Transformation sequence

Samanta SK, AK Chakraborti, RK Jain (1999) Degradation of phenanthrene by different bacteria evidence for novel transformation sequences involving the formation of 1-naphthol. Appl Microbiol Biotechnol 53 98-107. 

REAL PART OF TRANSFORMED SEQUENCE IMAGINARY PART OF TRANSFORMED SEQUENCE... 

MacMillan s catalysts 56a and 61 allowed also the combination of the domino 1,4-hydride addition followed by intramolecular Michael addition [44]. The reaction is chemoselective, as the hydride addition takes place first on the iminium-activated enal. The enamine-product of the reaction is trapped in a rapid intramolecular reaction by the enone, as depicted in Scheme 2.54. The intramolecular trapping is efficient, as no formation of the saturated aldehyde can be observed. The best results were obtained with MacMillan s imidazolidinium salt 61 and Hantzsch ester 62 as hydride source. As was the case in the cyclization reaction, the reaction affords the thermodynamic trans product in high selectivity. This transformation sequence is particularly important in demonstrating that the same catalyst may trigger different reactions via different mechanistic pathways, in the same reaction mixture. 

Figure 1.6 Partition of the transformation sequence into groups with the kinetic "irreversibility" between them. When the reaction goes left to right, transformations Y2 Y3, Y3 Y4, Y4 Y5, Y5 —> Ye, Ye Yy, and Yy P can be considered as kinetically irreversible, whereas the kinetic reversibility must be taken into consideration for transformations R Yq, Yq Yy, and Y5 Yg. The differences of stationary chemical potentials of intermediates are greater than RT for the former groups but smaller than RT for the latter groups (the scale is shown at the top right).

In addition to the visualization of topographic transformations, sequences of in-situ images yield a measure of the local kinetics of the reaction. The etch rate of Si has been evaluated in [20] by using the expression R = (AS/S) /i/Af, with AS/S the surface area of terraces removed per cm of electrode in one sequence, h the step height (3.14 A) and At the time elapsed. The quantity (AS/S)h in fact represents the volume of material which has been removed per cm of electrode, because the dissolution occurs layer by layer. The experimental determination of AS is sketched in Fig. 22 f, in which the hatched area represents AS. In other sequences AS includes the surface of eventual pits. The bias dependence of the etch rate and the current voltage curve are shown in Fig.26 for n-Si(lll) in a 2M NaOH solution [20]. 

Even though the differences between the Al NMR spectra of the transition aluminas are subtle, the technique has been used to smdy the thermal transformation sequences of the hydrated aluminas gibbsite, Al(OH)3 (Slade et al. 1991, Meinhold et al. 1993), boehmite, 7-AIOOH (Slade e a/. 1991a, Meinhold era/. 1993, Pecharromm eta/. 1999), pseudoboehmite (Meinhold et al. 1993) and bayerite, Al(OH)3 (Meinhold et al. 1993, Pecharroman et al. 1999). 

The reason for the sluggish development of ligand synthesis in this case lies in the fact that standard transformation sequences of organic chemistry have to be modified substantially whenever phosphane groups are part of an organic precursor. Many methods to overcome these inherent difficulties have been developed and many novel tripod ligands are hence waiting for their use in coordination chemistry. 

To some extent this may be subjective, imposing a human interpretation on the observations, but it also reflects an objective reality some reactions carry a greater flux of metabohtes than others, some are active in a wider range of cell types than others, and so on. To understand the entire chart, therefore, it is useful to collect the reactions into groups of transformation sequences known as metabolic pathways. The number of steps considered to be one pathway can be very small if very few steps are needed to convert one important metabolite into another. For example, serine biosynthesis is a three-step pathway, in which the aminoacid serine is synthesized from 3-phosphoglycerate. At the other extreme, beta-oxidation, the process that converts fatty acids from the form in which they are stored in fat cells into the form in which they are metabolically active, involves seven repetitions of the same four types of step, making an unbranched pathway of nearly 30 reactions. 

Figure 4[A] diows the XRD patterns of the cobalt precursors precipitated at 35 °C followed by aging for various times. The HTL phase was initially firrmed (aging for 0.5 h) and it was converted into cobalt carbonate (1-2 h) and finally into the KBL phase (more than 3 h). This transformation sequence was exactly same as what was observed by varying the precipitation temperature thM is, fire HTL phase, cobalt carbonate and the KBL phase were successively formed ly increasing the precipitation temperature. This result suggests that at higher temperatures the transformafion proceed quickly and therefore the KBL phase was obtained.

Bromine derivatives of thiophene are the most widely used for the preparation of isomeric thienothiophenes and related systems. A classical example is illustrated by the following transformation sequence formylation of 3,4-dibromothiophene (1) through lithium derivatives, repeated metallation and treatment with elemental sulfur and methyl bromoacetate. Ring closure of the second heterocycle occurs in the present of sodium alkoxide. Decarboxylation of the resulting 4-bromothieno[2,3-Z ]thiophene-2-carboxylic acid (2a) affords 3-bromothieno[2,3-Z)]thiophene (3a) (74IZV1570). The reaction with selenium instead of sulfur produced 4-bromosele-nolo[2,3-Z ]thiophene-2-carboxylic acid (2b) and 4-bromoselenolo[2,3-Z)]thiophene (3b). 

Table 13.2. Thermal transformation sequence of proton-containing -alumina type compounds... 

Stork and Ponaras have developed a method, involving the intermediacy of P-hydroxyhydrazones, for the introduction of alkyl and aryl groups on the a-carbon of an aP-unsaturated ketone, based on principles discussed previously. The method is particularly useful in situations where the thermodynamic enolate ion (y-carbon) cannot, or must not for the sake of other sensitive groups, be formed. Acid-catalysed dehydration of substituted cyclohex-3-ene-l,2-diols, readily formed from dienone precursors, yielded mixtures of cyclohex-2-enones and substituted benzenes. The product ratio was strongly dependent on reaction conditions and on the substituent group at C-1, but independent of the initial diol configuration, thus indicating a common cationic intermediate. Conditions were found which allow the synthetic use of the transformation sequence shown (Scheme 2) in up to 90 % yield. 

---