A - Terpineol

Rerfision Problem 2 a-Terpineol also occurs widely in plants and was one of the first natural products to be isolated pure. There was originally some doubt as to whether its structure was TM 394A or TM 394B. Suggest syntheses of both these compounds so that they can be compared with the natural product. 

In the eyent, TM 394A proyed to be a-terpineol and the shortest synthesis is by Alder and Vogt (Annalen. 1940, 564, 109) using the Diels-Alder reaction ... 

Dehydration. Dehydration of hydroxy fatty acids is quite common. Other compounds undergo the same reaction, eg, elymoclavine [548-43-6] to agroclavine [548-42-5] chanoclavine [2390-99-0] and other compounds and <7j -terpin hydrate [2541-01-63] to a-terpineol [98-55-5] (19). 

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). 

Synthetic pine oil is produced by the acid-catalyzed hydration of a-pinene (Fig. 1). Mineral acids, usually phosphoric acid, are used in concentrations of 20—40 wt % and at temperatures varying from 30—100°C. Depending on the conditions used, alcohols, chiefly a-terpineol (9), are produced along with /)-menthadienes and cineoles, mainly limonene, terpinolene, and 1,4- and 1,8-cineole (46—48). Various grades of pine oil can be produced by fractionation of the cmde products. Formation of terpin hydrate (10) from a-terpineol gives P-terpineol (11) and y-terpineol (12) as a consequence of the reversible... 

Pine oils can be fractionally distilled to produce a higher a-terpineol product, but usually contain bomeol and y-terpineol, along with small amounts of other components. High grade perfumery a-terpineol can be made by the partial dehydration ofyvmenthane-l,8-diol (terpin hydrate) under mildly acidic conditions (117,118). 

Hydrogenation of a-terpineol gives dihydroterpineol [498-81-7] a mixture of cis- and trans-compounds (120). Dihydroterpinyl acetate [80-25-1] produced from the mixture is also a useful fragrance compound. 

Linalool has been used to prepare a mixture of terpenes useful for enhancing the aroma or taste of foodstuffs, chewing gums, and perfume compositions. Aqueous citric acid reaction at 100°C converts the linalool (3) to a complex mixture. A few of the components include a-terpineol (34%) (9), Bois de Rose oxide (5.1%) (64), ocimene quintoxide (0.5%) (65), linalool oxide (0.3%) (66), tij -ocimenol (3.28%) (67), and many other alcohols and hydrocarbons (131). 

The glucosides of menthol, citronellol, nerol, geraniol, cw-myrtenol, L-borneol, linalool and a-terpineol yielded yellow-green fluorescent chromatogram zones in long-wavelength UV light (2 = 365 nm). The same applied to arbutin (hRf 45 — 50). 

Loss of a proton from the tertiary carbocation fonned in this step gives limonene, an abundant natural product found in many citrus fruits. Capture of the carbocation by water gives a-terpineol, also a known natural product. 

Mondello et al. (2, 20-23) have used a multidimensional gas chromatographic system based on the use of mechanical valves which were stable at high temperatures developed in their laboratory for the determination of the enantiomeric distribution of monoterpene hydrocarbons (/3-pinene, sabinene and limonene) and monoterpene alcohols (linalol, terpinen-4-ol and a-terpineol) of citrus oils (lemon, mandarin, lime and bergamot). Linalyl acetate was also studied in bergamot oil. The system consisted of two Shimadzu Model 17 gas chromatographs, a six-port two-position valve and a hot transfer line. The system made it possible to carry out fully... 

Woker points out that the closing of a chain compound to form a ring compound does not affect the odour much, thus the aliphatic terpineol of W. H. Perkin, Jr., 2 3 di-methyl 5 hexenol 2 has a verj similar odour to a-terpineol, their respective formulae being —... 

Terpineol (that is a-terpineol) has been prepared synthetically by Perkin and his pupils, his method being described under the synthesis of limonene. 

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