Hydroxy ketones from epoxides

The oxidation of alkylsilyl alkoxides has been extended to the preparation of a-hydroxy ketones from alkenylsilanes. Thus, 5-decenyltrimethylsilane was treated with meta-chloroperbenzoic acid and the resulting a-silyl epoxide oxidized with hydrogen peroxide in the presence of fluoride ion to give 6-hydroxy-5-decanone in 88% yield145 (equation 149). 

It should finally be pointed out that the mild reaction conditions typically employed in dioxirane-mediated oxidations enable the asymmetric epoxidation of enol ethers and enol esters. With the silyl ethers, work-up provides enantiomeri-cally enriched a-hydroxy ketones. As summarized in Table 10.1, quite significant enantiomeric excesses were achieved by use of catalyst 10 at loadings ranging from 30 [30] to 300 mol% [31]. Enol esters afford the intact acyloxyepoxides enantiomeric purities are, again, quite remarkable. 

Cleavage of a,[i-epoxy ketones.1 The reagent (1), possibly (CH3)2SC1, formed from DMSO and ClSi(CH3)3, cleaves a,(3-epoxy ketones to 2-chloro-3-hydroxy ketones with high regioselectivity (equation I). If the epoxide has a phenyl substituent, 3-chloro-2-hydroxy ketones are formed (equation II). 

Cross-coupling reactions of ArCOAr. Reaction of Yb(0) with diaryl ketones changes the reactivity of the carbonyl group from electrophilic to nucleophilic. Thus in the presence of this lanthanoid metal, diaryl ketones couple with other ketones, nitriles, and epoxides to give pinacols, a-hydroxy ketones, and 1,3-diols, respectively, via the intermediate a. 

Addition of lithium enolates derived from esters and ketones to epoxides has been the object of some consideration, because it offers a direct route for the synthesis of y -hydroxy esters and y-hydroxy ketones, very useful for difunctionahzed organic compounds" . In fact, more complex molecules can be synthesized by using these compounds as synthons... 

P-Hydroxy selenides are conveniently prepared from epoxides by treatment with sodium phenylse-lenide (Scheme 32) and by the addition of benzeneselenenic acid and its derivatives to alkenes (Scheme 33), - -" although in some cases these reactions are not regioselective. Useful phenylseleno -etherification and -lactonization reactions have been developed which can be regioselective (equation 42 and Schemes 34 and 35). -" " Selenide- and selenoxide-stabilized carbanions have been used in addition reactions with aldehydes and ketones, - and the reduction of a-seleno ketones also provides a route to P-hydroxy selenides. ... 

Dihydroxy allenes are generated from ketoenes and ethynyl epoxides. a,P-Epoxy ketones undergo reductive cleavage but the P-hydroxy ketones thus obtained can react further, for example with an e, -double bond to give cyclic 1,3-diols. Note that the double bond does not have to be activated, and furthermore, a silylalkyne moiety and a tricarbonylchromium-complexed arene can play the same role, although in the latter case the net result is a cine-substitution. 

Grieco and his co-workers have completed a most notable total synthesis of /-quassin (130) (Scheme 3).1 The synthesis of the hydroxy-lactone (127) was outlined last year (see Vol. 11, p. 121). The conditions developed for the conversion of the bis-(a-hydroxy-ketone) (128) into the bis-(O-methyldiosphenol) (129) also achieved the crucial inversion of configuration at C-9.74 The close proximity of the C-7 oxygen atom to the C-ll carbon atom in the 9-epiquassin skeleton is evident from a series of reactions in which intramolecular participation occurs. Thus, for example, treatment of the epoxide (131) with lithium aluminium hydride gave the ether (132) whose structure and stereochemistry were established by A -ray analysis.75 Synthesis of the tetracyclic (133), a possible intermediate for quassinoid synthesis, involved the intramolecular cycloaddition of a quinonedimethane as the key step.78... 

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