Precursor Preparation

Scheme 7.32 Microwave-mediated thioalkylation to prepare precursors for macrocyclic peptide synthesis.

One of these methods involves intramolecular cyclization of specially prepared precursors A or B (Eq. 1). These precursors can be synthesized from readily available AN by classical reactions, many of which will be considered in detail below. 

Trying to prepare precursors for the synthesis of 3-substituted [l,2,4]triazolo[5,l- ]benzothiazoles, 2-hydrazino-4-methylbenzothiazole 393 was submitted to reaction with formic acid, urea, carbon disulfide, and acetic anhydride to give compounds 230, 238, 89, and 394 (Scheme 45) <1998IJC(B)921>. 

B. Francois, S. Izzillo, and P. Iratcabal, Substituted PPV block copolymer from anionically prepared precursor, Synth. Met., 102 1211-1212, 1999. 

The olefm ring-closing metathesis (RCM) has been developed into an important synthetic method which is frequently employed in many synthetic endeavors. In many applications the popular Grubbs compound 204 or advanced complexes such as 205 and 206 were used as catalysts [117]. Rutjes and co-workers [118] and Okuro and Alper [119] elegantly used oxygen-substituted allenes to prepare precursors for the... 

Vanadium phosphate catalysts were prepared by heating V2O4, phosphorus acid, either H3PO4 or H4P2O7, and water together in an autoclave at 145°C for 72 hours. Afterwards, the solid produced was recovered, washed with distilled water and dried in air at 120°C for 16 hours. Detailed preparation procedure is described in [79]. Such prepared precursors were activated in n-butane/air at 400°C to form the final catalysts. TEM and EELS are used to study the catalysts in Philips CM200 PEG microscope. 

For mixed metal oxides obtained from their hydroxide or carbonate precursors after calcination, it is generally difficult to determine whether the as-prepared precursor is a single-phase or multiphase solid solution [35]. Non-aqueous solvents appear superior for achieving two dissimilar metal oxides such as MM Oz or MM 04 precipitates such reactions cannot be carried out simultaneously in aqueous solution due to the large variations in pH necessary to induce precipitations [41,42]. Table 6.1 summarizes some of the nanoparticulate semiconducting metal oxides and mixed metal oxides prepared via co-precipitation techniques. The general procedure of achieving metal loaded nanoparticles on an oxide support is shown in Fig.6.5. 

F. Fuchtner, P. Angelberger, H. Kvaternik, F. Hammerschmidt, B. Peric-Simovc, J. Steinbach, Aspects of 6-[ F]fluoro-L-DOPA preparation Precursor synthesis, preparative HPLC purification and determination of radiochemical purity, Nucl. Med. Biol. 29 (2002) 477-481. 

The procedure demonstrates a safe and simple method for the generation of benzyne from a stable and easily prepared precursor and its use in the synthesis of a hitherto difficultly accessible hydrocarbon. 

Although the first preparation of pentacene reported by Clar in 1929 proceeded by the dehydrogenation of a difficult to prepare precursor molecule (9, Scheme 3.1) [23], most modern syntheses of pentacene and its end-functionalized derivatives proceed via the reduction of pentacenequinones, typically by the action of aluminum amalgam in cyclohexanol (Scheme 3.2) [24]. 

The spin-echo phosphorus NMR spectra indicate that there are differences between the three materials that depend on their preparation routes (Figure 25). The sharp peak at 0 ppm characterizing the VPA-prepared and VPO-prepared precursors is ascribed to the presence of VOPO4 phases, whereas, the broad peak at ca. 2600 ppm in the spectrum characterizing the VPD-prepared precursor is attributed to crystallized ( 0 2 207, along with a minor amount of VOPO4 phases. In the case of the precursors made by VPO and VPD, shoulders were observed in the spectra that are indicative of a disordered structure. 

BaTiOj precursor can be formed by sol-gel method under appropriate condition, and nanometer BaTiOj is prepared by further calcining the precursor at 973K. Solvent, temperature, pH value and water volume for hydrolysis have great effect on the quality of BaTiOj powders in the process of preparing precursor. Nanometer BaTiOj is used as support to prepare Ni based catalyst, and the catalyst has high activity in the reaction of CO2 reforming CH4 to syngas. 

Besides the assembly around the matrix support, the other components of a matrix-isolation instrument also require some comment. Great emphasis is placed on the appropriate design of the gas handling system and on the devices for sample preparation. Precursor substances or matrix samples with a vapor pressure of more than 1 mbar at... 

EPR signals increases gradually. In the preparation of (VO)2P207 obtained from an aqueously prepared precursor, it is clear that both moment quotients and coupling... 

Enzyme lacking these modifications is catalytically inactive, and the oxygenation occurs spontaneously when the anaerobically prepared precursor protein is exposed to O2. The... 

The products of co-precipitation reactions are usually amorphous at or near room temperature. It is difficult to determine experimentally whether the as-prepared precursor is a single-phase solid solution or a multi-phase, nearly homogeneous mixture of the constituent metal hydroxides, carbonates and oxides that react to form a single phase mixed metal oxide when heated. 

The Cp(/ ,f )Ti[All] and the Cp(S,S)Ti[All] reagents have been used to prepare precursors of 1,2-diols from a-oxygenated aldehy-... 

Additions of the transient y-OMOM crotyl indium chloride reagents to a-oxyge-nated aldehydes are strongly reagent-controlled. Thus the (R) and (S) reagents add to protected threose and erythrose aldehydes with high diastereoselectivity (Eq. 59) [75]. These additions are complementary to those previously effected with the y-BusSn counterparts (Eq. 45). It is thus possible to prepare precursors to the eight diastereomeric hexoses and their enantiomers from threose- and erythrose-derived aldehydes and their enantiomers plus the a-OMOM crotyl tributylstannane enantiomers. 

Stereoselective syntheses of polyhydroxycyclohexyl (a,a-difluoromethyl)-phosphonic acid (DFMPA)- esters - (229 a-b), which are stable analogues of inositol phosphates, involved Diels-Alder and conjugated addition reactions to prepare precursors. ... 

In a similar vein, De Lucchi and co-workers [24] prepared precursors to anti-inflammatory atrolactic acid derivatives. They used l,r-binaphthalene-2,2 -diol as an auxiliary for hydroxy aldehydes [Eq. (9)]. The symmetry of the diol ensures that only one isomer can be formed. Reaction of the diol with dibromoacelophenone produced the ketone (65 % yield). Grignard addition and hydrolysis gave only the R-hydroxyaldehyde. 

Acylated or sulfonylated a-amino ketones such as (106) are smoothly converted via N-nitrosation to the corresponding a-diazo ketones (107 equation 44). A limitation to this a q >roach has been the difficulty of preparing precursors such as (106), especially given the instability of the precursor a-amino ketones. A recent report that an azide is smoothly converted to the corresponding cartoxamide on exposure to triethylphosphine and a carboxylic acid might nicely sidestep this difficulty. 

Cyclopropanation with zinc-copper couple and diiodomethane is still the method of choice for preparing precursor cyclopropyl compounds for further studies, as in the case of mercury(II)-mediated cyclization of (hydroperoxyalkyl)cyclopropanes." ... 

Stevens, J.C. Timmers, F.J. Rosen, G.W. Lai, S.Y. Constrained Geometry Addition Polymerization Catalysts, Processes for Their Preparation, Precursors Therefore, Methods of Use, and Novel Polymers Formed Therewith. European Patent Application EP 0416815 A2, Mar 13, 1991. 

Two kinds of catalyst precursors were mainly employed in this work. One was the mixed oxide of Cu60sLn(N03), prepared by pyrolysis of the mixture of corresponding nitrates under O2 flow at 673 K, referred to in previous reports [7,11,12], The other was the homogeneous mixture of the corresponding oxides, which was prepared by calcination of the hydroxide coprecipitate, obtained by the hydrolysis of the mixed nitrate solution added with NaaCOs solution (0.05 mol/1) and washing the obtained coprecipitates with distilled water several times until the pH of the supernatant solution was dropped to 7, at prescribed temperature. The copper-oxide catalyst systems were obtained by the reduction of the prepared precursors under H2 (300 Torr 1 Torr=l33.32 Pa) at prescribed temperatures before the reaction. 

They demonstrated the effectiveness of the alkylation of enolates (160) + (161) - (162) for preparing precursors of penam and cepham derivatives. The penam synthesis by Schutz and Ugi, as well as the cepham synthesis (168) - U73) profited from Just et at. immensely. ... 

Figure 1. Thermogravimetric analysis and differential scanning calorimetric of the y-AljOj-supported 3Ni-19W oxidic precursors. (A) UMxD-prepared precursor, (B) UMxC-prepared sample, (C) ChM-prepared catalyst precursor.

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