The initial synthesis developed by Sheehan was not appropriate for mass production of penicillins .
One of the intermediate compounds in Sheehan 's synthesis was 6 - aminopenicillanic acid ( 6 - APA ) , the nucleus of penicillin .
Although the initial synthesis developed by Sheehan was not appropriate for mass production of penicillins , one of the intermediate compounds in Sheehan 's synthesis was 6 - aminopenicillanic acid ( 6 - APA ) , the nucleus of penicillin .