In terpenes

Carbon-Carbon Bond Formation in Terpene Biosynthesis... 

CARBON-CARBON BOND FORMATION IN TERPENE BIOSYNTHESIS... 

Isoprene unit (Section 26 7) The characteristic five carbon structural unit found in terpenes... 

The isoprene unit exists extensively in nature. It is found in terpenes, camphors, diterpenes (eg, abietic acid), vitamins A and K, chlorophyll, and other compounds isolated from animal and plant materials. The correct stmctural formula for isoprene was first proposed in 1884 (7). 

The catalytic oxidation of isophorone (259—261) or P-isophorone (262,263) to ketoisophorone [1125-21 -9] (2,6,6-trimethyl-2-cyclohexen-l,4-dione) has been reported. Ketoisophorone is a building block for synthesis in terpene chemistry and for producing compounds of the vitamin A and E series. 

In contrast to the wide application that nitrosyi chloride has found in terpene chemistry, nitrosyi fluoride has only recently been discovered. Its first use in steroid chemistry was reported by Boswell who treated cholesterol acetate with nitrosyi fluoride at 0° in methylene dichloride and obtained the 5a-fluoro-6-nitrimine (31) already mentioned in section VI (page 461) in connection with the synthesis of 6,6-difluoro steroids. This... 

The value of spruce-oil chemistry in sorting out problems of hybridization and introgression—major factors in Picea taxonomy—was succinctly summarized by von Rudloff who defined three situations (1) Terpene variation is limited such that it is not possible to use these characters in studies of introgression this is the case in eastern North America where the ranges of black spruce and red spruce overlap. (2) Sufficient variation in terpene profiles exists for the compounds to be useful markers in systematic studies as seen in white spruce. Brewer s spruce, and Sitka spruce. (3) Tree-to-tree variation in terpene content is so variable that use in che-mosystematic studies is precluded, or at least requires very large sample sizes for statistical reliability, as seen with Engelmann s spruce. 

The use of nitrogen fertilization results in higher content of N-containing compounds, including free amino acids, and also increases in terpene content in wood plants, whilst starch, total carbohydrates, phenylpropanoids and total carbon-based phytochemicals decreased (Koricheva et al., 1998). Higher levels of nitrogen favoured its uptake and increased the nitrate content of the crop, which is critical for salad vegetables and baby foods. 

The three main platform molecules employed in terpene chemistry are a-pinene and / -pinene, which are extracted from turpentine oil (350000 t a-1) a co-product of paper pulp industry, and limonene extracted from citrus oil (30000 t a-1). 

Recently it was shown by radiolabeling studies that the formation of the serrulatane skeleton is catalyzed by the pseudopterosin diterpene cyclase, which can be considered as a key enzyme in terpene biosynthetic pathways (Scheme 2). The elisabethatriene cyclase is a monomer with a molecular mass of47kDa [25]. 

WaUach, O. Justus Liebigs Ann. Chem. 1892, 272, 99. Otto Wallach (1847—1931), bom in Konigsberg, Prussia, studied under Wohler and Hofmann. He was the director of the Chemical Institute at Gottingen from 1889 to 1915. His book Terpene und Kampfer served as the foundation for future work in terpene chemistry. Wallach was awarded the Nobel Prize in Chemistry in 1910 for his work on alicyclic compounds. Moore, M. L. Org. React. 1948, 5, 301. (Review). 

Microbial reduction of prochiral cyclopentane- and cyclohexane-1,3-diones was extensively studied during the 1960 s in connection with steroid total synthesis. Kieslich, Djerassi, and their coworkers reported the reduction of 2,2-dimethylcyclohexane-l,3-dione with Kloeokera magna ATCC 20109, and obtained (S)-3-hydroxy-2,2-dimethylcyclohexanone. We found that the reduction of the 1,3-diketone can also be effected with conventional baker s yeast, and secured the hydroxy ketone of 98-99% ee as determined by an HPLC analysis of the corresponding (S)-a-methoxy-a-trifluoromethylphenylacetate (MTPA ester).(S)-3-Hydroxy-2,2-dimethy1cyc1ohexanone has been proved to be a versatile chiral non-racemic building block in terpene synthesis as shown in Figure 1. 

Wagner-Meerwein type rearrangements have also been widely reported in terpene chemistry [127, 128]. One well-known transformations involves the... 

Prenylation, the key step in terpene biosynthesis, is catalyzed by prenyltransferases. These enzymes are responsible for the condensation of isopentenyl pyrophosphate (IPP) with an allyl pyrophosphate, thus yielding isoprenoids. Numerous studies have been performed with fluorinated substrates in order to determine the mechanism of the reactions that involve these enzymes prenyltransferases, farnesyl diphosphate synthase (FDPSase), famesyltransferase (PFTase), and IPP isomerase. These studies are based on the potential ability of fluorine atoms to destabilize cationic intermediates, and then slow down S l type processes in these reactions. 

Ergosterol, the predominant sterol in yeast cells, plays an important role in membrane fluidity, permeability and the activity of many membrane-bound enzymes. In terpene-treated cells, ergosterol synthesis was strongly inhibited, and a global upregulation of genes associated with the ergosterol biosynthesis pathway was described in response to terpene toxicity [80, 121]. 

Polyene cyclization in terpene and steroid synthesis is critically dependent on the terminator in order to generate useful functionalities for further modification of the products. Allyl- and propargylsilanes have proven their value in facilitation of the cyclization and generation of an exocyclic methylene and allene, respectively. Thus, a concise approach to albicanyl acetate [126] and the rapid construction of a tetracyclic precursor of steroids [127] are sufficient to demonstrate the concept. Again, a comparison of the substrates with a silyl group with those having a simple alkyl moiety is very enlightening. 

Complete Assignment of Proton Chemical Shifts in Terpenes 69... 

The first rearrangement of an oxaspiropentane probably occurred in terpene l39 which was isolated from Zieria smithii and later,40 but mistakenly,41 thought to be chrysanthenone (2), to which it readily rearranges. Interestingly, this rearrangement now constitutes one of the most powerful instruments for the construction of cyclobutanones. 

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