Studies toward the total synthesis of naturally occurring pyranonaphthoquinones from <em>Ventilago harmandiana</em>

Kongsriprapan, Sopanat; Kuhakarn, Chutima; Deelertpaiboon, Pramchai; Panthong, Kanda; Tuchinda, Patoomratana; Pohmakotr, Manat; Reutrakul, Vichai

doi:10.1351/PAC-CON-11-08-24

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.