Enone Rearrangements

Srinivasan, Abstracts, 156th National Meeting of the American Chemical Society, San Francisco, 

Detailed studies of the cis-trans and trans-cis isomerization of chalcone and some of its derivatives and analogues (123) have been reported. Perkins and his co-workers have reported an irradiative method of reducing enone systems such as chalcone to the corresponding saturated compounds. The cis-trans isomerization of chalcone has been studied in CCI4 solution.  

A solvent dependence has been reported for the photocyclization of the chalcones (124). Efficient cyclization takes place yielding the flavanones (125) 

Murato, B. Frei, W. B. Schweizer, H. R. Wolf, and O. Jeger, Helv. Chim. Acta, 1980, 63, 1856. 

The oxiran system in (136) undergoes bond cleavage on excitation (nn ) and yields the furan derivative (137) as the major product (13%). Several other minor products were also isolated and identified. The irradiation of the enone (138) has been reported. 

The photochemical isomerization of the E-isomers (53) to the corresponding Z-isomers32 and of the furylacrylic acids (54) has been reported.88 

The conjugated ester, methyl cyclopent-l-en-l-carboxylate, undergoes complete deconjugation forming methyl cyclopent-l-en-3-carboxylate upon irradiation in methanol.81 An intermolecular path is suggested since deconjugation reactions of this type are not likely to occur easily in cyclic systems. The photochemistry of 

The principal product from irradiation of the enones (56) in propan-2-ol, cyclohexane, or benzene is 1,1,4,4-tetraphenylbutadiene.38 The authors propose that this product arises from the singlet state by addition of an excited-state molecule to a ground-state molecule to afford a biradical (57) which fragments to product and acyl radicals. A reduction product (e.g. 1,3,3-triphenylpropan-1-one) is also formed, but only from irradiations in propan-2-ol. 

The rionone (58) undergoes reversible cw-tmns-isomerization to the Z-isomer 

The enones (68) can be induced to cyclize to (69) using Pyrex-filtered light when BF3-Et20 is added to the system.39 An attractive mechanistic rationalization would involve a Cope process but this was excluded by the failure of the enol ether (70) to cyclize. This system undergoes cis-trans-isomerization, which is an efficient method of excited-state relaxation in these systems since the enone 

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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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