R,â-unsaturated iminium salts 46 involves a Knoevenagel
condensation followed by a 6π-electron electrocyclic ring-
closure of 1-oxatrienes 5, providing a convergent synthesis
of 1-oxadecalins 6 (Scheme 1).7 This sequential anionic-
The synthesis of daurichromenic acid 2a is outlined in
Scheme 2. Aldehyde 8,15 the precursor for preparing the
Scheme 2a
Scheme 1
pericyclic strategy8 constitutes a stepwise formal [3 + 3]
cycloaddition9,10 in which two σ-bonds are constructed in
addition to a new stereocenter adjacent to the oxygen atom.11
More recently, we demonstrated that this methodology can
serve as a useful entry to chromenes 7 via oxidative
aromatization of 6,6b,12 thereby allowing us to explore
syntheses of those natural chromenes and chromanes with
important medicinal values.13,14 We report here concise total
syntheses of (()-rhododaurichromanic acids A and B and
methyl (()-daurichromenic ester.
a Reaction conditions: (a) DMSO, CH2Cl2, SO3-pyridine, Et3N
(94%). (b) Piperidine, Ac2O, 90 °C, EtOAc, 1 h; this mixture was
then added to 5-methyl-1,3-cyclohexanedione in toluene, 90 °C,
12-18 h (70%). (c) LDA, THF, -78 °C; this was then added to
NCCO2Me (71%). (d) DDQ, toluene, reflux (44%). (e) KOH,
NaOH, LiOH, AlCl3, or BBr3; decarboxylation occurred.
appropriate R,â-unsaturated iminium salt, was obtained
nearly quatitatively via SO3-pyridine oxidation of (E,E)-
(4) Hsung, R. P.; Shen, H. C.; Douglas, C. J.; Morgan, C. D.; Degen, S.
J.; Yao, L. J. J. Org. Chem. 1999, 64, 690.
(5) For pyranyl heterocycle construction and related natural product
syntheses, see: (a) Hsung, R. P.; Cole, K. P.; Zehnder, L. R.; Wang, J.;
Wei, L. L.; Yang, X.-F.; Coverdale, H. A. Tetrahedron 2003, 59, 311. (b)
Cole, K. P.; Hsung, R. P.; Yang, X.-F. Tetrahedron Lett. 2002, 43, 3341.
(c) Wang, J.; Cole, K. P.; Wei, L. L.; Zehnder, L. R.; Hsung, R. P.
Tetrahedron Lett. 2002, 43, 3337. (d) Cole, K. P.; Hsung, R. P. Tetrahedron
Lett. 2002, 43, 8791. (e) McLaughlin, M. J.; Hsung, R. P. J. Org. Chem.
2001, 66, 1049. (f) McLaughlin, M. J.; Shen, H. C.; Hsung, R. P.
Tetrahedron Lett. 2001, 42, 609. (g) Zehnder, L. R.; Wei, L.-L.; Hsung, R.
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J.; Zificsak, C. A. Org. Lett. 2001, 3, 2141. (h) Zehnder, L. R.; Hsung, R.
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(15) All new compounds are characterized by 1H NMR, 13C NMR, FTIR,
and LRMS.
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