Acyclic analog

Onium ions of small and large heterocyclics are usually produced by electrophilic attack on a heteroatom. In three- and four-membered rings nucleophilic attack on an adjacent carbon follows immediately, in most cases, and ring opening stabilizes the molecule. In large rings the onium ion behaves as would its acyclic analog, except where aromaticity or transannular reactions come into play (each with its electronic and steric pre-conditions). A wide diversity of reactions is observed. 

Meldrum s acid, pK 7.4, is exceptionally acidic in comparison to an acyclic analog such as dimethyl malonate, pK 15.9. For comparison, 5,5-dimethyl-1,3-cyclohexane-dione is only moderately more acidic than 2,4-pentanedione (11.2 versus 13.43). The pK values are those for DMSO solution. It is also found that the enhanced acidity of Meldrum s acid derivatives decreases as the ring size is increased. Analyze factors that could contribute to the enhanced acidity of Meldrum s acid. 

Podand Open-chained (acyclic) analogs of either coronands or cryptands. 

It is interesting to note that the acyclic analog, nitroguanidine, exists in the symmetrical form 288 rather than as 289. Structure 288 has been established by ultraviolet and proton nuclear magnetic resonance spectroscopy. X-ray crystallography, dipole moments, and ipK measurements (see reference 367 and references therein). 

Wingerath, T. et al. (1999). Analysis of cyclic and acyclic analogs of retinol, retinoic acid, and retinal by laser desorption ionization-, matrix-assisted laser desorption ionization-mass spectrometry, and UV/Vis. spectroscopy. Anal. Biochem. 272(2) 232-242. 

The high in vivo stability of DOTA complexes makes it a desirable ligand framework for BFCAs relative to acyclic analogs however, complex formation with DOTA and its analogs can be slow. The slow kinetics of complex formation with DOTA-type ligands does not pose problems with nuclides such as 177Lu (t /2 6.64 d) however, improved reaction conditions may... 

The same authors [38] further refined these catalysts by developing very bulky cyclophane diimine palladium(II) catalysts (Fig. 5), which show greater activity and stability than their acyclic analogs. The incorporation levels of acrylates (methyl and tert-butyl acrylate were exemplified) are unusually high (up to around 25... 

TABLE 1. Enthalpies of formation of bis-allenes, the related bis-olefins and bis-acetylenes and their acyclic analogs... 

Cyclic Nitronates The chemistry of cyclic nitronates substantially differs from the chemistry of their acyclic analogs. Cyclic nitronates are involved predominantly in various rearrangements rather than in elimination reactions. The character and pathways of these rearrangements are determined not only by the nature of the reagent used but also by the character of the heterocycle and the nature of the substituents attached to the heterocycle. 

Given these surprising experimental facts, it is obviously important to inquire why the cyclic conjugation patterns (3.122) and (3.123) should lead to such diametrically opposite changes in conjugative stability compared with acyclic analogs. A... 

Cyclic five-membered sulfates and sultones are known to undergo hydrolysis from 105 to 107 faster than their acyclic analogs. Thus, for example, catechol sulfate [54] undergoes alkaline hydrolysis 2 x 107 faster than diphenyl sulfate (Kaiser et al., 1965), and l-naphthol-8-sulfonic acid sultone [55] hydrolyzes 5 x 105 faster than phenyl cr-toluenesulfonate (Kaiser et al., 1967). In contrast... 

Macrocyclic polyamines have been extensively studied by Lehn s group67 and the protonation constants of polyamines [24]ane-N6 (5), [32]ane-Ns (6) and [27Janc-Nfi()3 (7) have been evaluated and compared with those of the acyclic analog (8) and of the macrocycles 124 Nf,02 (9) and [18]-N6 (10)68 (Table 7). 

The thiazofurin acyclic analog 925 was prepared from 924 as shown in the scheme (87H947). 1,3-Dipolar cycloaddition of the acetylenic derivative 927 to the diazo derivative 926 gave the pyrazole 928, whose amidation and debenzylation gave 929 (93MI11). 

The enthalpies of formation of the cycloalkylmagnesium bromides that have been determined by reaction calorimetry are listed in Table 3. As with other functionalized cycloalkanes and the cycloalkanes themselves, there is no regularity to these values with respect to carbon number as there are for their acyclic analogs because of the influence of ring strain on the enthalpies. Unfortunately, there are no enthalpies of formation for the bromocycloalkanes with which to compare these values there are, however, enthalpies of formation for liquid phase cycloalkanes. Figure 3 is a plot of the enthalpies of formation for the cycloalkyl-MgBr vs. those for cycloalkyl-H. There is a linear relationship with... 

Reaction of cyclic lithiate (47) with ZnCl2 affords a nniqne l-zinca-3,6-disilacyclo-heptane (48) (equation 17) . Like its acyclic analogs 39-44 (vide supra), compound 48 is air-stable and no hydrolysis was observed after treatment with aqueous THF during a period of 5 h at 80 °C. 

The hydrolysis in acid or base of five-membered cyclic esters of phosphoric acid proceed some 10 times faster than their acyclic analogs or the six- and seven-membered cyclic phosphates 55 56. Unlike that of dimethyl phosphate, the hydrolysis of hydrogen ethylene phosphate is accompanied by rapid oxygen exchange into the unreacted substrate57, viz-... 

Table 1 Thermodynamic Data (kJmol-1) for the [18]crown-6 and Acyclic Analog 11...

Gas phase studies reveal size selective effects on binding of ammonium cations to crown ethers and acyclic analogs [2.111]. 

Other examples suggest the generality of this selectivity for cyclic diazoacetates and a limited number of acyclic analogs [124], and the composite provides, in an absolute sense, the reality of diastereotopic reaction considerations. 

FIGURE 5.10 Assessment of stability constant (log K) values of the complexes of crown ethers and their acyclic analogs with K+in methanol.25-77 Linear correlations between predicted and experimental log K values for the test set (a) of crown ethers and for the set of acyclic ligands (b) in Figure 5.6a. As QSPR models were built on the training set containing exclusively crown ethers, it is not surprising that predictions for the test set (a) are more reliable. 

An important argument against any SN2 mechanism in the process is the observation that hydride transfer from a silicon atom in a strained ring (kx 1.63 x 102 at 25°C) proceeds at approximately the same rate as the transfer from its acyclic analog (, 1.64 x 102 at 25°C) under the same conditions. 

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