Half-chair conformation, and

More recently, Janda has described the production of a galactopyranosidase antibody in response to hapten [96]. This was designed to accommodate several features of the transition state for glycoside hydrolysis notably a flattened half-chair conformation and substantial sp2 character at the anomeric position. Some 100 clones were isolated in response to immunization with [96] and used to generate a cDNA library for display on the surface of phage (Appendix entry 7.3) (Janda et al., 1997). Rather than proceed to the normal screening for turnover, Janda then created a suicide substrate system to trap the catalytic species. 

The spectra of chromans are more complex than those of chromenes. The heterocyclic ring adopts a half-chair conformation and consequently substituents at the 2-, 3-, and... 

The different conformations of indolo[2,3-a]quinolizidines play an important role in predicting the thermodynamically more stable epimer (see below). The indolo[2,3-a]quinolizidine system can exist in three main conformations one C/D trans ring juncture (conformation a) and two C/D cis ring junctures (conformations b and c). These are in equilibrium by nitrogen inversion and cw-decalin type ring interconversion. Ring C is considered to be in a half chair conformation and ring D in a chair conformation, Scheme (16) [40]. 

Heating TbtPhSiH2 with sulphur at 280 °C gives 23 while the mesityl derivative gives TbtMesSi(SH)OH. However, the silatetrathialane 24 does form at 230 °C and the crystals have a distorted half-chair conformation and Si—S bonds of 216 and 222 pm70. It also results from the silylene and sulphur at 70 °C71. 

They found that the compounds exist in half-chair conformations and that their boat conformers are the transition states for ring interconversion. 

Miller et al [21], basing on competitive inhibition studies with Arthrobacter sialophilus neuraminidase, postulated that a salt bridge is formed between a positively charged group on the enzyme and the carboxylate group of the bstrate, which results in the distortion of the substrate to the half-chair conformation and the formation an oxocarbonium ion with a positive charge at C-2. 

The crystal and molecular structures of bicuculline (175) have been determined by 2f-ray analysis.N.m.r. studies of the N-methylbicuculline cation indicate that the N-containing ring is in a half-chair conformation and that the phthalide ring adopts a pseudo-axial arrangement staggered with the N-methyl function. ... 

The mechanistic work on the hydrolysis of acetals provides a stereoelectronic rationale for the earlier suggestions that this essential conformational change shonld be involved in the enzyme-catalyzed p-glycosyl transfer reactions either as a part of the initial binding process or in one of the subsequent isomerization steps. The lone pair-a overlap gradually increases as the ring flattens into the half-chair conformation and an alternative mode of stereoelectronic assistance becomes possible. 

Scheme 4.6 (a) D-Culosyl oxocarbenium ion half-chair conformers and (b) rate of hydroly-... 

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