CHAPTER 10: Question 7

 

2D Explanations

This question is based on the chemistry shown in Scheme 10.6. In each case, a prochiral alkene is converted into an enantiomerically pure alcohol by the addition of an OH group to the less hindered end and less hindered face of the alkene. This transformation can be carried out by reaction with a suitable borane derivative followed by sodium hydroxide / hydrogen peroxide. To obtain a non-racemic product, it is necessary to use a chiral borane derivative. As discussed in the text, reagent 10.22 reacts well with trisubstituted and trans-disubstituted alkenes such as those present in the two acyclic examples in this question. Furthermore, the configuration (and relative configuration in the case of the trisubstituted alkene) of the products from the acyclic alkenes are identical to that illustrated in Scheme 10.6 for the product from 1-phenylpentene. Thus, reagent 10.22 would achieve both of these transformations.

The cyclohexene derivative however, is a cis-disubstituted alkene so, reagent 10.22 will not give good asymmetric induction with this substrate. However, as discussed in the text, reaction of 10.22 with another equivalent of a-pinene gives reagent 10.23 which does give good asymmetric induction with cis-disubstituted alkenes. Again, the configuration of the product from the cyclohexene derivative is identical to that illustrated in Scheme 10.6 for the product from 1-phenylpentene. Thus, reagent 10.23 would achieve this transformation.



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