Enones rearrangement

It is obvious that much further work is needed to unravel all the mysteries of enone rearrangements. Not the least of the problems to be explained is the retention of absolute configuration about C-9 in the rearrangement of 35. The shift of C-l from C-9 to C-10 and the shift of C-9 from C-I to C-4 must occur almost synchronously, and the existence as discrete intermediates of either biradical or charge-separated species with a planar C-9 seems unlikely. 

A requirement for the enone rearrangement is the presence of a rc-moiety at carbon-4 (i.e. at the 7-carbon). The rearrangement proceeds with migration of this group to C-3 (the -carbon). The preferred stereochemical course leads preferentially to the 6-endo phenyl configuration (i.e. trans phenyl groups here). The overall reaction is shown in equation 3... 

Hydroxycyclopent-2-enones rearrange to diastereoisomeric mixtures of 2-oxa-bicyclo[3.1.0]hexan-3-ones in low yields if the alcohol function at C5 is secondary. In contrast, the tetracyclie lactone 4 is obtained in 70% yield upon sunlight irradiation of the... 

Scheme 8.12 Mechanisms of Type B and Type C enone rearrangements for enone 43.

See also the Type B enone rearrangement in TADDOL inclusion compounds H. E. Zimmerman, 1. V. Alabugin, V. N. Smolenskaya, Tetrahedron 2000, 56, 6821. 

A similar synthesis starts from commercially available 1,5-hexadiyne and 2-methyl-cyclopent-2-enone. The benzocyclobutene is obtained from a bis-acetylene in a cobalt-catalyzed reaction. It rearranges regio- and stereoselectively to a 3-deoxy steroid derivative. The overall yield from the cyclopentenone was 40% (R.L. Funk, 1977). 

Conjugate addition of vinyllithium or a vinyl Grignard reagent to enones and subsequent oxidation afford the 1.4-diketone 16[25]. 4-Oxopentanals are synthesized from allylic alcohols by [3,3]sigmatropic rearrangement of their vinyl ethers and subsequent oxidation of the terminal double bond. Dihydrojasmone (18) was synthesized from allyl 2-octenyl ether (17) based on Claisen rearrangement and oxidation[25] (page 26). 

Cyclic a,) -unsaturated ketones present a rich array of photochemical reactions, some of which are of considerable synthetic value (see Section 6.4 of Part B). For cyclohex-enones, two prominent reactions are the di-rr-methane rearrangement (path A) and the lumiketone rearrangement (path B). 

Until recently, pyridine-type bases have been commonly used to produce conjugated enones from 2-halo ketones yields are usually poor °° and these reactions are frequently accompanied by rearrangement, reduction and salt formation. Thus, Warnhoff found that dehydrobromination of (28) with 2,4-lutidine gave a mixture of (29), (30) and (31) in the ratio 55 25 20. Collidine gave a ratio of 38 25 37, whereas pyridine gave mainly the salt (32). 

Cyclohex-2-enone- bicyclo [3.1.0] hexan-2-one rearrangement, 318 3a,5o -Cyclo-6/3,19-oxidocholestane, 268 18,20-Cyclo-5a-pregnane-3, 20-diol 3-acetate, 264... 

Photochemical cyclohex-2-enone bicyclo (3.1.0)hexan-2-one rearrangement, 320 Photochemical a-cyclopropyl ketone isomerization, 313... 

The key step in this sequence, achieved by exposure of 46 lo a mixture of sulfuric acid and acetic anhydride, involves opening of the cyclopropane ring by migration of a sigma bond from the quaternary center to one terminus of the former cyclo-l>ropane. This complex rearrangement, rather reminiscent of the i enone-phenol reaction, serves to both build the proper carbon. keleton and to provide ring C in the proper oxidation state. 

Clearly compound 26 (R = Ac) had undergone a reaction analogous to the glycal rearrangement. It has been demonstrated that the rearrangement of this compound also occurs at room temperature in acetic anyhydride in the presence of zinc chloride (34). Under these conditions, however, a further slower isomerization takes place and a third product, assigned the acetylated enone-hydrate structure 29, was isolated. As noted later this structure has been shown to be incorrect. 

In this synthesis, we have witnessed the dramatic productivity of the intramolecular enone-olefin [2+2] photocycloaddition reaction. This single reaction creates three contiguous and fully substituted stereocenters and a strained four-membered ring that eventually provides the driving force for a skeletal rearrangement to give isocomene. 

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