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. 2016 Jan 27;138(3):798-801.
doi: 10.1021/jacs.5b12778. Epub 2016 Jan 19.

Asymmetric Syntheses of the Flavonoid Diels-Alder Natural Products Sanggenons C and O

Chao Qi  1 , Yuan Xiong  1 , Vincent Eschenbrenner-Lux  1 , Huan Cong  1 , John A Porco Jr  1
Affiliations

Asymmetric Syntheses of the Flavonoid Diels-Alder Natural Products Sanggenons C and O

Chao Qi et al. J Am Chem Soc. .

Abstract

Metal-catalyzed, double Claisen rearrangement of a bis-allyloxyflavone has been utilized to enable a concise synthesis of the hydrobenzofuro[3,2-b]chromenone core structure of the natural products sanggenon A and sanggenol F. In addition, catalytic, enantioselective [4+2] cycloadditions of 2'-hydroxychalcones have been accomplished using B(OPh)3/BINOL complexes. Asymmetric syntheses of the flavonoid Diels-Alder natural products sanggenons C and O have been achieved employing a stereodivergent reaction of a racemic mixture (stereodivergent RRM) involving [4+2] cycloaddition.

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Figures

Figure 1
Figure 1
Structures of sanggenon-type natural products.
Scheme 1
Scheme 1
Retrosynthetic Analyses for Sanggenons C and O
Scheme 2
Scheme 2
Synthesis of Morin Di-allyl Ethera aReagents and conditions: (a) MOMCl (5 equiv), DIPEA (5 equiv), CH2Cl2, 0 °C to rt, 55%; (b) allyl-Br (1.2 equiv), Cs2CO3 (1.2 equiv), DMF, 65 °C, 12h, 93%; (c) NaI (0.8 equiv), 1 N HCl (0.1 equiv), 40 °C, 12h, 73%; (d) allyl-Br (2.0 equiv), Cs2CO3 (2.0 equiv), DMF, 65 °C, 12h, 96%; (e) conc. HCl, MeOH, 40 °C, 93%.
Scheme 3
Scheme 3
Syntheses of Sanggenol F and Sanggenon Aa aReagents and conditions: (a) TBSOTf (3.2 equiv), NEt3 (3.0 equiv), CH2Cl2, rt, 6h; (b) Grubbs second generation catalyst (10 mol%), isobutene, 40 °C, 24h, 92%, 2 steps; (c) NEt3·3HF (8.0 equiv), CH3CN, 0 °C, 3h, 91%; (d) DDQ (1.2 equiv), THF, 60 °C, 12h, 78%.
Scheme 4
Scheme 4
Syntheses of (±)-Sanggenons C and Oa aReagents and conditions: (a) DDQ (1.5 equiv), THF, 60 °C, 73%; (b) 20 (1.2 equiv), AgNP (0.25 mol%), AcOH (2 equiv), DCE, 65 °C, 3d; (c) sat. NaHCO3, MeOH, rt, 12h; (d) NEt3·3HF (8.0 equiv), CH3CN, rt, 12h, 36% (3 steps), sanggenon C (3):sanggenon O (4) = 4:1.
Scheme 5
Scheme 5
Model Reaction and Proposed Active Complex for [4+2] Cycloaddition
Scheme 6
Scheme 6
Asymmetric Syntheses of Sanggenons C and Oa aReagents and conditions: (a) B(OPh)3 (20 mol%), 23 (22 mol%), 20 (1 equiv), PhCF3, 80 °C, 48h; (b) sat. NaHCO3, MeOH, rt, 12h; (c) NEt3·3HF (8.0 equiv), CH3CN, rt, 12h.
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