taxonID	type	description	language	source
63358663FFBE5C166663E1ECFCC37974.taxon	description	Biosynthetically, the biogenesis of gem-dimethyl cyclobutanes can be traced back to geranyl pyrophosphate and / or its longer chain analogues, which ultimately originate from isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP). The biosynthesis of the historic precursor β- amyrin (10) commences at the chair – chair – - chair – boat conformation of (3 S) - 2,3 - oxidosqualene A (Scheme 1). This conformation allows it to cyclize to the corresponding 6.6.6.5 - fused tetracyclic dammarenyl C 20 cation B from which a group of dammarane terpenes were isolated (Scheme 2) (Fattorusso et al., 1985). A series of enzymatically driven reactions included a concerted hydride shift from C 13 to C 17, a C 18 methyl group shift from C 14 to C 13, a C 30 methyl group shift from C 8 to C 14 and the base abstraction of the C 9 proton to construct the double bond between C 8 and C 9, leading to tirucallane-type structures and consequently the isolation of a few tirucallic acids (Scheme 3) (Pardhy and Bhattacharyya, 1978 a). Approximately 34 lupane structures, including lupeolic acids, were isolated from various Boswellia species (Schemes 4 and 5) (Proietti et al., 1981). Their biosynthetic formation involved ring D expansion, electrophilic addition on the terminal double bond of the tetracyclic baccharenyl cation C, and the construction of five-membered ring E to form lupanyl cation D. This reaction cascade proceeds to oleanyl cation F and then the oleanane derivatives (Scheme 6) (Mannino et al., 2016) by ring E enlargement, which occurs by the C 30 shift from C 20 to C 19. Oleanyl cation F undergoes three major steps, including sequential 1,2 - hydride shifts from C 18 to C 19 and from C 13 to C 18 and an elimination of H- 12 to afford 6.6.6.6.6 - fused pentacyclic ursane derivatives (Schemes 7 and 8) (Reddy et al., 2003). Notably, β- amyrin (10), the precursor for α- boswellic acid, is oleanane and α- amyrin (9), and the precursor for β- boswellic acid is ursane. The C- 24 oxidation of 9 and 10 then afforded β- BA and α- BA, respectively (Schemes 9 – 11) (Culioli et al., 2003; Schweizer et al., 2000). In a similar manner, the C- 24 oxidation of 9 afforded 3 - epi -- β- boswellic acid (1) (Schemes 9 – 11) (Shah et al., 2007). It is envisioned that a similar oxidation of 10 will furnish 3 - epi - α- boswellic acid (5). However, this compound has not been reported to date (Al-Harrasi et al., 2018 b).	en	Al-Harrasi, Ahmed, Khan, Abdul Latif, Rehman, Najeeb Ur, Rene, Csuk (2021): Biosynthetic diversity in triterpene cyclization within the Boswellia genus. Phytochemistry (112660) 184: 1-23, DOI: 10.1016/j.phytochem.2021.112660, URL: http://dx.doi.org/10.1016/j.phytochem.2021.112660
