Epoxidation With mCPBA

The 1,2-carbonyl transposition takes place through the enJo-epoxide 18 easily prepared through the tosylhydrazone 16, followed by regioselective cleavage to the less substituted double bond (17) with 2 equivalents of methyllithium [4] and epoxidation with MCPBA in chloroform from the more accesible convex face of the decalin system. 

Attempts to functionalize the homoallylic alcohol 15 quickly revealed that this product of an intramolecular aldol condensation was sensitive to base. Fortunately, heating with thiocarbonyldiimidazole effected clean dehydration to give predominantly the desired regioisomer of the diene. Methanolysis followed by oxidation then gave the triketone 1, which on epoxidation with MCPBA gave 2 as the minor component of a 3 1 mixture. 

Whereas the trans-diols on epoxidation with MCPBA give anti-diol epoxides, the ds-diols give a 1 1 mixture of anti- and syn-diol epoxides.69 Thus... 

Initially, the products of these reactions suggested radical ions were involved138. In particular, when hexamethyl Dewar-benzene was epoxidized with MCPBA, the nature of the products depended on whether or not the iron(III) porphyrin hemin was added to the reaction mixture1381 . Furthermore, when Z-stilbene was epoxidized with dioxygen, catalyzed by (tetraphenylporphorinato)iron(III) chloride, -stilbene appeared in the reaction mixture139. 

The group of G. Hofle, the discoverer of the epothilones, was able to isolate sufficient epothilones from fermentations to examine classic de-rivatization. They examined the products of Oxidative and reductive transformations of epothilone A" [54], Derivatizations of the C12-C13 functional groups of epothilones A, B and C [55] as well as Substitutions at the thia-zole moiety of epothilone [56]. They were among the first to discover that the epoxide can be formed with predominantly correct diastereo-selectivity in a final transformation from (protected) epothilones C and D. Recently a more thorough investigation of the epoxidation with mCPBA to the epothilones A and B was finally... 

For the large scale preparation of epoxides, reagents which are cheaper than MCPBA are available. Though many electron-deficient alkenes have been epoxidized with MCPBA at elevated temperatures, reagents which are more reacdve than MCPBA have been used when these reagents are employed the reacdons can be carried out under comparatively mild conditions, leading to improvements in yields and selecdvides. 

During the synthesis of such compounds, the epoxide is often installed at an early stage for example, dihydroquinoline 31 (Scheme 18) is readily epoxidized with MCPBA, and subsequent formation of the bis-alkynyl iodides followed by bis-intramolecular Stille coupling occurs with complete chemoselectivity <2006ARK261>. 

A recent application of the furan-carbonyl photocycloaddition involved the synthesis of the mycotoxin asteltoxin (147)." Scheme 16 shows the synthetic procedure that began with the photoaddition of 3,4-dimethylfuran and p-benzyloxypropanal to furnish photoaldol (148), which was epoxidized with MCPBA to afford the functionalized product (149) in 50% overall yield. Hydrolysis (THF, 3N HCl) provided the monocyclic hemiacetal which was protected as its hydrazone (150). Chelation-controlled addition of ethylmagnesium bromide to the latent a-hydroxy aldehyde (150) and acetonide formation produced compound (151), which was transformed through routine operations to aldehyde (152). Chelation-controlled addition of the lithium salt of pentadienyl sulfoxide (153) followed by double 2,3-sigma-tropic rearrangement provided (154) as a 3 1 mixture of isomers (Scheme 17). Acid-catalyzed cyclization of (154) (CSA/CH2CI2) gave the bicyclic acetal (155), which was transformed in several steps to ( )-asteltoxin (147). ... 

Sample Problem 12.2 Draw the stereoisomers formed when cis- and frans-2-butene are epoxidized with mCPBA. 

During the first total synthesis of briarellin diterpenes, briarellins E and F, L.E. Overman and co-workers utilized the large reactivity difference between a triple and a double bond in peroxyacid oxidations to selectively epoxidize a trisubstituted double bond in the presence of a terminal alkyne." The epoxidation with mCPBA was carried out in DCM in the presence of a base to afford the a-epoxide in a 9 1 diastereomeric ratio. 

A Peterson elimination, necessarily in acid solution as the OH and SiMe3 groups are anti (see chapter 15) gives a new alkene 126 that is allylic to the remaining OH group. Now epoxidation with mCPBA occurs on the face of the alkene syn to the OH group to give 127. 

For synthesis of the (24S)-configurated 2a,3a-epoxy compound 38 the known70 diacetyl ketone 59 was used. Hydrolysis to the (22/ ,23/ )-diol 60 followed by epoxidation with MCPBA gave 38. 

Epoxidation Conversion of alkenes to epoxides with MCPBA, using (MeCN)4CuPFft as catalyst proceeds at low temperature (e.g., —20°). 

For the second approach, (37 )-3-hydroxy-p-cyclocitral (77) was protected with dihydropyran and the aldehyde function reduced to give 181. The hydroxy group was further reduced to give the unsaturated alcohol 182 which was transformed to the acetate 183. The double bond was epoxidized with MCPBA to form a mixture of the cis- and rra/i5-epoxides (184) in a ratio of 2 1. After separation, trans-184 underwent a pinacolic rearrangement and ring contraction with boron trifluoride as a catalyst saponification of the acetate then gave the desired building block 775 [57] (Scheme 41). 

Epoxidation with MCPBA led to concomitant Baeyer-Vllliger oxidation... 

Another modification of Route B requires enantioselective reduction of ketones (E)-27 or stereoselective carbon-carbon bond formation at C-1 of (E)-27 (R = H) with appropriate organometallic species in the presence of chiral additives, both of which successfully supply the optically active (E)-26. The resulting chiral allylic alcohols (E)-26 are subjected to hydrogen bond-directed epoxidation with mCPBA, leading to the diastereoselective formation of syn-epoxy alcohols. In conplementary fashion, antz-selective epoxidation is possible using the Sharpless protocol. ... 

The peroxy-acid epoxidation of olefins in the presence of base may prevent further reaction of acid-sensitive epoxides. Thus when protoadamantene is epoxidized with MCPBA in the presence of sodium carbonate the endo-epoxide (347) is isolable but the ex [Pg.59]

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