Cyclooct-2-enone

Treatment of aminoazines, in particular, 6-aminouracil 305, with cyclic oc,(3-unsaturated ketones was described in [235]. The authors showed that 1,3-dimethyl-6-aminouracil reacts easily with cyclooct-2-enone 313, cyclonon-2-enone 316 and other cyclic unsaturated carbonyls (cyclododec-2-enone, cycloundec-2-enon, etc.), forming the appropriate tricyclic compounds 314, 317 or analogues (Scheme 3.86). The interaction of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) with compound 314 leads to a rearrangement of the carbon skeleton and formation of another tricyclic structure 315, while in the... 

In contrast to cyclopentenones and cyclohexenones, medium-ring vinylogous esters are not suitable for photocycloaddition reaction unless the olfin coupling partner is substituted. For example, when ( -3-(but-3-enyloxy)cyclooct-2-enone 78a is irradiated under a variety of conditions, no intramolecular cycloadduction occurs. However, upon substitution of a vinyl or phenyl on the olefin, the cycloaddition proceeds efficiently to give diastereomeric mixtures 79b/c, and 80b/c, respectively. The dramatically enhanced yields and rates of the photoaddition reactions upon olefin substitution result from the stabilization of the 1,4-biradical by a vinyl or phenyl. A mixture of diastereomers is formed presumable because the rotational relaxation of the intermediate 1,4-diradical is faster than [2 + 2] ring closure. 

The primary and fastest photochemical reaction of acyclic and some cyclic a, P-unsaturated ketones is cis-trans isomerisation. This isomerisation is not expected for cyclopent-2-enone or cyclohept-2-enone because of high strain energy, but it has been reported for cis-cyclooct-2-enone and cis-cyclohept-2-enone. It is thought to occur from triplet excited state. 

Cyclooct-2-enone and cyclohept-2-enone undergo Z —> isomerization on irradiation. While E-q cloocten-2-one can be characterized spectroscopically at room temperature, E-cyclohep-2-enone is stable only at temperatures around -ISO C, but can be trapped with dienes in a dark reaction at -78°C. The photoisomerization of these medium-ring cycloalkenones appears to occur much faster than inter-molecular processes, e.g., cycloaddition to alkenes or energy transfer to triplet quenchers. Another seven-membered ring enone, 2,3-benzocyclohepta-2,6-dienone (55) was found to afford trans-fased [4+2]... 

An illustrative example for the potency of catalyst C is Paquette s highly efficient total synthesis of the natural products teubrevin G 180 and teubrevin H 181, which feature a cyclooctane core fused and spiroannulated to smaller oxygen containing rings. In the retrosynthetic analysis, the viability of an RCM step for annulation of a cyclooct-enone ring to the furan played a central role. Despite the presence of a conformational constraint by the furan ring in... 

When 5,6-epoxycyclooctene was exposed to excess lithium diethylamide in refluxing diethyl ether for two days, it rearranged to a mixture of cycloocta-2,4- and -3,5-dienols which underwent facile thermal conversion to cyclooct-3-enone by a pathway involving a 1,5-hydrogen shift. ... 

When we extended the activated olefins to cyclohept-2-en-l-one and cyclooct-2-en-l-one we found that the reaction is very complicated, because the Lewis bases, solvents and the ring-size of the cyclic enone can all significantly affect the MBH reaction rate and even the reaction product. The reaction of cyclohept-2-en-l-one with A-arylidene-4-methylbenzenesulfonamides afforded the usual MBH adducts 203 along with abnormal adducts 204, whereas the corresponding reaction of cyclooct-2-en-l-one provided different aldol products (205-207) depending upon the Lewis base employed (no MBH adduct was formed in any of the cases) (Scheme 2.104). Moreover, the formation of aldol products 209 along with the usual MBH adducts 208 has been observed in the reaction between cyclopent-2-en-l-one and aromatic aldehydes in the presence of BU3P (Scheme 2.105). 

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