Diterpenes, acid alcohols

Chemistry of rosin. All three types of rosin consist primarily of C20 mono-carboxylic diterpene resin acids, the most common of which have the molecular formula C20H20O2. In addition, rosins contain small amounts of neutral and other acidic components (e.g. fatty acids in tall oil rosin). The neutral components of rosins are diterpene alcohols, hydrocarbons and aldehydes, and their contents generally vary between 5 and 15 wt%. 

Provided that the silanolate elimination proceeds with anti selectivity, it must be concluded, that the intermediate homoallylic alcohol has an anti configuration, and thus the reagent has an ( -configuration. Acidic hydrolysis of the enol ether leads to enones the overall sequence consists of a nucleophilic acroylation. This has also been applied in the total synthesis of the marine diterpene ( )-aplysin-2067. 

Essential oils may comprise volatile compounds of terpenoid or non-terpe-noid origin. All of them are hydrocarbons and their oxygenated derivatives. Some may also contain nitrogen or sulphur derivatives. They may exist in the form of alcohols, acids, esters, epoxides, aldehydes, ketones, amines, sulphides, etc. Monoterpenes, sesquiterpenes and even diterpenes constitute the composition of many essential oils. In addition, phenylpropanoids, fatty acids and their esters, or their decomposition products are also encountered as volatiles [1-16, 21-33, 36-38]. 

Two important diterpene alcohols are vitamin A (Section 28-7) and phytol, which occurs as an ester of the propanoic acid side-chain of chlorophyll (Figure 20-6) ... 

The most general method for the synthesis of tetrahydrofurans based upon the IMSC methodology was developed by Overman et al. [53, 54, 94—96] For example, condensation of alcohol 221 with an aldehyde or a ketone in the presence of a Lewis acid leads to the formation of the carbocations 222a,b. The tertiary carboca-tion 222a undergoes a pinacol rearrangement and forms the desired heterocycle 224 (Scheme 13.82). Overman et al. used this approach during the synthesis of the various cladiellin diterpenes, which possess the core skeleton 224 [53]. 

Other complementary methodologies include the preparation of substituted cyclopropanes from glycals using rhodium acetate carbenoid additions [65,66]. Additionally, acid catalyzed cyclopropane opening reactions in alcoholic solutions afford the 2-C-branched-glycosides. These combined reactions were used to prepare a key intermediate in marine diterpene norrisolide synthesis from D-mannose [67]. 

Stypodiol, epistypodiol and stypotriol are secondary diterpene metabolites produced by the tropical brown algae Stypopodium zonaie. These compounds display diverse biological properties, including strong toxic, narcotic, and hyperactive effects upon the reef-dwelling fish. In the laboratory of A. Abad an efficient stereoselective synthesis of stypodiol and its C14 epimer, epistypodiol, was accomplished from (S)-(+)-carvone. The key transformations in the synthesis of these epimeric compounds were an intramolecular Diels-Alder reaction, a sonochemical Barbier reaction and an acid-catalyzed quinol-tertiary alcohol cyclization. 

Terpenes are are a class of compounds whose chemical structures are based on a number of isoprene units , derived from the hydrocarbon CH2=C(CH3)-CH=CH2 they may themselves be hydrocarbons, but may also contain alcohol (OH), aldehyde/ketone (CO) and carboxylic acid (COOH) groups. Monoterpenes are Cm compounds derived from two isoprene units, sesquiterpenes (Cu) are derived from three isoprene units, diterpenes (C20) from four and tritepenes (C30) from six. Terpenes are widespread in plants, where they are largely responsible for the odor, and they are the major constituents of plant-derived essential oUs . Among the best known terpenes are 3-pinene (turpentine), camphor, menthol and citroneUal (aU monoterpenes) and farnesol (a sesquiterpene that is a constituent of the essential oils of many plants). Certain terpenes have important biological roles vitamin A, for example, is a diterpene, and steroid hormones have a structure related to triterpenes (and are biosynthesized by a similar route). 

Hindered secondary and tertiary alcohols can be silylated with TBSOTf (bp 60 C/0.9 kPa) using 2,6-lutidine as the base as illustrated by the silylation of the secondary alcohol 61.1 in a synthesis of the diterpene Zoapatanol [Scheme 4,61] and the tertiary alcohol 62.1 [Scheme 4.62] originating in a synthesis of Erythronolide A. The reactions can be run at molar concentrations using dichloromethane as a co-solvent and the recommended ratio of reactants is alcohol TBSOTf 2,6-lutidine = 1 1.5 2. TBSOTf is both expensive and very moisture sensitive and in our experience commercial samples do not store well consequently, it may be best to prepare it fresh from the reaction of /er/-butyldi-methylsilyl chloride and triflic acid. For the silylation of p-hydroxybutyrolac-lones, which are prone to dehydration during silylation with TBSOTf under the standard conditions, 2,6-di-/m-butylpyridine has been recommended as the base. Owing to their higher acidity, phenols can be silylated selectively in the presence of alkanols [Scheme 4.63]. ... 

Cembranoids The first macrocyclic diterpene isolated from an Eremophila species was the triol (99) produced by E. clarkei (96). The resin obtained from this plant was a complex mixture of compounds from which the crystalline triol could be obtained with difficulty from the neutral fraction. Larger quantities of triol were available after methylation and lithium aluminium hydride reduction of the acidic fraction. The plane structure of 99 was established by conversion of the triol to a crystalline stereoisomer of cembrane (100) as shown in Scheme 25. The two dihydrotriols (101 and 102) obtained in this sequence also served to prove the location of the hydioxymethylene groups on the cembrane skeleton. Since both 101 and 102 are optically active, the symmetrical 4,12-cis arrangement of the primary alcohol groups can be excluded. Furthermore, the 4, 2-trans-disposition of these groups can also be excluded since elimination of the asymmetry at Cl leads to two different olefins (103 and 104). 

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