S-cis configuration

The Diels-Alder reaction is stereo specific. The stereochemistry of the dienophile is retained in the product i.e., cis and trans dienophiles produce different diastereoisomers in the product. For example, freshly distilled cyclopentadiene, having s-cis configuration, reacts with maleic anhydride to give c/ -norbornene-Sjh-endo-dicarboxylic anhydride. 

Cyclic dienes usually react more readily than open-chain dienes, probably because they have their double bonds fixed in the proper conformation for [4 + 2] cycloaddition, consequently the price in energy of achieving the s-cis configuration already has been paid ... 

A Pariser-Parr-Pople (PPP) calculation indicates that the long-wavelength absorption band is what would be expected for the trans-s-cis-s-cis configuration shown by crystal-structure analyses to occur in the indigo crystal [2],... 

There are three possibilities for the location of the enyl units in the 2,3,4,5-tetramethyl-2,5-hexadiene-l,4-diyl frame (149), E,E, E,Z, and Z,Z. Moreover, the enyl units may adopt s-trans or s-cis configurations. On steric grounds,... 

At his point, it seems worthwhile to revisit the peculiar intramolecular bond that appears in the most abundant isomer of glycolaldehyde, which shows an OO-s-cis configuration, since it is relevant to SAR predictions. The glycolaldehyde -I-OH rate constant that predicted by SAR is 2.3 x 10 cm /(molecules), while the average obtained from the available experimental data is 9.25 x 10 cm / (molecules). The reaction profiles of the acetaldehyde, ethanol and glycolaldehyde -I- OH reactions have been modeled at the same level of theory. 

Next, Evans et al. [15] reported that Cu-based catalysts were superior in the Diels-Alder reaction of the oxazolidinone 9 with cyclopentadiene 8. ITie (5,5)-bis(oxazo-line)-Cu(II) and -Zn(II) complexes were very effective catalysts of the reaction. The optimum tert-butyl ligand 13-Cu(II) complex afforded (2S)-endo-ll with > 98 % ee. In contrast, the optimum catalyst system for the phenyl-substituted ligand 12-Zn complex afforded the enantiomeric (R) product, (2R)-endo-ll, with 92 % ee. The different direction of asymmetric induction was explained in terms of the geometry of cata-lyst-dienophile complexes at the corresponding metal centers. The bis(oxazoline)-Zn(II) complex-catalyzed reaction proceeded via the tetrahedral chiral Zn-dienophile complex VIII, in a manner similar to the bis(oxazoline)-Mg catalyst reported by Corey [13], whereas the reaction catalyzed by the cationic bis(oxazoline)-Cu complex proceeded via the square-planar Cu(II)-dienophile intermediate VII, so the diene preferred to approach from the opposite si face of the bound dienophile with s-cis configuration, avoiding steric repulsion by one of the tert-butyl substituents on the oxazoline rings. 

Crystal structures of 3,5,6-tri-t-butyl-2-hydroxypyrazine (1093) 2,5-dichloro-3-methoxypyrazine (915) and 2,3-dichloro-5-ethylamino-6-methoxypyrazine (916) have been determined. The dipole moment of 2-amino-5-bromo-3-methoxypyrazine (2.94D in benzene) is consistent with the S-cis configuration (52) (749) and the polarographic behavior of 3-hydroxy(and methoxy)-2,5-dimethylpyrazine (and their A(-oxides) at various pH values has been investigated (588). 

The spectra obtained upon adsorption of 1,3-butadiene over the Mg-ferrite catalyst are reported in Fig. 3. The spectrum obtained is similar to that previously reported for the adsorption of 1,3-butadiene over a-Fe203 [19]. The detection of the symmetric C=C stretching mode at 1636 cm (IR inactive in the gas) shows that the molecule is either in the non-centrosymmetric s-cis configuration, or its symmetry is strongly lowered by interaction with... 

In every case for copper catalyst 31, the absolute stereochemistry of the cycloadducts is accounted for by the intervention of the substrate-catalyst complex depicted in Fig. 23, in which the s-cis configured dienophile is bound to the catalyst in the plane of the ligand in a bidentate fashion. The ferf-butyl group shields the top face and cycloaddition occurs from the exposed si enantioface. Support for this model derives from X-ray structures of aquo complexes of catalysts 31a and 31b which show that the complex possesses a distorted square planar geometry EPR spectroscopy on the binary catalyst-dienophile complex indicates that this geometry carries over from the solid state into solution. Calculations at the PM3 level of theory further favor the indicated reactive assembly [85]. 

Since the CD spectrum of the one isomer of the [Co(i-sdda)-en]+ (Figure 12) shows a positive dominant CD peak and an overall CD curve similar to that of (+)-s-cis-[Co(edda)( -stien)]+, it might be expected to have the A-configuration. However, the A-s-cis-configuration would require the jt-stilbene-diamine backbone to have the X-conformation, forcing the large... 

As indicated in these equations, the main factor determining the stereochemistry of enchainment is the mode of coordination of the transition metal center with the monomer to form either a syn or anti r-allyl type of intermediate. In general, the coordination with two double bonds of the 1,3-diene in an s-trans configuration (see (b) Eq. (2.89)) is less common than the coordination in an s-cis configuration shown in Eq. (2.88) (Zohuri et al., 2012). This interpretation is complicated by the fact that the syn and anti 7r-allyl complexes are in equilibrium. These simple... 

The dienes of Diels-Alder reaction Dienes can be either chain or circular structure and may contain different substituent groups. The limitation is that diene must contain s-cis configuration. Bulky substiment groups may affect the most stable conformation of diene, which leads to the failure of the reaction. For cricoid... 

Scheme 1 Selected examples of structures of 1,4-dilithio-1,3-butadienes, which favor a double dilithium bridged structure (s-cis configuration) in both solution and solid states...

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