Special Topic Three-Membered Rings in Biochemistry

Normal hydrogenation of an alkyne adds two equivalents of H2 to give the alkane. Use of a poisoned Lindlar catalyst leads to absorption of one equivalent of H2 and formation of the cis alkene. The trans alkene can he made by reducing the alkyne with Na in ammonia. 

12 Special Topic Three-Membered Rings in Biochemistry 

Despite the strain inherent in cyclopropanes, Nature finds ways to make them. Bacteria, in particular, contain surprising amounts of cyclopropanated fatty acids. Even more remarkable is the conversion of such molecules into cyclopropenes. The source of the single extra carbon is 5-adenosylmethionine, the same agent involved in the methyl transfers discussed in Chapters 3 and 7 (pp. 142, 288 Fig. 10.85). The mechanism of this intriguing change hasn t been worked out yet. 

FIGURE 10.85 Cyclopropanation of a fatty acid is followed by dehydrogenation. 

In Chapter 12 we will see other three-membered rings found in Nature. Epoxides are used as triggers in the construction of the biologically active molecules called steroids, and, amazingly enough, one of the most prominent molecules in interstellar space is not only a cyclopropene, but a carbene as well. This molecule is known as cyclopropenylidene  

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