Prochiral structure

In this exfflTipIe, addition to the double bond of an alkene converted an achiral molecule to a chiral one. The general term for a structural feature, the alteration of which introduces a chirality center in a molecule, is prochiral. A chirality center is introduced when the double bond of propene reacts with a peroxy acid. The double bond is a prochiral structural unit, and we speak of the top and bottom faces of the double bond as prochiral faces. Because attack at one prochual face gives the enantiomer of the compound formed by attack at the other face, we classify the relationship between the two faces as enantiotopic. 

Preeclampsia, Viagra and, 164 Prelog, Vladimir, 181 Prepolymer, epoxy resins and, 673 Priestley, Joseph, 245 Primary alcohol, 600 Primary amine, 916 Primary carbon. 84 Primary hydrogen, 85 Primary structure (protein), 1038 Primer strand (DNA), 1108 pro-R prochiralitv center, 316 pro-S prochirality center, 316 Problems, how to work, 27 Procaine, structure of, 32 Prochirality, 315-317 assignment of, 315-316 naturally occurring molecules and, 316-317... 

Reaction of lithium enolate 2 with prochiral 3-buten-2-one (4) proceeds with minimal selectivity to produce nearly equal amounts of the two diastereomers of structure 540,41. 

It is not easy to control the steric course of photoreactions in solution. Since molelcules are ordered regularly in a crystal, it is rather easy to control the reaction by carrying out the photoreaction in a crystal. However, molecules are not always arranged at an appropriate position for efficient and stereoselective reaction in their crystals. In these cases inclusion chemistry is a useful technique, as it can be employed to position molecules appropriately in the host-guest structure. Chiral host compounds are especially useful in placing prochiral and achiral molecules in suitable positions to yield the desired product upon photoirradiation. Some controls of the steric course of intramolecular and intermolelcular photoreactions in inclusion complexes with a host compound are described. 

Such enantioselective deprotonations depend upon kinetic selection between prochiral or enantiomeric hydrogens and the chiral base, resulting from differences in diastere-omeric TSs.27 For example, transition structure E has been proposed for deprotonation of 4-substituted cyclohexanones by base D.28 This structure includes a chloride generated from trimethylsilyl chloride. 

Chirality at surfaces can be manifested in a number of forms including the intrinsic chirality of the surface structure and even the induction of chirality via the adsorption of achiral molecules onto achiral surfaces. The ability of STM to probe surfaces on a local scale with atomic/molecular resolution has revolutionized the understanding of these phenomena. Surfaces that are globally chiral either due to their intrinsic structure or due to the adsorption of chiral molecules have been shown by STM to establish control over the adsorption behavior of prochiral species. This could have profound consequences for the understanding of the origin of homochirality in life on Earth and in the development of new generations of heterogeneous chiral catalysts that may, finally, make a substantial impact on the pharmaceutical industry. 

New chiral oxazaborolidines that have been prepared from both enantiomers of optically active inexpensive a-pinene have also given quite good results in the asymmetric borane reduction of prochiral ketones.92 Borane and aromatic ketone coordinate to this structurally rigid oxazaborolidine (+)- or (—)-94, forming a six-membered cyclic chair-like transition state (Scheme 6-41). Following the mechanism shown in Scheme 6-37, intramolecular hydride transfer occurs to yield the product with high enantioselectivity. With aliphatic ketones, poor ee is normally obtained (see Table 6-9). 

All the new chiral centers are controlled by the rigid structure of the dithia-decalin system. Taking the NaBH4 reduction of 14 as an example, the orientation of the fused rings forces the H- to attack from the Sl -prochiral face of the carbonyl group (Fig. 7-1). 

Some structures of this type of compound were established by X-ray diffraction analysis. For example, the structure of compound 84 resulting from the enantioselective Norrish-Yang cyclization of prochiral imidazolidinone was... 

Haining RL, Hunter AP, Veronese ME, et al. Allelic variants of human cytochrome P450 2C9 baculovirus-mediated expression, purification, structural characterization, substrate stereoselectivity, and prochiral selectivity of the wild-type and I359L mutant forms. Arch Biochem Biophys 1996 333(2) 447 t58. 

The results of the study of the last-mentioned reaction, DCA-PhCOCH3 —> 167, provided a surprise. X-Ray analyses of the structure of the clathrate before and after partial reaction, and of the final product, 167, showed that the prochiral Re face of the ketone, initially the face more distant from the to-be-attacked host, and not the close-lying Si face, is the one that adds to the steroid. A rationalization of this extraordinary sterochemical effect, which results in formation of a new chiral center with quantitative asymmetric induction, has been proposed (241). 

An element of prostereoisomerism is a partial structure that can be converted into an element of stereoisomerism not otherwise present, by considering one of a pair of homomorphic groups to be different from the other. The groups involved in this operation are necessarily heterotopic. Depending on the character of the element of stereoisomerism thus produced, one can divide the elements of prostereoisomerism into centers, lines, and planes and subdivide them, as appropriate, into those that are (fully) prochiral, only prographochiral, only... 

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