Pyrazolidine, structure

Chemical Name 1,2-Diphenyl(3,5-dioxo-4-(3 -methyl-2 -butenyl)-pyrazolidine Common Name Phenylprenazone, prenazone Structural Formula ... 

Voigt-Martin et al. [13] have used MICE to solve the stmcture of 4-(4 -(N,N-dimethyl)aminobenzylidene)-pyrazolidine-3,5-dione at 1.4A in projection using 42 reflections. The potential maps do not resolve atoms with these data and models have to be fitted to the map density in a way reminiscent of macromolecular crystallography. This can pose problems in structure validation which were overcome in this case by simulation calculations. There is an excellent agreement between the solution and independent model building and high resolution electron microscopy studies. 

Replacement of carbon atoms in the cyclic aliphatic side chain of proline with nitrogen leads to the azaprolines where incorporation of an additional imino group may affect molecular recognition processes via electrostatic interactions. While the proline analogues imidazoli-dine-2-carboxylic acid (3-azaproline, 15) and pyrazolidine-3-carboxylic acid (5-azaproline, 16) allow for additional side-chain modifications in peptidomimetic structures, with pyr-azolidine-2-carboxylic acid (2-azaproline, 17) (Table 2), where the a-CH is replaced by nitrogen, the peptide backbone is modified with consequently strong conformational effects. 

The reactions of the structurally related M-allyl acethydrazides 185 gave similar results [100]. As indicated in Scheme 28, at - 30 °C the cyclization afforded the six-membered 1,3,4-oxadiazines 186 and at 20 °C gave the five-membered M-acetyl pyrazolidines 187. 

Pyrazolinones and pyrazolidinones are oxo derivatives of pyrazo-lines and pyrazolidines, respectively, and are so named in Chemical Abstracts at present. However, the usual method of naming in the earlier literature is the pyrazolone-pyrazolidone system. Although a large number of tautomeric structures are possible for pyrazolinones, the usual assignment of ring structures is as shown in (I), (II) and (III). The Chemical Abstracts names for these are (I) 2-pyrazolin-5-one, (II)... 

In this review an aromatic compound is any five- or six-membered cyclic structure that is formally aromatic or that one of its tautomers or resonance forms is aromatic (76M11), that is, not only p nidine but also pyridones, and not only thiazole but also thiazolinethiones. We will include systems, like phthalimide, that although nonaromatic have all its ring atoms in a plane or close to it, that is, the critical condition is the absence of sp atoms in the ring, thus 3,5-dihydrox5 yrazoles will be considered but their tautomers pyrazolidine-3,5-diones will not. 

To overcome this drawback the core structure of the molecules was modified. We switched to other cyclic ketoenol subclasses, e.g., tetronic acids, pyrazolidin-3,5-diones, 4-hydroxy-pyrones and 6-hydroxy-thiazinones. 

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