Laurenanes:  Fenestranes with a Twist

Abstract

Four isomeric alcohols derived from the diterpene lauren-1-ene (1) have been examined by NMR methods (nuclear Overhauser enhancements, coupling constants, and variable-temperature studies) and by molecular mechanics using the MM3 force field to investigate a conformational twisting of the [5.5.5.7]fenestrane ring system. Results have been correlated with a concurrent study of remote functionalization reactions induced by iodobenzene diacetate/iodine under ultrasonic irradiation. Three of the laurenan-2-ols, 6, 7, and 8, lead to the same tetrahydrofuran derivative, 2beta,14beta-epoxylaurenane (9), and evidence for beta-cleavage of the alkoxy radical intermediate is obtained through the isolation of a ring-cleavage product 11 with a rearranged carbon skeleton. The products from the remaining alcohol 3 demonstrate solution dynamics involving the conformational twisting of the laurenane skeleton.

Fenestranes in Synthesis: Unique and Highly Inspiring Scaffolds

“…The structure was confirmed by X-ray analysis of the brominated derivative 4, and the angles around the central carbon atom were determined to be 117.98 and 118.98. [24] Laurenene (2) represents the first, and so far only, isolated natural product with an all-carbon fenestrane scaffold. Since its discovery, three total syntheses of laurenene have been completed: those of Crimmins and Gould [25] and Tsunoda et al [26] were published nearly simultaneously in 1987, and the third one by Wender et al [27] one year later.…”

Unprecedented [5.5.5.6]Dioxafenestrane Ring Construction in Fungal Insecticidal Sesquiterpene Biosynthesis

“…Numerous total syntheses of 1 have been achieved recently, [15][16][17] but the genetic and biosynthetic basis for 1 remain unknown. Ap revious hypothesis regarding the biosynthesis of 1 suggested that silphinene (6), an angular triquinane sesquiterpene isolated from various plants and used as an antifeedant, may be an intermediate. [18] It has been proposed that oxidation of 6 at the C15 methyl group yields carboxylic acid 7,w hich can undergo cleavage of the C1-C2 olefin to form dialdehyde intermediate 8.S ubsequent cyclization and oxidation of 8 to al actol (9)a nd d-lactone completes the biosynthesis of 1 (Figure 1b).…”

“…)isoprenoid diphosphates,and is subsequently cyclized by terpene cyclases (TCs) to generate structures that often contain multiply fused ring systems. [5] Among the types of chemical structures observed in cyclized terpenes,avery unique subgroup is those that contain the fenestrane core,a sf ound in the plant diterpene laurenene, [6] which contains four condensed rings and ad istorted tetracoordinated central carbon atom (Figure 1a). [3,4] During the course of this reaction, up to half of the carbon atoms in the linear substrate can undergo changes in C À Cc onnectivity and hybridization to produce quaternary and chiral centres.…”

“…[3,4] During the course of this reaction, up to half of the carbon atoms in the linear substrate can undergo changes in C À Cc onnectivity and hybridization to produce quaternary and chiral centres. [5] Among the types of chemical structures observed in cyclized terpenes,avery unique subgroup is those that contain the fenestrane core,a sf ound in the plant diterpene laurenene, [6] which contains four condensed rings and ad istorted tetracoordinated central carbon atom (Figure 1a). [7] Because of this unusual structure, fenestrane-based natural products have emerged as attractive targets in total chemical synthesis.…”

The Knowledge Base of the Structure Elucidator CASE System

“…Changes were not made in those cases where the contradiction was caused by the presence of ''anomalous'' chemical shifts in a structure, a so-called ''exotic'' structure. An example of such a structure 8 In structure 7.5 the chemical shift of the quaternary carbon atom that is common to all four cycles is 71.91 ppm, a value which is typical for a carbon neighboring an oxygen atom.…”