p149b

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When the rings of a steroid are represented as projections onto a plane in the conventional manner, an atom or group is called

if it projects below the plane and

if it projects above the plane. This '

-system' has been extended to other cyclic natural products and related products (see, for instance, Section E, Stereochemistry and Stereoparents, pp. 192I-212I, in 'Chemical Substance Index Names', Appendix IV, in the 1992 Index Guide, Chemical Abstracts Services, Columbus, Ohio 1992).

This HTML reproduction is as close as possible to the published version [see IUPAC, Commission on Nomenclature of Organic Chemistry. A Guide to IUPAC Nomenclature of Organic Compounds (Recommendations 1993), 1993, Blackwell Scientific publications, Copyright 1993 IUPAC]. If you need to cite these rules please quote this reference as their source.

Published with permission of the IUPAC by Advanced Chemistry Development, Inc., www.acdlabs.com, +1(416)368-3435 tel, +1(416)368-5596 fax. For comments or suggestions please contact webmaster@acdlabs.com