Allyl esters, isomerisation

Triethylborane is also an excellent initiator for intramolecular iodine atom transfer reactions. For example, cyclisation of the propargyl a-iodoacetal depicted in Scheme 6.5 gives the corresponding bicyclic vinyl iodide in high yield.10 Allyliodoacetamides (Scheme 6.5) and allyl iodoacetates (Scheme 6.5) cyclise cleanly under Et3B-02 initiation. In the case of the ester, the reaction has to be run in refluxing benzene in order to allow Z/E-ester isomerisation prior to... 

Conclusive evidence for the participation of 7r-allylic intermediates in double bond migration has been obtained from a study of the nickel-catalysed hydrogenation of the isomeric olefinic esters methyl oleate and methyl elaidate using tritium as a tracer [147]. It was also concluded that in this system cis—trans isomerisation occurred by an addition—abstraction mechanism. 

Reaction of cyclopropenes with bases such as alkoxide or amide ions often leads to a methylenecyclopropene by removal of an allylic hydrogen and reprotonation 6 9-71) though other reactions such as nucleophilic addition (see Section 5) or metallation at a vinylic position (see Section 2) may compete. Thus the ester (209) is isomerised by KOH to (210), and under more vigorous conditions to (211)144) ... 

The highly reactive allyl-iron hydride [HFe(CO)3(T 3-CH2CHCH2)], first proposed over 30 years ago as a key intermediate in the [Fe(CO)s]-catalysed isomerisation of alkenes, has been characterised by NMR spectroscopy for the first time53. A procedure has been reported for the synthesis of tetrahydrofuran esters based on a formal [3 + 2] cycloaddition reaction of [CpFefii -C3H5)(CXD)2] with unactivated carbonyl compounds. 

Building on these results, Hanessian and co-workers devised an efficient protocol for the isomerisation of terminal olefins with minimal self-dimerisation or cross-metathesis by employing methanol to generate hydride complex 33 in situ from its parent 10. The procedure was successfully applied to a variety of allylic compounds, including O- and A-allyl ethers, and cleanly afforded the corresponding propenyl species as EjZ isomeric mixtures, without any further isomerisation or conjugation in the cases of ketones, esters and lactams. 

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