7-6-5 Tricarbocyclic Diterpenes

Peyssonnosides A–B, Unusual Diterpene Glycosides with a Sterically Encumbered Cyclopropane Motif: Structure Elucidation Using an Integrated Spectroscopic and Computational Workflow

“…33,34 A series of hydride and methyl shifts is expected to form a tertiary carbocation at C-11 (intermediate viii ), which would be intercepted by an incipient allylic carbanion at C-1, leading to a highly substituted cyclopropane ring. 35 It is plausible to speculate that the sterically encumbered cis-fusion of rings A and B at C-10 and C-11 in ix arises because the carbanion formed at C-1 in intermediate viii is conformationally constrained and consequently can attack C-11 from only one direction. As shown in Scheme 1, only structure a shown in Figure 5 adequately accounts for a reasonable diterpene biosynthetic pathway.…”

“…Subsequent hydride shift is envisioned as a concerted process with a ring closing reaction and would form intermediate iv . , The cyclopentane ring in iv would undergo a Wagner–Meerwein ring expansion reaction leading to the formation of the secondary carbocation v which consequently forms the cyclized intermediate vi . It is important to note that although secondary carbocations have been proposed in reaction coordinates of isoprenoid biosynthetic pathways, they often are not intermediates that exist as energy minima. , A series of hydride and methyl shifts is expected to form a tertiary carbocation at C-11 (intermediate viii ), which would be intercepted by an incipient allylic carbanion at C-1, leading to a highly substituted cyclopropane ring . It is plausible to speculate that the sterically encumbered cis-fusion of rings A and B at C-10 and C-11 in ix arises because the carbanion formed at C-1 in intermediate viii is conformationally constrained and consequently can attack C-11 from only one direction.…”

Access to Natural Valparanes and Daucanes: Enantioselective Synthesis of (−)-Valpara-2,15-diene and (+)-Isodaucene

“…The secondary alcohol 13 could be recycled into 14 via mesylation and S N 2′ hydrolysis, and considering this recycling, the primary alcohol 14 was obtained from 11 in a 36% overall yield. Since natural valparadiene ( 1 ) has been reported to be the ent- form, chiral (−)-diethyl tartrate was used to perform the key asymmetric Sharpless epoxidation, which afforded 15 in 77% yield. The silyl ether protection of the resulting asymmetric epoxy alcohol 15 afforded the key intermediate 8 .…”

“…Five-, six-, and seven-membered carbocycles co-occur in natural terpenes possessing different molecular backbones. Among them, diterpene valparanes, isolated from the Cistaceae family of plants, contain a cyclohexane fused between five- and seven-membered rings. A large majority of these diterpenes share the stereochemistry seen in valpara-2,15-diene ( 1 ) as well as the presence of an isopropenyl group at C-14 of the cyclopentane ring.…”