Anti-syn conformation

The syn-anti conformational problem of a- and /3-pyrazofurins (756 one of the rare naturally occurring pyrazole compounds, see Section 4.04.4.4.3), which involves a rotation around a pyrazolic sp carbon atom and a sugar sp carbon atom, has been studied theoretically using the PCILO method (81MI40403). In agreement with the experimental observations, the /3 anomer is energetically more favourable than the a anomer, the preferred conformations being anti and syn, respectively. 

A prototype of such phenomena can be seen in even the simplest carboxylic acid, acetic acid (CH3CHOOH). Acidity is determined by the energy or free energy difference between the dissociated and nondissociated forms, whose energetics usually depend significantly on their conformation, e.g., the syn/anti conformational change of the carboxyl-ate group in the compound substantially affects the acid-base equilibrium. The coupled conformation and solvent effects on acidity is treated in Ref. 20. 

Finally, it should be mentioned that all our arguments and examples have centered on the syn conformer of the o-methylbenzoy1 group. In small molecules the syn-anti conformational equilibrium is known to play an important role in the photochemistry of the... 

The analysis of these 25 compounds confirms the flexibility of dpg/dpg+. It can adopt a propeller like conformation and induce the formation of chiral crystals. It establishes predominantly two-dimensional H-bonding networks with the counter ion. From the perspective of the crystal engineer, an anticlinal conformation would maximize the non-linear properties of the molecule. However the non-linear optical response depends greatly on the molecular alignment. Non-centrosymmetric dispositions have not yet been achieved for dpg with an anti-anti conformation. The best strategy to attain NLO samples is by inducing a syn-anti conformation where a propeller structure might induce a chiral disposition of molecules and thus a non-centrosymmetric order. Another successful approach is to force crystallization with a chiral counterion. 

These spectra were routinely obtained for most of the compounds discussed in forthcoming sections. Specific studies were performed on selected compounds such as (i) (+)-(S )-4,5,6,7-tetrahydroimidazo-9-chloro-5-methyl-6-(3-methyl-but-2-enyl)imidazo [4,5,1 -jk [1,4] benzodiazepin-2(l//)-thione (9-chloro-TIBO)187 and (ii) series of metal chelates derived from l,2-dithiol-3-thion-4,5-dithiolate (dmt)188. The equilibria between 2-ethoxycarbonylthiolane-3-thiones and their (Z)-enethiol tautomers were also studied by these techniques189. It was found that the equilibrium is largely shifted in favor of the enethiols. NMR was also used to study the syn-anti conformational equilibria of seven A-(l-methoxycarbonylethyl)-A4-thiazoline-2-thiones with S conformation of the chiral rotor (43)190. A variety of substituents were used. 

Figure 3.13 Self-sorting of syn-anti conformations about the glycosidic bonds in Gottarelli s octamer.

An x-ray study of trans-1,8-dinitrosooctahydronaphthyridine (18) reveals a syn, anti-conformation of the nitroso groups. The rings in compound (18) adopt a chair, twist-boat conformation. 

The gas-phase thermodynamic (van t Hoff) parameters AH°, A5°, and AG029g for the syn-anti conformer interconversion of these gaseous alkyl nitrites are presented in Table 2. For MeONO this data compared well with those obtained by direct integration of slow exchange spectra. [27,22] The agreement of these gas-phase NMR thermodynamic parameters with microwave and theoretical data reinforce the validity of this technique applied to the syn-anti equilibria. Comparison of these gas-phase data with solution values yields information on the effects solvents have on the conformer equilibria, which in the particular case of these alkyl nitrites reflect a balance of steric and dielectric effects. 

Pressure-dependent rate constants for the syn-anti conformational process in larger alkyl nitrites provide a further test of the ability of RRKM theory to successfully model the kinetics of the internal rotation process in these molecules. Solution of the Lindemann mechanism shows that at the pressure where the rate constant is one-half of its limiting high-pressure value, Pm, the frequency of deactivating collisions is comparable to , the average rate that critically... 

Dihydroxy benzoic acid crystallizes in two polymorphic forms monoclinic and orthorhombic. The molecule in the monoclinic form adopts the syn-anti conformation (20) while it adopts the syn-syn conformation in the orthorhombic form (21). 

Ultrasonic relaxation techniques have been used to estimate the energy barrier to syn-anti conformational interconversion in cytidine 2, 3 -mono-phosphate, in the presence and absence of ethidium bromide. Raman spectroscopic measurements on a number of dinucleoside monophosphate crystals of precisely known sugar-phosphate structure have been compared with those of B- and A-form DNA, to give information on the comparative rigidity of the backbone in the nucleic acids. The same technique, applied to poly(dA-dT) and poly[d(A-T)] at different temperatures, suggests that some of the furanose... 

Gorenstein Ue have also found evidence in the CMP complexes with RNase A that the first complex undergoes an Isomerization to a second distinct complex with a rate - 200 sec / and that this isomerization is pH dependent. Do you find any pH effects to your N-15 spectra of the UMP-RNaseA complex Do you have any evidence that these two isomers represent syn/anti conformational isomers ... 

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