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Synthesis 2008(8): 1182-1192  
DOI: 10.1055/s-2008-1042950
PAPER
© Georg Thieme Verlag Stuttgart · New York

Synthesis of Siloxy-α-Lapachone Derivatives by Chemo- and Regioselective Diels-Alder Reactions of 3-Methylene-1,2,4-naphthotriones with Silyl Enol Ethers

Da-Quan Penga,b, Yun Liua, Zhi-Feng Lua, Yong-Miao Shena, Jian-Hua Xu*a
a Department of Chemistry, Nanjing University, Nanjing 210093, P. R. of China
Fax: +86(25)83317761; e-Mail: xujh@nju.edu.cn;
b Department of Chemistry, Chongqing Normal University, Chongqing 400047, P. R. of China
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Publication History

Received 1 May 2007
Publication Date:
18 March 2008 (online)

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Abstract

o-Quinone methides, generated in situ from the Knoevenagel condensation of 2-hydroxy-1,4-naphthoquinone with aliphatic and aromatic aldehydes, take part in chemoselective hetero-Diels-Alder reactions with silyl enol ethers to give a series of siloxy-containing naphtho[2,3-b]pyran-5,10-dione (α-lapachone) derivatives in moderate to high yield. These reactions regio­selectively gave α-lapachone derivatives with an acetal structure. This regioselectivity can be rationalized by considering the frontier molecular orbital interactions of the o-quinone methide with the silyl enol ether, and by taking into account the energetically more favorable pathway leading to a zwitterion-like transition state of lower energy in a Michael addition between the two reactants.

Key words

α-lapachone - o-quinone methide - silyl enol ether - Diels-Alder reaction - regioselectivity