A9-THC

The chemical name of A9-THC according to the dibenzopyrane numbering system is 3-pentyl-6,6,9-trimethyl-6fl,7,8,10fl-tetrahydro-6H-dibenzo-[b, d]pyran-l-ol as depicted in 1.1 (Fig. 1). 

On the market are two drugs imder the trade names of Dronabinol, which is the generic name of trans-A9-THC, and Marinol, which is a medicine containing synthetic dronabinol in sesame oil for oral intake, distributed by Unimed Pharmaceuticals. 

A9-THC (2.1 in Fig. 2) is the only major psychoactive constituent of C. sativa. It is a pale yellow resinous oil and is sticky at room temperature. A9-THC is hpophihc and poorly soluble in water (3 p,g mL ), with a bitter taste but without smell. Furthermore it is sensitive to light and air [4]. Some more physical and chemical data on A9-THC are fisted in Table 1. Because of its two chiral centers at C-6a and C-lOa, four stereoisomers are known, but only (-)-trans-A9-THC is foimd in the Cannabis plant [5]. The absolute configuration of the... 

It must be noted that A9-THC is not present in C. sativa, but that the te-trahydrocannbinolic acid (THCA) is almost exclusively found. Two kinds of THCA are known. The first has its carboxylic function at position C-2 and is named 2-carboxy-A9-THC or THCA-A (2.2) the second has a carboxylic function at position C-4 and is named 4-carboxy-A9-THC or THCA-B (2.3). 

THCA shows no psychotropic effects, but heating (e.g., by smoking of Cannabis) leads to decarboxylation, which provides the active substance A9-THC. A9-THC is naturally accompanied by its homologous compounds containing a propyl side chain (e.g., tetrahydrocannabivarin, THCV, THC-C3,2.4) or a butyl side chain (THC-C4,2.5). 

Seventy cannabinoids from C. sativa have been described up to 2005 [2j. Mostly they appear in low quantities, but some of them shall be mentioned in the following overview - especially because of their fimctions in the biosynthesis of A9-THC and their use in medicinal applications. 

The lUPAC name of cannabidiol is 2-[(lS, 6iI)-3-methyl-6-prop-l-en-2-yl-l-cyclohex-2-enyl]-5-pentyl-benzene-1,3-diol. Cannabidiol (CBD, 2.9) in its acidic form cannabidiolic acid (CBDA, 2.10) is the second major cannabinoid in C. sativa besides A9-THC. As already mentioned for A9-THC, variations in the length of the side chain are also possible for CBD. Important in this context are the propyl side chain-substituted CBD, named cannabidivarin (CBDV, 2.11), and CBD-C4 (2.12), the homologous compound with a butyl side chain. Related to the synthesis starting from CBD to A9-THC as described in Sect. 3.1, it was accepted that CBDA serves as a precursor for THCA in the biosynthesis. Recent publications indicate that CBDA and THCA are formed from the same precursor, cannabigerolic acid (CBGA), and that it is unlikely that the biosynthesis of THCA from CBDA takes place in C. sativa. 

This compound and its related acidic form, A8-tetrahydrocannabinolic acid (A8-THCA, 2.13) are structural isomers of A9-THC. Although it is the thermodynamically stable form of THC, A8-THC (2.14) contributes approximately only 1% to the total content of THC in C. sativa. In the synthetic production process, A8-THC is formed in significantly higher quantities than in plants. 

Cannabinidiol (CBND, 2.18) and cannabinol (2.19) are oxidation products of CBD and A9-THC formed by aromatization of the terpenoid ring. For the dehydrogenation of THC a radical mechanism including polyhydroxylated intermediates is suggested [10,11]. CBN is not the sole oxidation product of A9-THC. Our own studies at THC-Pharm on the stability of A9-THC have shown that only about 15% of lost A9-THC is recovered as CBN. 

In this section, the latest developments and recent publications on the biosynthesis of A9-THC and related cannabinoids as precursors are discussed. Special points of interests are the genetic aspects, enzyme regulation, and the environmental factors that have an influence on the cannabinoid content in the plant. Because of new and innovative developments in biotechnology we will give a short overview of new strategies for cannabinoid production in plant cell cultures and in heterologous organisms. 

As depicted in Fig. 3, in glandular trichoma the cannabinoids are produced in the cells but accumirlate in the secretory sac of the glandular trichomes, dissolved in the essential oil [17-21]. Here, A9-THC was found to accumulate in the cell wall, the fibrillar matrix and the surface feature of vesicles in the secretory cavity, the subcutilar wall, and the cuticula of glandular trichomes [19]. 

After identification of A9-THC as the major active compound in Cannabis and its structural elucidation by Mechoulam and Gaoni in 1964 [66], a lot of work was invested in chemical synthesis of this substance. Analogous to the biosynthesis of cannabinoids, the central step in most of the A9-THC syntheses routes is the reaction of a terpene with a resorcin derivate (e.g., olivetol). Many different compounds were employed as terpenoid compounds, for example citral [67], verbenol [68], or chrysanthenol [69]. The employment of optically pure precursors is inevitable to get the desired (-)-trans-A9-THC. 

A general problem during the syntheses of A9-THC is the formation of the thermodynamically more stable A8-THC, which reduces the yield of A9-THC. It is formed from A9-THC by isomerization under acidic conditions. While the usage of strong acids such as p-TSA or TEA leads mainly to A8-THC, the yield of A9-THC can be increased by employment of weak acids, e.g., oxalic acid [70]. 

Recently the most employed method for the production of A9-THC on industrial scale is the condensation of (+)-p-mentha-2,8-dien-l-ol (5.1 in... 

Fig. 5) with olivetol (5.2) in the presence of boron trifluoride etherate, BF3 0C(C2H5)2 with CBD as a key intermediate. This one-step synthesis of A9-THC is also used for the production of synthetic dronabinol, which is used in the medicinal application named Marinol. The mechanism of this synthesis is particular described by Razdan et al. [71] and is shown in Fig. 5... 

Synthesis of A9-Tetrahydrocannabinol from Natural Cannabidiol (Semisynthetic A9-THC)... 

As discussed, the cultivation of C. sativa with high content of A9-THC (drug-type) is not allowed in many coimtries. Because of this, there is no opportunity to harvest a high amoimt of the medicinally important substance A9-THC directly from plant material. In the synthesis route for semisynthetic A9-THC, natural CBD from fiber hemp plants is employed. It can be extracted with non-polar solvents such as petroleum ether and purified by recrystalUza-fion in n-pentane. This procedure avoids the formation of abnormal CBD and gives the opportunity to produce A9-THC from fiber hemp. Semisyn-fhetic A9-THC is disfinguishable from the synthetic compound because it contains, besides the major product, small amounts of A9-THC-C3 and A9-THC-C4, which are not available in the synthetic product. 

Most relevant for the affinity for A9-THC and analogs to CB-receptors are the phenolic hydroxyl group at C-1, the kind of substitution at C-9, and the properties of the side chain at C-3. Relating to the structure-activity relationships (SAR) between cannabinoids and the CB-receptors, many different modified strucfures of fhis subsfance group were developed and fesfed. The most important variations include variations of the side chain at the olivetolic moiety of the molecules and different substitutions at positions C-11 and C-9. One of the most popular analogous compounds of A9-THC is HU-210 or (-)-trans-ll-OH-A8-THC-DMH, a cannabinoid with a F,l-dimethylheptyl side... 

Direct oxidation of A9-THC at position C-11 involves mainly an isomerization to A8-THC another opportimity in the synthesis of A9-THC-metabolites is the pretreatment of terpenoid synthons by introduction of protective groups, e.g., 1,3-dithiane (6.1 in Fig. 6) followed by the condensation with olivetol (6.2) [76]. The formed product is a protected derivate... 

Fig.6 Synthesis of main metaboiites of A9-THC a CH3SO3H, b (C2H50)20/pyridine, c Hg0/BF3 0(C2H5)2, d NaCN/CH3C00H/Mn02, e NaOH/THF,/ NaBH4/EtOH...

When [ H]-labeled precursors are employed the resulting compounds can be used as internal standards for analysis, especially by utilization of mass spectrometric methods. Appropriate deuterated standards are shown in Fig. 7. The introduction of deuterium into the A9-THC precursors can be done with Grignard reagents such as C[ H3]MgI or reducing substances such as LiAl[ H4]. The general procedures for the synthesis with these [ Hj-labeled precursors are the same as described above for the unlabeled compounds [76,78]. 

The chemical composition of C. sativa is very complex and about 500 compounds in this plant are known. A complete list can be found in [81] with some additional supplementations [2,82]. The complex mixture of about 120 mono- and sesquiterpenes is responsible for the characteristic smell of C. sativa. One of these terpenoic compounds, carophyllene oxide, is used as leading substance for hashish detection dogs to find C. sativa material [83]. It is a widespread error that dogs that are addicted to drugs are employed for drug detection. A9-THC is an odorless substance and cannot be sniffed by dogs. 

The aim of the analysis of cannabinoids in plants is to discriminate between the phenotypes (drug-type/fiber-type). Quantification of cannabinoids in plant material is needed if it will be used in medicinal appHcations, e.g., in C. sativa extracts. The ratio between A9-THC and CBN can be used for the determination of the age of stored marijuana samples [84]. 

Analytical Methods for Detection of A9-THC and Other Cannabinoids in Plants... 

A9-THC and its main metabolites are detected and quantified in forensic samples. Determination of these compounds in human beings is needed to make decision on abuse of A9-THC-containing drugs by individuals. A careful interpretation of the results is very important to avoid fallacies with regard to the behavior of individuals. The Cannabis influence factor (GIF), for example, is an useful tool for distinguishing between acute and chronic intake of A9-THC [98]. 

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