Substitution with vinyl epoxides

Vinyl epoxides are highly useful synthetic intermediates. The epoxidation of dienes using Mn-salen type catalysts typically occurs at the civ-olefin. Epoxidations of dienes with sugar-derived dioxiranes have previously been reported to react at the trans-olefin of a diene. A new oxazolidinone-sugar dioxirane, 9, has been shown to epoxidize the civ-olefin of a diene <06AG(I)4475>. A variety of substitution on the diene is tolerated in the epoxidation, including aryl, alkyl and even an additional olefin. All of these substitutions provided moderate yields of the mono-epoxide with good enantioselectivity. 

The 1,3-diene moiety in 227 which included the carbon atoms and CVC was oxidized to the l,4-dihydroxy-2-ene moiety in 238 that was further exploited to functionalise the A-ring as well as for the annulation of the C-ring (Scheme 37). The transformation of 227 into 238 was realized by a diastereoselective epoxidation of 227 to afford a vinyl epoxide (241) that was subjected to the conditions for a Palladium(O)-catalysed allylic substitution with the acetate ion [126]. The mechanism and the stereochemical course of the allylic substitution may be explained as depicted in Scheme 37. Sn2 ring opening of the protonated vinyl epoxide 241 by an anionic Pd complex proceeded with a (3Si) topicity to the r-allyl Pd com-... 

Trimethylsilyl alkyl and aryl sulfides react with allyl carbonates and vinyl epoxides to deliver the alkyl or aryl sulfide to the allyl unit. These species show typical regioselectivities by (i) adding to the less substituted end of alkyl substituted allyls (equation 290) (ii) adding to the allyl terminus more distant from remote oxygen functionality (equation 291) and (iii) showing substantial endocyclic addition to methylenecyclohexane-derived allyls (equation 292).223... 

Gallium(III) triflate catalysed the rearrangement of 2-substituted vinyl epoxides into /3,y-unsaturated carbonyl compounds with high regio- and chemo-selectivity with low catalyst loading. Alkyl-substituted trimethylsilylvinyl epoxides gave /i,y-unsaluralcd ketones, but aryl-substituted vinyl epoxides gave the aldehydes.80... 

Vinyl epoxides can be regioselectively opened depending on the substitution at the 2-position of the 2-lithio-l,3-dithiane. Thus, the Sw2 adducts anti-204 were obtained with unencumbered dithiane anions, whereas Sa 2 adducts 205 were produced with sterically encumbered ones (Scheme 58)293. 

Alternatively, carbamate anions like those involved in the formation of 13 can be generated efficiently by trapping the alkoxide of zwitterionic 7t-allylpalladium complexes from vinyl epoxides with isocyanates55 57. Table 6 shows that cyclic vinyl epoxides lead exclusively to 2-oxazolidinones with retention of configuration which is independent of isocyanate substitution. Triisopropyl phosphite is the ligand of choice to assure a high turnover number. 

Vinyl epoxides can be cross-coupled with vinylstannanes <2001JOC589>. This reaction proceeds through an (if-allyOpalladium complex (Equation 57) <2001JOC589>. Similar reactivity can be observed using bismuth reagents (Equation 58) <2001SC2365>. Vinyl oxiranes react with substituted allenes to form functionalized allyl alcohols <2004JOC4686>. 

On treatment with a palladium(O) catalyst, vinyl epoxides undergo facile unimolecular rearrangement to give dienols or enones depending on the substitution pattern of the substrate. In the presence of an active methylene compound in the reaction system, however, a single alkylation product is formed. Cyclic and acyclic vinyl epoxides participate equally well. The reaction proceeds with clean alkylation from the same face as the oxygen of the epoxide, and proceeds with allyl inversion (Scheme 22). ... 

The biphasic double carbonylation of phenyloxiranes under PT conditions affords 4,5-dihydro-4-phenylfuran-2,3-diones [162]. A similar reaction involving 2-phenylthiirane gives the ) -mercapto-substituted acid [163]. The PTC reaction of vinyl epoxides with CO and Mel in the presence of catalytic amounts of Co2(CO)h and TDA-1 affords /ff-hydroxy acids [164]. 

The resulting derivatives were applied with success in the standard asymmetric allylic alkylation (up to 97 % ee) [134, 136] or in transformations involving either specific allylic substrates (2-cycloalkenyl derivatives, up to >99% ee) [135, 137], unsymmetrical substrates (monosubstituted allyl acetate, up to 83% ee) [140], or especial nucleophiles (nitroalkanes [141], iminoesters [138 a], or diketones [139, 140, 142]). Such ligands were also effective in the formation of quaternary chiral carbon through allylic substitution (eq. (6)) [138, 143], deracemiza-tion of vinyl epoxides (up to 99% ee) [144], or alkylation of ketone enolates [138 b], and deracemization of allylic derivatives [145]. 

In their reaction with bases, epoxides appear to undergo typical S 2 attack. Ammonia,18 amines,19 alkoxides,20 and sodio derivatives21 react at the less substituted carbon atom (except in vinyl systems, see p. 222), and at least in one case which has been studied inversion of configuration results (p. 202). 

Reductive cyclization. Epoxides substituted with a carbon chain terminated (at a proper length) in an allenyl group are converted into cyclic products containing vicinal hydroxymethyl and vinyl substituents. 

Hydration of vinyl epoxides. 1,2-Diols are formed from the Pd(0)-catalyzed ring opening with bicarbonate ion, in contrast to conventional allylic substitutions of such substrates. 

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