Asymmetric Total Syntheses of Two 3-Acyl-5,6-dihydro-2H-pyrones: (R)-Podoblastin-S and (R)-Lachnelluloic Acid with Verification of the Absolute Configuration of (−)-Lachnelluloic Acid

Expedient asymmetric total syntheses of both (R)-podoblastin-S and (R)-lachnelluloic acid, representative of natural 3-acyl-5,6-dihydro-2H-pyran-2-ones, were performed. Compared with the reported total synthesis of (R)-podoblastin-S (14 steps, overall 5% yield), the present study was achieved in only five steps in an overall 40% yield and with 98% ee (HPLC analysis). In a similar strategy, the first asymmetric total synthesis of the relevant (R)-lachnelluloic acid was achieved in an overall 40% yield with 98% ee (HPLC analysis). The crucial step utilized readily accessible and reliable Soriente and Scettri’s Ti(OiPr)4/(S)-BINOL‒catalyzed asymmetric Mukaiyama aldol addition of 1,3-bis(trimethylsiloxy)diene, derived from ethyl acetoacetate with n-butanal for (R)-podoblastin-S and n-pentanal for (R)-lachnelluloic acid. With the comparison of the specific rotation values between the natural product and the synthetic specimen, the hitherto unknown absolute configuration at the C(6) position of (−)-lachnelluloic acid was unambiguously elucidated as 6R.


Results and Discussion
Screening of synthetic racemate analogues of natural (R)-podoblastins 2a-2c was carried out by one of the authors (Y.T.). This optimization, by changing the carbon long chain length of the 3-acyl moiety by settling the substituent in the 6-position as the n-Pr group, revealed that unnatural podoblastin-S (2d) had two-to three-fold stronger anti-fungal activity [12], using Fthalide as a representative fungicide reference [10] (Figure 2). On the other hand, the terminal double bond, as exemplified by podoblastin B, was found to be unimportant for anti-fungal activity. Thus, we selected asymmetric synthesis of (R)-podoblastin-S.
The first and sole chiral synthesis of (R)-podoblastin-S (2d) was performed by Ichimoto's group, starting from (S)-1,3-dioxolane-4-methanol, a highly expensive chiral synthon, through 14 steps with an overall yield of 5% [17]. Our synthesis of 2d involved a catalytic asymmetric Mukaiyama aldol addition as a crucial step, and was completed in a total of five steps. Moreover, we performed the first asymmetric total synthesis of (R)-lachnelluloic acid (3) containing the same 3-acyl-5,6-dihydro-2H-pyran-2-one structure as is in podoblastins. The unknown absolute configuration of natural (-)lachnelluloic acid (3) was unambiguously verified as (R), based on our synthetic strategy.

Results and Discussion
Screening of synthetic racemate analogues of natural (R)-podoblastins 2a-2c was carried out by one of the authors (Y.T.). This optimization, by changing the carbon long chain length of the 3-acyl moiety by settling the substituent in the 6-position as the n-Pr group, revealed that unnatural podoblastin-S (2d) had two-to three-fold stronger anti-fungal activity [12], using Fthalide as a representative fungicide reference [10] (Figure 2). On the other hand, the terminal double bond, as exemplified by podoblastin B, was found to be unimportant for anti-fungal activity. Thus, we selected asymmetric synthesis of (R)-podoblastin-S.
The first and sole chiral synthesis of (R)-podoblastin-S (2d) was performed by Ichimoto's group, starting from (S)-1,3-dioxolane-4-methanol, a highly expensive chiral synthon, through 14 steps with an overall yield of 5% [17]. Our synthesis of 2d involved a catalytic asymmetric Mukaiyama aldol addition as a crucial step, and was completed in a total of five steps. Moreover, we performed the first asymmetric total synthesis of (R)-lachnelluloic acid (3) containing the same 3-acyl-5,6-dihydro-2H-pyran-2-one structure as is in podoblastins. The unknown absolute configuration of natural (−)-lachnelluloic acid (3) was unambiguously verified as (R), based on our synthetic strategy.
Encouraged by the successful outcome, we next focused our attention on the relevant and first asymmetric total synthesis of (−)-lachnelluloic acid (3), which is also a natural anti-fungal product isolated from Lachnellula fuscosanguinea (Rehm) Dennis, as disclosed by Ayer and Villar [12]. (−)-Lachnelluloic acid (3) exhibits specific antagonistic activity against Dutch elm disease [22]. The first total synthesis of the racemic form of 3 was achieved by Ayer and Villar [9]. Later, a formal synthesis of racemate 3 was reported by Mineeva [23].
Encouraged by the successful outcome, we next focused our attention on the relevant and first asymmetric total synthesis of (−)-lachnelluloic acid (3), which is also a natural anti-fungal product isolated from Lachnellula fuscosanguinea (Rehm) Dennis, as disclosed by Ayer and Villar [12]. (−)-Lachnelluloic acid (3) exhibits specific antagonistic activity against Dutch elm disease [22]. The first total synthesis of the racemic form of 3 was achieved by Ayer and Villar [9]. Later, a formal synthesis of racemate 3 was reported by Mineeva [23].
Encouraged by the successful outcome, we next focused our attention on the relevant and first asymmetric total synthesis of (−)-lachnelluloic acid (3), which is also a natural anti-fungal product isolated from Lachnellula fuscosanguinea (Rehm) Dennis, as disclosed by Ayer and Villar [12]. (−)-Lachnelluloic acid (3) exhibits specific antagonistic activity against Dutch elm disease [22]. The first total synthesis of the racemic form of 3 was achieved by Ayer and Villar [9]. Later, a formal synthesis of racemate 3 was reported by Mineeva [23].
Notably, the present strategy was applied as a promising process route of the key common component of well-known HMG-CoA reductase inhibitors (statin drugs) [24], such as pravastatin, simvastatin, atorvastatin, and pitavastatin (Scheme 3). Scheme 3. Synthesis of statin drugs utilizing the asymmetric Mukaiyama aldol addition with alkyl 1,3-bis(trimethylsiloxy)diene 4 as the key step.

General
All reactions were carried out in oven-dried glassware under an argon atmosphere. Flash column chromatography was performed with silica gel Merck 60 (230-400 mesh ASTM, Darmstadt, Germany). TLC analysis was performed on 0.25 mm Silicagel Merck 60 F254 plates. Melting points were determined on a hot stage microscope apparatus (AS ONE, ATM-01, Osaka, Japan and were uncorrected. NMR spectra were recorded on a JEOL DELTA 300 (Tokyo, Japan) or JEOLRESONANCE ECX-500 spectrometer (Tokyo, Japan), operating at 300 MHz or 500 MHz for 1 H-NMR and 75 MHz 120 MHz for 13 C-NMR. Chemical shifts (δ ppm) in CDCl3 were reported downfield from TMS (= 0) for 1 H-NMR. For 13 C-NMR, chemical shifts were reported in the scale relative to CDCl3 (77.00 ppm) as an internal reference.IR Spectra were recorded on a JASCO FT/IR-5300 spectrophotometer (Tokyo, Japan). Mass spectra were measured on a JEOL JMS-T100LC spectrometer (Tokyo, Japan). HPLC data were obtained on a SHIMADZU HPLC system (consisting of the following: LC-20AT, CMB20A, CTO-20AC, and detector SPD-20A measured at 254 nm, Kyoto, Japan) using Daicel Chiracel AD-H or Ad-3 column (25 cm) at 25 °C. Optical rotations were measured on a JASCO DIP-370 (Na lamp, 589 nm, Tokyo, Japan).
nBuLi (1.45 M in hexane, 18 mL, 26 mmol) was added to stirred solution of iPr2NH (3.7 mL, 26 mmol) in THF (16 mL) at 0-5 °C under an Ar atmosphere, and the mixture was stirred for 5 min. The mixture was cooled down to −78 °C and ethyl 2-(trimethylsilyl)oxybut-2-enoate (4.05 g, 20 mmol) in THF (2.0 mL) was added dropwise over 3 min to the mixture, which was stirred at the same temperature for 0.5 h. TMSCl (3.0 mL, 26 mmol) in THF (2.0 mL) was added dropwise for 10 min to the mixture at the same temperature and the mixture was allowed to warm up to 0-5 °C over a period of 2 h. The mixture was concentrated using a rotary evaporator and filtered through Celite ® (No.503) using a glass filter, being washed with hexane (10 mL × 3). The filtrate was concentrated under reduced pressure to give the crude product 4 (5.03 g, 92%) [13,14], which was used for the next reaction without any purification.  −1 2961, 1649, 1443, 1391, 1250, 1196, 1090, 1015, 982, 835. 1 H-NMR and 13 C-NMR spectra: see supporting information.

General
All reactions were carried out in oven-dried glassware under an argon atmosphere. Flash column chromatography was performed with silica gel Merck 60 (230-400 mesh ASTM, Darmstadt, Germany). TLC analysis was performed on 0.25 mm Silicagel Merck 60 F 254 plates. Melting points were determined on a hot stage microscope apparatus (AS ONE, ATM-01, Osaka, Japan and were uncorrected. NMR spectra were recorded on a JEOL DELTA 300 (Tokyo, Japan) or JEOLRESONANCE ECX-500 spectrometer (Tokyo, Japan), operating at 300 MHz or 500 MHz for 1 H-NMR and 75 MHz 120 MHz for 13 C-NMR. Chemical shifts (δ ppm) in CDCl 3 were reported downfield from TMS (= 0) for 1 H-NMR. For 13 C-NMR, chemical shifts were reported in the scale relative to CDCl 3 (77.00 ppm) as an internal reference.IR Spectra were recorded on a JASCO FT/IR-5300 spectrophotometer (Tokyo, Japan). Mass spectra were measured on a JEOL JMS-T100LC spectrometer (Tokyo, Japan). HPLC data were obtained on a SHIMADZU HPLC system (consisting of the following: LC-20AT, CMB20A, CTO-20AC, and detector SPD-20A measured at 254 nm, Kyoto, Japan) using Daicel Chiracel AD-H or Ad-3 column (25 cm) at 25 • C. Optical rotations were measured on a JASCO DIP-370 (Na lamp, 589 nm, Tokyo, Japan). TMSCl (13.0 mL, 0.10 mol) was added to a stirred solution of ethyl acetoacetate (13.0 g, 0.10 mol) in THF-hexane (1 / 5, 150 mL) at 0-5 • C under an Ar atmosphere. After being stirred for 0.5 h, Et 3 N (14.0 mL, 0.10 mol) was added to the mixture, which was stirred at the same temperature for 0.5 h. The mixture was allowed to warm up to room temperature and the mixture was stirred for 14 h. The resulting mixture was reversely quenched with ice-water, which was extracted twice with hexane. The combined organic phase was washed with water, brine, dried (Na 2 SO 4 ), and concentrated. The obtained crude oil was purified by distillation (bp 40-42 • C/3.0 kPa) to give the desired methyl 2-(trimethylsilyl)oxybut-2-enoate (17.6 g, 87%).

4-Ethoxy
nBuLi (1.45 M in hexane, 18 mL, 26 mmol) was added to stirred solution of iPr 2 NH (3.7 mL, 26 mmol) in THF (16 mL) at 0-5 • C under an Ar atmosphere, and the mixture was stirred for 5 min. The mixture was cooled down to −78 • C and ethyl 2-(trimethylsilyl)oxybut-2-enoate (4.05 g, 20 mmol) in THF (2.0 mL) was added dropwise over 3 min to the mixture, which was stirred at the same temperature for 0.5 h. TMSCl (3.0 mL, 26 mmol) in THF (2.0 mL) was added dropwise for 10 min to the mixture at the same temperature and the mixture was allowed to warm up to 0-5 • C over a period of 2 h. The mixture was concentrated using a rotary evaporator and filtered through Celite ® (No. 503) using a glass filter, being washed with hexane (10 mL × 3). The filtrate was concentrated under reduced pressure to give the crude product 4 (5.03 g, 92%) [13,14], which was used for the next reaction without any purification.  −1 2961, 1649, 1443, 1391, 1250, 1196, 1090, 1015, 982, 835. 1 H-NMR and 13 C-NMR spectra: see supporting information.