Helicity rule

The helicity rule for s-trans-enones is shown in Figure 752 54... 

Figure 7. Helicity rule for j-trafl.s-a./1-unsaturated ketones (signs of Ac are reversed for > > 0).

All cisoid a,fi-enones studied, including compounds 93-98, obey the helicity rule for n —> it transition Cotton effects49,233,234. The sign of the 222-272 mn tt — tt band is also in accord with the helicity rule for unsubstituted enones, while the polar C—O... 

The n — n and n -+ it CD transitions of aza- (119) and thiaflavonone (120) were identified. Opposite signs were found for the Cotton effects of (—)-119 and (—)-120 as compared to those of known (—)-(S)-118. According to the helicity rule, the heterocyclic ring must also adopt the opposite conformation because the phenyl substituent is equatorially oriented in all three flavonones. Thus, the (2R) absolute configuration was assigned to (—)-119 and (—)-120146. 

Chiral alkoxy allenes derived from 1,3-alkylidene-L-erythritol and -D-threitol have been used in cycloaddition reactions to provide the 4-substituted /3-lactams 418 (R = Me, Ph). Intramolecular alkylation at nitrogen was achieved by the action of potassium carbonate and tetrabutylammonium bromide in dry acetonitrile. The absolute stereochemistry of the product 419 (R = Me, Ph) was assigned on the basis of the CD helicity rule (see Section 2.04.3.5) and NMR spectroscopy. The [2+2] cycloaddition of CSI to threitol vinyl ethers was found to have low stereoselectivity in contrast to the findings with erythritol derivatives <2004CH414, 2005EJ0429>. 

Chiral cages are such examples of molecules containing several centers of chirality but existing only in two stereoisomeric (enantiomeric) forms the absolute configuration of which can be described according to helicity rules. Thus, the two enantiomers of 4,9-twistadiene are the (lS 3S 6S 8S)-( + )- and (l/ 3/J 6/J 87 )-(—)-isomer,... 

A dramatic example of prevalence of y-alkylation over a-alkylation is provided by cyanoethylation of 3)S-acetoxy-5a-pregn-17-en-21-al (186) with acrylonitrile in benzene (see Part II, Chapter 1, refs. 213—216). The signs of the Cotton effects exhibited by the isomeric homoannular dienes (187) produced in this reaction violate the helicity rule for skewed dienes. ... 

FIGURE 9. Graphical representation of the helicity rule for inherently chiral vinylcyc-lopropanes. Reproduced by permission of The Weizmann Science Press of Israel... 

FIGURE 11. The Snatzke helicity rule for the (n, tc ) Cotton effect of inherently chiral cyclopropyl ketones... 

X-Ray study of 3,3-dimethoxy-19-norandrosta-5(10),6-dien-17-one (3), which exhibits a negative c.d. curve, confirms that the cisoid diene system has right-handed helicity the c.d. behaviour is therefore contrary to the simple diene helicity rule, in accordance with a recent report. ... 

A helicity rule has been proposed for 1-thio-D-glucopyranosides which allows the c.d. band at 220 nm n a of sulphur) to be interpreted in terms of the stereochemical relationship at C-1 and C-2. The o.r.d. and c.d. characteristics of the thioanhydro-compound (6) and the disulphides (7) and (8) have been measured at different temperatures and discussed. 

The assignment by semiempirical and empirical rules known as sector and helicity rules. Sector rules can be applied to inherent symmetric chrom-ophores. Helicity rules have been deduced for inherent dissymmetric chromophores. 

Whereas sector rules and helicity rules have been very successfully applied for the CD of electronic transitions (ECD), their applicability in VCD spectroscopy is very limited. 

Quantum mechanical computation for electronic circular dichroism and vibrational optical activity With the exception of the exciton chirality method for the development of sector and helicity rules, reference compounds are needed. Their absolute configuration has been usually obtained by X-ray analyses. The progress in numerical techniques of quantum mechanics nowadays allows performing computations of the rotational strength and its sign and, thus, the CD spectra with high reliability for CD... 

The three diagonal elements Ae P(v) (1=1, 2, 3) are proportional to products of electric dipole times electric quadrupole transition moments. They do not contribute to the isotropic CD because the sum over the three coordinates (v) (1 = 1, 2, 3) is zero. Asu, measured for oriented guest molecules in ordered liquid crystal phases, yield spectroscopic and structural information and, has been used, especially for the check of sector and helicity rules. First numerical quantum mechanical calculations of the CD tensor coordinates Asu have been published recently. 

The ring oxygen helicity rule has been extended to phenyl- and phenylthio-glycosides. ... 

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