Isomerism pairs

The stereochemistry of pyrazolines and pyrazolidines has already been discussed (Section 4.04.1.4.3). Optically active A - and A -pyrazolines have seldom been described (77JA2740, 79CJC360), but cis-trans isomeric pairs are common. The C-4 acid-catalyzed epimerization involves the mechanism shown in Scheme 38 (70TL3099), but in spite of some inconclusive arguments the C-5 epimerization has never been established with certainty. 

Also other examples affording isomeric pairs that are easy, difficult, or impossible to separate.5 25,35,37,296,374,389,756,769.839,843,849... 

Both optical isomers of AMPH are aetive (Seheehter 1978). In general, for the few isomeric pairs that have been examined, the S isomers of AMPH-like agents are slightly more potent than the raeemates and about 3 times more potent than the R isomers (Young and Glennon 1986). Y(-i-)AMPH, for example, is 3 times more potent than R(-)AMPH (table 2) 5 (-)cathinone is 2.5 times more potent than raeemie eathiione, but (unexpectedly) is nearly 15 times more potent than R(+)cathinone. 

Mixtures of alkenes are formed when more than one type of (3-hydrogen is present. In acyclic compounds the product composition often approaches that expected on a statistical basis from the number of each type of hydrogen. The E-alkene usually predominates over the Z-alkene for a given isomeric pair. In cyclic structures, elimination is in the direction that the cyclic mechanism can operate most favorably. 

A similar reaction of (-)-ephedrine (42) was reported to give 2-chloro-l,3, 2-oxazaphospholidine 2-oxide either as a single diastereomer (25,45,5R)-73a [51-53] or as the isomeric pair 73a,b, which could be separated by chromatography over silica gel (Scheme 23) [32, 54],... 

Treatment of (-)-ephedrine (42) with a variety of aIkyl(aryl)phosphonic dichlorides 106a-d afforded the isomeric pairs of the corresponding 2-alkyl(aryl)-2-oxo-l,3,2 oxazaphospholidines (Scheme 31 and Table 1) [32, 62-65],... 

Apart from this one-reaction type, the routine use of metal template procedures for obtaining a wide range of macrocyclic systems stems from 1960 when Curtis discovered a template reaction for obtaining an isomeric pair of Ni(n) macrocyclic complexes (Curtis, 1960). Details of this reaction are discussed later in this chapter. The template synthesis of these complexes marked the beginning of renewed interest in macrocyclic ligand chemistry which continues to the present day. 

Oxidation of oxazolidine derivatives (275) gives isomeric pairs (A) and (B) of a-methoxy-substituted oxazolidine nitroxyl radicals (276a-d, f, g) and a,a-dimethoxy-substituted nitroxyl radical (276e) (Scheme 2.102) (515). 

We can also examine isomeric pairs of H-bonded complexes in which the roles of electron-pair donor and acceptor are reversed, and the mutual dipole orientations are therefore fundamentally altered. As an example, Fig. 5.4 shows the isomeric pair of complexes between ammonia and hydrogen fluoride,... 

The observed differences in the fragmentation pathways of the isomeric pairs were further enhanced under chemical ionization conditions. Thus, elimination reactions became even more pronounced in the o/t/ o-nitro-substiluted compounds than in the para isomers83. 

As points of reference, we will take two well-established hydrogen-bond donor/ acceptors, H2O and NH3. Their computed gas-phase Vs,max and Vs,mm are in Table 5, along with the same data for all of the molecules that have been discussed hydroxylamine (5), dimethylhydroxylamine (6), acetoxime (7), acetohydroxamic acid (8), and the isomeric pairs of oximes examined in the last section. Finally, we included an additional hydroxamic acid, 11, to see the effects of the strongly electron-withdrawing cyano group. 

Systematic investigations of twofold additions of malonates to C70 revealed that the second addition takes place at one of the five a-bonds of the unfunctionalized pole [17, 26], With achiral, C2v-symmerical malonate addends, three constitutionally isomeric bisadducts are formed An achiral one (C2v-symmetrical 1), and two chiral ones (C2-symmetrical 2 and 3), which are obtained as pairs of enantiomers with an inherently chiral addition pattern (Figure 13.5). Twofold addition of chiral malonates leads to the formation of five optically active isomers, two constitutionally isomeric pairs of C2-symmetrical diastereomers and a third constitutional C2-symmetrical isomer (Figure 13.5). Twofold additions of azides to C70 lead to diazabis[70]homo-fullerenes, which served as starting material for the synthesis of bis-(aza[70]-fullerenyl) (Cg9N)2 (Chapter 12) [27]. As further bisadditions, addition reaction to C70 [2+2]cycloaddition of electron-rich bis(diethylamino)ethyne and 1-alkylthio-2-(diethylamino)ethynes [28] and the addition of transition metal fragments have been reported [29-32],... 

The UV spectra of isomeric pairs of pyrazolo[3,4-d]pyrimidine-4,6-diones and pyrazolo[4,3-type ring fusion absorbs at longer wavelength (red shift) compared to its [3,4-d] isomer (82IJC(B)585). 

The compound [Mo(NO)(NH20)(NCS)2(phen)J on heating under reflux in acetophenone or on photolysis at 80 °C gives the five-coordinate, paramagnetic isomeric pair [Mo(NCS)2(NO)(phen)] (Table 4) a square pyramidal analogue [Mo(NCS)2(NO)(bipy)] was also obtained. In all compounds the NCS is N-bonded.51... 

The chemical shifts of a range of cis and trans isomeric pairs of 2-alkoxytetrahydropyrans (73) and (74) show a consistent upheld shift of the three carbon atoms attached to oxygen in the axial isomers. The effect is greatest at C-6 and smallest at the alkoxy carbon atom (81JOC4948). 

Analysis of the 13C chemical shifts of C-2 and C-6 in a series of 2-substituted 6-methoxy-3,6-dihydropyran-3-ones (82) allows the assignment of configuration to the isomeric pairs. C-2 is shielded by ca. 5 p.p.m. and C-6 by ca. 2.5 p.p.m. in the trans isomer compared with the corresponding signals of the cis compound (79MI22200). The upheld shift is associated with the 1,3-diaxial disposition of the substituent and the proton on the y-carbon atom. This y-effect has also been noted in the dihydropyran ring system (74MI22200). 

In (Z)-( )-isomeric pairs of alkenes, the olefinic carbons of the cis isomers are slightly shielded when compared with those in the trans compounds. A much stronger shielding is experienced by the carbon atoms a to the double bond in the cis isomers due to the... 

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