A program for finding common three-dimensional substructures within a set of chemical compounds is described. The program allows a user to define what constitutes commonality of substructures by providing control over the importance of degree of substitution, atom type, aromaticity, and hybridization. Simple examples are used to illustrate various phases of the search process, and an application of the program to a structure/activity problem is used as a more realistic example.

A new A-nor sterol, 3/3-(hydroxymethyl)-A-nor-5a-cholest-15-ene, with an unusual unsaturation in the 15 position, has been found in the Pacific sponge Homaxinella trachys together with ten other A-nor sterols. The structure was elucidated by 360-MHz NMR, 13C NMR, and mass spectral analysis as well as by chemical interconversion and comparison with synthetic 5a-cholest-15-en-3/3-ol. 13C NMR spectra of the title compound and of various 3/Mhydroxymethyl)-A-nor-5a-cholestanes were assigned as an aid in future structure elucidation of this class of sterols.Sterols with normal skeletons have been isolated from several sponges of the family Axinellidae,2 3456"4 but this family is not homogeneous, as sterols with a contracted A-ring, the 3/3-(hydroxymethyl) -A-norsteranes, have been found as the only type or as the major sterols of Axinella verrucosa? Hymeniacidon perlevis? Stylotella agminata? Teichaxinella morchella? Axinella sp.,7 Phakellia aruensis?& and Acanthella aurantiaca? which belong to the same family. It has been shown10 that A-nor sterols are formed in the sponge by conversion of dietary sterols; intermediates are Á4-cholesten-3-one and analogues with different side chains.Our ongoing investigations of marine invertebrates have now uncovered another source of novel A-nor sterols, the Pacific sponge Homaxinella trachys.

Computer programs for interpretation and prediction of 13C resonance spectra are described. These programs utilize a data base containing representations of the substructural environments of resonating nucleii together with their chemical shifta. These representations capture both molecular constitution and contiguration, permitting for the first time in a computer program a comprehensive treatment of configurational stereochemical influences on 13C chemical shifts. Substructural features of an unknown structure are derived directly and automatically by '3c interpretive procedures. These features, together with additional structural information, are used to construct structural candidates for the unknown. 13C predictive procedures permit rank ordering of the candidates on the basis of agreement between predicted and observed 'Y! spectra. Applications of these programs to organic structure determination are illustrated through analyses of the structures of some diterpenes.(1) Part

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Computer programs for encoding canonical representations of stereochemical substructural environments are presented. Applications of these substructure codes in the study of relationships between molecular structure and chemical shifts observed in '"CNMR spectra are described, Using natural products as examples. The utility of the codes for the detection of erroneous spectral assignments is illustrated.