Achiral additives dramatically enhance enantioselectivities in the BINOL–Ti(IV) complex catalyzed aldol condensations of aldehydes with Chan’s diene

Synthesis of functionalized δ-hydroxy-β-keto esters and evaluation of their anti-inflammatory properties

δ‐Hydroxy‐β‐keto esters and δ,β‐dihydroxy esters are characteristic structural motifs of statin‐type natural products and drug candidates. Here, we describe the synthesis of functionalized δ‐hydroxy‐β‐keto esters in good yields and excellent enantioselectivities using Chan's diene and modified Mukaiyama‐aldol reaction conditions. Diastereoselective reduction of δ,β‐dihydroxy esters afforded the respective syn‐ and anti‐diols, and saponification yielded the corresponding acids. All products were evaluated for their anti‐inflammatory properties, which uncovered a surprising structure‐activity relationship.

Asymmetric Total Syntheses of Both Enantiomers of Plymuthipyranone B and Its Unnatural Analogues: Evaluation of anti-MRSA Activity and Its Chiral Discrimination

Chiral total syntheses of both enantiomers of the anti-MRSA active plymuthipyranone B and all of the both enantiomers of three unnatural and synthetic analogues were performed. These two pairs of four chiral compounds are composed of the same 3-acyl-5,6-dihydro-2H-pyran-2-one structure. The starting synthetic step utilized a privileged asymmetric Mukaiyama aldol addition using Ti(OiPr)₄/(S)-BINOL or Ti(OiPr)₄/(R)-BINOL catalysis to afford the corresponding (R)- and (S)-δ-hydroxy-β-ketoesters, respectively, with highly enantiomeric excess (>98%). Conventional lactone formation and successive EDCI-mediated C-acylation produced the desired products, (R)- and (S)-plymuthipyranones B and three (R)- and (S)- synthetic analogues, with an overall yield of 42–56% with a highly enantiomeric excess (95–99%). A bioassay of the anti-MRSA activity against ATCC 43300 and 33591 revealed that (i) the MICs of the synthetic analogues against ATCC 43300 and ATCC 33591 were between 2 and 16 and 4 and 16 μg/mL, respectively, and those of vancomycin (reference) were 1 μg/mL. (ii) The natural (S)-plymuthipyranone B exhibited significantly higher activity than the unnatural (R)-antipode against both AACCs. (iii) The natural (R)-plymuthipyranone B and (R)-undecyl synthetic analogue at the C6 position exhibited the highest activity. The present work is the first investigation of the SAR between chiral R and S forms of this chemical class.

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.

Enzymatic reductions for the regio- and stereoselective synthesis of hydroxy-keto esters and dihydroxy esters

Ketoreductases were utilized for the stereoselective synthesis of δ-hydroxy-β-keto esters, β-hydroxy-δ-keto esters, and β,δ-dihydroxy esters. Seven out of eight possible stereoisomers were obtained from the enzymatic reduction of the corresponding β,δ-diketo ester in high enantio- and diastereomeric excess.