NMR-pure 3,4-benzocyclodeca-1,5-diyne (2). This photochemical
reaction was also found to be quite efficient with a quantum yield
at 300 nm of W300 5 0.45 (in methanol).
In summary, we have shown the feasibility of the in situ
activation of enediynes by the photochemical generation of one of
the triple bonds. The cyclopropenone-containing enediyne pre-
cursor 1 does not undergo cycloaromatization even at elevated
temperatures and survives prolonged heating without decomposi-
tion. UV irradiation of the substrate 1, on the other hand, results
in an efficient and clean decarbonylation reaction producing
benzannulated enediyne 2. The latter is known to undergo
Bergman cyclization at 84 uC.
The authors thank the National Institutes of Health (grant
CA91856-01A1) and the Ohio Cancer Research Associates for the
support of this project. A. P. thanks the McMaster Endowment
for the research fellowship.
Scheme 3 Reagents and conditions: (a) HCCSiMe
PPh , Et N, THF, 40 uC; (b) K CO , MeOH, rt, 84% over two steps; (c)
n-BuLi, I(CH I, THF, rt, 40%; (d) CHCl , n-BuLi, THF, 278 uC, 67%.
3 3 2 2
, Pd(PPh ) Cl , CuI,
3
3
2
3
2
)
4
3
of the resulting 1,2-bis(29-trimethylsilyl ethynyl) benzene (3) was
accomplished by treating the substrate with potassium carbonate
in methanol to give 1,2-diethynyl benzene (4). The reaction of the
dianion of 4 with of 1,4-diiodobutane in THF at 278 uC produced
3,4-benzocyclodeca-1,5-diyne (2) in a 67% yield (Scheme 3).
The reaction of benzannelated enediyne 2 with dichlorocarbene,
followed by the hydrolysis in concentrated hydrochloric acid
produced a target cyclopropenone-containing macrocycle, 2,3-
benzobicyclo[8.1.0]undec-1(10)-ene-4yn-11-one (1c), in a 67% yield.
It is interesting to note that benzannelated enediyne analog of 2,
which contains TBDMS-protected hydroxy group in propargylic
position, is inert under these cyclopropanation conditions. We
believe that this phenomenon can be explained by the high acidity
of the propargylic proton The same very low reactivity towards
dichlorocarbene was demonstrated by 6-methoxy-1-[2-[(trimethyl-
silyl)ethynyl]phenyl]hexyne and several other ortho-dialkynylben-
zenes. We attribute the low reactivity of these substrates to steric
effect of the o- substituent, since m-(1-pentynyl)anisole produces a
good yield of corresponding cyclopropenone.
Andrei Poloukhtine and Vladimir V. Popik*
Center for Photochemical Sciences, Bowling Green State University,
Bowling Green, Ohio, 43403, USA. E-mail: vpopik@bgnet.bgsu.edu;
Fax: +1 (419) 372 9809; Tel: +1 (419) 372 2470
Notes and references
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A. Ishii and S. Nishimoto, J. Med. Chem., 2002, 45, 758–61;
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The enediyne precursor 1 is a surprisingly stable compound. No
apparent changes were observed in the UV spectra of the
cyclopropenone 1 even after heating for 7 d at 84 uC and the
substrate was quantitatively recovered. The corresponding ben-
zannulated enediyne 2, on the other hand, undergoes Bergman
12
cyclization with a half life time of 6 h at this temperature.
The UV photolysis of 2,3-benzobicyclo[8.1.0]undec-1(10)-ene-4-
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quantitatively producing enediyne analog 2 (Fig. 1). Thus,
irradiation of 2 mg of the cyclopropenone 1 for 20 min at
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4
Fig. 1 UV spectra of 300 nm photolysis of ca. 5 6 10 M methanol
solutions of cyclopropenone 1. Spectra were recorded ca. every 1 min.
6
18 | Chem. Commun., 2005, 617–619
This journal is ß The Royal Society of Chemistry 2005