Diene chirality rule

The diene chirality rule (hereafter referred to as DR) constitutes a simple tool for correlating the sign of the lowest energy tt —> n transition (] A —> 1B in C2 symmetry) of the distorted diene to the chirality (left or right-handed) of the chromophore. The validity of this rule is based on the assumption that the CD of the distorted chromophore is determined by its intrinsic helicity alone and that external dissymmetric perturbations have only minor effects on the optical activity. 

The diene chirality rule (hereafter referred to as DR) constitutes a simple tool for correlating the sign of the lowest energy r — r transition ( A — in C2 symmetry)... 

Fresh evidence is presented that the lowest-energy singlet electronic c.d. band in several chiral mono-olefins is associated with the -> 35 Rydberg transition. Inadequacies of the diene chirality rule have become apparent from recent data. C.d. effects of cyclohexa-1,3-dienes seem to be due as much to allylic axial substituents as to the diene chirality. The effect is well illustrated by the series of... 

The intrinsic contribution, accounted for by the diene rule, seems to be easily outweighed by the perturbations arising from the allylic axial substituents, which in turn define the allylic axial chirality rule. This latter can be formulated in two ways The olefin-picture , where chirality is referred to the nearest double bond, and the diene-picture ,... 

Mason and coworkers10 studied the chiral bicyclic derivatives 2, 3, 33 and 34, having known absolute configuration. These molecules possess a planar s-cis diene chromophore and formally their chirality is due to the presence of the D or CH3 substituents, which rule out all the symmetry planes. However, it is interesting to point out a peculiar structural... 

In this situation the two end-ring double bonds define a positive chirality. Application of the twisted diene rule gives a positive sign for the lowest-energy 1A - 1B transition, and in the case of 15-cA-(3R,3 R)-zeaxanthin this is confirmed by experiment. 

The cyclocondensation of the diene (1) with (R)-glyceraldehyde acetonide (9) results in high asymmetric induction at C5 of the dihydropyrone (10). The configuration (S) was established by degradation to 2-deoxyribonolactone (11). The result is in accord with the Cram rule for addition to chiral carbonyl compounds. The paper also describes conversion of the pyrone (10) to chiral 2,4-dideoxy-D-glucose. 

A recent example of an intermolecular [3 + 4] cycloaddition starts with an allylic acetal, as shown in Eq. (167) [419,420]. Other Lewis acids, for example AlEt Cl3 (n = 0-3), TMSOTf, TfOH, SbCL, SnCL were less effective. Although the exact nature of the transition state is still uncertain, the stereochemistry of the product might be explained on the basis of the rule of endo addition with the least hindered approach of the diene. The possibility of asymmetric synthesis starting with the same substrate with a chiral acetal moiety has been mentioned (see Table 15). 

Since its discovery over sixty years ago [1] the Diels-Alder reaction has lost none of its attraction. [2, 3] It enables, in a one-step inter- or intramolecular reaction, the rapid preparation of cyclic compounds having a six-membered ring. During the course of the [4 -I- 2] cycloaddition four new stereocenters can be introduced directly, and their stereo-control is a topic of major interest in modem synthetic chemistry. [4-6] In addition, in intermolecular reactions, the relative positions of the reaction partners (regiochemistry) must be taken into account. If a concerted reaction is assumed, both a cis addition (suprafacial mode) and a preferred endo orientation (Alder rules) can be expected. But how can the absolute configuration of the desired product be controlled There are three basic possibilities the use of a chirally modified diene, a chirally modified dienophile, or a chiral catalyst. Although the first successes resulted from the attractive, hut difficult, catalytic route, [4b, 7] the majority of the investigators are concerned with the stoichiometric... 

The chiroptical behaviour of cisoid conjugated dienes, already known to be controlled by a combination of diene helicity and axial chirality effects, is further influenced by methyl substituents on the unsaturated carbon atoms. The observed methyl group contributions are not susceptible to a simple interpretation, for in some cases [e.g. at C-2 or C-3 in a 2,4-diene (24), or at C-3 in a 19-nor-l,3-diene (25)] the signs of methyl group contributions to Ae and to the rotatory strength (R) are opposite. Computer resolution showed that the c.d. and u.v. absorption curves are of composite form, with up to five vibronic components. Moreover the wavelength shifts accompanying methyl substitution do not accurately follow the Fieser-Woodward rules. It is concluded that... 

The retrosynthetic plan (Scheme 10) indicates that the dienophile would be a carboxyketene equivalent, CII, which implies the diastereofadal selectivity would be controlled entirely by the diene component Cl. At the outset of this work, there were few examples where chiral dienes had been employed in intermolecular Diels-Alder reactions upon which the investigators could rely for precedents [28, 29]. However, seminal studies had been undertaken by Franck and co-workers who reported that the sorbaldehyde diene CHI reacted with N-phenylmaleimide to give a 7.3 1 mixture of cycloadducts CIV and CV (Scheme 10b) [29]. Thus, the major product resulted from a like process (i.e., preferential attack from the r -face) and these workers developed a set of rules based on their experimental observations. 

Reaction rules (1) Syn-addition principle Diels-Alder reaction is triggered by overlapping dienophile and the p track of n bond in diene. Therefore, Diels-Alder reaction is a stereospecific reaction, and the initial latent chiral center substituent remains unchanged. (2) Endo-addition principle In the cycloaddition reaction, the electron-withdrawing group of dienophile heads to the internal direction of diene. It can be explained by the secondary track overlap, but there are some limitations, so this rule is not suitable for all cases [1, 2]. 

The ultraviolet spectra of crowded olefins and substituted butadienes have been studied the sign of the Cotton effect has been related to the chirality of a series of w-molecular complexes of olefins with tetracyanoethylene the sign of the chiral-optical effects of non-planar heteroannular cisoid dienes is opposite to that predicted from the diene rule. Studies of linear and circular dichroism in mono-olefins and magnetic circular dichroism of conjugated olefins have been made, and the experimental ionization potentials of fourteen alkenes have been correlated with the inductive effects of substituents. ... 

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