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Pure Appl. Chem., 2012, Vol. 84, No. 6, pp. 1435-1443

http://dx.doi.org/10.1351/PAC-CON-11-08-24

Published online 2012-02-14

Studies toward the total synthesis of naturally occurring pyranonaphthoquinones from Ventilago harmandiana

Sopanat Kongsriprapan1, Chutima Kuhakarn1, Pramchai Deelertpaiboon1, Kanda Panthong2, Patoomratana Tuchinda1, Manat Pohmakotr1 and Vichai Reutrakul1*

1 Department of Chemistry and The Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Rama 6 Road, Bangkok 10400, Thailand
2 Department of Chemistry and The Center of Excellence for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Songkhla, 90112, Thailand

Abstract: The enantioselective synthesis of a naturally occurring pyranonaphthoquinone isolated from a Thai endemic plant, Ventilago harmandiana, has been achieved. L-Rhamnose and gallic acid were used as the starting materials. A new C-1-glycosidation of L-rhamnal with trimethyl aluminum in the presence of a catalytic amount of ytterbium(III) triflate was developed. A new reagent, PhSCF2H/SnCl4, for the formylation of partially deactivated and hindered aromatic compounds has been introduced. Phenylthiophthalide was used efficiently as a cycloannulating agent in the Hauser cycloannulation reaction, employing slightly excess lithium t-butoxide as a base with a catalytic amount of lithium chloride. The synthetic route developed is applicable for the synthesis of other analogues with substituents at the aromatic and pyran rings.