- la ct ams ▁containing ▁ 3 , 6 - d ih ydro - 2 H - 1 , 3 - ox azine ▁rings ▁are ▁named ▁ox ace ph ems . ▁ β - la ct ams ▁not ▁f used ▁to ▁any ▁other ▁ring ▁are ▁named ▁mon ob act ams . ▁ ▁By ▁convention , ▁the ▁b icy clic ▁ β - la ct ams ▁are ▁number ed ▁starting ▁with ▁the ▁position ▁occupied ▁by ▁sul fur ▁in ▁the ▁pen ams ▁and ▁ce ph ems , ▁regardless ▁of ▁which ▁atom ▁it ▁is ▁in ▁a ▁given ▁class . ▁That ▁is , ▁position ▁ 1 ▁is ▁always ▁adjacent ▁to ▁the ▁ β - car bon ▁of ▁ β - la ct am ▁ring . ▁The ▁number ing ▁continues ▁clock wise ▁from ▁position ▁one ▁until ▁the ▁ β - car bon ▁of ▁ β - la ct am ▁is ▁reached , ▁at ▁which ▁point ▁number ing ▁continues ▁counter clock wise ▁around ▁the ▁la ct am ▁ring ▁to ▁number ▁the ▁remaining ▁to ▁car b ons . ▁For ▁example , ▁the ▁nit ro gen ▁atom ▁of ▁all ▁b icy clic ▁ β - la ct ams ▁f used ▁to ▁five - member ed ▁rings ▁is ▁label led ▁position ▁ 4 , ▁as ▁it ▁is ▁in ▁pen ams , ▁while ▁in ▁ce ph ems , ▁the ▁nit ro gen ▁is ▁position ▁ 5 . ▁ ▁The ▁number ing ▁of ▁mon ob act ams ▁follows ▁that ▁of ▁the ▁I UP AC ; ▁the ▁nit ro gen ▁atom ▁is ▁position ▁ 1