Lactone sector rule

The negative signs for the (n, tt ) CD bands of the lactones 124 and 125 follow also from Snatzke s lactone sector rule with the sign pattern for the upper sectors (above the plane of the paper) depicted below . This last rule is based on the nodal surfaces of the riy and nf... 

The use of the lactone and modified lactone sector rules (Jennings et a/., 1965b Keller and Snatzke, 1973 Polonski, 1975) has also been suggested for predicting the sign of the n-7c Cotton effects of a-amino acids. 

FIGURE 7. The sector rule of Weigang and cowor kers for the lactone chromophore... 

A sector rule for the rotatory strength of low-symmetry chromophores such as lactones, lactams, and peptides (Fig. 17) has been described by Ong et al (1977) and Weigand (1979, and references therein) and compared with the quadrant rule (Schellman, 1968). One of the most important conclusions is that the longest-wavelength singlet absorption (based on the perturbation theory) of an ester group and that of an amide arise from basically different... 

The A-factor contains two asymmetric centres C-2 and 0-3. In its spectrum of circular dichrolsm two positive Cotton s effects (at 285 and 225 ani) are seen, the second of them is connected with a lactone cbromophore. According to the sector lactone rule the sign of this effect is determined mainly by the S-conflguration of the substituent at C-3. As to the orientation of the acyl substituent, the more stable trans-isomer, S-configura-tion, (as well as enol form, which causes a shoulder at 250 nm in the UV-spectrum of the A-factor) must be predominant in solution in consequence of easy epi-merlzation of 0-2. Thus, the spatial structure of the A-factor can be expressed by the formula in fig. 2. 

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