A novel iodine-mediated tandem cyclization

Chem. Commun., 2009DOI: 10.1039/b910232a

A novel iodine-catalyzed tandem cyclization–cycloaddition reaction of ortho-alkynyl-substituted benzaldehydes leading to polyoxacyclic ring systems has been developed, which represents a useful approach towards the synthesis of the oxabicyclo-[3.2.1]octane ring skeleton found in a variety of natural products.


mechanism:

bromination with POBr3 POCl3

usual condition use Phosphorus(V) oxybromide or Phosphorus(V) oxychloride
in toluene, xylene and heat to over 120 C.
during the rxn, HBr/Br2 can be generated.
sometimes bring side reactions and low yields.

the following paper has a better procedure:
J. Chem. Soc., Perkin Trans. 1, 2002, 529–532

The use of phosphorus oxychloride to prepare compound 12 was sometime fraught with side reactions. Although a very efficient procedure has been recently reported,16 we developed a robust method, also using phosphorus oxychloride or bromide, but in the presence of potassium carbonate in boiling acetonitrile. Thus the troublesome hydrolysis of the reaction mixture takes
place under neutral conditions, without heat evolution, and avoids unwanted decomposition. The 1-halogenated derivatives 13 or 14 were thus routinely prepared in 82–85% yields.

Bromination on aromatic ring

o-Bromination of phenol.
: iPr-NH, slow addition of 1 eq. of NBS in DCM at 0 C.

transformation of alcohol to iodide

1. TEA/MsCl/DCM, rt, then NaI/acetone, reflux.
2 steps, good yield.
2. PPh3/I2/imidazole
good yield, one step rxn, sometimes separation of pph3/pph3O is problematic.