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.
The key intermediate compound produced during Sheehan's initial penicillin synthesis that served as a foundation for mass production was 6-aminopenicillanic acid (6-APA).