Priority order, of groups

Know the rules for establishing priority orders of groups in the R-S convention. 

Given a compound with a stereogenic center, assign the priority order of groups attached to it. 

The stereogenic center is marked with an asterisk. The priority order of groups at this center is... 

Give the priority order of groups in compound A (Problem 11.31), and draw a dash-wedge formula for its R-isomer. 

In the third model (finite chain with different terminal groups) no reflection symmetry element exists in the Fischer projection. The individual macromolecules are, therefore, chiral and all the tertiary atoms are asymmetric and different. The stereochemical notation for a single chain, depending on the priority order of the end groups, can be R, R2, R. . . R -2, R -i, Rn or R, R2, R3... 

Determine the priority order of the groups at position 4 of the 1,4-dihydro-pyridine ring in the calcium antagonist amlo dipine and state whether the compound is chiral. 

In the determination of the absolute configuration of cysteine it is important to note that the sulfur atom has a higher priority than the oxygen atom. The priority order of the atoms and groups directly attached to the chirality centre by application of the CIP rules is N > C(S,H,H)) > C(0,0,(0)) > H. Thus L-cysteine has the R configuration in contrast to other L-amino acids which have the S configuration. 

Adrafinil has a chirality centre since the sulfur atom of the sulfinyl group still possesses an unshared electron pair and consequently can exist with either an R or S configuration. From the sequence rules, the unshared electron pair, which in the S enantiomer shown below is pointing towards the observer, has the lowest priority. The priority order of the groups attached to the sulfur atom is O > C(C,C,H) > C(C,H,H) > e . The mirror image of this formula represents the R isomer. Note that sulfoxides with two different groups attached always have a chirality centre, since the sulfur atom cannot oscillate through the plane (i.e. there is no pyramidal inversion). 

The formula is that of the R isomer of debropol. The priority order of the groups attached to the chirality centre is Br > N02 > CH2OH > CH3. Since the methyl group is already directed away from the observer, the configuration is readily deduced. 

The compound has the R configuration. Note that the unshared electron pair on the sulfur atom in the formula shown below is oriented towards the observer. Contributions of the sulfur d-orbitals to the S-0 double bond are not considered in the determination of the priority order of the groups attached to the chirality centre. Thus the decyloxy residue has precedence over the doubly bonded oxygen atom. This compound has been used in the synthesis of liquid crystalline substances [6]. 

Doxycycline contains six chirality centres whose configuration is indicated in the structural formula shown below. The chirality centre at position 12a requires careful consideration since the priority order of the groups attached to this atom may be dependent upon which tautomer is present. As an illustration, if one of the possible tautomers is considered (right hand formula) then the priority order can only be established in the fourth sphere as indicated in the digraph depicted below. 

Draw a Fischer projection for L-leucine. What is the priority order of the groups attached to the stereogenic center What is the absolute configuration, R or S ... 

B) Stereochemical assignment of the stereoisomer. Recall that the color scheme indicates the priority order of the groups around each stereocenter red > blue > green > black. 

Groups attached to the chiral centre are given an order of priority according to the sequence rules. For an enantiomeric carbon compound the group of lowest priority is... 

Listed in order of decreasing priority for citation as principal group or parent name. 

For nomenclature purposes, substituents can be divided into those that are always cited as prefixes (e.g. chloro-, alkoxy-) and those which can be cited as either prefixes or suffixes e.g. oxo-/-one, carbamoyl-/-carboxamide). If any of the latter are present, the name will normally contain a suffix. However, only one kind of group can be cited as suffix, and this is chosen according to a strict order of priorities. The group thus selected is known as the principal characteristic group . 

Table 7 lists some of the commonest substituent groups and the corresponding prefixes and/or suffixes, in order of priority for selection as principal group. Some examples of their use are given in (148)-(152). 

These three groups of parameters are arranged in order of their priority, so that the necessity for detailed investigation of succeeding parameters depends on the results from the preceding group. 

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