Open form

The equations are presented in their primitive form to keep them more universal. Consistent units must be used, as appropriate, at the time of application. The example problems will include conversion values for the units presented. The symbol g will be used for the universal gravity constant to maintain open form to the units. 

The study of structure and reactivity of tertiary heterocyclic enamines is associated with the problem of equilibrium of the cyclic enamine form (70) and the tautomeric hydration products 173,174) quaternary hydroxide (71), pseudo base (so-called carbinolamine) (72) and an opened form of amino aldehyde or amino ketone (73). 

AddRedundant Input Section epoxide form open form... 

The structures are quite similar for all of the model chemistries we are considering. The second isomer is characterized by a very long bridging carbon distance, indicating that the two carbons are no longer bonded (which is why we refer to it as the open form). 

CDCI3, DMSO-d, pyridine-ds), the equilibrium content of the ring-opened form 273 increases with increase in solvent polarity and electron-donor ability of the aryl substituent R. In the sohd, the cyclic tautomers are preferred as proven by IR spectroscopy (97ZOR418). 

For the isoxazolines 284 substituted at position 3, ring-chain tautomerism is depicted by the equilibrium 284 and 285-287 (Scheme 103). In general the cyclic tautomers 284 are strongly preferred. The ring-opened forms exist in equilibrium with 284 in rare cases [95ZOB705 96AHC(66)1, p. 21]. The equilibrium of the oxazolidinones 288 [78MI1, p. 107] is affected by the nature of the solvent. 

An equilibrium involving reversible dissociation of the N-N bond in N-nitrobenzotriazole 299a has not been directly registered, although occurrence of the ring-opened form 299b was inferred on the basis of isolation of its derivative 300 (Scheme 112) [96CHEC-II(4)1]. 

The asymmetrical anhydrobase (20) constitutes the first step of the formation of trimethine thiazolocyanine when a 2-methylthiazolium salt reacts either with a benzothiazolium or its opened form [which is bis-o-(formylmethylamino)(diphenyl disulfide] (Scheme 26). In a second step, 20 is protonated by a second molecule of 2-methylthiazolium. It results in cleavage of the benzothiazoline ring, which gives 21 together with the formation of the monomeric anhydrobase (22). Cleavage of the C-S bond of 20 can be explained by the important electronic desaturation of the C atom observed in NMR spectrum and the great polarizability of the C-S bond in this type of ring (48). 

In 1986, Schaefer et al. reported the first extensive ab initio calculations of S3 [61]. SCF and CISD calculations indicated that the global minimum is the ring D3h form. The more sophisticated CASSCF and MR-CISD calculations, on the other hand, favoured the open bent structure of 2 symmetry. The preference of the open form was subsequently confirmed by several other theoretical investigations [62-65]. The I>3h structure is calculated to be 31-44 kj mol less stable than the open C2V structure, depending on the method of calculation and the level of theory applied [56, 59-65]. The best estimate of the S-S bond length of the Czv form is 193.2 pm (CCSD(T)) [65] and the bond angle is 117°. Compared with the multireference calculations, DFT methods adequately describe the structure and PES of S3. 

Fig. 4. E1-E2 reaction cycle of the Na,K-pump with four major occluded conformations and ping-pong sequential cation translocation. The phosphoforms can occlude Na" and dephosphoforms can occlude or Rb. Na and K without brackets are cations bound to an open form such that they can exchange with medium cations [Na ] or [K ] within brackets are occluded and prevented from exchanging with medium cations. It is proposed that release of Na cxt accompanies transition from EiP[3Na] to E2P[2Na], since the capacity for occlusion of Na in the ouabain-stabilized E2P form is lower than in the EjP form prepared by incubation with CrATP [29] or oligomycin [89].

The first approach applied for [cinchonidine (CD) - a-keto ester] complex was also unsuccessful. In the open conformation CD cannot provide the required steric shielding. In open form either the quinuclidine or the quinoline moiety of CD will interact with the substrate. It has already been demonstrated that the quinuclidine moiety has a crucial role both in the rate acceleration and the induction of ED [13]. 

In earlier kinetic and computer modeling [1,2, 14] the open form of CD (CDopen) was used to illustrate the adsorbed [CD - a-keto ester] complex. In this complex the quinuclidine nitrogen was involved in the interaction with the substrate directly or via a proton bridge. 

The spiroindolinobenzopyran 2 is a classical example of spiropyran and is easily prepared by the condensation of l,3,3-trimethyl-2-methyleneindo-line (Fischer s base) and salicylaldehyde in anhydrous ethanol or benzene (Scheme 2).ia The nucleophilic attack of Fischer s base on the carbonyl group (like an enamine) gives an aldol product, which undergoes ring closure followed by dehydration. This condensation is reversible therefore, an exchange of the salicylaldehyde component of spiropyran with a different salicylaldehyde is possible. For example, when a solution of spiropyran 2 (Scheme 2) was refluxed with 3,5-dinitro-substituted salicylaldehyde, the open form of 6,8-dinitro-BIPS was obtained.2... 

The enolanil form (150) of ambuside shows some similarity to the open form of some of the cyclic thiazide diuretics (151) which have been speculated to be the active form of these molecules. c. Sulfonylureas... 

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