A regioselective Wagner–Meerwein rearrangement directed towards the six-membered ring of the longipinane skeleton

Novel Sesquiterpene Skeletons by Multiple Wagner–Meerwein Rearrangements of a Longipinane-1,9-diol Derivative

“…The reaction mechanisms for the formation of 8 , 11 , 14 , 16 , 18 , and 22 (Schemes – ) were outlined according to the extensive knowledge of the transformation of terpenoids, which include site-specific deuterium labeling − to reinforce the existence of 1,2- and 1,3-hydride shifts. ,− In addition, the presence of nonclassical carbocations, ,,− which have been widely studied in the rearrangement of terpenes containing a norbornyl moiety, was taken into account. It is reasonable to assume that formation of these new compounds occurs through pathways that resemble those proposed for uruapanes and jiquilpanes according to the reaction mechanism depicted in Schemes –. For the formation of 8 (Scheme ), Lewis acid coordination at the C-1 hydroxy group of 7 induces migration of the anti-periplanar C-5–C-11 bond to form the C-5–C-1 bond.…”

“…A solution of rastevione ( 1 ) (400 mg) in MeOH (10 mL) was treated with NaBH 4 (250 mg) at room temperature for 15 min. The reaction mixture was poured over ice–H 2 O and extracted with EtOAc.…”

“…Their phytochemical studies started almost four decades ago with the structure elucidation of rastevione ( 1 ), which is easily available from the hexanes extracts of the roots of S. serrata . Since then, the longipinane skeleton has inspired the study of molecular rearrangements, − since it possesses a central strained four-membered ring simultaneously holding a six- and a seven-membered ring in 1,3 dispositions. The chemical exploration of these substances has led to the discovery of new carbocyclic scaffolds obtained through a series of molecular rearrangements promoted by the strategic location of functional groups on the tricyclic skeleton, which act as leaving groups when the four-membered ring strain is released.…”

“…The chemical exploration of these substances has led to the discovery of new carbocyclic scaffolds obtained through a series of molecular rearrangements promoted by the strategic location of functional groups on the tricyclic skeleton, which act as leaving groups when the four-membered ring strain is released. Thus, when 1 was hydrolyzed to triolone 2, esterified to diacetate 3, and treated with Et 2 O−BF 3 , a Wagner− Meerwein rearrangement involving the seven-membered ring produced the moreliene derivative 4, 11 while triesterification of 2, followed by C-1 carbonyl reduction, afforded 5, 8 which led to the jiquilpane derivative 6 by treatment with Et 2 O−BF 3 via two successive Wagner−Meerwein shifts directed toward the six-membered ring. 9 As a result of these antecedents, in the present study the chemical behavior of rasteviol (7) was explored.…”

Novel Sesquiterpenoid Skeletons by Wagner–Meerwein Rearrangements of Longipinane-9,13-diol-1-one Derivatives

“…It is known that the degree of functionalization and stereochemistry of the functional groups present in the longipinane skeleton are decisive in directing the rearrangements toward the six- or the seven-membered ring. This allows rearrangements in highly functionalized longipinanes to generate various novel skeletons and provide valuable information about their chemical behavior, which is very useful for the design of new synthetic strategies. − …”