Circular dichroism—LI : Circular dichroism of derivatives of hop constituents containing a five-membered ring

The conformation of the cyclopentenones 5 to 8 is determined from their CD-spectra. The site of enolization and the conformation of cis- and trans-isohumulone (1 and 2) is determined with the help of the CD-spectra, the direction of enolization for the humulinic acids 3 and 4 is tentatively determined by the same method.     

Circular Dichroism and Absolute Conformation: Application of Qualitative MO Theory to Chiroptical Phenomena

Although chiroptical methods have been used in chemistry for some 150 years, and the basic theory was developed by Rosenfeld 50 years ago, their success in determining absolute conformation rests mainly on empirical rules. Some of these can be proved theoretically; however, in the case of more complex molecules this is not yet always possible. In what follows, an attempt is made to bridge the wide gap between theory and empiricism: “qualitative MO-theory”, which has already been successfully used to explain reaction mechanisms, the shape of molecules, or photoelectron spectra, can also be applied to circular dichroism (CD). In general we shall have to be content with a reasonable interpretation of the experimentally determined sign correlation, but even this will be of great value since it can yield general relations between the geometry of molecules and the sign of CD. Such relations become particularly important when only a few examples of a given chromophore are known. If first and second sphere of a molecule are achiral then sector rules can be put forward, and in other cases helicity or chirality rules.     

Über triterpene—VIII Zuordnung von triterpenen zu den grundtypen durch ir spektroskopie

By comparison of the IR spectra of a number of triterpenes in two spectral ranges (“A” between 1392–1355 K and “B” between 1330–1245 K) bands characteristic of oleanolic and ursolic acid derivatives could be identified. The former have two bands (1392–1379 and 1370–1355 K) in the A- and three of increasing intensity (1330–1315, 1306–1299 and 1267–1250 K) in the B-region; the latter possess in every region three bands (1392–1386, 1383–1370, 1364–1359) K and with increasing intensity 1312–1308, 1276–1270, 1250–1245 K). Tetracyclic triterpenic acids or their esters show only one stronger absorption in these two ranges. Acids which do not belong to the types mentioned above possess their intensive bands outside the described limits. In the case of neutral triterpenes the characteristic absorption of the B-range are missing, and assignments based merely on the A-absorptions are possible only with reservation. From the study of the spectra the skeleton structure for some novel triterpenes have been tentatively proposed.