2.05.6.5.5 Reactions with halides
4-Dimethylaminopyridine (DMAP) reacts with POCl3 or PCl3 to form a complex and liberate a chloride ion. It has been shown that when this reaction is carried out in situ in the presence of an oxetane, a ring-opening reaction proceeds to give chlorohydrins in good yields and under relatively mild conditions. The reactions can also be highly regioselective for asymmetric oxetanes, particularly when using POCl3, with chloride attack at the carbon with either the most favorable electronic effects or the least steric hindrance. Conditions have been developed for the ring opening of oxetanes, with ether moieties at C-2, in a highly regioselective fashion. The reaction between samarium diiodide and acyl chloride or anhydride gives a reagent which can form a bidentate chelate with oxetanes (e.g., 56), making the addition of iodide to the unsubstituted α-carbon (C-4 rather than C-2) far more favorable (Scheme 9). Although some alkyl- or ether-substituted oxetanes showed moderate regioselectivities, one regioisomer only (57) was formed when there was a CH2OBn moiety at C-2.