Abstract
Phytoalexins have attracted much attention due to their health-promoting effects and their vital role in plant health during the last years. Especially the 6a-hydroxypterocarpans glyceollin I and glyceollin II, which may be isolated from stressed soy plants, possess a broad spectrum of bioactivities such as anticancer activity and beneficial contributions against western diseases by anti-oxidative and anti-cholesterolemic effects. Aiming for a catalytic asymmetric access to these natural products, we establish the asymmetric syntheses of the natural isoflavonoids (−)-variabilin, (−)-homopterocarpin, (−)-medicarpin, (−)-3,9-dihydroxypterocarpan, and (−)-vestitol by means of an asymmetric transfer hydrogenation (ATH) reaction. We successfully adapt this pathway to the first catalytic asymmetric total synthesis of (−)-glyceollin I and (−)-glyceollin II. This eight-step synthesis features an efficient one-pot transformation of a 2′-hydroxyl-substituted isoflavone to a virtually enantiopure pterocarpan by means of an ATH and a regioselective benzylic oxidation under aerobic conditions to afford the susceptible 6a-hydroxypterocarpan skeleton.
Highlights
Phytoalexins have attracted much attention due to their health-promoting effects and their vital role in plant health during the last years
The glyceollins are synthesised de novo from the isoflavone daidzein in response to pathogen attack such as fungi, bacteria, UV irradiation, or abiotic elicitors such as silver nitrate or aluminium chloride8–11. 1 and 2 belong to a group of at least eight
Due to the interesting bioactivity of glyceollins and the difficulties to isolate them in pure form from soy, the asymmetric total synthesis of these compounds became desirable
Summary
Phytoalexins have attracted much attention due to their health-promoting effects and their vital role in plant health during the last years. Aiming for a catalytic asymmetric access to these natural products, we establish the asymmetric syntheses of the natural isoflavonoids (−)-variabilin, (−)-homopterocarpin, (−)-medicarpin, (−)-3,9-dihydroxypterocarpan, and (−)-vestitol by means of an asymmetric transfer hydrogenation (ATH) reaction We successfully adapt this pathway to the first catalytic asymmetric total synthesis of (−)-glyceollin I and (−)-glyceollin II. In 2008, the first total synthesis of (−)-glyceollin I (1), which yielded minor amounts of (−)-glyceollin II (2), was published by the group of Erhardt and scaled up in 2011 to a multigram synthesis[17,18,19,20] These studies utilised a Sharpless asymmetric dihydroxylation of an isoflav-3-ene with stoichiometric amounts of osmium tetroxide and the chiral ligand[21]. A strategy relying on the epoxidation of an isoflav3-ene led to racemic 1 and 2, only[24]
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