C O M M U N I C A T I O N S
91,9 followed by a 1,2-H shift13 to form alkenylgold(I) complex
1014 (Scheme 4). The isomer of 10 with AuL in the convex face
would be formed similarly from the C1 epimer of intermediate 9.
Retro-cyclopropanation via 11, presumably by stepwise cleavage
by electrophilic [AuL]+,5a would then give naphthalenes 3 and free
gold(I) carbene 5. Experimental support for this proposal was
obtained by treatment of enol ether 12 with catalyst 6 (CH2Cl2,
23 °C, 1.5 h), which led cleanly to a mixture of 1-methoxy-3-
phenylnaphthalene (3a) and biscyclopropane 7a.
Scheme 6
Scheme 4
times were required in these cases.15 This reaction presumably
proceeds by 6-endo-dig cyclization to form 15 followed by
intramolecular attack of the OR group at the cyclopropane3 to form
16. Fragmentation of intermediate 16,19 followed by protodeaura-
tion,20 would give naphthalenes 4.
Scheme 7
1,3-Disubstituted naphthalenes are a class of compounds not
readily available by current methods such as electrophilic substitu-
tion/cross-coupling. This annulation allows the preparation of these
compounds from enynes 1, which are readily assembled in a
modular form.15 To illustrate the potential of this method, a formal
synthesis of the cytotoxic benzo[c]phenanthridine alkaloid mac-
arpine16,17 was performed (Scheme 5). Thus, gold(I)-catalyzed
cyclization of 1′k gave biaryl 3k (79% yield), which upon
hydrogenolysis provided naphthol 3l, an intermediate in the total
synthesis of macarpine.16
In summary, we have developed new gold(I)-catalyzed annula-
tions of enynes for the synthesis of 1,3-disubstituted naphthalenes
that proceed by fragmentation of the alkene. One of these
transformations involves the first example of a retro-cyclopropa-
nation promoted by gold(I) in solution.
Acknowledgment. We thank the MICINN (CTQ2010-16088/BQU,
Consolider Ingenio 2010 Grant CSD2006-0003, and an FPI predoctoral
fellowship to C.R.S.-A.), the AGAUR (2009 SGR 47), and the ICIQ
Foundation for support and V. Mart´ınez for technical assistance.
Scheme 5
Supporting Information Available: Additional data, experimental
details, and characterization data. This material is available free of
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9
11882 J. AM. CHEM. SOC. VOL. 132, NO. 34, 2010