Irones - The Chemistry of Iris

In order to illustrate the significant increase in difficulties involved in the synthesis of single enantiomers rather than mixtures of stereoisomers, we will now look at a more recent irone synthesis. This synthesis was carried out by Brenna and co-workers8 23 and also illustrates the use of an enzyme to achieve a stereospecific synthesis. This is a tactic which 

The synthesis started with the bromination of tetramethylethane (8.99) using N-bromosuccinimide (NBS) and dibenzoyl peroxide. This gave the monobromide (8.100) which was used to alkylate the dianion (8.101) of 

Resolutions such as this are inevitably lengthy and require numerous stages with handling and purification issues at each one. This makes them expensive to carry out on a large scale. Thus, chiral synthesis or synthesis from a homochiral (only one enantiomer) precursor are always preferable, if they can be found. It is not surprising therefore that the Nobel Prize for chemistry in 2001 was awarded to William S. Knowles, Ryoji Noyori and K. Barry Sharpless for their pioneering contributions to research on chiral catalysts which can be used to introduce chirality into a synthesis starting from achiral precursors. We saw some of the fruits of Noyori s work in the chiral synthesis of menthol described in Chapter 4. 

Mookherjee and R.R. Patel, Proceedings of the Seventh International Conference of Essential Oils, Kyoto, 1977, paper 137, 479. 

Lederer, in Fortschritte Der Chemie Organischer Naturstoffe, Springer, Berlin, 1950, Vol. 6, pp. 120. 

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