Reaction center, chirality remote

RECOGNITION OF CHIRALITY REMOTE FROM REACTION CENTER... 

One of the most distinguishable features of the biocatalytic reactions compared with the chemical ones is that the biocatalysts can recognize a remote chiral center apart from the reaction center of substrates. As shown in Figure 4, biocatalysts can recognize a chirality separated by six bonds as well as at the reaction center. 

Phosphoric acids 32 and 35 have been successfully employed as chiral Bronsted acid catalysts for vinylogous Mannich reactions of acyclic silyl dienolates with imines (Table 5.5 and Scheme 5.10). Why does the exchange of a para methyl group for a tert butyl group on the 3,3 aryl groups within the BINOL backbone have such a pronounced effect on the enantioselectivity of the reaction although this position appears to be quite remote from the reaction center ... 

Stereogenic centers close to the coordinating metals (usually the reaction center) are not prerequisite. Equally effective can be the induction of a chiral metal vicinity via remote stcrcogcnic centers. 

The effect of a remote chiral center in this reaction has also been investigated. In most cases, the reaction was found to be controlled mainly by the chiral catalyst (e.g., 58), whereas the resident center generally exercised a weak to modest influence [61]. 

Phosphonates and phosphonamidates, which mimic the tetrahedral geometry and anionic character of the transition state for hydrolysis, have proven especially reliable as haptens. Antibodies generated against such compounds readily promote the cleavage of esters and, in a few cases, amides. High levels of stereospecificity are attainable even at chiral centers remote from the reaction site [11]. 

Fig. 12. The four diastereoisomeric 1 1 adducts which can be formed on reaction of the bisdiene 37 with the syn-bisdienophile 38. The stereoelectronic control in operation during the reaction is such that only the syn/endo-H isomer 40 has been detected [21, 38, 110, 112]. The remote stereochemical descriptors, syn and anti, refer to the relative configurations of the endoxide bridges across each newly-formed cyclohexene ring. The close stereochemical descriptors, exo-H and endo-H, refer to the relative configuration adopted by the hydrogen atoms at the ring junctions associated with the newly created chiral centers...

For example, addition of ketone 202 to complex aldehyde 214 led to adduct 215 with high stereocontrol (90% ds). The a-chiral aldehyde 214 could be expected to show a small preference for the und-Felkin adduct 215, and presumably there is a strong facial bias from chiral ketone 202, despite the relatively remote C3-stereo-center. When the silyl-protected ketone 216 was used for this reaction, adduct 217 was also obtained with good stereocontrol [78]. 

Chiral-auxiliary control in radical addition reactions is presented in a general format in Eqs. (3), (4) and (5). In these equations, a or d represents a resident group which has a stereogenic center that controls the configuration of the new center formed in the transformation. In Eq. (3), the resident stereogenic center resides on the radical. In Eq. (4), the resident center is attached to the radical trap at the site of unsaturation undergoing reaction, while in Eq. (5), the center resides on the unsaturated radical trap at a site remote from the unsaturated center undergo-... 

We will discuss the structural and mechanistic basis of Cram s rule in Chapter 3. As would probably be expected, the influence of a chiral center on the diastereoselec-tivity of reaction is diminished when the chiral center is more remote from the reaction site. 

---