136 THE FORMATION OP CYCLIC KETONES Phosphorus trichloride also has been employed, but this reagent has serious disadvantages. 86 Purification of the acid chloride by distillation or crystallization is generally unsatisfactory. Both techniques are likely to be wasteful, and few of the polycyclic acid chlorides can be distilled even under reduced pressure without considerable decomposition. It has been found generally expedient to* treat the acid in such a manner that the by-products on formation of the chloride are readily removed, thus excluding all material which might interfere with the cyclization. Procedures involving the use of phosphorus pentachloride and thionyl chloride are described in the following discussions of stannic and alu-minum chloride in cyclizations. STANNIC CHLOBIDE Stannic chloride has been shown to be generally effective in the Friedel-Crafts reaction in bringing about ring closures into polycyclic nuclei. Of thirty-eight acid chlorides which have been cyclized (Table
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