Cyclohexenones 3 + 2 cycloaddition reactions

Investigation of the photochemistry of protonated durene offers conclusive evidence that the mechanism for isomerization of alkyl-benzenium ions to their bicyclic counterparts is, indeed, a symmetry-allowed disrotatory closure of the pentadienyl cation, rather than a [a2a -f 7r2a] cycloaddition reaction, which has been postulated to account for many of the photoreactions of cyclohexadienones and cyclohexenones (Woodward and Hoffmann, 1970). When the tetramethyl benzenium ion (26) is irradiated in FHSO3 at — 90°, the bicyclo[3,l,0]hexenyl cation (27) is formed exclusively (Childs and Farrington, 1970). If photoisomerization had occurred via a [(r2a-t-772 ] cycloaddition, the expected... 

Asymmetric induction has been observed in some of these cycloaddition reactions with the use of chiral enamines52-54. For example, reaction of L-proline derived enamines (e.g. 95) with methyl vinyl ketone (equation 17) afforded, after hydrolysis, chiral cyclohexenones (e.g. 96) with optical yields up to 50%52. 

The importance of the ethylene ketal described above with respect to stereocontrol of the de-Mayo reaction is emphasized by later published works of two other teams. Fetizon and co-workers obviously followed a similar concept and carried out photocycloaddition reactions with 96a in model studies (Scheme 23) (Si). As can be seen from retroaldol product 98, exclusive a attack of the cyclohexene has taken place. Thus, the relative stereochemistry of the BC ring connection is opposite to that of taxane. Totally comparable results were obtained by Berkowitz et al. in the course of cycloaddition reactions of cyclopentene, cyclohexene, or those of a cyclohexenone ketal to the camphor derivative 96b (.84). 

The stabilizing effect of the secondary orbital interactions of the trifluoromethyl substituent with dienes is exemplified in the cycloaddition reaction of a-(trifluorornethyl)styrene (7) with Danishefsky s diene 4. A phenyl substituent is known to stabilize the endo-phenyl transition state. Furthermore, the steric hindrance of a trifluoromethyl group should favor its exo approach. However, cycloadducts 8 are obtained in a 1 1 mixture of transicis stereoisomers which are converted into the cyclohexenone 9. Under the same thermal conditions, a-methyl-styrene is unreactive.5... 

The ABCD ring system of the diterpene alkaloid atisine was constructed by T. Kametani et al using an intramolecular Diels-Alder cycloaddition reaction as the key step. The dienophile was obtained by the traditional Hofmann degradation of the corresponding dimethylamino precursor. The diene was prepared by the kinetic enolization of the cyclohexenone system with LDA. 

Direct Cycloaddition Reaction of Aldimines with Cyclohexenone... 

Scheme 3.25 Direct cycloaddition reaction of imines with cyclohexenone.

Inter molecular Additions to Cyclohexenones and Related Systems. Calculations have been reported that deal with a transition state analysis of the regioselectivity encountered in triplet-state (2 + 2)-cycloaddition reactions of cyclohex-2-enone. Diastereoselective (2 + 2)-cycloaddition of ethene to cyclohexenone carboxylates in the presence of chiral auxiliaries has been described. Yields of bicyclo[4.2.0]octanone derivatives can be obtained with de as high as 81%. The enone (16) can be tethered to a polymer substrate via the tether group R. Irradiation of this material in toluene with ethene at —78°C gives a 68% yield of the adduct (17). The de of the product is reasonable at 72%. Addition of 1,1-diethoxyethene to the cyclohexenone (18, R = Ph) results in the formation of the two stereoisomeric head-to-tail regioisomers (19) and (20). The outcome of the reaction is dependent on the rate of formation of 1,4-biradical intermediates. This can be seen in the dependence of the cisjtrans ratio on the solvent and on the temperature at which the reaction is carried out. Thus with enone (18, R = Ph) in acetone at 3°C, a cjt ratio of 4.4 is obtained, and this changes to 2.1 at —40°C. In acetonitrile the cjt ratio is only 1.9 at the same temperature. With the other derivative of (18), the cjt ratio is 1.3 in acetone and 0.8 in acetonitrile. Photoaddition of ethene to the enone carboxylate [21, R = (-)-8-(2-naphthyl)menthyl] results in the formation of the diastereoisomers (22) and (23) with a de of 56% at —78°C. The diastereo-selectivity can be enhanced by the addition of naphthalene derivatives to the solution. Thus with naphthalene, a de of 71% is obtained, and this can be increased to 83% with 1-phenylnaphthalene. ... 

Both inter- and intramolecular [2-1-2] cycloaddition reactions have been used in synthesis. In intermolecular reactions, a common problem is that mixtures of regioisomers (sometimes referred to as head-to-tail and head-to-head products) in addition to more than one stereoisomer may be formed. In general, the head-to-tail regioisomer is the major product using an electron-rich alkene, whereas the head-to-head regioisomer is favoured using an electron-deficient alkene. For example, the first step in Corey s synthesis of caryophyllene involved addition of cyclohexenone to isobutene to give predominantly the trans-cyclobutane (head-to-tail) derivative 151 (3.107). 05... 

Similarly, substituted cyclohexenones undergo the light-induced [2+2] cycloaddition reaction to afford the bicyclic adduct 168". ... 

The remarkable selectivity of the cycloaddition reaction appears to be due to intervention of the enol form of the cyclohexenone as the true dienophilic partner ... 

However, in the photoaddition of cyclopentenone to 1,6-heptadiene, the initial 1,4-biradicals were observed, consistent with the lower rate of rearrangement of 1-hexenyl to cyclopentyhnethyl radicals, <10 sr. The initially formed triplet 1,4-biradicals in the cycloaddition reactions must therefore have lifetimes substantially less than 10 ps, consistent with the results on the cyclopropyl-substituted systems. A similar observation was made in intramolecular photoadditions of cyclohexenones possessing a tethered diene moiety. ... 

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