A Unifying Bioinspired Synthesis of (−)-Asperaculin A and (−)-Penifulvin D

The first syntheses of the isomeric dioxafenestrene natural products (−)-asperaculin A and (−)-penifulvin D are reported. Each target is formed selectively by choice of oxidant in a final divergent bioinspired Baeyer–Villiger (BV) reaction. Density functional theory calculations reveal that electrostatic interactions between the oxidant leaving group and the lactone motif accounts for a reversal of selectivity with H2O2/H3O+ compared to peracids. Synthetic features include forging the polycyclic carbon framework with a diastereoselective meta-photocycloaddition biased by an ether substituent at the aryl α-position. The encumbered tertiary alcohol was installed by cyanation of a ketone intermediate followed by nonaqueous hydrolysis of the resulting delicate cyanohydrin.

conducted on a small scale. We recommend that application of these conditions on a larger scale is preceded by a detailed risk assessment with respect to potentially explosive intermediates.

I. General Procedures.
Equipment: All reactions were conducted in dry glassware. Drying was accomplished by evacuation of the vessel followed by heating with a hot air gun for >5 minutes. The vessels were then backfilled with dry nitrogen or argon gas, and allowed to cool to ambient and the filtrate concentrated under reduced pressure. The resulting oil was diluted water and pentane. The aqueous phase was extracted with pentane (x 3) and the combined organic layers were washed with brine, dried (Na2SO4, then filtered) and the filtrate concentrated under reduced pressure to give a yellow oil, which was purified by flash chromatography (2 -6 % ethyl acetate/heptane) to give alcohol 6.
Yield: 2.74 g (37%). Isolated as a pale yellow oil containing 60 wt% ethyl acetate (4.54 g total mass). To avoid mass-loss during further concentration, the mixture was typically used directly in the subsequent step. Analytical data is given for a >95% homogeneous sample.
Rf: 0.19 in 10% diethyl ether in pentane. UV active and stains blue with CAM.
Rf: 0.22 in 3% Ethyl acetate in pentane. UV active and stains blue with CAM.

Linear cycloadduct 9b
Yield : 166 mg (19%). Isolated as a pale yellow oil, >95% pure by NMR spectroscopy and a single spot by TLC.

UV/Vis
The resulting mixture first turned red, then purple, and finally a deep persistent blue color emerged. The mixture was then stirred at -78 °C for 3 h after which ammonium chloride (sat. aq.) and ether were added carefully until the blue coloration faded and the mixture was warmed to 0 °C under a positive flow of nitrogen. Ammonium chloride (sat. aq) was then added in small portions until the remaining pieces of lithium metal were completely consumed. The aqueous phase was extracted with ether (x 3) and the combined organic layers were washed with ammonium chloride (sat. aq), brine, dried (Na2SO4, then filtered), and the filtrate concentrated under reduced pressure. The resulting crude oil (5.68 g) contained a mixture of angular product 10a along with its double bond isomer 10b 4 in a 77 : 23 ratio as determined by 1 H NMR spectroscopy. Purification by flash chromatography (0 -5% ether/pentane) gave olefin 10a as a single isomer.
Rf: 0.33 in 5% diethyl ether in pentane. Faintly UV active at very high concentrations and stains blue with CAM.

HRMS (ESI-QTOF) (m/z):
A mass ion corresponding to the expected m/z could not be found.
The mixture was then neutralized with solid NaHCO3. Water was added, and the aqueous phase was extracted with dichloromethane (x 3) and the combined organic layers were washed with brine, dried (Na2SO4, then filtered) and the filtrate concentrated under reduced pressure to give Strand and co-workers 2021 -S12 -an amber oil. This oil was purified by flash chromatography (7 -10% ethyl acetate/pentane) and carefully concentrated to give the volatile alcohol S3. 5 Yield: 227 mg (95%). Isolated as a yellow oil as a solution in pentane (529 mg total, 52 wt%).
To avoid mass-loss during further concentration, the mixture was typically used directly in the subsequent step. Analytical data are given for a >95% homogeneous sample.
Rf: 0.33 in 10% ethyl acetate in pentane. Faintly UV active at very high concentrations and stains blue with CAM. 189.1642. 5 It is recommended that the compound is concentrated carefully to near dryness after chromatography and used directly in the next step to avoid unnecessary loss of mass.
To avoid mass-loss during further concentration, the mixture was typically used directly in the subsequent step. Analytical data are given for a >95% homogeneous sample.
Rf: 0.30 in 3% ethyl acetate in pentane. Faintly UV active and stains blue with CAM.
Yield: 107 mg (47%). Isolated as a viscous yellow oil, ˃95% pure by NMR spectroscopy and one spot by TLC.

Penifulvin D (2).
To a solution of ketone 1b (5 mg, 20 µmol) and sodium bicarbonate (38 mg, 450 µmol) in dichloromethane (4 mL) was added meta-chloroperbenzoic acid (25 mg, 150 µmol) in one portion and the mixture was stirred for 3 days. Sodium bisulfite (sat. aq.) was then added and the aqueous phase extracted with dichloromethane (x 3) and the combined organic layers were washed with NaHCO3 (sat. aq), dried (Na2SO4, then filtered), and the filtrate concentrated under reduced pressure to give a 95 : 5 mixture of the penifulvin D (2) and Strand and co-workers 2021 asperaculin A (3) as shown by 1 H NMR spectroscopy of the reaction crude. The mixture was purified with flash chromatography (40 -100% ether/pentane) to give penifulvin D (2). Yield: 2.3 mg (46%). Isolated as an amorphous colorless solid, >95% pure by NMR spectroscopy and a single spot by TLC.

Asperaculin A (3).
To a solution of ketone 1b (5 mg, 20 µmol) in dichloromethane (5.0 mL) was added 50% aqueous hydrogen peroxide (0.10 mL, 1.80 mmol), followed by a solution of triflic acid in dichloromethane (3.8 M, 0.50 mL). The resulting mixture was stirred for 2 min, after which sodium bisulfite was added. The aqueous phase was extracted with dichloromethane (x 3) and the combined organic layers were washed with NaHCO3 (sat. aq.), Strand and co-workers 2021 -S21 -dried (Na2SO4 then filtered) and the filtrate concentrated under reduced pressure to give a yellow oil containing a 77 : 23 mixture of asperaculin A (3) and penifulvin D (2) as measured by 1 H NMR spectroscopy. The mixture was purified by flash chromatography (40 -100% Et2O/pentane) to give asperaculin A (3).

III.
Comparison of synthetic and natural 2 and 3.

Single crystal X-ray diffraction (scXRD) analysis of 2 and 3
Single crystals covered in paratone oil were cut to size and mounted on a MiTeGen micromount loop. For low temperature data-collection, the mounted crystals were rapidly transferred to the nitrogen cold stream of the diffractometer. Data collection was performed on an Agilent Xcalibur Sapphire3 or an Agilent Enhance diffractometer equipped with a MoKα highbrilliance IµS radiation source (λ = 0.71073 Å) and an Oxford Cryosystems low temperature device. Absorption was corrected for using multi-scan empirical absorption correction with spherical harmonics as implemented in the SCALE3 ABSPACK scaling algorithm. 10 The structures were solved in WinGX 11 using SUPERFLIP 12 or SHELXL 2016/4 13 and refined using SHELXL 2016/4. Non-hydrogen atoms were refined anisotropically.
The vessel was capped and degassed as described above. The sample was irradiated in a Rayonet RPR-100 photoreactor at the specified frequency. At the stated time points the tube was removed and an aliquot (0.50 mL) was drawn. The remaining mixture was degassed, and irradiation continued. The aliquot was carefully concentrated (700 mbar at 40 °C) and then 1-Strand and co-workers 2021 -S27 -methoxynaphthalene (25 μL, 170 μmol) was added as an internal standard together with CDCl3.
The sample was homogenized and the amounts of 8, 9a, and 9b were quantified by 1 H NMR spectroscopy.
The sample was irradiated in a Rayonet RPR-100 photoreactor at the specified frequency. At the stated time points the mixture was concentrated and analysed by 1 H NMR spectroscopy.
The mixture was then re-dissolved in pentane, degassed, and subjected to irradiation. Further irradiation beyond 72h at 300nm or 2h at 254 nm led to significant decomposition/by-product formation.

VII. Comparison of heat of formation between model acetal S4 and bis-lactone S5
To gain further insight into the origin of the difference in free energy between asperaculin A and penifulvin D we used 1,4-dioxane-2,5-dione (S4) and 1,3-dioxane-4,6-dione (S5) as simplified models capturing differences between the key acetal/bis-lactone motif. The gas phase energies were calculated using the Jaguar workflow for Heat of Formation, employing the DFT/M06-2x functional and the 6-311g-3df-3pd++ basis set, both for geometry optimizations and for energy calculations. The results show that the acetal is thermodynamically favored over the corresponding bis-lactone motif by 9.8 kcal/mol at 298K.