Intermediates, isolation isomerisation reactions

The importance of the isolation of intermediates in these reactions cannot be overemphasised since compounds (8), (13) and (14) could all be rationalised by single tff transformations of the appropriate overcrowded, pre-isomerised, icosahedral species. 

Butatrienylamines 41 must occur as intermediates but they isomerise too fast to give 39 in t-BuOK/DMSO or KNH2/NH3 mixtures. They are indeed isolated if BuLi/t-BuOK is used to achieve a rapid deprotonation of 38 followed by kinetic quenching with tert. butanol at —100°. This sequence not only clarifies the reaction mechanismus but provides also a new class of very reactive intermediates (75)124 125). 

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). 

The original reaction mentioned in the introduction is shown in Scheme 1. In the NMR of the reaction mixture, two apparently isomeric products could be identified in a ratio of ca. 9 1. In an attempt to separate them by column chromatography, the isomer ratio was inverted to 1 9. From this it was obvious that 4 was an intermediate and 5 was the final product. Compound 5 could be isolated and fully characterised, including by X-ray structural analysis. So it was concluded that the isomerisation consisted of a reductive elimination to form the... 

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