A-Santalene

Two distinct sesquiterpenes exist in the essential oil of Santalum album, known as a-santalene and /3-santalene. 8-santalene is probably a bicyclic and a-santalene a tricyclic sesquiterpene. These sesquiterpenes were discovered by Guerbet. From the method of their preparation it is doubtful whether they have been obtained in a state of purity, so that the characters assigned to them must be accepted with some reserve, and as probably being only approximate. 

Further studies on the biosynthesis of ovalicin (57) have demonstrated the use of deuterium magnetic resonance in biosynthetic studies. Compounds (59)—(62), structurally related to (-)-a-santalene (58), have been isolated from the essential oil of Lavandula ojficinalis and L. hybrida " An alternative synthesis of a-santalol... 

As an example, when phenylpotassium is allowed to react with 8-bromonortricyclene, a-santalene is produced which is then submitted again to metallation with LIC-KOR followed by borylation and oxidation.76 The allowance for torsional equilibration produces the (Z)-a-santalol, one of the main constituents of the highly prized sandalwood oil, with no trace of the (A l-isomcr. The entire sequence can also be contracted to a one-flask protocol still giving quite acceptable yields.76... 

Opopanax Commiphora guidottii /3-Ocimene (82), (S,Z)-a-bisabolene (76), a-santalene (83) 109... 

These variously substituted camphor derivatives have in turn been used in a number of syntheses of natural products, some of which also involve Wagner-Meerwein rearrangements in their later stages. Examples drawn from the syntheses of (+)-a-santalene (34),(-)-3-santalene (35), (-)-copacamphene (36) and (-)-sativene (37) are given in Schemes 12-14. 

Allylic halogenation of aldehyde enol acetates, e.g. 23, derived from norbornadiene provided an elegant and efficient construction of the tricyclic structure 24 found in many sesquiterpenes such as cyclosativene, cyclocopacamphene, longicyclene, a-santalene and its derivatives. This rearrangement in fact arises from an electrophilic addition to a multiple bond with participation of the homoallylic bond. The tricyclic skeleton was also obtained successfully by treatment of 5-methylenenorborn-2-ene with A-bromosuccinimide in aqueous dimethyl sulfoxide which gave 5-bromo-2-hydroxymethyltricyclo[2.2.1.0 ]heptane (25) in high yield. 

Sand-sodium base. See Sodium-sand base a-Santalene, 42 Sobering process, 118-119 Schiff bases. See Imines 9,10-Secosteroids (calciferols and tachysterols), 31-32, 34, 281-283, 289 Seebach-Frater alkylation, 27. AT-Selectride = potassium hydrotris(l-mcthylpropyl)borate(l —) (KBHBVl), 107 XS-Selectride 1 — potassium tris(l,2-dimethyl-propyl)hydroborate(l —) (KBHSiay), 107 Z-Selectride = lithium hydrotri (l-... 

The bisabolyl cation (P10.1) is the key intermediate in the biosynthesis of the bergamotane, a- and (3-santalane families of terpenes. The biogenetic reactions involved in converting (P10.1) to the final products, are all enzyme mediated, but their chemistry follows the basic principles of carbocation mechanisms. Draw mechanisms to account for the formation of a-bergamotene (P10.2), a-santalene (P10.3) and (3-santalene (P10.4) from the cation (P10.1) and propose a structure for the intermediate (X) (Figure P10). 

The two a-santalene derivatives (79) and (80) are constituents of the aerial parts of Ayapana amygdalina. (+)-Epi-c/5-/S-santalol (81) has been identified as a new minor component of East Indian sandalwood oil. Treatment of (+)-a-santalyl acetate (82) with hydrogen chloride followed by dehydrOchlorination with basic alumina produces a mixture of /3-santalyl acetate (83) and the acetate of (81). Last year Christenson and Willis reported the acid-catalysed rearrange-... 

In solution in polar solvents the complexes couple with primary and secondary alkyl, aryl, vinyl and allyl bromides and iodides at or below room temperature. This was exploited by Corey as early as 1956 in his classic synthesis of a-santalene. Coupling via the Grignard reagent gave low yields. 

Cross-coupling of the complex (XCIII) and the iodide (CXX) leads to the formation of a-santalene (CXXI) in 88% yield (Corey and Semmelhack, 1967). j -Santalene (CXXII) has been also prepared by applying this process... 

Of the other skeletal types shown in Fig. 8.1.10 bergamotane type (161) has not, apparently, yet been isolated from woody tissues. Santalane type is represented by sesquiterpenoids from Indian sandalwood oil, which is obtained by steam distillation of wood of Santalum album. Important constituents are a- and )ff-san-talene (182,183), and the derived allylic alcohols 184,185 which constitute almost 90% of the essential oil (71, 98, 238, 242). It may be noted that a-santalene was the first sesquiterpene for which the correct structure was proposed (344). 

Semmler F W 1910 Constituents of ethereal oils. Constitutions of the a-santalol and of the a-santalene series, and of sesquiterpene alcohols and of sesquiterpenes. Ber Deutsch Chem Ges 40 1893-1898... 

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