Dimethyldioxirane direct epoxidation

Murray s finding that dimethyldioxirane (DMDO) can be readily prepared with acetone and oxone [54] allowed the development of epoxidation reactions under essentially neutral conditions [55]. Such DMDO oxidations were utilized by Danishefsky in an efficient direct epoxidation of glycals, as illustrated by the formation of epoxide 10 (dr >99 1, Equation 5) [56]. 

All attempts to achieve a direct transformation of the carbazomadurins A (253) and B (254), as well as the disilyl-protected carbazomadurins A (769a) and B (769b), into the epocarbazolins A (258) and B (259) were unsuccessful and resulted in complete decomposition. Therefore, prior to the epoxidation, the disilyl-protected carbazomadurins A (769a) and B (769b) were transformed to the corresponding trisilyl-protected carbazomadurins A (770) and B (771) by treatment with TPS chloride in the presence of stoichiometric amounts of 4-(dimethylamino)pyridine (DMAP). Epoxidation of the fully protected carbazomadurins A (770) and B (771) with dimethyldioxirane at — 20°C, followed by desilylation, provided racemic epocarbazolin A (258) and epocarbazolin B (259) (605) (Scheme 5.82). 

Direct oxidation of aromatics with m-CPBA in a two-phase system affords the most stable so-called K-region arene oxides in moderate yields.805 Careful control of pH is necessary to avoid acid-catalyzed rearrangement of the acid-sensitive product epoxides. The new powerful oxidants, dimethyldioxiranes,81,82 have also been used to convert arenes to arene oxides.806... 

The most widely used and, presumably, the most chemoselective reagents for the epoxidation of nucleophilic C—C double bonds are the peroxycarboxylic acids (see Houben-Weyl, Vol. IV/ 1 a, p 184, Vol. Vl/3, p 385, Vol. E13/2, p 1258). Using chloroform as solvent, epoxidation rates are particularly high79. Reactive or acid/base sensitive epoxides can often be obtained with dimethyldioxirane (see Houben-Weyl, Vol. R13/2, p 1256 and references 15, 16, 87-90), peracid imides (see Houben-Weyl, Vol. IV/1 a, p 205, Vol. VI/3, p 401, Vol. E13/2, p 1276) (prepared in situ from nitriles and hydrogen peroxide), hydroperoxy acetals (see Houben-Weyl, Vol. El3/2, p 1253) or peroxycarbonic acid derivatives (see Houben-Weyl, Vol. IV/la, p 209 and references 17-19) as oxidants. For less reactive alkenes, potassium hydrogen persulfate is a readily available reagent for direct epoxidation20. 

Epoxidation of allylic phosphonates is achieved wilh success at room temperature witli MeCOjH in Et O, CF3CO3H in CHCI3, MCPBA in ( HT I- or MOO5/HMPA complex in CH2CI2 to give the corresponding 2,3-epoxyalkylphosphonates as a mixture of diastereomers. s- Allylic phosphonates may also be converted into 2,3-epoxyphosphonates via the 1,1,1-trifluorodimethyl-dioxirane-mediated oxidation. Dimethyldioxirane (DMD) in acetone at room temperature or methyl(trifluoromethyl)dioxirane (TED) in CHjClj at low temperature can be used instead of MCPBA. Because the reaction is quantitative, evaporation of acetone and excess of DMD allow the direct isolation of the pure product. 2,i55... 

Stability as compared with taxol, and it was approximately one third as active as taxol in the HCTl 16 and A2780 cell lines (727). The syntheses of 1-deoxybaccatin III and 1-deoxytaxol have also been claimed in a patent application 128). Finally, going in the opposite direction, an interesting hydroxylation of a 2-deacetoxytaxinine J derivative by dimethyldioxirane has been reported. Reaction of the 5-(triethylsilyl) derivative of 2-deacetoxytaxinine J with dimethyldioxirane gave the 1-hydroxy derivatives 3.5.8 and 3.5.9, together with the normal un-hydroxylated epoxides (729). 

The VAE has also been used to generate nitrosoalkenes from a-chloro ketoximes and to accelerate Claisen rearrangements of appropriately substituted substrates. An interesting example of the remote effect on reactivity was found in the epoxidation of 4-deoxypentenosides (4-DPs) by dimethyldioxirane (DMDO) where competing directing effects can be imposed by the anomeric and allylic substituents. ... 

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