Epoxy acylates, rearrangement

Iy2-Acyl rearrangement of a, -epoxy ketones. This rearrangement can be used for synthesis of cyclic spiro-1,3-ketones. Thus the 2-cycloheptylidenecyclopenta-none oxide 1 rearranges in the presence of BF3 etherate at 25° within one minute to the spiro-1,3-diketone 2 in 91% yield.1... 

In an interesting and unusual reaction, Paquette and Maynard have shown that peracid oxidation of l,2-disiIyl-3,3-dimethylcyclopropene gives rise to an o -silyl-aq/i-unsaturated acyl silane, through rearrangement of the intermediate epoxycyclopropane. Further treatment with peracid gave the a-epoxy acyl silane (Scheme 54)145. 

This rearrangement reaction was extended to fA-epoxy acylates derived from monocyclic alkenes (Table 16) <1997JOC4991>. 

Table 16 Rearrangement of c/s-epoxy acylates of monooyolio alkenes...

In addition to (3-elimination, epoxides are also known to undergo rearrangement to a carbonyl group, and this reactivity can be synthetically useful as well. For example, Kita and co-workers <97JOC4991> have used a novel acid-promoted rearrangement of cyclic a,(3-epoxy acylates e.g., 110) for the stereoselective synthesis of spirocyclanes (e.g.. Ill), a technique which is... 

The encouraging result of the trans-epoxy acylates with the chiral spiro compounds was appUed to the optically active system (Scheme 15). Asymmetric reduction of the enone 31 by Corey s method [72] afforded the allyl alcohol (-)-34 (90% ee). Epoxidation of (-)-34 by the stereoselective Sharpless epoxidation [73] afforded the cts-epoxy alcohol, cfs-(-)-35, as the sole product. The Mitsunobu reaction [74] of czs-(-)-35 with benzoic acid gave the trans-epoxy benzoate, trans- -)-36, (90% ee) in 89% yield. Treatment of trans-(-)-36 with BF3-Et20 afforded the optically active spiro compound (+)-37 in 89% yield with retention of the optical purity (90% ee). This means that the rearrangement occurs stereospecifically. The optically pure epoxy camphanate (-)-38 could be obtained after one recrystallization of the crude (-)-38 (90% de), which was obtained by the Mitsimobu reaction of cfs-(-)-35 with D-camphanic acid. The optically pure spiro compoimd (+)-39 (100% de) was obtained from the optically pure (-)-38 in 89% yield. 

Scheme 15 Synthesis and rearrangement of optically active epoxy acylates...

Kita and coworkers have performed a study of BF3 OEt2 promoted rearrangements of 2-aryl-2,3-epoxy acylates and have shown that cyclic 2-aryl-2,3-epoxy acylate... 

Equation 35). On the other hand, the acyclic ary 1-2,3-epoxy acylate (57) undergoes rearrangement through C3-cleavage of the oxirane ring via a phenonium intermediate to give ester (58) (Equation 36) [36]. 

Kita Y, Yoshida Y, Kitagaki S, Mihara S, Fang D-F, Furu-kawa A, Higuchi K, Fujioka H. Regio- and stereoselective rearrangement reactions of various a,p-epoxy acylates suitable combination of acyl groups and Lewis acids. Tetrahedron 1999 55 4979-4998. 

Scheme 36 Bi(0Tf)3-xH20-catalyzed rearrangement of 16a,17a-epoxy-20-oxosteroids in the presence of an acylation agent...

Acylation of hydroxylamine with 8-2 Rearrangement of epoxy silyl ethers... 

Treatment of 2,3-epoxy -2-methyl -4 butanone with boron iri-fluorido etherate in benzene (Kq. 471) gives a good yidd of 2,2-dimethyl-S-oxobutyraidehyde.6 7 This product can only be formed by niigr.it ion of an acyl group. When rearrangement waa conducted at 230 over alumina, methyl isopropyl ketone was isolated, not 4-methyJpenUi>< 2,3-dione as claimed previously.71 Deformylation appears U occur readily on the surface of the alumina catalyst. 

Attempted peroxy acid epoxidation of the bicyclic ketone (31 equation 13) gave the lactone (33), instead of several possible rational alternatives. The epoxide (32) was implicated as an intermediate when it was independently synthesized from the epoxy alcohol, and shown to give (33) on treatment with aqueous acid.- A mechanism involving scission of the acyl bridgehead bond via the hydrated 1,1 -diol form of the ketone was proposed to account for the formation of this unexpected product. The rearrangement of the isolongifolene derivative (34 equation 14) appears to be mechanistically related. The product (35) is formed by brief treatment with dilute HCIO4 in dioxane as a mixture of isomers believed to arise by acid-catalyzed epimerization of the carbinol center. ... 

---