Abstract
Chiral epoxides—such as ethyl and methyl (S)-3-(oxiran-2-yl)propanoates ((S)-1a/1b)—are valuable precursors in many chemical syntheses. Until recently, these compounds were synthesized from glutamic acid in four steps (deamination, reduction, tosylation and epoxide formation) in low to moderate overall yield (20%–50%). Moreover, this procedure requires some harmful reagents such as sodium nitrite ((eco)toxic) and borane (carcinogen). Herein, starting from levoglucosenone (LGO), a biobased chiral compound obtained through the flash pyrolysis of acidified cellulose, we propose a safer and more sustainable chemo-enzymatic synthetic pathway involving lipase-mediated Baeyer-Villiger oxidation, palladium-catalyzed hydrogenation, tosylation and treatment with sodium ethoxide/methoxide as key steps. This route afforded ethyl and methyl (S)-3-(oxiran-2-yl)propanoates in 57% overall yield, respectively. To demonstrate the potentiality of this new synthetic pathway from LGO, the synthesis of high value-added (S)-dairy lactone was undertaken from these epoxides and provided the target in 37% overall yield from LGO.
Highlights
Chiral epoxides are widely used as intermediates in organic synthesis
Starts from an enantiopure glycidyl alkene and involves an involves an alkynylation with heptynyl lithium followed by an osmium tetroxide-mediated oxidative be synthesized prior to cuprate formation and that its synthesis involves the use of CuCN and low alkynylation with followed by an osmium tetroxide-mediated oxidative cleavage temperature
The efficiency and green aspects of these new synthetic pathways to ethyl and methyl (S)-3-(oxiran-2-yl)propanoates as well as (S)-dairy lactone will be discussed with regard to the route involving involving L-glutamic acid
Summary
Chiral epoxides are widely used as intermediates in organic synthesis. For example, methyl (S)-3-(oxiran-2-yl)propanoate (1a) has been employed for the synthesis of many compounds such as elicitor [1], Streptrubin B [2] or chiral β-3-substituted homopropargyl [3], thiobutyrolactone [4], γ-hydroxyesters [5], diol-γ or δ-lactones [6], keto-esters [7], (+/ ́)-4-alkanolides [8] (Figure 1). Glutamic (S)-γ-carboxy-γ-butyrolactone, which was first reported by Austin et al using nitrous acid [11]. Three different nitrous routes have been reported for the reduction of (S)-γ-carboxy-γ-butyrolactone into 2. The third and route consists of aconsists one-stepofreduction of reduction the carboxylic acid into the alcohol in presence of borane-methylsulfide (BH3-Me2S) [3,17] or borane-THF [18]. Noteworthy thatofthe substitution of borane-methylsulfide borane-THF to obviate on resulted in over-reduction to the lactol [16]. These three routes provide 2 in good yields, they resulted in [12,19]. The epoxides good yields, require steps or use a carcinogenic reagent
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