Benzothiazine and benzothiazepine derivatives: structures of N-p-bromophenyl-6,7-dimethoxy-1,2-benzothiazin-3(4H)-one 1,1-dioxide (BBTZ) and 4,5-dihydro-8,9-dimethoxy-N-(5-methyl-2-pyridyl)-1,2-benzothiazepin-3-one 1,1-dioxide (MPTE)