The use of the enol trifluoroacetates of cyclohex-2-en-l-ones for initiating alkene cyclisation is compared with results from cyclisation of the related 2,3-epoxycyclohexanones.HARDING'S demonstration1 that protonation of cyclohexadienol acetates provided efficient initiating cations for monocyclisations prompted us to t r y this route where our Lewis acid-epoxide-induced cyclisations had given poor results., The success of dienol trifluoroacetate in our hands and, independently, in Harding's3 prompted us to make a more extensive comparison of the alternative methods. Compounds (3), ( 5 ) , and (6) gave comparable results in both types of cyclisation. Reaction of (3) with (CF,CO),O-CF,-C0,H at 0 "C gave, after hydrolysis, the cisand tramisomers? of (8) (56%) as an inseparable 2 : 1 mixture [7 (CDCl,) 2.8-3.5 (3H, m), 8.66 ( s ) , and 8.94 (s)] while the 2,3-epoxide with SnC1,-CH,Cl, a t -20 OC, gave the related /3-hydroxy compound in 88% yield [vmax 1710 cm-1;
M 13 9PL 2-Substituted 3-methylcyclohex-2-enones and the derived epoxides have been cyclised to a variety of hydrindanone derivatives. When the 2-substituents are pent-3-ynyl or 4-chloropent-3-enyl the cyclisations take a predictable course ; however, with 4-methylpent-3-enyl in the enone, the initial cyclisation product undergoes hydride and methyl migration. In the related epoxide an unexpected 'ene ' reaction occurs.Cation-induced cyclisation of appropriate electronically unbiased alkenes predominantly favours six-membered ring formation (6-endo-Trig) rather than five-membered ring formation (5-exo-Trig). When substitution on the alkene is electronically biased in favour of the latter such cyclisation does occur but is frequently followed by secondary processes such as hydride or alkyl migrations.2 In order to test the feasibility of 5-membered ring formation using cyclohexenone derivatives as initiators, we examined the cyclisation of compounds (1)-( 6). Reaction of (4) with SnCl4-CH2CI2 at -20 "C gave a product (9479, the analytical and spectroscopic properties of which [ v, , .
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