Alkenes in nature

Strained alkenes in natural product synthesis 13AG(E)4078. Supramolecular construction of optoelectronic biomaterials 13ACR1527. Suzuki reaction in total synthesis 12T9145. 

Many reviews on general ethylene physiology and fruit ripening are available (Burg, 1962 Pratt and Goeschl, 1969 Abeles, 1973 Pratt, 1975). In this chapter we shall deal with the biochemical mechanism by which the double bond is introduced in the biosynthesis of ethylene, the simplest alkene in nature. 

PROBLEM 12.32 Notice the Z configuration of the alkenes in natural rubber. 

An alkene, sometimes caJled an olefin, is a hydrocarbon that contains a carbon-carbon double bond. Alkenes occur abundantly in nature. Ethylene, for instance, is a plant hormone that induces ripening in fruit, and o-pinene is the major component of turpentine. Life itself would be impossible without such alkenes as /3-carotene, a compound that contains 11 double bonds. An orange pigment responsible for the color of carrots, /3-carotene is a valuable dietary source of vitamin A and is thought to offer some protection against certain types of cancer. 

The addition of (TMS)3SiH to a number of monosubstituted acetylenes has also been studied in some detail. These reactions are highly regioselective (anti-Markovnikov) and give terminal (TMSlsSi-substituted alkenes in good yields. High cis or trans stereoselectivity is also observed, depending on the nature of the substituents at the acetylenic moiety. For example, the reaction of the alkynes 23 and 24 with (TMSlsSiH, initiated either by EtsB at room temperature (method or by thermal decomposition of di-ferf-butyl peroxide at 160 °C... 

Subshtuted 3-alken-l-ynes can be hydroaminated with primary or secondary aliphahc or aromatic amines at the alkynyl sites or at the alkynyl and at the alkenyl sites in the presence of Hg(ll) salts. However, the reachon is essentially stoichiometric in nature, even if the mercury compound can be recycled without apparent loss of achvity [262-264]. 

Several alkenes occur naturally in living organisms. Some of these alkenes act as hormones and control biological functions. Plants produce ethene as a hormone to stimulate flower and seed production and to ripen fruits. Ethene stimulates enzymes in the plants to convert starch and acids of unripe fruit into sugars. The enzymes also soften fruit by breaking down pectin in cell walls. 

Much progress has been made on regioselective hydroformylation of terminal alkenes in favor of the linear product. In particular bidentate phosphine or phosphite ligands, which have a natural bite angle 9 of about 110°, will favor the linear product. The most successful ligand types are BISBI [49, 50], BIPHEPHOS [51,52], and XANTPHOS systems (Scheme 8) [53]. 

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