Stereocenters numbering groups

Most substrates, with the exception of hydroxypymvate, have a threo configuration of hydroxyl groups at the C-3 and C-4 positions (139). The new stereocenter formed in TK-catalyzed addition is formed in the threo configuration with high diastereo-selectivity (151). Using TK-catalyzed condensations of hydroxypymvic acid with a number of aldehydes a practical preparative synthesis of L-idose [5934-56-5], L-gulose [6027-89-0], 2-deoxy-L-xylohexose, and... 

Now that we can find stereocenters, we must now learn how to determine whether a stereocenter is R or S. There are two steps involved in making the determination. First, we give each of the four groups a number (from 1 to 4). Then we use the orientation of these numbers to determine the configuration. So, how do we assign numbers to each of the groups ... 

EXERCISE 7.16 In the compound below, find the stereocenter, and label the four groups from 1 to 4 using the system of priorities based on atomic number. 

The syntheses in Schemes 13.45 and 13.46 illustrate the use of oxazolidinone chiral auxiliaries in enantioselective synthesis. Step A in Scheme 13.45 established the configuration at the carbon that becomes C(4) in the product. This is an enolate alkylation in which the steric effect of the oxazolidinone chiral auxiliary directs the approach of the alkylating group. Step C also used the oxazolidinone structure. In this case, the enol borinate is formed and condensed with an aldehyde intermediate. This stereoselective aldol addition established the configuration at C(2) and C(3). The configuration at the final stereocenter at C(6) was established by the hydroboration in Step D. The selectivity for the desired stereoisomer was 85 15. Stereoselectivity in the same sense has been observed for a number of other 2-methylalkenes in which the remainder of the alkene constitutes a relatively bulky group.28 A TS such as 45-A can rationalize this result. 

A number of nonprotein amino acids with unsaturated side chains have been isolated. Many of these contain alkene side chains, but some alkyne side chains containing amino acids have also been identified. Nonprotein dehydroamino acids do not have an a-stereocenter these amino acids are still classified under this category. Dehydroamino acids are generally biosynthesized by the enzymatic elimination of a leaving group at the /3-carbon. For example, serine and threonine are enzymatically dehydrated to give dehydroalanine and dehydrobutyrine, respectively. A similar biosynthetic pathway is hypothesized for dehydroamino acids found in nonribosomal peptides, such as nodularins and microcystins. ... 

We have reported one of the shortest total syntheses of epothilone D so far. A large moiety of the 16-membered macrocycle is derived from the easily available isoprenoid nerol. Only four C-C bond formations and three protective groups are required. Only one step requires the use of a (recoverable) chiral auxiliary. The other stereocenters are constructed by catalytic hydrogenation (C8), resolution (C15), and induction (C6, C7). In particular the northern half synthesis is well suited for large-scale preparation because of the combination of cheap reagents and catalytic processes. Using essentially the same method, a number of epothilone D5 analogues were synthesized. 

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