Cyclopentadienes isomerisation

The sodium salt of cyclopentadiene has been reported to condense readily with ethylene oxide, but the reaction appears to be exceedingly complex, 1,w The initial adduct shown in Eq. (894) can react further with ethylene oxide, undergo DieLa-Alder dimerization, or isomerise to give two more j -hydroxyethy]cyolopeDtadienee. Each of... 

The photo-Nazarov cyclisation of l-cyclohexenyl phenyl ketone (285) yields the hexahydrofluorenone (286). Schaffner s group has examined the mechanism of this reaction and have found evidence for some novel intermediates. They propose that the excited state of (285) decays by cis-trans isomerisation of the cyclohexenyl double bond the tx-ans-cyclohexenyl derivative (287) cyclises to the oxallyl species (288) which collapses to the relatively stable enol (289) or reacts with accumulated (289) to give the isolable enol dimer (290). When the ketone derived from (290) is photolysed it gives back (289) along with (286). The enol (290) and its corresponding ketone were both isolated and characterised, while (287) and (288) were trappable with cyclopentadiene. The enol (289) was observable by H-NMR before it ketonised and gave the isolated product (286). 

An autoxidative procedure has been described by the initial alkylation of phenol with cyclopentadiene in the presence of phosphoric add at ambient temperature giving 4-(cyclopenten-2-yl)phenol, in more than 80% yield followed by isomerisation in 91% yield to the 1-isomer during 2 hours in refluxing benzene solution with a catalytic quantity of dichlorobis(benzonitrile)palladium(ll). The conjugated product with 30% hydrogen peroxide and hydrochloric add upon stirring at 50°C for 3 hours afforded 1,4-dihydroxybenzene in 92% yield accompanied by cyclopentanone (ref.46). 

A similar synthesis of tropolone started from cyclopentadiene and tetrafluoroethylene the bicyclic adduct was thermally isomerised into a tetrafluorocycloheptadiene which was in turn converted into tropolone, the overall yield being 20% [254]. 

In another approach by ICI chemists [26-28], the aldehyde (lb) has been synthesized by a modification of the original route used by Corey via the bicycloheptene (17a) which was constructed by Diels—Alder addition of 2-chloroacetonitrile and 5-methoxymethyl-l,3-cyclopentadiene [1, p. 329]. In the ICI method the latter was replaced by 6-acetoxyfulvene to give the adduct (17b) which was then hydrolysed with hydrochloric acid in acetone to the antialdehydes (17c) and the latter isomerised to the s> -aldehydes (17d) by... 

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