Synthesis of arenediynes via the vinylidenecarbene-acetylene rearrangement

Article abstract of DOI:10.1016/j.tetlet.2005.02.101

A convenient method for the two-step synthesis of arenediynes from 1,2-arenedialdehydes is reported. Dibromomethylenation of dialdehydes under Corey-Fuchs conditions (CBr₄, Ph₃P, Zn) provides the tetrabromides in excellent yields. Treatment of the tetrabromides with n-BuLi or LDA affords 3,4-unsaturated 1,5-diynes, the key structural moiety present in several naturally occurring antitumour antibiotics, in varying yields. The key intermediates in these transformations appear to be vinylidenecarbenes or carbenoids, generated in situ via metal-halogen exchange and elimination.

Full text of DOI:10.1016/j.tetlet.2005.02.101

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 2593–2597
Synthesis of arenediynes via the
vinylidenecarbene–acetylene rearrangement
Bichismita Sahu,a Irishi N. N. Namboothiria,* and Rachel Perskyb
aDepartment of Chemistry, Indian Institute of Technology, Bombay, Mumbai 400 076, India
bDepartment of Chemistry, Bar-Ilan University, Ramat Gan 52900, Israel
Received 26 December 2004; revised 7 February 2005; accepted 15 February 2005
Dedicated to Professor A. P. Marchand on the occasion of his 65th birthday
Abstract—A convenient method for the two-step synthesis of arenediynes from 1,2-arenedialdehydes is reported. Dibromomethyl-
enation of dialdehydes under Corey–Fuchs conditions (CBr4, Ph3P, Zn) provides the tetrabromides in excellent yields. Treatment of
the tetrabromides with n-BuLi or LDA affords 3,4-unsaturated 1,5-diynes, the key structural moiety present in several naturally
occurring antitumour antibiotics, in varying yields. The key intermediates in these transformations appear to be vinylidenecarbenes
or carbenoids, generated in situ via metal–halogen exchange and elimination.
Ó 2005 Elsevier Ltd. All rights reserved.
The enediyne natural products containing a (Z)-3-hex-
en-1,5-diyne moiety belong to five different classes of
natural products such as calicheamicin, esperamicin,
dynemicin, kedarcidin and C-1027 and are amongst
the most potent antitumour agents known to date.1
The key steps in the antitumour activity of enediynes
are Bergman cyclization1,2 to give 1,4-dehydroarene
biradicals and abstraction of hydrogen from DNA by
the latter thereby inflicting permanent damage on the
genetic material.
for highly sensitive and often toxic organometallic re-
agents. Thermal (Diels–Alder),8 photochemical (Norrish
type II)9 and several elimination10 methods employed to
introduce a double bond between and in conjugation
with a 1,5-diyne are specific to a narrow range of
substrates.
In this context, a fundamentally novel approach to the
synthetic analogues of the enediyne antibiotics based
on the rearrangement of a vinylidenecarbene to its cor-
responding acetylene (Fritsch–Buttenberg–Wiechell
rearrangement)11 is reported here. Although the FBW
rearrangement (Scheme 1),11–14 has been employed for
the synthesis of acetylenes15 and its applications to
numerous systems in which the migrating group is an
alkyl, aryl or heteroaryl group have been reported,16,17
there is no report, to our knowledge, of the syn-
thesis of 3,4-unsaturated 1,5-diynes (enediynes) 7 from
Given the structural complexity of the naturally occur-
ring enediyne antitumour agents, design and synthesis
of simple structures that can mimic the biological activ-
ity of these natural products is important. The methods
available for the construction of Z-enediyne subunits, in
both linear and cyclic form, include the Pd catalyzed
alkynylation3 of 1,2-dihalides3,4 or 1,2-ditriflates5,6 of
alkenes or arenes. While 1,2-dihalides are often expen-
sive and/or difficult to prepare, complex mixtures are
often encountered in the case of triflates.5 Other related
metal-mediated methods7 are essentially multistep reac-
tion sequences and are complicated by the requirement
R
X
Y
R
geminal
1,2-shift
R
R'
C
C:
elimination
R'
R'
3
1
2
1a: X = H, Y = Br, NH2, OTf, N=NTs; 1b: X = SiMe3, Y = Cl, OTf, IPhOTf;
1c: X = Cl, Y = S (O)Tol; 1d: X = Y = Br; 1e: XY = N2;
1f: XY = -COOC(CH3)2OCO-
Keywords: Enediynes; Vinylidenecarbene; Fritsch–Buttenberg–Wiec-
hell rearrangement.
*
Corresponding author. Tel.: +91 22 2576 7196; Fax: +91 22 2576
7152; e-mail: irishi@iitb.ac.in
Scheme 1. 1a–f: R,R0 = alkyl, aryl, heteroaryl or H.
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.02.101

2594
B. Sahu et al. / Tetrahedron Letters 46 (2005) 2593–2597
X
Y
C:
R3
R4
O
O
R1
R2
R1
R2
R1
R2
R1
R2
geminal
R3
R4
R3
R4
chain
bis-1,2-shift
R3
R4
extension
elimination
C:
X
Y
4
5
6
7
Scheme 2.
2,3-unsaturated 1,4-dicarbonyl compounds 4 via a 2,3-
unsaturated 1,4-bis-vinylidenecarbene 6 (Scheme 2).18
The use of complex organometallic reagents and cum-
bersome reaction conditions are also obviated in this
method.
In view of the above, we have subjected a variety of sym-
metrical and unsymmetrical 1,2-arenedialdehydes 8a–g
to the dibromomethylenation conditions (Table 1, col-
umns 1–3). Optimization of the dibromomethylenation
procedure using phthalaldehyde 8a as the model sub-
strate showed that 4 equiv of CBr4, 8 equiv of Ph3P
and 8 equiv of Zn dust were necessary to obtain the best
yield (94%) of the tetrabromide 9a (Table 1, entry a, col-
umns 1–3). Exclusion of Zn dust or lowering the amount
of the reagents (e.g., 3 equiv of CBr4, 6 equiv of Ph3P
and 6 equiv of Zn dust) led to substantial reduction in
the yields (68% and 56%, respectively). Subsequently,
1,2-dialdehydes 8b–g were converted to their corre-
sponding tetrabromides 9b–g under the optimized con-
ditions in good to excellent yields (Table 1, entries b–g,
columns 1–3).
In fact, a variety of geminal elimination methods such as
deprotonation–elimination from 1a, desilylation–elimi-
nation from 1b, metal–halogen exchange–elimination
from 1c19 or 1d, elimination of N2 from 1e,20 cycloeli-
mination via pyrolysis from 1f, etc., is available for the
generation of vinylidenecarbenes 2.12,13 Among these,
the metal–halogen exchange–elimination from 1d ap-
pears to be the most convenient method as 1d is easily
accessible via dihalomethylenation of an aldehyde21–23
or a ketone.24
Table 1. Preparation of tetrabromides 9 from 1,2-dialdehydes 8 and subsequent n-BuLi- and LDA-mediated transformation to enediynes 10a
R
n-BuLi, n-hexane
0 oC-rt, 1 h, Method A
CHO
CH=CBr2
CH=CBr2
CBr4, Ph3P
CHO
Zn, CH2Cl2
or LDA, toluene
-78 oC-rt, ~3 h
0 oC-rt, 12 h, Method B
R
8
10
9
Entry
1
2
Tetrabromides 9
3
% Yield of 9
4
Enediynes 10
5
6
1,2-Dialdehydes 8b
% Yield of 10
Method A
Method B
CH=CBr2
CH=CBr2
CHO
CHO
a
94
96
88
CH=CBr2
CH=CBr2
CHO
CHO
b
62
46
88
84
71
67
68
84
44
MeO
MeO
MeO
MeO
MeO
MeO
CHO
CHO
CH=CBr2
cc
CH=CBr2
CH=CBr2
CH=CBr2
CHO
CHO
d

B. Sahu et al. / Tetrahedron Letters 46 (2005) 2593–2597
2595
Table 1. (continued)
Entry
1
2
Tetrabromides 9
3
% Yield of 9
4
Enediynes 10
5
6
1,2-Dialdehydes 8b
% Yield of 10
Method A
Method B
MeO
MeO
MeO
CHO
CHO
CH=CBr2
CH=CBr2
e
66
91
87
R
CH=CBr2
CH=CBr2
CHO
CHO
fd
73
95
33
43
26
31
O
O
O
R
R
CHO
CH=CBr2
CH=CBr2
gd
CHO
S
S
S
R
Me
he
—
—
—
62
—
Me
a All tetrabromides 9a–g and enediynes 10a–h provided satisfactory IR, 1H NMR, 13C NMR and HRMS data. Enediynes 10a–e and 10g–h are
known compounds; see Ref. 26.
b The dialdehydes 8a and 8g are commercially available. All other dialdehydes, that is, 8b–f, were prepared in the laboratory following published
procedures; see Ref. 25.
c Heated at 70 °C for 4 h.
d The parent enediyne, due to its instability, was isolated as its bis-trimethylsilyl derivative (R = TMS).
e 10h was formed after 10a was treated with an additional amount of n-BuLi (2 equiv) and then quenched with MeI (R = Me).
The transformation of tetrabromides 9a–g to enediynes
10a–g was performed under two different sets of condi-
tions, viz. using n-BuLi (Method A, Table 1, column 5)
and LDA (Method B, Table 1, column 6). Under the
optimized conditions of Method A, that is, with 6 equiv
of n-BuLi in n-hexane as solvent at ꢀ78 °C, tetrabromide
9a afforded the desired enediyne 10a in excellent yield
(96%, Table 1, entry a, column 5). While the yield was
23% when THF was used as solvent, toluene provided
the enediyne 10a in 78% yield. Similarly, the yields were
<20% and 55% when 2 and 4 equiv of n-BuLi, respec-
tively, were used. Other tetrabromides 9b–g were subse-
quently subjected to the n-BuLi mediated transform-
ation to enediynes 10b–g (Table 1, entries b–g, column
5). Although tetrabromides 9b–e provided the corres-
ponding enediynes 10b–e, respectively, in very high yields
(Table 1, entries b–e, column 5), complex mixtures were
isolated in the cases of tetrabromides 9f and 9g due to
poor stability of the parent enediynes. The enediynes
were subsequently isolated as their bis-trimethylsilyl
derivatives by quenching the reaction mixtures with
TMSCl (R = TMS, Table 1, entries f–g, column 5).
by screening different solvents and by varying the
amount of LDA. Among the solvents screened (THF,
toluene and n-hexane), toluene was found to give the
best yield (88%, Table 1, entry a, column 6). Excess
LDA (6 equiv) was also necessary to achieve complete
conversion of the starting material. Therefore, LDA
(6 equiv) in toluene at –78 °C–rt was employed for the
conversion of tetrabromides 9b–g to enediynes 10b–g.
As in the case of the n-BuLi-mediated reactions of 9f
and 9g (Table 1, entries f–g, column 5), the parent
enediynes were transformed in situ to their TMS deriva-
tives for isolation and characterization.
Finally, the enediynes synthesized by our methodology
could be transformed to 1,6-disubstituted enediynes by
generation and trapping of the acetylides in the same
reaction vessel with appropriate electrophiles. For in-
stance, enediyne 10a, after generation from tetrabro-
mide 9a, was treated in situ with an additional amount
of n-BuLi (2 equiv) and then the reaction mixture was
quenched with MeI (Table 1, entry h, column 5). The
resultant dimethylated enediyne 10h was isolated in
62% yield.
Comparable results were obtained when tetrabromides
9a–g were reacted with LDA (Method B, Table 1, col-
umn 6). Optimization of the LDA-mediated transforma-
tion of tetrabromide 9a to enediyne 10a, was carried out
In conclusion, an easy synthesis of arenediynes from 1,2-
arenedialdehydes has been developed. Efforts towards
expanding the scope of this strategy for the synthesis

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B. Sahu et al. / Tetrahedron Letters 46 (2005) 2593–2597
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The authors thank DST, India, for financial assistance
and SAIF, IIT Bombay, for NMR data. B.S. thanks
CSIR, India, for a scholarship.
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