Isoprene alcohol

Table 1. Aerosol yields in seeded ozone-isoprene-alcohol systems (Czoschke et al, 2003).

We began our work with isoprene alcohol (see Scheme 11). While the Prilezhaev reaction with monoperoxyphthalic acid provided the appropriate dimethyl glycidol in good yield, we failed in our attempts to prepare the corresponding chiral epoxide by Sharpless epoxidation. Obviously, the reaction does not work in the case of this tertiary allylic alcohol. When the solution is allowed to warm up to room temperature a slow epoxidation takes place, however, the isolated dimethyl glycidol does not show any optical activity. Very recently the asymmetric epoxidation of a tertiary allylic alcohol in reasonable optical and chemical yield has been described [21]. 

Hemiterpenes, by definition, consist of only one isopentane unit and are, thus, the simplest members of terpenoids. The basic building blocks of isoprenoids, namely IPP and DMAPP, must be universally present in green plants, being the progenitors of the phytyl side chain of chlorophyll, which is essential for biological activity (Sect. 8.1.4). These do not appear to be accumulated by plants (3), though the related prenyl alcohol (3,3-dimethylallyl alcohol, 9) and its isomer isoprene alcohol (10) have been found to occur in a few essential oils (229). Isoprene (11) itself is an important emittant of pine forests (165). 

Cholesterol was isolated m the eighteenth century but its structure is so complex that Its correct constitution was not determined until 1932 and its stereochemistry not verified until 1955 Steroids are characterized by the tetracyclic ring system shown m Figure 26 9a As shown m Figure 26 9b cholesterol contains this tetracyclic skeleton modified to include an alcohol function at C 3 a double bond at C 5 methyl groups at C 10 and C 13 and a C Hn side chain at C 17 Isoprene units may be discerned m var lous portions of the cholesterol molecule but the overall correspondence with the iso prene rule is far from perfect Indeed cholesterol has only 27 carbon atoms three too few for It to be classed as a tnterpene... 

Continuous processes have been developed for the alcohols, operating under pressure with Hquid ammonia as solvent. Potassium hydroxide (206) or anion exchange resins (207) are suitable catalysts. However, the relatively small manufacturing volumes militate against continuous production. For a while a continuous catalytic plant operated in Raveima, Italy, designed to produce about 40,000 t/yr of methylbutynol for conversion to isoprene (208,209). 

Isoprene [78-79-5] (2-methyl-1,3-butadiene) is a colorless, volatile Hquid that is soluble in most hydrocarbons but is practically insoluble in water. Isoprene forms binary azeotropes with water, methanol, methylamine, acetonitrile, methyl formate, bromoethane, ethyl alcohol, methyl sulfide, acetone, propylene oxide, ethyl formate, isopropyl nitrate, methyla1 (dimethoxymethane), ethyl ether, and / -pentane. Ternary azeotropes form with water—acetone, water—acetonitrile, and methyl formate—ethyl bromide (8). Typical properties of isoprene are Hsted in Table 1. 

Terpenes are characterized as being made up of units of isoprene in a head-to-tail orientation. This isoprene concept, invented to aid in the stmcture deterrnination of terpenes found in natural products, was especially useful for elucidation of stmctures of more complex sesquiterpenes, diterpenes, and polyterpenes. The hydrocarbon, myrcene, and the terpene alcohol, a-terpineol, can be considered as being made up of two isoprene units in such a head-to-tail orientation (1). 

The hyoscine system occurs alone in certain northern Duboisia myo-poroides, and forms scopine, -tropine, dihydroxytropane and, in Datura meieloides, teloidine. Of these amino-alcohols, all the scopine is esterified by tropic acid and the minor bases by tiglic, methylbutyric or isovaleric acid, which contain the isoprene skeleton and are presumed to arise from that source. 

Long-chain polyisoprenoid. molecules with a terminal alcohol moiety are called, polyprenols. The dolichols, one class of polyprenols (Figure 8.18), consist of 16 to 22 isoprene units and, in the form of dolichyl phosphates, function to carry carbohydrate units in the biosynthesis of glycoproteins in animals. Polyprenyl groups serve to anchor certain proteins to biological membranes (discussed in Chapter 9). 

FIGURE 8.18 Dolichol phosphate is an initiation point for the synthesis of carbohydrate polymers in animals. The analogous alcohol in bacterial systems, undecaprenol, also known as bactoprenol, consists of 11 isoprene units. Undecaprenyl phosphate delivers sugars from the cytoplasm for the synthesis of cell wall components such as peptidoglycans, lipopolysaccharides, and glycoproteins. Polyprenyl compounds also serve as the side chains of vitamin K, the ubiquinones, plastoquinones, and tocopherols (such as vitamin E). 

Isoprene (2-methyl-1,3-butadiene) is a colorless liquid, soluble in alcohol but not in water. Its boiling temperature is 34.1 °C. 

Fluoboric acid is also an efficacious promoter of cyclic oxo-carbenium ions (Scheme 4.24) bearing an activated double bond which, in the presence of open-chain and cyclic dienes, rapidly undergo a Diels-Alder reaction [91]. Chiral a, -unsaturated ketones bearing a -hydroxy substituents, protected as acetals, react with various dienes in the presence of HBF4, affording Diels-Alder adducts that were isolated as alcohols by hydrolysis of the acetal group by TsOH. Some examples of reactions with isoprene are reported in Table 4.23. The enantios-electivity of the reaction is dependent on the size of the substituent R on the of-carbon high levels of asymmetric induction were observed with R = z-Pr (90 1) and R = t-Bu (150 1) and low levels with R = Me (2.7 1) and R = Ph (3.0 1). Scheme 4.24 shows the postulated reaction mechanism. 

Figure 47-6. The structure of dolichol. The phosphate in dolichol phosphate is attached to the primary alcohol group at the left-hand end of the molecule. The group within the brackets is an isoprene unit (n = 17-20 isoprenoid units).

No dimerization-carbonylation takes place with isoprene, irrespective of the catalytic species (78). Selective formation of 4-methyl-3-pentenoate (75) was observed in alcohol by the catalysis of either PdCl2 or Pd(OAc)2 and PPh3 ... 

Asymmetric telomerization of isoprene and methanol by using chiral phosphines, such as menthyldiphenylphosphine, gave an optical yield of 17.6%. The telomerization of methanol and isoprene using w-allylpalla-dium chloride and PBu3 in the presence of sodium methoxide in a mixed solvent of methanol and isopropyl alcohol at room temperature for 2 days produced l-methoxy-2,6-dimethyl-2,7-octadiene (89) (80%) and 1-meth-oxy-2,7-dimethyl-2,7-octadiene (91) (15%) (91). After 2 days, the reaction mixture was heated at 80°C for 8 hours, and 2,6-dimethyl-l,3,7-octatriene (88) (75%) and 2,7-dimethyl-1,3,7-octatriene (85) (14%) were obtained. Also, NiCl2(Bu3P)2 was used as a cocatalyst for the formation of 88. 

The results in Table VI were obtained by the reactions at 80°C with Pd(acac)2 using different ligands in a mixture of methanol and isopropyl alcohol. From these results, it seems likely that reaction temperature has large influence on the regiospecificity of the telomerization. As another example, a mixture of isomeric telomers was obtained by the reaction of methanol and isoprene at 100° (95). 

The biosynthesis of monoterpenes, the major components of peppermint essential oils, can be divided into four stages (Fig. 9.4). Stage 1 includes the formation of isopentenyl diphosphate (IPP) and dimethylallyl alcohol (DMAPP). In plants, two separate pathways are utilized for the synthesis of these universal C5 intermediates, with the cytosolic mevalonate pathway being responsible for the formation of sterols and certain sesquiterpenes, and the plastidial mevalonate-independent pathway being involved in the biosynthesis of isoprene, monoterpenes, certain sesquiterpenes, diterpenes, tetraterpenes, as well as the side chains of chlorophyll and plastoquinone.16 In peppermint oil gland secretory cells, however, the mevalonate pathway is blocked and the biosynthesis of monoterpenoid essential... 

---