Isomer isomerisation

In the olivanic acid series of carbapenems the ( )-acetamidoethenyl grouping can be isomerised to the (Z)-isomer (19) (22) and reaction with hypobromous acid provides a bromohydrin that fragments to give a thiol of type (20) when R = H, SO H, or COCH. The thiol is not isolated but can react to provide new alkyl or alkenyl C-2 substituents (28). In the case of the nonsulfated olivanic acids, inversion of the stereochemistry at the 8(3)-hydroxyl group by way of a Mitsunobu reaction affords an entry to the 8(R)-thienamycin series (29). An alternative method for introducing new sulfur substituents makes use of a displacement reaction of a carbapenem (3)-oxide with a thiol (30). Microbial deacylation of the acylamino group in PS-5 (5) has... 

The synthesis of confound 16 was reported. Valence isomerisation of 16 to the isoannulenofuran 17 could be achieved either photochemically or thermally with 16 as the thermodynamically more stable isomer <96JOC935>. 

The reaction of 2-bromo-5-nitrothiazole with weakly basic secondary aliphatic amines gave the expected 2-amino products. The isomeric 5-bromo-2-nitrothiazole with such amines gave mixtures of the expected 5-amino products along with 2-aminated 5-nitrothiazole rearrangement products. A mechanism was proposed which involves the slow thermal isomerisation of the 5-bromo-2-nitrothiazole to the much more reactive 2-bromo isomer which competes, in the case of relatively weak amine nucleophiles, with direct but slow displacement of the 5-bromo group to form the normal displacement product <96JHC1191>. 

The JV-phosphino-l-azadiene 66 undergoes cycloaddition with DMAD in ether at -20 °C to form the bridged structure 67 <96AG(E)896>. The new compound is thermally unstable and isomerises to the alternative bridged structure 68 at 25 °C. X-ray analysis data have been interpreted to suggest that the isomer 69 co-exists in the solid state. The same ring system is also formed by protonation at the sp carbon atom neighbouring phosphorus in 67 (Scheme 15). 

With a view to determining the equilibrium constant for the isomerisation, the rates of reduction of an equilibrium mixture of cis- and rra/i5-Co(NH3)4(OH2)N3 with Fe have been measured by Haim S . At Fe concentrations above 1.5 X 10 M the reaction with Fe is too rapid for equilibrium to be established between cis and trans isomers, and two rates are observed. For Fe concentrations below 1 X lO M, however, equilibrium between cis and trans forms is maintained and only one rate is observed. Detailed analysis of the rate data yields the individual rate coefficients for the reduction of the trans and cis isomers by Fe (24 l.mole sec and 0.355 l.mole .sec ) as well as the rate coefficient and equilibrium constant for the cw to trans isomerisation (1.42 x 10 sec and 0.22, respectively). All these results apply at perchlorate concentrations of 0.50 M and at 25 °C. Rate coefficients for the reduction of various azidoammine-cobalt(lll) complexes are collected in Table 12. Haim discusses the implications of these results on the basis that all these systems make use of azide bridges. The effect of substitution in Co(III) by a non-bridging ligand is remarkable in terms of reactivity towards Fe . The order of reactivity, trans-Co(NH3)4(OH2)N3 + > rra/is-Co(NH3)4(N3)2" > Co(NH3)sN3 +, is at va-... 

Isomerisation of 15-c/5-phytoene to the all-/ra x configuration must occur during the desaturation steps, since most desaturated carotenes are in the all-trans form. The CRTI type desaturases appear to be able to carry out this isomerisation themselves (Fraser et al, 1992 Bartley etal, 1999), but mutants of PDS/ZDS-type organisms accumulate cis isomers of unsaturated carotenes, suggesting the presence of a separate isomerase (Clough and Pattenden, 1983 Ernst and Sandmann, 1988). Three recent publications have reported the cloning of a carotene isomerase (CrtlSO) from tomato (Isaacson et al, 2002), Arabidopsis (Park et al, 2002) and Synechocystis 6803 (Breitenbach... 

This results in the preferential formation of non-terminal alkenes, in reactions which could lead to either these or their terminal isomers on introduction of the double bond (p. 256), and to the fairly ready isomerisation of the less to the more stable compound, e.g. (31) — (30). 

This is a situation that must rise where the alternative products are mutually interconvertible under the conditions of the reaction, either by direct isomerisation or by reversal of the reaction to form the starting material which then undergoes new attack to yield a more thermodynamically stable isomer. It is important to emphasise that the relative proportions of alternative products formed will be defined by their relative thermodynamic stabilities under the conditions of the reaction, which may possibly differ from those of the isolated molecules. Thus if m-dimethylbenzene is heated at 82° with HF and a catalytic amount of BF3 the proportions of the three isomeric dimethylbenzenes in the product resemble very closely those calculated thermodynamically ... 

Once the FBA has been identified, ultraviolet absorption spectroscopy affords a rapid and accurate method of quantitative analysis. Care must be taken when interpreting the spectra of stilbene-type compounds, since turns to cis isomerisation is promoted by ultraviolet radiation. Usually, however, a control spectrum of the turns isomer can be obtained before the compound undergoes any analytically significant isomerisation. FBAs are often marketed on the basis of strength comparisons determined by ultraviolet spectroscopy. 

The diamines and their mono- and di-protonated ions can exist in various conformations, in which the nitrogen lone pairs and the protons on nitrogen are directed in (i or i+) or directed out (o or o+) from the molecular cavity. Diprotonated l,10-diazabicyclo[8.8.8]hexacosane, for example, may exist in either of the three forms in equation (79). When the out-out isomer of l,10-diazabicyclo[8.8.8]hexacosane bis hydrochloride (o+ o+) is dissolved in aqueous acidic solution, isomerisation to the in-in isomer (i+ i+) occurs so... 

This stereospecific oxidation does not occur for all dioximes, probably due to isomerisation of the dioxime during the reaction or to different reaction mechanisms involved in the use of different oxidants. When the lipophilic-hydrophilic balance of the two furoxan isomers is appropriate, they are easily separated by chromatography or fractional crystallisation. For example, the synthesis of 4-hydroxymethyl-3-furoxancarboxamide (CAS 1609), one of the most promising furoxancarboxamide vasodilators (see later), passes through the intermediate formation of a mixture of the two isomeric methyl hydroxymethylfuroxancarboxylic esters, which can easily be separated by recrystallisation from isopropyl acetate [18]. 

In the bilirubin molecule there are two isomerisable double bonds, designated as C4=C5 and C15=C16, which normally exist as the cis,cis isomer (Figure 8.4). 

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