Syn-form

Substituent Pyrrole Percent of syn form (51) Furan Thiophene Selenophene Tellurophene Phase... 

Compute the energy difference between the anti (left) and syn forms of furfuraldehyde in a solution of dimethyl ether (e=12.0), using either the Onsager (MP2/6-31+G(d)) ortheSCIPCM (B3LYP/6-31+G(d)) SCRF models. The observed energy difference is -0.53 kcal-moT. ... 

In the above-mentioned reactions with hydrazine and hydroxylamine, the actual reducing species is diimide (NH=NH), which is formed from N2H4 by the oxidizing agent and from NH2OH by the ethyl acetate.The rate of this reaction has been studied.Although both the syn and anti forms of diimide are produced, only the syn form reduces the double bond, at least in part by a cyclic mechanism ... 

In the brosylate acetolysis sets, conjugative stabilization of the transition state is geometrically excluded in the syn form, but is anticipated in the anti form (13). That is, on structural grounds, is expected to be the parameter of choice for the syn set and for the anti set. The available data (for the OMe, Me, Cl and NO2 substituents) do indeed conform to this expectation ... 

Cfs-butene should lead initially to the anti form trrms-butene should lead initially to the syn form and 1-butene should give rise initially to both. The equilibrium distribution of syn and anti forms usually differs greatly from the equilibrium distribution of cis- and frans-butene for cobalt complexes 59, 60) the syn form, precursor of irans-butene, is by far the most stable. By way of contrast for the corresponding carbanion, the cis anion seems by far the more stable. This preference for the cis carbanion is presumed to be the source of the high initial cis-to-trans ratio in the initial products of base catalyzed isomerization. In the base catalyzed isomerization of more complex cf-s-olefins (cfs-S-methyl-stilbene), the ions corresponding to syn and anti are not interconvertible and cis-trans isomeriza-... 

BaP is metabolically activated predominantly to the 7R,8S-dihy-drodiol-9S,10R-epoxide (anti form) and to a minor extent to 7R,8S-dihydrodiol-9R,lOS-epoxide (syn form) via 7R,8S-epoxide and 7R,8R-dihydrodiol (Figure 1). Evidence for the formation of the 7S,8S-dihydrodiol and the 7S,8R-dihydrodiol-9R,10S-epoxide from the metabolism of BaP in vivo on mouse skin has been reported (5). 

Similarly, bicyclic systems of the type shown below enjoy sigma nonbonded attractive interactions in the syn form. 

Experimental results indicate that steric effects dominate and the an ft form is more stable by 2.1 kcal/mol230. However, by following the strategy illustrated for the cyclic system discussed above, we can design systems standing a good chance of being thermodynamically more stable in the syn form. 

Paleoproterozoic Hurwitz Group siliciclastic and lesser carbonate rocks, which are currently distributed as erosional remnants of regional scale syn forms. 

Racemic mixtures of dadt ligands with substituents such as A -benzylpiperazinyl on the carbon backbone between the two nitrogen donors were reacted directly with [Tc04] in the presence of [8204], to yield mixtures of the corresponding syn and anti isomers. The syn form showed higher brain uptake and resided longer than the anti isomer, but with a phenylpiperi-dine substituent the syn and anti forms showed the reverse uptake/excretion behavior. ... 

It is suggested that there are two possible conformations for the bridging cyclopropyl groups the syn form (Vila) observed in the crystal and the anti form (Vllb) that appears from minimum energy calculations to be more stable in solution (100). These differences and some of the angles seem to result from crystal packing energies in the solid state versus the less constricted form in solution. 

The C—Br bond is anticlinal (ac) to the ring bisector in the prevailing conformer of2764 the syn form has higher energy by 30 kJ mol. The distal C—C bond in the ring is shorter by about 0.03 A than the adjacent bonds from a joint ED and ab initio analysis. The ac forms have been found to be predominant in 28 and 29 from vibrational spectra and ab initio calculations65. 

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