Journal of the American Chemical Society
COMMUNICATION
Scheme 3. Synthetic Transformations
’ ASSOCIATED CONTENT
S
Supporting Information. Experimental procedures and
b
characterization data for all reactions and products, including
copies of 1H NMR and 13C NMR spectra and single-crystal X-ray
diffraction analysis (CIF). This material is available free of charge
’ AUTHOR INFORMATION
Corresponding Author
’ ACKNOWLEDGMENT
We are grateful to Pfizer Inc. for a postdoctoral fellowship to
K.S., the China Scholarship Council for a grant to W.Q., and S.-L.
Zheng for the X-ray crystallographic analysis.
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The syntheses of the substrates in entries 1À3 of Table 1
started with geraniol and those in entries 4 and 5 with
farnesol and geranylgeraniol, respectively. For each case in
Table 1, InBr3-catalyzed cyclization occurred cleanly and
stereoselectively to give the product shown. The cyclization
reactions were easily monitored by TLC analysis. The reac-
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2À4 h with 20 mol % InI3 (the yields were comparable). The
cyclization shown in entry 1 of Table 1 occurred somewhat
more rapidly with the corresponding free alcohol, but the
yield of tricyclic product was slightly lower than with the
TBS ether.
The In(III)-promoted cyclization of unsaturated pro-
pargylic alcohols (or TBS ethers) is tolerant of oxygen-
containing subunits, as shown by the six examples listed in
Table 2, all of which employed InI3 as the catalyst and used
racemic substrates. It is evident that a phenolic hydroxyl
substituent (entry 1), an ester function (entries 3 and 4),
and skeletal ether oxygens (entries 5 and 6) did not interfere
with the polycyclization process.
The effectiveness and facility of the In(III)-catalyzed cycli-
zation at À20 °C appears to be due to the highly electrophilic
reactivity of the indium complex with the ethynyl subunit
rather than the degree of In(III)/CtC complexation (which
is clearly low, but higher than with any CdC linkages in the
substrate). The ease of the cyclizations reported herein con-
trasts sharply with the forcing conditions (100 °C) for the
known addition of enolizable 1,3-dicarbonyl compounds to
acetylenes.19 Further research is needed to ascertain the full
scope and utility of the powerful new In(III)-mediated con-
struction outlined herein.
The cyclization products obtained in this research are espe-
cially interesting because they can be transformed into a wide
range of other structures using well-known synthetic methods.
The A-ring subunit, for example, can serve to provide such
compounds as are shown in Scheme 3.
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