in ethylene glycol dimethyl ether; hexane byproducts: LiCl; soln. of B-P-compd. in DME is added to soln. of BCl2N(i-Pr)2 in hexane at -78°C (under N2), stirring for 2 h, warming to 23°C, stirring for 16 h; filtration, evapn. of solvent from the filtrate, redissolving in hexane, cooling to -10°C (crystn. of the 6-membered-ring compd.), decanting, concg., cooling to -10°C, recrystn. from hexane, elem. anal.;