The Two-step Procedure

The new N-terminal residue can then be removed as a phenylhydantoin in a second cycle of phenylcarbamoylation and cyclization. This method of stepwise degradation, while discovered several decades earlier (Bergmann et al. 1927), reached practicality only in 1950 when Edman modified the reagent and applied chromatographic procedures for identification of the cyclic products. The improved reagent, phenylisothiocyanate, smoothly converts the peptide to the phenylthiocarbamoyl derivative which is cyclized and cleaved by the action of hydrochloric acid (dissolved in an organic solvent such as dioxane)  

The phenylthiohydantoins formed in the process of Edman degradation were compared with authentic phenylthiohydantoins prepared from amino acids and thus identified. Yet, in spite of numerous further improvements, such as application of thin layer chromatography, vapor-phase chromatography and even mass spectrometry for the identification of phenylthiohydantoins, the method 

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The two-step procedure includes formation of a N-substituted phthalimide 3, and its subsequent cleavage to the primary amine 5. Phthalimide (which can be obtained from reaction of phthalic acid with ammonia) shows NH-acidity, since the negative charge of the phthalimide anion (the conjugated base) is stabilized... 

Two principle approaches from thiopyrylium salts to thiopyrans accompanied by C-substitution have been found, e.g., the one-step additions of C-nucleophiles or the two-step procedures involving primary conversions of the salts to nucleophilic intermediates followed by attacks with appropriate electrophiles. 

The combination of PCA and LDA is often applied, in particular for ill-posed data (data where the number of variables exceeds the number of objects), e.g. Ref. [46], One first extracts a certain number of principal components, deleting the higher-order ones and thereby reducing to some degree the noise and then carries out the LDA. One should however be careful not to eliminate too many PCs, since in this way information important for the discrimination might be lost. A method in which both are merged in one step and which sometimes yields better results than the two-step procedure is reflected discriminant analysis. The Fourier transform is also sometimes used [14], and this is also the case for the wavelet transform (see Chapter 40) [13,16]. In that case, the information is included in the first few Fourier coefficients or in a restricted number of wavelet coefficients. 

Data from several laboratories within the Interregional Research Project No. 4 (IR-4) in the USA have been evaluated for determining the values of MDL and MQL. These data have been presented in Table 1. The two-step procedure described in the EPA guideline was used to calculate the values of MDL and MQL. For the first step, the slope, intercept and RMSE values for the first three calibration curves of each study were separately calculated, then the IDL and IQL values calculated and the value of LQQ estimated for the method. These values were compared with the actual values of LLMV. The standard deviation of the spike recoveries at the LLMV (xllmv) was used to calculate the MDL and MQL. The values of LLMV were separately determined by the laboratory not using any of the methods described in this article. 

In preparing elastomers by the two-step procedure, the entire quantities of LHT-240 and TDI were mixed in the resin flask under high-purity nitrogen for 2 hours at 50°C. Even without catalyst, this period was sufficient to obtain complete reaction, or essentially so, because of the relatively high reactivity of the para isocyanate group in the TDI. The... 

Scheme 4. The two-step procedure for the synthesis of PAHs (HBC is given as an example). Hexaphenylbenzenes are afforded in the first step either by cobalt-catalyzed eye-lotrimerization of suitable diphenylethynylenes 22 or by...

Preparation of Glucan Synthase Fractions. Microsomal and plasma membranes were isolated by differential and density-gradient centrifugation. CHAPS-solubilized glucan synthase (CSGS) was prepared by the two-step procedure (4,9). In step 1, contaminating proteins were extracted with... 

The two-step procedure starts with the separation of the cyclotron-generated n.c.a. (no carrier added) [ F]fluoride, mediated by an anion exchanger, and... 

Morphological characteristics. Iron powders obtained by the two-steps procedure described earlier are formed of polyhedral, and even often cubic, nonagglomerated particles with a rather narrow size distribution and a mean size in the... 

The synthesis of a triptan with a chiral side chain begins by reduction of the carboxylic acid in chiral 4-nitrophenylalanine (15-1). The two-step procedure involves conversion of the acid to its ester by the acid chloride by successive reaction with thionyl chloride and then methanol. Treatment of the ester with sodium borohy-dride then afford the alanilol (15-2). Reaction of this last intermediate with phosgene closes the ring to afford the oxazolidone (15-3) the nitro group is then reduced to the aniline (15-4). The newly obtained amine is then converted to the hydrazine (15-5). Reaction of this product with the acetal from 3-chloropropionaldehyde followed by treatment of the hydrazone with acid affords the indole (15-6). The terminal halogen on the side chain is then replaced by an amine by successive displacement by means of sodium azide followed by catalytic reduction of the azide. The newly formed amine is then methylated by reductive alkylation with formaldehyde in the presence of sodium cyanoborohydride to afford zolmitriptan (15-7) [15]. 

In this case, however, the yields are lower than those obtained by the two-step procedure (21-59% isolated yields). This is likely due to the occurrence of several competing reduction steps. The Pd(0) species generated by a two-electron reduction may thus react with the alkyl iodide by oxidative addition preferentially to the aryl halide. 

Members of the she same compound class, arylethenylpurines, can also be prepared in a two step sequence. The cross-coupling of a halopurine with vinyl-tributylstannane leads to the formation of a vinylpurine, which in turn can undergo palladium catalyzed Heck reaction with a series of aryl halides (8.20.),28 The two step procedure is of particular interest, since the alternate approach, the Heck reaction of halopurines and arylethenes is of very limited scope. 

Return now to the first part of the two-step procedure for characterizing an adsorption column. Our model represented by Equations 1, 2, 3, 4, and 5 shows that the following space and point rate coefficients define the problem ... 

Exercise 16-37 An elegant modification of the two-step procedure to prepare ketones from alkenes by hydroxylation and oxidative cleavage of the diol formed uses a small amount of potassium permanganate (or osmium tetroxide, 0s04) as the catalyst and sodium periodate as the oxidizing agent ... 

The two-step procedure that was followed used a Wittig reaction to form the carbon-carbon bond, then catalytic hydrogenation of the resulting alkene. 

Figure 1. A. The synthetic route that is used for the synthesis of L-Asn-Sepharose CL 6B affinity adsorbent. B SDS-polyacrylamide gel electrophoresis of caL-ASNase purification. Lane 1 E. coli crude extract after induction with 1 mM IPTG Lane 2 EcaL-ASNase purified according to the two-step procedure using cation-exchange and affinity chromatography.

The nonlinear nature of the Hamiltonian implies a nonlinear character of the Cl equations which must be solved through an iteration procedure, usually based on the two-step procedure described above. At each step of the iteration, the solvent-induced component of the effective Hamiltonian is computed by exploiting the first-order density matrix (i.e. the expansion Cl coefficients) of the preceding step. In addition, the dependence of the solvent reaction field on the solute wavefunction requires, for a correct application of this scheme, a separate calculation involving an iteration optimized on the specific state (ground or excited) of interest. This procedure has been adopted by several authors [17] (see also the contribution by Mennucci). 

In addition to information compiled elsewhere 1-7> the preparation of N-trimethylborazine has been described in Inorganic Syntheses 20> and the standard procedures have been used to make several isotopically labeled species of borazine 21> and N-methylborazine 22.28). Recently, the direct synthesis of N-organoborazines by the interaction of nitriles with diborane or alkali metal hydroborates, rather than the two-step procedures involving the reduction of the corresponding B-chloroborazi-nes, has received much attention. Besides the extensive study by Wade et al. 24>, it has been shown 25> that borazines can be prepared in high yield according to the reaction depicted in [Eq. (4)]. 

The [2 + 2]-photocycloaddition chemistry of a,(3-unsaturated lactones has been widely explored. The factors governing regio- and simple diastereoselectivity are similar to what has been discussed in enone photochemistry (substrate class Al, Section 6.2). The HT product is the predominant product in the reaction with electron-rich alkenes [84]. A stereogenic center in the y-position of ot,P-unsaturated y-lactones (butenolides) can serve as a valuable control element to achieve facial diastereoselectivity [85, 86]. The selectivity is most pronounced if the lactone is substituted in the a- and/or P-position. The readily available chiral 2(5H)-furanones 79 and 82 have been successfully employed in natural product total syntheses (Scheme 6.30). In both cases, the intermediate photocycloaddition product with 1,2-dichloroethylene was reductively converted into a cyclobutene. In the first reaction sequence, the two-step procedure resulted diastereoselectively (d.r. = 88/12) in product 80, which was separated from the minor diastereoisomer (9%). Direct excitation (Hg lamp, quartz) in acetonitrile solution was superior to sensitized irradiation (Hg lamp, Pyrex) in acetone, the former providing the photocycloaddition products in 89% yield, the latter in only 45%. Cyclobutene 80 was further converted into the monoterpenoid pheromone (+)-lineatin (81) [87]. In the second reaction... 

Fig. 22. The two-step procedure to anchor the thread 33 onto the surface of a sol-gel film.

These acids were coupled to the appropriate olefinic alcohols and subsequent cyclization produced a small library of differentially-linked (i-D-C-disaccharides (Table 4). The yields for the two-step procedure are good and entry 1 gives a side-by-side yield comparison of both catalysts. In essence, we were able to walk around the ring and install acid functionality at any carbon atom and selectively prepare the corresponding C-disaccharide (18). 

We would like to stress the similarity between Eqs. (5) and (18). The main difference is that the poles of the linear response function are excitation energies rather than energy eigenvalues (c./. Eq. (11)) but in both cases, the residues correspond to transition moments between the ground state and excited states. The two-step procedure for evaluating the linear response function is now (c.f. Eqs. (6) and (7) ) solve... 

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