Abstract
Two different strategies are presented to synthesize mollugin, based upon a close
investigation of possible natural precursors. The best total synthesis of mollugin,
a natural product isolated from rubiaceous herbs, is achieved in an overall yield
of 61% starting from 1,4-dihydroxynaphthalene-2-carboxylic acid. The key reaction
is the prenylation and spontaneous pyran ring formation. Subsequent oxidation of the
intermediate 3,4-dihydromollugin with DDQ afforded mollugin.
Key words
quinones - natural products - oxidations
References and Notes
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3-Bromomollugin 7: IR (KBr): 1650 cm-1 (C=O). 1H NMR (CDCl3, 300 MHz): δ = 1.59 (6 H, s, 2 × CH3), 4.04 (3 H, s, OCH3), 7.54 (1 H, ddd, J = 8.2, 6.9, 1.4 Hz, CH-8 or CH-9), 7.56 (1 H, s, CH), 7.63 (1 H, ddd, J = 8.2, 6.9, 1.4 Hz, CH-8 or CH-9), 8.13 (1 H, ddd, J = 8.2, 1.4, 0.8 Hz, CH-7), 8.37 (1 H, ddd, J = 8.2, 1.4, 0.8 Hz, CH-10), 12.27 (1 H, s, OH). 13C NMR (CDCl3, 75 MHz): δ = 25.82 (2 × CH3), 52.62 (OMe), 78.73 (=CBr), 101.24 (Cquat), 113.15 (Cquat), 121.85 (CH-7), 123.80 (Cquat), 124.29 (CH-10), 125.25 (Cquat), 125.31 (=CH), 126.73 (CH-8 or CH-9), 128.81 (Cquat), 129.77 (CH-8 or CH-9), 140.32 (Cquat), 157.27 (Cquat), 172.20 (C=O). MS (ES+): m/z (%) = 362/364 [M]+(100).
