Esters restricted rotation

Alkylamino crotonic esters (22) all show in solution the presence of the isomers of both E and Z configurations, and there is also some evidence for rotamers involving restricted rotation about the (C=C)-N and C-C02R bonds (64). In all cases studied, when only one crystal form was found the molecular configuration was Z about the double bond. For R = CH2Ph, R = CJI, two crystal forms were obtained, one with E and one with Z molecular configuration. 

The 7-azaquadricyclane (77), like its precursor (12), shows evidence in the NMR spectrum of restricted rotation about the N-CO bond. All the 7-heteroquadricyclanes, 76-79, are thermolabile, and they rearrange very readily to the corresponding azepines (80) or oxepin (81). (In another instance the 7-oxabicyclo[4.1.0] valence-bond tautomer was obtained instead. ) In appropriate conditions the addition of acetylenic esters (methyl propiolate and dimethyl acetylene-dicarboxylate) competes successfully with the isomerization and gives the ca o-tricyclic adducts 82 or 83. " °... 

At 60 °C, 50% of the ester groups, trapped in constrained environments with high activation energy barriers, do not performed 7r-flips at a frequency higher than 10 kHz and their motions are limited to restricted rotations (rocking) around the local chain axis with an average amplitude of 7°. 

The planarity of the amide linkers in the protein backbone restricts rotation around the carbon-nitrogen bond. This provides some restrictions on the number of conformers that can be adopted. The linkage joining amino acids in a polymer is quite stable, but not infinitely so, and it can be relatively easily hydrolyzed by enzymes to allow turnover of proteins within cells. This propitious combination of properties is conferred by the amide bonds linking the amino acids in the polymer polymers linked by ester, thioester, ether, or carbon-carbon bonds lack one or more these properties. 

N-Substituted 3-aminocrotonic esters (67) tend to adopt a planar or near-planar structure. As a consequence of their push-pull nature these molecules show an increased facility for Z-67-is-67 isomerization around the C=C double bond as well as restricted rotation around the C—N and C—COOR2 single bonds. The IR and XH-NMR spectra of simple 3-(alkylamino)crotonic esters have shown that these substances exist either in the liquid state or in solution as equilibrium mixtures of the Z and E configurations, respectively. The position of the equilibrium is solvent-dependent, and the energy difference between the isomers varies from ca 7.3 kJ mol-1 in non-polar solvents to ca 0.8 kJ mol"1 in dimethyl sulphoxide, the intramolecularly bonded Z-form 68 or 69 being the most stable111-113. 

The other systems, esters 2.71, enamines 2.72, and enol ethers 2.73, similarly have restricted rotation about the bond drawn as a single bond, but the barrier is... 

Restricted rotation was also introduced into the succinic acid moiety of succinyldicho-line by preparation of the choline esters of cis-and m iS Cyclopropane-l,2-dicarboxylic acids... 

Examination of models indicated that there should be restricted rotation about the N(l)-C(11 ) bond due to steric congestion and the observed H(12)/H(10) and H(9 ),H(10 )/Me (ester) NOE interactions suggested a preferred conformation in which the approximately planar dihydroindole portions of the two monomers are nearly mutually orthogonal to each other. This proposal was also supported by the unusual upfield shift of the ester methyl which can be accounted for by the anisotropic influence of the other aromatic ring as a result of the methyl ester function being placed above the aromatic ring due to the preferred conformation adopted. 

The alkaloids are chiral as a result of restricted rotation of the biaryl system as well as of the possession of asymmetric carbon atoms the methyl at C-3 is on the a-face in all alkaloids and that at C-1 on the B-face in most, but not all cases. The structure of ancistrocladine has been determined by oxidation to the acid (13) the methyl ester of which is prepared by two routes, and by Hofmann degradation of its 0,n-dimethyl-derivative successively to the methine base (14) and the nitrogen-free products (15) and (16), the second of which on ozonolysis gives an aldehyde which yields the lactone (17) on oxidation. The position of the phenolic hydroxyl group in the alkaloid has been confirmed by Claisen rearrangement of the allyl ether (T.R. Govindachari and P.C. Parthasarathy, Tetrahedron, 1971, 1013). 

Chemical Aspects of the Restricted Rotation of Esters, Amides, and Related Compounds... 

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