Chirality centers configuration

Woerdenbag et al also evaluated the influence of chiral center configurations present in artemisinin (1) structure on the proliferation of Ehrlich ascites tumor (ETA) cells. Compounds 11-hydroxyartemisinin (47) and 11-hydroxy-11-epi-artemisinin (48) (Fig. 4) were synthesized and the... 

There are also rules specifying chiral centers, configurations... 

Figure 7.2. Generation of a second chiral center. Configuration at original chiral center unchanged. Chlorine becomes attached via (a) or b) to give diastereomers, and in unequal amounts.

Addition of dialkyl phosphites to aldehydes and imines are important synthetic methods for the preparation of a-hydroxyl and a-amino phosphonates, which represent phosphoms analogs of a-hydroxyl and a-amino acids. These compounds were investigated in terms of their biological activity [146-156], especially the role of the chiral center configuration on the biological activity [157-170]. Both transition-metal-catalyzed as well as thermal additions were studied. Utilization of... 

Figure 2-69. The two enantiomers of lactic acid assignment of R and S configurations to the enantiomers of lactic acid after ranking the four ligands attached to the chiral center according to the Cl P rules (OH > COjH > Me > H).

Benzyl bromide can be converted into ethylbenzene (731) by the reaction of Me4Sn. The use of HMPA as a solvent is important. Overall inversion of configuration takes place at the chiral center of deuterated benzyl bro-mide[598]. The cyanomethyiation[599] and methoxymethyiation[600] of aromatic rings are carried out by the reaction of cyanomethyltributyltin (732) and methoxymethyltributyltin. 

The order of decreasing precedence is counterclockwise The configuration at the chirality center is S... 

Compounds in which a chirality center is part of a ring are handled in an analo gous fashion To determine for example whether the configuration of (+) 4 methyl cyclohexene is R or S treat the right and left hand paths around the nng as if they were independent substituents... 

To verify that the Fischer jjrojection has the R configuration at its chirality center rotate the three dimensional representation so that the lowest ranked atom (H) points away from you Be careful to maintain the proper stereochemical relationships during the operation... 

To convert a molecule with two chirality centers to its enantiomer the configura tion at both centers must be changed Reversing the configuration at only one chirality center converts it to a diastereomeric structure... 

Erythro and three describe the relative configuration (Section 7 5) of two chirality centers within a single molecule... 

Eleven chirality centers may seem like a lot but it is nowhere close to a world record It is a modest number when compared with the more than 100 chirality centers typ ical for most small proteins and the thousands of chirality centers present m nucleic acids A molecule that contains both chirality centers and double bonds has additional opportunities for stereoisomerism For example the configuration of the chirality center m 3 penten 2 ol may be either R or S and the double bond may be either E or Z There fore 3 penten 2 ol has four stereoisomers even though it has only one chirality center... 

Section 7 13 Addition reactions of alkenes may generate one (Section 7 9) or two (Sec tion 7 13) chirality centers When two chirality centers are produced then-relative stereochemistry depends on the configuration (E or Z) of the alkene and whether the addition is syn or anti... 

Section 7 15 Certain polymers such as polypropylene contain chirality centers and the relative configurations of these centers affect the physical properties of... 

All the double bonds are cis and the absolute configuration of the chirality center is S Wnte a stereochemically accurate representation of ectocarpene... 

The same cannot be said about reactions with alkyl halides as substrates The conver Sion of optically active 2 octanol to the corresponding halide does involve a bond to the chirality center and so the optical purity and absolute configuration of the alkyl halide need to be independently established... 

The acyl group of the carboxylic acid acyl chloride or acid anhydride is trans ferred to the oxygen of the alcohol This fact is most clearly evident m the esterification of chiral alcohols where because none of the bonds to the chirality center is broken m the process retention of configuration is observed... 

None of the bonds to the chirality center is broken when hydroxide attacks the carbonyl group Had an 8 2 reaction occurred instead inversion of configuration at the chirality center would have taken place to give (8) (—) 1 phenylethyl alcohol... 

The particular aldotetrose just shown is called d erythrose The prefix d tells us that the configuration at the highest numbered chirality center is analogous to that of d (+) glyceraldehyde Its mirror image is l erythrose... 

Highest numbered chirality center has configuration analogous to that of D glyceraldehyde -... 

As shown for the aldotetroses an aldose belongs to the d or the l series accord mg to the configuration of the chirality center farthest removed from the aldehyde func tion Individual names such as erythrose and threose specify the particular arrangement of chirality centers within the molecule relative to each other Optical activities cannot be determined directly from the d and l prefixes As if furns ouf bofh d eryfhrose and D fhreose are levorofafory buf d glyceraldehyde is dexfrorofafory... 

Another name for glucitol obtained by reduction of d glucose is sorbitol it is used as a sweetener especially in special diets required to be low in sugar Reduction of D fructose yields a mixture of glucitol and mannitol corresponding to the two possi ble configurations at the newly generated chirality center at C 2... 

The reaction is used for the chain extension of aldoses in the synthesis of new or unusual sugars In this case the starting material l arabinose is an abundant natural product and possesses the correct configurations at its three chirality centers for elaboration to the relatively rare l enantiomers of glucose and mannose After cyanohydrin formation the cyano groups are converted to aldehyde functions by hydrogenation m aqueous solution Under these conditions —C=N is reduced to —CH=NH and hydrolyzes rapidly to —CH=0 Use of a poisoned palladium on barium sulfate catalyst prevents further reduction to the alditols... 

What are the R S configurations of the three chirality centers in D nbose" (A molecular model will be helpful here )... 

Glycine is the simplest ammo acid and the only one m Table 27 1 that is achiral The a carbon atom is a chirality center m all the others Configurations m ammo acids are normally specified by the d l notational system All the chiral ammo acids obtained from proteins have the l configuration at their a carbon atom meaning that the amine group IS at the left when a Fischer projection is arranged so the carboxyl group is at the top... 

A novel technique for dating archaeological samples called ammo acid racemiza tion (AAR) IS based on the stereochemistry of ammo acids Over time the configuration at the a carbon atom of a protein s ammo acids is lost m a reaction that follows first order kinetics When the a carbon is the only chirality center this process corresponds to racemization For an ammo acid with two chirality centers changing the configuration of the a carbon from L to D gives a diastereomer In the case of isoleucme for example the diastereomer is an ammo acid not normally present m proteins called alloisoleucme... 

Absolute configuration (Section 7 5) The three dimensional arrangement of atoms or groups at a chirality center Acetal (Section 17 8) Product of the reaction of an aldehyde or a ketone with two moles of an alcohol according to the equation... 

Syndiotactic polymer (Section 7 15) Stereoregular polymer in which the configuration of successive chirality centers alternates along the chain... 

Generally polymers involve bonding of the most substituted carbon of one monomeric unit to the least substituted carbon atom of the adjacent unit in a head-to-tail arrangement. Substituents appear on alternate carbon atoms. Tacticity refers to the configuration of substituents relative to the backbone axis. In an isotactic arrangement, substituents are on the same plane of the backbone axis that is, the configuration at each chiral center is identical. 

Double Fiiedel-Ciafts alkylation of configuiationally pure pyiocene (a substituted lactone) with aiomatics results in the formation of cycloalkylation product with retention of configuration at the chiral center (63). 

---