The terpenes are secondary metabolites of plants that are produced, in part, as a defense against microorganisms and insects in addition to their pollinator-attractive properties.1 In mammals, terpenes contribute to stabilizing cell membranes, participate in metabolic pathways, and act as regulators in some enzymatic reactions.2 Members of this class of chemicals have carbon structures which can be decomposed into isoprene (C5H8) residues and are classified, based on the number of carbons in the molecule, as monoterpenes (ten carbons), sesquiterpenes (fifteen carbons), diterpenes (twenty carbons), triterpenes (thirty carbons), and tetraterpenes or carotenes (forty carbons).3 The simpler terpenes (monoand sesquiterpenes) are the major constituents of essential oils and are widely used in the perfumery industry, while diand triterpenes are less volatile and are obtained from plant gums and resins.4 Carotenes are synthesized by bacteria, algae, fungi, and green plants and comprise more than 600 known structures.5 The most important terpenes and their oxygenated derivates (terpenoids) cited in this study may be seen in Figures 1-3. Terpenes are a good starting material for the synthesis of many fine chemicals due to their similar carbon skeleton. R-(+)-Limonene (2), for example, is the most abundant monocyclic monoterpene in nature, and it represents more than 90% of the orange peel oil; thus, it is an inexpensive precursor.6,7 The oxygenated derivatives of limonene, e.g. carveol (24), carvone (25), perillyl alcohol (26), menthol (39), and R-terpineol (29), are recognized for their pleasant fragrances,8 and some of them also present bioactivity against certain types of tumor cells, not only preventing the formation or progression of cancer but also regressing existing malignant tumors.9,10 R-(6) and -pinene (7), in turn, are found in high concentrations in turpentine, a paper and pulp industry residue, and they are, therefore, also available in bulk at a low price. These bicyclic monoterpenes are used as a fragrance substance that is used to improve the odor of industrial products and are also precursors of important flavor compounds, such as terpineols, borneol (45), camphor (46), citronellol (11), geraniol (14), menthol (39), verbenol (48), and verbenone (49).6,7 The tetraterpene -carotene (62), an orange pigment found mainly in tropical vegetables, is a precursor of norisoprenoid ionones, molecules responsible for desirable fruity and floral flavors.7,11 Volatile carotenoid breakdown products have been long known as important flavor compounds.12

Bio-oxidation of terpenes: an approach for the flavor industry.

Unter „En-Reaktionen” versteht man die indirekte substituierende Addition einer Verbindung mit Doppelbindung (Enophil) an ein Olefin mit allylischem Wasserstoff (En). Lange Zeit stand die En-Reaktion im Schatten der verwandten Diels-Alder-Addition. In diesem Aufsatz wird gezeigt, das das Anwendungsgebiet der En-Reaktion von industriellen bis zu biosynthetischen Prozessen reicht. Einer Zusammenstellung der praparativen Aspekte folgte eine Diskussion des Reaktionsmechanismus.

Die En‐Reaktion

Oxidation of alkenes to enones using tert-butyl hydroperoxide in the presence of chromium carbonyl catalysts

The mechanism of the allylic oxidation of 2-methyl-2-butene with selenium dioxide was explored by a combination of experimental and theoretical studies. A comparison of the experimental (13)C and (2)H kinetic isotope effects with predicted values shows that the observed isotope effects are consistent with an initial concerted ene step mediated by SeO(2). However, this comparison also does not rule out the involvement of a selenous ester in the ene reaction or a stepwise reaction involving reversible electrophilic addition of HSeO(2)(+) followed by rate-limiting proton abstraction. Becke3LYP calculations strongly favor SeO(2) over a selenous ester as the active oxidant, with the predicted barrier for reaction of 2-methyl-2-butene with SeO(2) being 21-24 kcal/mol lower than that for reaction with H(2)SeO(3). The possibility of a selenous ester being the active oxidant is also disfavored by the observation of oxidations in non-hydroxylic solvents. The involvement of HSeO(2)(+) does not appear consistent with a lack of dependence of the reaction on the basicity of the reaction mixture. A concerted ene reaction with SeO(2) as the active oxidant appears to be the major mechanistic pathway operative in these reactions.

Isotope effects and the mechanism of allylic hydroxylation of alkenes with selenium dioxide.

Allylic oxidation remains a reaction of considerable value in organic synthesis. Oxidation reactions in this section are divided into two types: reactions which produce allylic alcohols (equation 1) and those which produce α,β-unsaturated aldehydes or ketones directly (equation 2). Examples from the recent literature fall approximately equally into each type. Examples of allylic oxidations which occur with rearrangement (equation 3 and 4) or give mixtures are discussed individually within each reagent type; however, it is fair to say that many reagents and reaction conditions can give either direct oxidation or oxidative rearrangement, and almost no allylic oxidation system exclusively gives one course of reaction in all cases. In many examples the course of oxidation depends mostly upon substrate structure.

Howard P. Klein

Papers

Selenium dioxide oxidations. III. Oxidation of olefins