Abstract
γ-(E)-Vinylic and γ-alkylic γ-butyrolactones are two different types of lactones existing extensively in animals and plants and many of them show interesting biological activities. Nature makes alkylic γ-butyrolactones by many different enzymatic lactonization processes. Scientists have been mimicking the natural strategy by developing new catalysts. However, direct and efficient access to γ-(E)-vinylic γ-butyrolactones is still extremely limited. Here, we wish to present our modular allene approach, which provides an efficient asymmetric approach to (E)-vinylic γ-butyrolactones from allenoic acids by identifying a new gold complex as the catalyst. Based on this cycloisomerization strategy, the first syntheses of racemic xestospongiene and xestospongienes E, F, G, and H have been realized and the absolute configurations of the chiral centers in xestospongienes E and F have been revised. In addition, by applying a C–O bond cleavage-free hydrogenation, the syntheses of naturally occurring γ-alkylic γ-lactones, (R)-4-tetradecalactone, (S)-4-tetradecalactone, (R)-γ-palmitolactone, and (R)-4-decalactone, have also been achieved.
Highlights
We present the highly stereoselective cycloisomerization of optically active 5-monosubstituted 4,5-alkadienoic acids affording various non-natural and natural enantioenriched γ-(E)-alkenyl-γ-butyrolactones by using AuCl(LB-Phos) as the catalyst
AgOTs (0.01 mmol), AuCl(LB-Phos) (0.01 mmol), and solvent (2 mL) were stirred at room temperature for 15 min under nitrogen atmosphere; 0.2 mmol (Ra)-5a and solvent (1 mL) were added a Determined by 1H NMR of the crude product using 1,3,5-trimethylbenzene as internal standard b Determined by chiral high-performance liquid chromatography (HPLC) analysis c 2.5 mol% Au2Cl2(dppm) was used instead of AuCl(LB-Phos) d The reaction was conducted at 25 °C e 1,2-DCE: 1,2-dichloroethane f 92% recovery of (Ra)-5a g The reaction was conducted in the absence of AuCl(LB-Phos); 100% recovery of (Ra)-5a
AgOTs (0.01 mmol), [Au] (0.01 mmol), and CHCl3 (2 mL) were stirred at room temperature for 15 min under nitrogen atmosphere; 0.2 mmol of (Sa)-5m and CHCl3 (1 mL) were added; the reaction mixture was continuously stirred at 25 °C for 4 h a Determined by 1H NMR of crude product using 1,3,5-trimethylbenzene as internal standard b Determined by chiral high-performance liquid chromatography (HPLC) analysis c The reaction was conducted in the absence of AgOTs; 88% of (Sa)-5m was recovered d 2.5 mol% Au2Cl2(dppm) was used e Reaction time was 3.75 h f 2% recovery of (Sa)-5m
Summary
With this rather sterically bulky AuCl(LB-Phos), gladly, an E/Z selectivity of 97:3 with 100% yield and 97% ee was observed, indicating that the new catalyst was able to control both the C=C stereoselectivity and ensure the efficiency of chirality transfer (Table 1, entry 2). Inspired by this result, we explored the effect of solvents: CH2Cl2 gave a similar a.
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