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I have often seen that in many drugs chirality plays a role. So I was trying to analyze the structure of Penicillin V.

Penicillin V

What is number of chiral centers in the structure of penicillin V? Is the nitrogen which is a part of the four membered ring to be counted here because I have read that the nitrogen atom without pyramidal inversion is treated as chiral. So can the nitrogen at bridgehead position be treated as chiral? Because that makes the stereocenter count 4 while on pubchem website the count is mentioned to be 3.

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Yes, here Nitrogen has to be considered as a stereocenter. Also, you can assign R and S to it. The rule is that the lone pair has the least priority and then you can assign R and S as you do for other molecules.

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  • $\begingroup$ In terms of CIP-rules, I agree. For the triple substituted nitrogen, however, I assumed nitrogen inversion still could occur, thus discounting this site. Maybe N's simultaneous participation in a five and a four membered ring does not permit the large «umbrella flip» otherwise seen / taught with amines thrice substituted by alkyl chains so ChemSE users with experience in quantum chemistry can provide an estimate for the activation barrier to interconvert the two states into each other to support either perspective about this substrate. $\endgroup$ – Buttonwood Oct 9 '20 at 10:05

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