I am trying to solve this mechanism for a disfavoured 5-endo-trig cyclisation to generate the disfavoured product (4) from starting material (1). The favoured 5-exo-trig is given by (2), which I have got a mechanism for. My efforts take me to intermediate (3) as shown below, but I'm not too sure on how to generate the desired product without reagent reduction.

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1 Answer 1


Your mechanism to the 5-endo-trig product (4) is very close. Notice however that when you do the initial cyclization, the intermediate formed has a positive charge on nitrogen along with the negative charge on oxygen (as you showed). This cyclized intermediate then just needs to move a proton from the nitrogen to oxygen to form the neutral enol which then converts to the (ester) carbonyl - which is your desired product. No other reagents are required.

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  • 1
    $\begingroup$ Additionally, the OP's second step is not possible, the pi bond cannot move into the ring since the destination carbon already have four bonds $\endgroup$
    – Ben Norris
    Dec 2, 2014 at 20:55

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