Synthetic Study of 1,3-Butadiene-Based IMDA Approach to Construct a [5−7−6] Tricyclic Core and Its Application to the Total Synthesis of C8-epi-Guanacastepene O

Abstract

An efficient intramolecular Diels-Alder (IMDA) strategy for the construction of the [5-7-6] tricyclic core (18) of guanacastepenes has been developed from cis- and trans-1,3-butadiene-tethered 4-oxopent-2-ynoic acid ethyl esters 10 and 11. This method facilitates the synthesis of C8-epi-guanacastepene O (36) in a very efficient manner.

Chemoenzymatic Approach to Optically Active 4‐Hydroxy‐5‐alkylcyclopent‐2‐en‐1‐one Derivatives: An Application of a Combined Circular Dichroism Spectroscopy and DFT Calculations to Assignment of Absolute Configuration

“…The organic extracts were combined and solvent was evaporated. The residue was purified by column chromatography on silica gel (10 g, hexane:AcOEt, 6:4) to give (±)‐ 4b containing ~1% of 5b (406 mg, 42% yield): 1 H NMR (400 MHz) 7.51 (dd, J = 5.6, 2.4 Hz, 1H), 6.16 (dd, J = 5.6, 1.2 Hz, 1H), 4.80‐4.79 (m, 1H), 2.39 (brs, 1H, OH), 2.32‐2.24 (m, 1H), 2.19 (dd, J = 4.4, 2.4 Hz, 1H), 1.09 (d, J = 6.8 Hz, 3H), 0.83 (d, J = 6.8 Hz, 3H); 13 C NMR (100 MHz): 208.2, 162.2, 134.9, 72.8, 60.8, 27.1, 20.7 and 20.2, in agreement with reported data …”

“…The organic extracts were combined and solvent was evaporated. The residue was purified by column chromatography on silica gel (10 g, hexane:AcOEt, 6:4) to give (±)-4b containing~1% of 5b (406 mg, 42% yield): 7,21,22 An analogous experiment carried out over an extended reaction time afforded (±)-4b contaminated with a side product to which the structure of (±)-5b was tentatively assigned, 1 The stirred mixture was cooled to -78°C and n-butyllithium (2.32 M in hexane, 54 mL, 0.13 mol; 1 equiv) was added dropwise maintaining the temperature below -65°C. After completed addition, the mixture was allowed to warm to 0°C (a white cloudy precipitate appeared).…”

Diastereoselective Synthesis of Cyclopentanoids: Applications to the Construction of the ABCD Tetracyclic Core of Retigeranic Acid A

“…We then prepared substrate 23 (Scheme ), which has a fixed conformation that would allow the proposed Pauson–Khand reaction to proceed. Ketone 17 a was treated with LDA in THF at −78 °C and the resultant enolate was reacted with aldehyde 21 20 to afford alcohol 22 . This was then converted to its corresponding mesylate, followed by β‐elimination of the newly generated mesylate by using 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) as a base to give enyne 23 in 85 % yield over the two steps.…”

The Piancatelli Rearrangement: New Applications for an Intriguing Reaction

“…Bimatoprost (25) and travoprost (26) can be synthesized from the common intermediate 4-silyloxycyclopentenone 28, whose production was set-up on a kg-scale by exploiting a "Piancatelli rearrangement/chloral-mediated isomerization" sequence, starting from furfural, and subsequent enzymatic resolution of the resulting 4-hydroxycyclopentenone 27 [38] The rearrangement of 2-furylcarbinols has also been applied to the total synthesis of natural products [41,42]. Very recently, the core framework of the proposed structure of sargafuran (31) was accomplished via the Piancatelli rearrangement on intermediate 32 in the presence of MgCl 2 as the key-step [43].…”