TANDEM REARRANGEMENTS DURING FLASH VACUUM PYROLYSIS : CONVERSION OF 2,3,5,6-TETRACHLO-ROPENTACYCLO 5.4.0.02,6.03,10.05,9LUNDECANE-4,8,12-TRIONE TO 3,5 ,6-TRICHLOROINDENONE AND 2,6,7-TRICHLOROINDENONE