Dienyl ketones, preparation

A bis([2.2]paracyclophane)-annelated cyclopentadiene could be synthesized via a Nazarov cyclization of the bis([2.2]paracyclophane-l,9-dienyl) ketone 136 prepared by addition of monolithiodiene 133 to ethyl formate and subsequent oxidation of the carbinol 110. The synthesis was completed by diisobutylalumi-num hydride (DIBA1-H) reduction of the resulting cyclopentenone, and dehydration with p-toluenesulfonic acid. Treatment with methyllithium gave the corresponding cyclopentadienyllithium 136-Li, which was identified by NMR spec-... 

The tricarbonylcyclohexadienyliumiron salts (32) are synthetically equivalent to substituted phenyl cations/ and react with silyl enol ethers to give, after decomplexation, the a-(cyclohexa-l,3-dienyl) ketones, which are readily oxidized to a-aryl ketones. 2-Substituted cyclopent-2-enones can be prepared from the salts and l,2-bis(trimethyIsilyloxy)cyclopentene (Scheme 52).A... 

Pyrrolo[l,2-a]azepin-9-one (12), which in acid solution exists as the cation 13, is prepared by thermal cyclization of ( , )-4-(dimethylaniino)buta-l,3-dienyl pyrrol-2-yl ketone (ll)7. 

A useful building block 519 for vitamin D synthesis was prepared by reaction of the alkenyloxirane 517 [195], Under usual conditions of the elimination, both the dienyl alcohol 519 and the unsaturated ketone 520 were formed in equal amounts. Radinov and co-workers discovered an interesting and remarkable effect of the addition of fluorinated alcohols, such as l,3-bis-(l,l,l,3,3,3-hexafluoro-2-hydroxypropyl)benzene or perfluoro-t-butyl alcohol as a proton source, on the chemoselectivity, and the desired 519 was obtained with nearly complete chemos-electivity with these additives [195], They suggest that rapid protonation of the intermediate alkoxide 518 with these alcohols seems to be important. 

Cyclic l,3-dienyl-2-boronates, prepared via a ring-dosir metathesis reaction, have been described [86]. Treatment with an excess of dienophile (nitroethylene, acrolein, methyl vinyl ketone) furnished in all cases one major product the minor components being the P-epimer and the endo regioisomer (Scheme 9.40). 

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