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. Author manuscript; available in PMC: 2015 Dec 31.
Published in final edited form as: Nature. 2014 Dec 11;516(7530):181–191. doi: 10.1038/nature14007

Figure 1. Quaternary stereocenters are important structural features of many biologically active molecules as exemplified by the natural products cortisone and morphine.

Figure 1

a, Structures of the steroid cortisone and opioid morphine with their quaternary stereocenters highlighted. Me, methyl. b, Steric congestion, which presents a formidable challenge for chemical synthesis of molecules containing quaternary stereocenters, is illustrated in the molecular models of morphine, particularly in the space-filling model on the right in which its sterically congested quaternary center is barely visible at the end of the pointing arrow.