Stereocenters finding

EXERCISE 7.1 In the compound below, there is one stereocenter. Find it. 

Usually, stereocenters are drawn with dashes and wedges to show the configuration. If the dashes and wedges are not drawn, then we assume that there is a mixture of equal amounts of both configurations (which we call a racemic mixture). In fact, in the compound above, there is a second stereocenter. Can you find it Each of the two stereocenters in the compound above can be either R or S. Since there are two stereocenters, there will be four possibilities R,R and R,S and S,R and S,S. Since neither stereocenter has been drawn with dashes and wedges, we must assume that we have all four possible stereoisomers. 

In the previous problems, you knew that you were looking for just one stereocenter. Hopefully, you started to realize some tricks that make it faster to find the stereocenter (for example, ignore CH2 groups). So, now, we will move on to examples where you don t know how many stereocenters there are. There could be five stereocenters or there could be none. 

Answer If we go around the ring, we find that there are only six carbon atoms in this compound. Four of them are CH2 groups, so we know that they are not stereocenters. If we look at the remaining two carbon atoms, we see that each of them is connected to four different groups. They are both stereocenters. 

PROBLEMS For each of the compounds below, find all of the stereocenters, if any. 

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. 

PROBLEMS For each compound below, find all stereocenters, and determine their configuration. 

When you understand a mechanism, you will understand why the reaction took place, why the stereocenters turned out the way they did, and so on. If you do not understand the mechanism, then you will find yourself memorizing the exact details of every single reaction. Unless you have a photographic memory, that will be a very difficult challenge. By understanding mechanisms, you will be able to make more sense of the course content, and you will be able to better organize all of the reactions in your mind. 

Although a major drawback to this reaction was the destruction of the C3 stereochemistry, it represented the first successful attempt to open the peroxide in a productive manner. To regain the C3 stereocenter, we treated vinylogous ester 48 (from acid-promoted solvolysis of 49b) NaBH4 but only to find alcohols were isolated in a 4 1 ratio (Scheme 8.12). Fortunately, L-Selectride gave exclusively... 

The lack of any directing effect from the 4-methoxy and the 5-ethyl substituents at the two stereocenters already present in 71 is a remarkable finding, and points to strong catalyst-dependence in the stereocontrol (Scheme 7.20). On the basis of these findings, various stereoisomers of 3,4,4,5-tetrasubstituted cyclohexanones are now accessible through sequential catalytic 1,4-additions, with control over the relative and absolute configurations possible simply by judicious selection of the appropriate enantiomer of the chiral ligand in each step. 

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