Foreword

SPECIAL SUBJECT ISSUE ON CHIROPTICAL METHODS Part

Circular Dichroism of Bufadienolides

With the help of PMO theory a non-empirical sector rule has been derived for the n ~ 1t5* band CD (around 260 nm) of the pentadienolide chromophore of steroidal bufadienolides. The rule was verified by experiment using the preferred conformation for the 2-pyrone side chain found in most of the crystalline bufadienolides investigated by x-ray diffraction. Substituents in ring D do not drastically influence the Cotton effect of this transition, whereas the sign of the 1C4 ~ 1t5* band Cotton effect around 300 nm is strongly dependent on such substitutions. The Cotton effect of additional carbonyl groups can in general be detected without difficulty in the CD-spectra

Circular Dichroism of Optically Active 1,2-Disubstituted 1,2-Diphenyl Ethanes Part II: Compounds without COOR-group at Benzylic C

(R,R)-Hydrobenzoin and homochirally analogous thTeo-compounds of formula Ph-CHX-CHY-Ph (X = Y; X rO Y; Y = H) show anegative O-O - CD-line below 37200 crn? and/er a positive one just above it, which can be explained by conformational equilibria around the Ph-C - bonds. The Cotton effect at 220-210 nm is negative for this same configuration, if X rO Y (also Y = H), in these other cases there no such simple correlation. Cotton effects within the Elg- or the E2g-transitions might be the reason for this, although otherwise no indication of them could be found. The same rules hold if a ring is closed between X and Y, the Cotton effect at 220-210 nm has thus the same sign as the torsional angle (Ph-)C-C(-Ph), and the same is also true of the next Cotton effect at 198-191 nm. The temperature dependence of the CD can be explained only by assuming conformational as well as solvational equilibria. Introduction of asingle p-chloro substituent shifts the borderline between the two O-O - CD-lines of opposite signs to 36200 cm-lo It causes a sign inversion of the CD within the «-band of a »half\u27-molecule« of type CI-C6H4-C(H, Bn)-OZ, not however in the case of CI-C6H4(H, Bn)-CH20Z. Such p-chloro substitution does not influence the Cotton effects below 220 nm. p,p\u27-Disubstitution by the CH3CO- -group shifts the p-band strongly batbochromically, the corresponding intense CD-couplet prooves the preponderance of that conformation deduced already from NMR-spectra, and prooves as well the absolute configuration

Chiroptical Properties of Optically Active Thiazolidines Derived from Aldoses and Natural Mercapto-amino Acids

Optically active thiazolidines substituted at C(2) or/and C(4) show at least 2 Cotton effects below 260 nm, and different correlations between stereochemistry and CD hold for these compounds and their N-acyl derivatives. By such N-acylation all CD- -bands are enhanced, and the inherently chiral moiety -S-C- -N-C(=O)- of absolute conformation shown in Figure 4 always leads to a very strong negative Cotton effect near 205nm, regardless whether C(4) carries a C(=O)X group or not. Signs, positions, and magnitudes of the other CD bands are strongly influenced by other factors, too. Such a conformation is fixed in the lactones XXVIII through XXXIV, but is also preferred for 2,4-cis-disubstitution (with a polyhydroxyalkyl group at C(2) and a carboxylic group at C(4». Exciton interactions between the N-acyl and other (C=O)X - groups do not play a decisive role

Synthesis of Aryl-sulphinyl Acetic Acids and Sulphoximides, and CD of their in situ Complexes with [MO2(OAc)4]

The syntheses of several optically active aryl-sulphinyl acetic acids and sulphoximides are described. They all give two or more Cotton effects in the presence of the metal cluster [MO2(OAc)4]in DMSO solution above 300 nm, from which their absolute configuration can be determined unequivocally

Circular Dichroism of Optically Active 1,4-Benzodiazepines

Assignment of the CD-bands of 1,4-benzodiazepin-2-one derivatives (1-14, 26), and their 2-deoxo-congeners (15-25) has been performed by applying the qualitative MO theory and the exc1iton coupling theory. It has been found that the longest wavelength band (about 310 nm), as well as that around 250 nm correspond to the B2u and B1u transition of the chiral partial chromophore A, respectively, while the corresponding transitions for the partial chromophore C give rise to the bands at 285 nm and 250 nm (Figures 2-4). The CD-signs for several of these Cotton effects can be derived in a nonempirical manner. 7-Nitro derivatives, 23-25 scape above analysis, however, since the nitro-group is a very strong perturber, which has its own absorption bands in the same region. Model cyclic and acyclic compounds 27-36 have been prepared, and their CD-spectra analysed in the same way

Synthesis of Some Optically Active α-Pyridyl-carbinols and Determination of Absolute Configuration from the CD of Their in situ Complexes with [MO2(OAc)4 ]

Four new a-pyridyl-aryl-carbinols have been synthesized, and twelve such racemates have been for the first time resolved by standard procedures. From the positive CD-couplet of the complex between (S)-l and 23 an antiperiplanar arrangement of the H-C(OH, Ph)-C.(Py)-N-moiety could be derived as the preferred conformation. From the CD-spectra above 300 nm of this and seven other compounds of known absolute configuration in the presence of 23 the following correlation with stereochemistry was derived, which was then applied to determining the absolute configuration of fourteen other a-pyridyl-aryl-carbinols: for (R)-configuration negative CD-bands appear between 500 to 510 (sometimes detectable only as shoulder) and at 408 to 470 nm, between 335 to 380 nm apositive Cotton effect is registered, and another one shows up below 320 nm, which has mostly apositive sign for secondary, but anegative one for tertiary carbinols of the mentioned absolute sense of chirality