Optimization of the Reaction Conditions

Entry Equiv aldehyde Conditions Conv. (%) I-16a I-17a I-18 ratio  

In order to optimize the reaction conditions, a series of complexes were screened (Table 2.2). The best ratios were obtained with cationic gold(I) complexes (Table 2.2, entries 1-3). [AuCl(PPh3)]/AgSbF6 gave a nearly 1 1 mixture of dihydropyran I-16a and metathesis-type product I-17a (Table 2.2, entry 1). However, a similar ratios of I-16a/I-17a = 2.3 1 were observed with phosphine gold(I) 16 and phosphite gold(I) 20 (Table 2.2, entries 2-3). No difference between the results with the cationic phosphite gold(I) 20 and the in situ form 

19/AgSbFg were found (Table 2.2, entry 3). In the examples with gold(I) carbene complexes 14, 15, and 16 quantitative conversion was observed but with low 

In the reaction catalyzed by AuCl and platinacycle 24 [Ref. 29 in Chap. 1] the rearranged product 1-18 was observed as the major product (Table 2.2, entries 7 and 10), whereas very low conversions ( 7 %) were obtained when using PtCli, PtCl4, PdCl2, AgSbFg, InCls, or GaCls as catalysts (Table 2.2, entries 8,9,11-14). 

The choice of solvent influenced both the activity and the selectivity of the catalytic system. Consequently, the reactions with the best cationic complexes phosphine gold(I) 6, phosphite gold(I) 20, and IPr gold(I) 16 were tested in different solvents (Table 2.3). In CH2CI2, phosphine complex 6 gave a similar ratio to phosphite complex 20 (Table 2.3, entries 1 and 6), but in Et20 the major product was the hydroxycychzation product 1-19, due to traces of residual water (Table 2.3, entry 2). DMF completely inhibited the reaction of all the complexes (Table 2.3, entries 4, 9, and 13). 

Aerobic Palladium-Catalyzed Oxidation of Cyclohexene to 1,4-Dioxospiro-[4,5]-decane 11.2.1.1 Optimization of the Reaction Conditions 

It appeared that overoxidation decreased with ketal formation selectivity. Thus, whereas the reaction product obtained in the case of ethylene glycol as 

Apparently, the reoxidation of Pd(0) by CUCI2 alone is not sufficient enough to prevent Pd black formation. Reoxidation of Pd(0) by FeClj is more efficient, thus increasing the reaction rate. Furthermore, when the CuCl2/FeCl3 combination is 

1 mmol of both, 20 mmol cyclohexene in 10 ml ethylene glycol at40 C and 0.1 MPa oxygen 

---

Camphor derived acetates 1 (Helmchen-type65) and 3 (Oppolzer-lype66) add to aldehydes in a stereoselective manner when the Mukaiyama variation via the corresponding silylketene acetals is undertaken. Careful optimization of the reaction conditions provides predominantly... 

The Heck coupling reaction appeared to be a route of choice to achieve the synthesis of the modified-DIOP ligands. We previously studied the palladium-catalyzed coupling of acrolein and acrolein acetals with several polyaromatic and heteroaromatic bromides either in the presence of homogeneous or heterogeneous catalytic systems (6, 7). After optimization of the reaction conditions, high conversions and selectivities were achieved except with anthracenyl derivatives (8). Based on these results, we developed the synthesis of the desired ligands. The... 

Prechromatographic derivatization reactions re usually favored when it is desired to modify the properties of the sample to enhance stability during measurement (i.e., minimize oxidative and catalytic degradation, etc.), to improve the extraction efficiency of the substance during sample cleanup, to improve the chromatographic resolution, or to simplify the optimization of the reaction conditions [698-702]. As both pre- and postchromatographic methods enhance the sensitivity and selectivity of the detection process a choice between the two methods will usually depend on the chemistry involved, ease of optimization, and which method best overcomes matrix and reagent interferences. 

Formic acid is stable under the reaction conditions. Optimization of the reaction conditions led to a system that afforded 30 turnovers of 1 after 5 h (Table 2). Based on both IR and UV-vis, the Ce-POM retains its structure under the turnover conditions. The formic acid product inhibits the reaction, so procedures to remove it during reaction are needed to increase CH20 conversions. 

Implementation of the C5, C5 -hydroxy lation protocol as described in Scheme 7.19 above (71/72 to 73/74) provided further efficiencies. The C5,C5 -chlorination proceeded uneventfully, but the chloro to alkoxy interchange was difficult and required optimization of the reaction conditions. The catalyst system derived from Pd2dba3 and the X-phos(t-Bu) ligand proved to be effective in the coupling with KOH to provide the desired bisphenol. The resulting product was highly unstable and decomposed under a one-pot alkylation protocol. Isolation of the bisphenol under carefully controlled conditions followed by immediate benzylation (BnBr, NaH, DMF) furnished key intermediate 79 in 70 % yield. 

A flow injection optical fibre biosensor for choline was also developed55. Choline oxidase (ChOX) was immobilized by physical entrapment in a photo-cross-linkable poly(vinyl alcohol) polymer (PVA-SbQ) after adsorption on weak anion-exchanger beads (DEAE-Sepharose). In this way, the sensing layer was directly created at the surface of the working glassy carbon electrode. The optimization of the reaction conditions and of the physicochemical parameters influencing the FIA biosensor response allows the measurement of choline concentration with a detection limit of 10 pmol. The DEAE-based system also exhibited a good operational stability since 160 repeated measurements of 3 nmol of choline could be performed with a variation coefficient of 4.5%. 

Optimization of the reaction conditions was undertaken in order to find the best Sn2 /Sn2 ratio and the best substrate conversion. Initial formation of a lithium carbamate salt of 1 on treatment with MeLi, followed by treatment with a stoichiometric amount of MeGu in Et20 at 0 °G, produced clean Sn2 selectivity and isolation of the desired alkene in 75% yield. A variety of chiral carbamates 1 were investigated, the substrate with R = 1-naphthyl and X = OMe being chosen as the candidate for further studies. It is noteworthy that substrates in which X = H gave... 

Lipase-catalyzed transesterification of (3-nitroalcohol substrates had not previously been reported and required careful optimization of the reaction conditions. A series of enzymes were screened, followed by acyl donors. From these results, the lipase Pseudomonas cepacia (PS-C I) (for more... 

Ugi reaction of acid 88 with isonitrile 85, isobutyraldehyde and isopropylamine furnished dipeptide 89 in 67% yield. Similar Ugi reactions with other components afforded linear cyclization precursors in yields up to 98%. The final macrocyclization was not straightforward (no similar reactions were described in literature), but after optimization of the reaction conditions (varying base, solvent, concentration and reaction time) cyclopeptide alkaloid analogue 90 was obtained in 96% yield after treatment with K2CO3 and catalytic 18-crown-6 in acetone. 

The substituents on the alkyne and the cyano components can be widely varied so that we have been able to develop the basic reaction of Eq.(l) into a general synthetic method for preparing pyridines (see Section III). We have concentrated on the development of highly reactive organoco-balt(I) complexes and have tested a number of catalyst complexes under standard conditions. Both the influence of the controlling ligand Y and the neutral ligand L on the catalytic turnover number (TON) have been determined. The final step involves the optimization of the reaction conditions. 

Among the various polar solvents tested, acetonitrile, dichloromethane, and THF afforded good yields of the expected product, although with moderate diastereo-selectivity (Table 16, entries 1-3). The most suitable solvent was found to be diethyl ether. 5-[Hydroxy(phenyl)methyl furan-2(5//)-one 28a was obtained with the best yield and diastereoselectivity (Table 16, entry 4). With further optimization of the reaction conditions, we found that a lower catalyst loading (0.5 mol%) did not allow the reaction to proceed (Table 16, compare entries 5 and 4), although a higher catalyst loading (5 mol%) afforded the product with close diastereoselectivity but decreased yield (Table 16, compare entries 6 and 4). 

More recently, optimization of the reaction conditions ([Pd] = 0.001%, Pd/Imes. HCl = 1/10, NaOCHaCHaONa = 0.5%, THF, 80°C) allows the formation of monoether in good chemoselectivity (60-80%) and the TON reaches 250,000 [10, 13]. This optimized system was evaluated with different diols and a high selectivity towards the monoether could be achieved with frans-l,2-cyclohexanediol. 

Imbalance in the stoichiometry of polycondensation reactions of AA-BB-type monomers can be overcome by changing to heterofunctional AB-type monomers. Indeed, IIMU has been subjected to bulk polycondensation using lipases as catalyst in the presence of 4 A molecular sieves. At 70 °C, CALB showed 84% monomer conversion and a low molecular weight polymer (Mn 1.1 kDa, PDI 1.9). No significant polymerization was observed with other lipases (except R cepacia lipase, 47% conversion, oligomers only) and in reference reactions with thermally deactivated CALB or in the absence of enzyme. Further optimization of the reaction conditions (60wt% CALB, II0°C, 3 days, 4 A molecular sieves) gave a polymer with Mn of 14.8 kDa (PDI 2.3) in 86% yield after precipitation [42]. 

Gouverneur and coworkers showed that a,b-unsaturated esters can be cy-clized in good to excellent yields (Eq. 31) [156]. Optimization of the reaction conditions revealed that four equivalents of BQ were necessary to achieve good yields (73%). Aerobic conditions, without a copper cocatalyst, proved to be superior, resulting in 89% yield over extended reaction times. No cycli-zation product was observed with the Pd/pyridine catalyst system described above. 

Over recent years, the electrochlorination of a wide range of substrates has been reported. The direct halogenation of benzene has been studied at Pt and Rh electrodes and the in situ spectro-electrochemical monitoring of the process with confocal microprobe Raman methods allowed optimization of the reaction conditions [80]. Toluene has been chlorinated at DSAs and the kinetics of this process have been explored in detail [81]. The electrochlorination of 1,4-dimethoxy-2-tert-butylbenzene has been reported in CCI4 and in acetonitrile environments [82]. A difference in mechanism has been proposed to explain the observation of l,4-dimethoxy-2-tert-butyl-5-chlorobenzene and l,4-dimethoxy-2-tert-butyl-6-chlorobenzene, respectively, as the main products. Succinimide is electrochlo-rinated to give A -chlorosuccinimide at platinum electrodes, but the process has been reported to be relatively inefficient due to side reactions (when compared... 

During the optimization of the reaction conditions it was noticed that at higher temperatures the imination proceeded even in the absence of the metal. Subse-... 

As described above, the resolution of many types of secondary alcohols by hydrolase-catalyzed acylation in an organic solvent is usually possible after screening for a selective lipase and optimization of the reaction conditions. 

Because of industrial interest in these derivatives, several papers have appeared in the scientific literature, mainly focused on the optimization of the reaction conditions for the esterification of rutin (22) and naringin (23) [76]. When the best operating conditions were established, the conversion yield of the substrate could exceed 95%. Several pilot plants are at an advanced stage, and it seems that these acylated flavonoids will soon be marketed [21a]. 

---