 enol ethers 'I), and even the labile a-methy1ene-P-lactonesl2) (Scheme 1). Previously, it was reported that the in situ generated dimethyldioxirane (2 a) is effective for the epoxidation of cinnamic acids and esters 13), while the transformation of phenylpropiolic acid into phenylacetic acid was postulated to proceed through an oxirene intermediate14).In view of the convenience and efficiency of dimethyldioxirane (2a) as oxidant*), we decided to test it in the direct conversion of a$-unsaturated ketones into their epoxides. * ) Undergraduate Research Participant, University of Wurzburg, Spring 1990. In this paper we describe our results obtained by using isolated dimethyldioxirane (2a) (as acetone solution) and our new in situ method, which utilizes ethyl(methy1)dioxirane (2b) prepared from 2-butanone and Caroate.Scheme 1. Previous epoxidations of u,P-unsaturated carbonyl derivatives by dimethyldioxirane R R Results and DiscussionThe a$-unsaturated ketones 1 a -r are transformed by isolated dimethyldioxirane (2a) (as acetone solution) into the corresponding ep...

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