Group preference

The large sulfur atom is a preferred reaction site in synthetic intermediates to introduce chirality into a carbon compound. Thermal equilibrations of chiral sulfoxides are slow, and parbanions with lithium or sodium as counterions on a chiral carbon atom adjacent to a sulfoxide group maintain their chirality. The benzylic proton of chiral sulfoxides is removed stereoselectively by strong bases. The largest groups prefer the anti conformation, e.g. phenyl and oxygen in the first example, phenyl and rert-butyl in the second. Deprotonation occurs at the methylene group on the least hindered site adjacent to the unshared electron pair of the sulfur atom (R.R. Fraser, 1972 F. Montanari, 1975). 

Pyrazole Equivalent Electron-withdrawing group prefers 3-position... 

For these reasons many research groups prefer to dry the chromatograms in a vacuum desiccator with protection from light. Depending on the mobile phase employed phosphorus pentoxide, potassium hydroxide pellets or sulfuric acid can be placed on the base of the desiccator, to absorb traces of water, acid or base present in the mobile phase. 

Pyrazole Equivalent 1. 7T-Acceptor group prefers 3-position 2. Acceptor group (NH3", CF3) prefers 5-... 

Indazole IH > 2H in the gas phase. position. 3. Alkyl group prefers 5-position. 4. Preferential annelation of saturated rings is at 3,4-positions. Substituents do not affect predominance of... 

Imidazole solution, and solid state Equivalent the IH tautomer Electron-withdrawing group prefers 4-... 

Triazole IH > 2H solution IH > 2H solid IH >4H Electron-releasing group prefers 5-position... 

Tetrazole 2H > IH gas phase Electron-withdrawing group prefers 3-position Electron-releasing and aryl substituents... 

The preference for the axial position in unhindered thiane-1-oxides has been known for some time. The spectra of the cis and trans isomers of the 2-, 3- and 4-methyl thiane-1-oxides, 169-171, were also measured. It was concluded from the 13C chemical shifts that the methyl groups preferred the equatorial positions. A comparison of the 170 chemical shifts obtained for sulfoxides 169-174 with those obtained for the cis and tram sulfoxide isomers of trans- 1-thiadecalin, 175 and 176, was consistent with this proposal. Sulfoxide 175 with the S=0 axial gave a shift about 17 ppm upfield from that of its equatorial isomer 176. For sulfoxides 169-174, the conformers proposed to have the S=0 axial gave shifts that were upfield from those of the supposed equatorial conformers. For tram-3, (rans-5-dimethylthiane-1 -oxide (177) with the oxygen axial, the 170 signal was 21 ppm upfield from the signal observed for the equatorial oxygen in cis-3, cis-5-dimethylthiane-l-oxide (178). 

Note 2. To maintain integrity of carbohydrate names, it is sometimes helpful to overstep the strict order of principal group preference specified in general organic nomenclature [13,14]. For example, a carboxymethyl-substituted sugar can be named as such, rather than as an acetic acid derivative (see 2-Carb-31.2). 

An interesting anomaly is a//-rra j-l,2,3,4,5,6-hexaisopropylcyclohexane, in which the six isopropyl groups prefer the axial position, although the six ethyl groups of the corresponding hexaethyl compound prefer the equatorial position. The alkyl groups of these compounds can of course only be all axial or all equatorial, and it is likely that the molecule prefers the all-axial conformation because of unavoidable strain in the other conformation. 

The focus of this chapter is the development of a technique often called wholecell matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometry (MS) or whole-cell MALDI-TOF MS. Some groups prefer to use terms such as intact or unprocessed rather than whole, but the intended meaning is the same regardless of which word is used. As noted in the first chapter of this book, there are many different methods for the analysis of bacteria. However, for the analysis of intact or unprocessed bacteria, whole-cell MALDI-TOF MS is the most commonly used approach. This method is very rapid. MALDI-TOF MS analysis of whole cells takes only minutes because the samples can be analyzed directly after collection from a bacterial culture suspension. Direct MALDI MS analysis of fungi or viruses is similar in approach1,2 but is not covered in this chapter. MALDI-TOF MS of whole cells was developed with very rapid identification or differentiation of bacteria in mind. The name (whole cell) should not be taken to imply that the cells are literally intact or whole. Rather, it should be taken to mean that the cells that have not been treated or processed in any way specifically for the removal or isolation of any cellular components from any others. In whole-cell analysis the cells have been manipulated only as necessary to... 

Some research groups prefer IgAs because they are more stable than IgGs and more resistant to proteolysis. Plants can assemble secretory IgAs, which consist of... 

Second, insertions are very selective, and the nature of the bond between organic groups and nickel determines the type of molecule which can be inserted. For example, allylic groups prefer to react with acetylene rather than with carbon monoxide (example 33, Table VII) but the opposite is true for benzyl or aryl groups. 7r-Lactonyl groups do not react either with carbon monoxide or acetylene, but they do react with ketones or aldehydes (example 39, Table VII). In this way sequential reactions take place on nickel with high selectivity. 

The most conventional investigations on the adsorption of both modifier and substrate looked for the effect of pH on the amount of adsorbed tartrate and MAA [200], The combined use of different techniques such as IR, UV, x-ray photoelectron spectroscopy (XPS), electron microscopy (EM), and electron diffraction allowed an in-depth study of adsorbed tartrate in the case of Ni catalysts [101], Using these techniques, the general consensus was that under optimized conditions a corrosive modification of the nickel surface occurs and that the tartrate molecule is chemically bonded to Ni via the two carbonyl groups. There were two suggestions as to the exact nature of the modified catalyst Sachtler [195] proposed adsorbed nickel tartrate as chiral active site, whereas Japanese [101] and Russian [201] groups preferred a direct adsorption of the tartrate on modified sites of the Ni surface. 

The amphipathic compounds shown in Scheme 2 can form a disc-like micelle(7). The shape of a molecular aggregate depends on the shape of the constituent molecules(8). For instance, conical molecules with large polar head groups prefer to form spherical micelles while cylindrical molecules tend to give flat aggregates. Trans-azobenzene is a rod-like molecule whereas the cis-form is bent. 

Fig. 7.5. General formula summarizing the structural and stereochemical requirements, which an ester should meet to fit into the active site of pig liver esterase. A nucleophilic attack by the serine OH group seemingly occurs from this side B no or only small substituents allowed here C groups of small to medium size allowed in this area D space allowed in these areas for carbon chains, with polar substituents (e.g., another ester group) preferred in the upper part ...

In an asymmetric p-diketone there should be a preference by the enol proton for one of the carbonyls over the other and attempts have been made to determine which it is by C-nmr spectroscopy (Shapet ko et al., 1975 Lazaar and Bauer, 1983). With the additional help of O-nmr spectroscopy it has been possible to demonstrate convincingly that the enol group prefers the carbonyl with a p-group in the following order CF3 > Ph > Bu > Me (Geraldes et al., 1990). 

The selective dense layer of hydrophilic membranes is made from different polymers with a high affinity for water. These polymers contain ions, oxygen functions like hydroxyl, ester, ether or carboxylic moieties, or nitrogen as imino or imi-do groups. Preferred hydropilic polymers are polyvinylalcohol (PVA) [32], poly-imides, cellulose acetate (CA) or natural polymers like chitosan [33] or alginates. Organophilic membranes usually consist of crosslinked silicones, mostly polydimethyl siloxane (PDMS) or polymethyl octyl siloxane (POMS). 

Exposure for 28 days At least three dose levels plus control At least five males and females per group Preferred rodent species Rat... 

At least three dose levels plus control At least 50 males and females per group Satellite group At least 20 males and females per group Preferred species Rat Exposure for a minimum of 4 weeks (males) or from 2 weeks prior to mating until at least postnatal day 4 (females -at least 6 weeks of exposure)... 

In the conformational equilibria of 2,2-di-Me-5-R-5-R -l,3-dioxanes (R = Me, CH2OH, CH2OAC R = NHBz, NO2), both the NHBz and the nitro group prefer the axial position in the chair conformations in the case of R = CH2OH, intramolecular hydrogen bonding proved to further stabilize this conformational behavior (Scheme 10) (85KGS468). 

Contrary to Uthiated alkyl carbamates, which hold a pyramidalized sp -hybridized carb-anionic centre and thus generally react with electrophiles under retention of configuration (see Section n.B.l), lithiated ( )-allyl carbamates 310 have a high tendency for antarafa-cial reactions, which seem to be enforced by the bulky sparteine as the lithium ligand (equation 79). Fortunately, in these reactions, the Af,Af-diisopropylcarbamoyloxy group prefers the ewdo-position to lead to the a-product 311 (inversion) and to the y-product (Z)-312 [sometimes besides small amounts of (E)-ent-312)]. 

---