Friday, August 17, 2007

Radical cyclization

In general, halides can be reduced to radical by aibn. Bu3SnH is used to reduce the radical to R-H.

1. Mn(OAc)3
Mn3+ oxidizes the enone form of a beta-ketoester. A stablized carbon/oxygen radical is formed,
then it cyclizes to a double bond, the secondary carbon radical then can be oxidized by Cu2+, gives a cation, then finally, cation eliminates to give a double bond.
thermodynamically most stable isomer is the major product.
LiCl can be used instead of Cu(OAc)2 to give a chloride.
For some hindered double bond, Mn/Cu or Mn/LiCl do not work. O2 in the air reacted with the initial radical and R-OOH was the final product. if LiCl was used, R-Cl was formed.
In some case, push away O2 by Argon instead of N2 gives better yield.
Mn/EtOH gives reduced product instead of oxidized product. only works when you formed a methyl radical after cyclization.
AcOH is a good solvent, MeCN is also OK.

2. Iodine transfer reaction
treat a beta-ketoester with LDA/I2 gives the iodide. then aibn/heat/benzene gives cyclized product.
This method worked better for hindered double bond than Mn,
AIBN along with some heat is enough to initiate the rxn.
Lewis acid (for ex. Mg(ClO4)2 ) can be used to catalyze the reaction.
gives thermodynamic product.

3. selenide transfer rxn.
in my case, starting material - beta ketoester phenylselenide is not stable on silica gel.
usually, UV light, lewis acid can catalyze the reaction.

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