Photoannelation reactions of 3-(Alk-1-ynyl)cyclohept-2-en-1-ones

Article abstract of DOI:10.1002/hlca.200900202

Irradiation (350 nm) of the newly synthesized 3-(alk-1-ynyl)cyclohept-2-en- 1-ones 1 and 2 leads to the selective formation of tricyclic head-to-head dimers. In the presence of 2,3-dimethylbuta-1,3-diene, the (monocyclic) enone 1 affords trans-fused 7-alk

Full text of DOI:10.1002/hlca.200900202

Helvetica Chimica Acta – Vol. 93 (2010)
17
Photoannelation Reactions of 3-(Alk-1-ynyl)cyclohept-2-en-1-ones
by M. Robert J. Valle´e, Inga Inhꢀlsen, and Paul Margaretha*
Department of Chemistry, University of Hamburg, Martin-Luther-King Platz 6, D-20146 Hamburg
(phone: þ 49-40-428384316; fax: þ 49-40-428385592;
e-mail: Paul.Margaretha@chemie.uni-hamburg.de)
Dedicated to Prof. Jean-Marie Lehn on the occasion of his 70th birthday
Irradiation (350 nm) of the newly synthesized 3-(alk-1-ynyl)cyclohept-2-en-1-ones 1 and 2 leads to
the selective formation of tricyclic head-to-head dimers. In the presence of 2,3-dimethylbuta-1,3-diene,
the (monocyclic) enone 1 affords trans-fused 7-alkynyl-bicyclo[5.2.0]nonan-2-ones as major photo-
products, whereas photocycloaddition of benzocyclohept-5-en-7-one 2 to the same diene gives
preferentially the eight-membered cyclic allene 16 via ꢀend-to-endꢁ cyclization of the intermediate
allyl-propargyl biradical 22. On contact with acid, cycloocta-1,2,5-triene 16 isomerizes to cycloocta-1,3,5-
triene 18.
Introduction. – Whereas five- and six-membered cyclic enones undergo efficient
photocycloadditions to alkenes [1][2], similar reactions of their seven-membered ring
counterparts have so far remained unknown. This is most probably due to the fact that
the more flexible cyclohept-2-enones a) undergo efficient (Z) ! (E) photoisomeriza-
tion [3][4], and b) exhibit lower ET values than the corresponding smaller cycloenones.
The only established cyclobutane forming reaction of cyclohept-2-enones is photo-
dimerization to afford tricyclo[7.5.0.02,8]tetradecanediones. Interestingly, the config-
uration of the major dimer in all reported examples [5 – 7] is (transoid-anti-transoid),
corresponding to a 2a þ p2s ground-state cycloaddition between an (E)- and a (Z)-
p
diastereoisomer. We had previously shown that incorporating an alk-1-ynyl group at
C(3) of cyclohex-2-enones or cyclopent-2-enones has a strong influence on their
behavior in light-induced reactions, as such compounds, in the presence of alkenes,
undergo (triplet-) spin selective annelation reactions via cyclopentenyl carbenes as
intermediates [8][9]. Here, we report results on photochemical reactions of newly
synthesized alkynylcycloheptenones 1 and 2.
Results. – The common 1,3-diketone ! enol ether/ester! alkyne-coupling se-
quence for the synthesis of both 3-(3,3-dimethylbut-1-ynyl)cyclohept-2-enone (1) and
of 2,3-dimethoxy-5-(3,3-dimethylbut-1-ynyl)benzocyclohept-5-en-7-one (2) is depict-
ed in Scheme 1. Thus, cycloheptane-1,3-dione (3) was converted into enol ether 4,
which then reacted with (3,3-dimethylbut-1-ynyl)magnesium bromide to afford 1 in
12% overall yield. The synthesis of 2 started with the Friedel – Crafts-acylation of 5 to
6, followed by aldol cyclization to diketone 7, which, after transformation into toluene-
4-sulfonate 8, finally afforded 2 by a Sonogashira-type coupling reaction in 25% overall
yield.
ꢂ 2010 Verlag Helvetica Chimica Acta AG, Zꢃrich

18
Helvetica Chimica Acta – Vol. 93 (2010)
Scheme 1
Irradiation (350 nm) of 1 in benzene afforded tricyclic dimer 9 exclusively, the same
dimer being formed – again selectively – in other solvents as well. In the presence of a
tenfold molar excess of 2,3-dimethylbuta-1,3-diene, 1 afforded a 1:2 :2 :2 mixture of
1 þ 1 photoadducts 10, 11, 12, and 13, respectively (Scheme 2). In contrast, irradiation
of 2 in benzene gives a 3 :2 mixture of pentacyclic dimers 14 and 15, the ratio again not
being affected by the choice of the solvent, whereas, in the presence of 2,3-
dimethylbuta-1,3-diene, a 2 :1 mixture of photocycloadducts 16 and 17 is obtained. On
contact with silica gel or traces of acid, the cyclic allene 16 (13C-NMR shifts for the
allenic C-atoms in CDCl3: 126, 201, and 105 ppm) cleanly isomerized to – the fully
conjugated – cyclooctatriene 18 (Scheme 3). All the photoproducts were purified and
isolated by column chromatography and characterized spectroscopically. The struc-
tures of enone 2, of dimers 9, 14, and 15, as well as that of photocycloadduct 17 were
established by X-ray analysis. The assignment of a trans-fusion between the seven- and
the four-membered rings in bicyclononanones 12 and 13 was achieved on the basis of
the size of the vicinal coupling constants of the bridgehead H-atom (J ¼ 7 and 11 Hz),
identical to those observed in compound 17, which indicated a rigid puckered
conformation of the four-membered ring.
Discussion. – Regarding the conversion to tricyclic dimer 9, cycloheptenone 1
behaved exactly like its six-membered analogue, which also gave a head-to-head dimer
selectively. This can be explained by the stability of the bis-propargylic 1,4-biradical
resulting from the primary binding between the C(2)-atoms of the cycloalkenone units.

Helvetica Chimica Acta – Vol. 93 (2010)
19
Scheme 2
Scheme 3

20
Helvetica Chimica Acta – Vol. 93 (2010)
As for the higher flexibility of cycloheptenones as compared to that of cyclohexenones,
it is not surprising that 9 had trans-anti-trans configuration, whereas the corresponding
6/4/6 tricycle had cis-anti-cis configuration [10]. Similarly, 2 behaved like (the parent)
benzocyclohept-5-en-7-one [5] by affording a roughly 1:1 mixture of head-to-head
dimers with the two above mentioned configurations. In its reaction with 2,3-
dimethylbuta-1,3-diene, (monocyclic) enone 1 behaved very similarly to 2,3-dihydro-
6,6-dimethylthiin-4-one (19) [11] by affording both [2 þ 2] and [2 þ 4] photocycload-
ducts via biradicals 20 and 21, respectively. Again, the higher flexibility of 1 as
compared to that of thiacyclohexenone 19 is the reason for the relatively higher overall
ratio of trans-fused bicycles from 20 (56%) as compared to those from 21 (10%). The
ease of access to cyclic allene 16 in the photocycloaddition of 2 to 2,3-dimethylbuta-1,3-
diene is remarkable. The parent cycloocta-1,2,5-triene has been reported to be formed
by thermolysis of cis-1-ethenyl-2-ethynylcyclobutane and to dimerize rapidly in
solution at low temperatures [12]. Obviously, the bulky tBu group in 16 prevents such
a dimerization, as it does for 1-(tert-butyl)cycloocta-1,2-diene itself [13]. This cyclic
allene isomerizes to a mixture of conjugated cyclooctadienes in the presence of acid, as
observed here in the conversion 16 ! 18.
Examples of cyclizations of alkyl propargyl biradicals on C(3) of the propargylic
moiety to afford cyclic allenes are highly uncommon. A six-membered cyclic allene,
which undergoes a subsequent 1,3-H shift, has been proposed as intermediate in the
photocycloaddition of 2-methylcyclohex-2-enones to 2-methylbut-1-en-3-yne [14][15],
and a seven-membered cyclic allene, which cyclodimerizes, is formed by photo-
isomerization of 2,5,5-trimethyl-2-(prop-1-yn-1-yl)cyclopentanone [16]. Obviously,
allyl-propargyl biradical 22 strongly differs in its cyclization behavior from its
(monocyclic) counterpart 20 (Scheme 4), and the main reason for this seems to be
the – additional – benzylic character of the propargyl radical moiety which apparently
induces an increased spin density on the ꢀallenicꢁ C-atom [17].
Unfortunately, so far we have not been able to obtain crystals of 18 suitable for X-
ray analysis. For the moment, we propose that the C(5)¼C(6) bond in both cycloocta-
1,2,5- and -1,3,5-trienes, 16 and 18, respectively, has (Z)-configuration, as DFT studies
indicate that a (Z,Z,E)-cycloocta-1,3,5-triene should be more strained by ca. 80 kJ/mol
than the corresponding (all-Z)-isomer [18].
Experimental Part
1. General. Photolyses were conducted in a Rayonet RPR-100 photoreactor equipped with 350-nm
lamps and solvents of spectrophotometric grade. Column chromatography (CC): silica gel 60 (SiO2;
Merck; 230 – 400 mesh). 1H- and 13C-NMR spectra (including 2D plots): Bruker WM 500; at 500.13 and
125.8 MHz, resp.; d in ppm rel. to Me4Si as internal standard, J in Hz. GC/EI-MS: Varian MAT 311A at
70 eV. X-Ray analyses: Bruker SMART APEX II three-circle diffractometer at 153 K with MoKa
radiation (l 0.71073 ꢄ).
2. Starting Materials. Cycloheptane-1,3-dione (3) was synthesized according to [19], and methyl 3-
(3,4-dimethoxyphenyl)propanoate (5) was prepared from the commercially available acid via the
corresponding acid chloride.
2.1. Synthesis of 3-(3,3-Dimethylbut-1-yn-1-yl)cyclohept-2-enone (1). 2.1.1. Synthesis of 3-Ethoxy-
cyclohept-2-enone (4). A soln. of 12.8 g (0.1 mol) of 3 and 0.1 g of 4-TsOH in a mixture of EtOH
(100 ml) and CHCl3 (200 ml) was refluxed for 15 h on a Dean – Stark trap, and additional TsOH (0.1 g)

Helvetica Chimica Acta – Vol. 93 (2010)
21
Scheme 4
was added after 5 and 10 h, resp. Subsequent workup as described for similar 3-ethoxycycloalk-2-enones
[20] afforded 6.4 g (40%) of 4. Light yellow oil. NMR Data: identical to those reported in [21].
2.1.2. Synthesis of 1. In analogy to [22], the addition of (3,3-dimethylbut-1-ynyl)magnesium bromide
(from 4.2 g (0.05 mol) of 3,3-dimethylbut-1-yne and 6.2 g (0.05 mol) of EtMgBr) to 7.8 g (0.05 mol) of 4,
and subsequent workup, followed by CC (SiO2; pentane/Et2O 3 :2), afforded 2.85 g (30%) of 1 (Rf 0.36).
Light yellow liquid. 1H-NMR (CDCl3): 6.23 (s); 2.61 – 2.56 (m, 4 H); 1.88 – 1.77 (m, 4 H); 1.26 (s, 9 H).
13C-NMR (CDCl3): 203.2 (s); 141.4 (s); 135.6 (d); 106.3 (s); 81.3 (s); 42.5 (t); 34.9 (7); 30.6 (q); 28.3 (s);
25.1 (t); 21.5 (t). EI-MS: 190 (100, Mþ).
2.2. Synthesis of 5-(3,3-Dimethylbut-1-yn-1-yl)-5,6-dihydro-2,3-dimethoxy-7H-benzocyclohepten-7-
one (2). 2.2.1. Synthesis of Methyl 3-(2-Acetyl-4,5-dimethoxyphenyl)propanoate (6). To a soln. of 7.0 g
(31.2 mmol) of 5 in CH2Cl2 (140 ml) were added AcCl (3.4 ml, 46.8 mmol) and AlCl3 (6.2 g, 46.8 mmol),
and the mixture was stirred for 4 h at r.t. After addition of ice cold H2O (200 ml), the org. phase was
separated and washed twice with 10% HCl. The aq. phase was extracted with CH2Cl2, and the combined
org. phases were dried (Na2SO4). After evaporation of the solvent, the residue was purified by CC (SiO2;
hexane/AcOEt 6 :1) to afford 7.6 g (92%) of 6 (Rf 0.42). White solid. M.p. 71 – 728. 1H-NMR (CDCl3):
7.24 (s); 6.78 (s); 3.92 (s, 3 H); 3.91 (s, 3 H); 3.65 (s, 3 H); 3.17 (t, J ¼ 7.0, 2 H); 2.65 (t, J ¼ 7.0, 2 H); 2.57 (s,
3 H). 13C-NMR (CDCl3): 199.2 (s); 173.7 (s); 151.8 (s); 146.7 (s); 136.2 (s); 129.1 (s); 114.3 (d); 113.6 (d);
56.2 (q); 56.0 (q); 51.5 (q); 35.7 (t); 29.9 (t); 29.2 (q). EI-MS: 266 (90, Mþ), 43 (100).
2.2.2. Synthesis of 5,6,8,9-Tetrahydro-2,3-dimethoxy-7H-benzocycloheptene-5,7-dione (7). In anal-
t
ogy to [23], 6 (4.5 g, 17 mmol) was reacted with 2 equiv. of BuOK. After workup, the residue was
dissolved in CH2Cl2 and filtered from insoluble material. After evaporation, the residue was purified by
CC (SiO2; Et2O/AcOEt 9 :1) to afford 2.07 g (52%) of 7 (Rf 0.37). White solid. M.p. 131 – 1328.
1H-NMR (CDCl3): 7.54 (s); 6.70 (s); 4.13 (s, 2 H); 3.96 (s, 3 H); 3.91 (s, 3 H); 3.32, 2.73 (AA’XX’, JAA’
¼

22
Helvetica Chimica Acta – Vol. 93 (2010)
JXX’ ¼ 15.0, JAX ¼ JAX’ ¼ 7.0, 4 H). 13C-NMR (CDCl3): 203.7 (s); 189.5 (s); 153.4 (s); 148.2 (s); 136.9 (s);
128.3 (s); 112.7 (d); 112.1 (d); 60.3 (t); 56.1 (q); 56.0 (q); 41.6 (t); 30.9 (t). EI-MS: 234 (100, Mþ).
2.2.3. Synthesis of 5,6-Dihydro-2,3-dimethoxy-7-oxo-7H-benzocyclohept-5-yl 4-Methylbenzenesulfo-
nate (8). To a soln. of 7 (1.5 g, 6.3 mmol) in CH2Cl2 (75 ml) were added equivalent amounts of TsCl and
Et3N. After hydrolytic workup, drying (MgSO4), and evaporation of the solvent, the residue was purified
by CC (SiO2; Et2O/AcOEt 7:2) to afford 1.26 g (51%) of 8 (Rf 0.38). Yellow solid. M.p. 121 – 1228.
1H-NMR (CDCl3): 7.76 (d, J ¼ 8.3, 2 H); 7.31 (d, J ¼ 8.3, 2 H); 7.08 (s); 6.70 (s); 5.97 (s); 3.92 (s, 3 H); 3.80
(s, 3 H); 2.92, 2.66 (AA’XX’, JAA’ ¼ JXX’ ¼ 15.0, JAX ¼ JAX’ ¼ 7.0, 4 H); 2.43 (s). 13C-NMR (CDCl3): 199.7
(s); 156.8 (s); 151.1 (s); 147.7 (s); 135.2 (s); 133.4 (s); 129.9 (d); 128.4 (d); 123.7 (s); 120.7 (d); 111.8 (d);
111.7 (d); 56.1 (q); 56.0 (q); 42.6 (t); 29.9 (t); 21.7 (q). EI-MS: 388 (37, Mþ); 191 (100).
2.2.4. Synthesis of 2. To a soln. of 8 (1.26 g, 3.3 mmol) in dry MeCN (50 ml) were added 3,3-
dimethylbut-1-yne (0.95 ml, 7.5 mmol), EtNiPr2 (0.9 ml, 5 mmol), CuI (60 mg, 0.33 mmol), and
bis(triphenylphosphine)palladium chloride (230 mg, 0.33 mmol). After stirring the mixture overnight
at r.t., the suspension was filtered over Celite, and the solvent was evaporated. The residue was purified
by CC (SiO2; Et2O/AcOEt 9 :1) to afford 0.91 g (94%) of 2 (Rf 0.44). Yellow solid. M.p. 108 – 1098.
1H-NMR (CDCl3): 7.47 (s); 6.72 (s); 6.49 (s); 3.93 (s, 3 H); 3.90 (s, 3 H); 2.93, 2.69 (AA’XX’, JAA’ ¼ JXX’
¼
15.0, JAX ¼ JAX’ ¼ 7.0, 4 H); 1.34 (s, 9 H). 13C-NMR (CDCl3): 200.9 (s); 149.9 (s); 147.2 (s); 135.5 (s); 135.0
(s); 131.2 (d); 127.1 (s); 114.5 (d); 111.7 (d); 106.4 (s); 81.0 (s); 56.0 (q); 55.8 (q); 43.4 (t); 30.7 (t); 29.2
(q); 28.4 (s). EI-MS: 298 (100, Mþ).
X-Ray Crystal-Structure Determination of 21). Pale colorless blocks (0.46 ꢀ 0.26 ꢀ 0.07 mm) from
hexane, C19H22O3, Mr 298.37; monoclinic, space group P2(1)/c; Z ¼ 4, a ¼ 13.5310(17), b ¼ 9.8353(12),
c ¼ 14.0853 ꢄ, b ¼ 116.567(2)8; V¼ 1676.6(4) ꢄ3, Dx ¼ 1.182 g cmꢁ3
.
3. Photochemical Reactions. Ar-Degassed solns. of 1 or 2 were irradiated (concentration, solvent,
added reaction partner, duration, degree of conversion, and workup as described).
3.1. Photodimerizations. Solns. of either 1 or 2 in benzene (0.5 mmol/ml) were irradiated for 5 – 6 h.
3.1.1. Photodimerization of 1. Leads to selective and quantitative conversion into (1a,2b,8a,9b)-8,9-
bis(3,3-dimethylbut-1-yn-1-yl)tricyclo[7.5.0.02,8]tetradecane-3,14-dione (9). White crystals. M.p. 55 – 578.
1H-NMR (CDCl3): 3.58 (s, 2 H); 2.53 – 2.33 (m, 4 H); 2.06 – 1.99 (m, 2 H); 1.99 – 1.96 (m, 2 H); 1.89 – 1.83
(m, 4 H); 1.81 – 1.78 (m, 2 H); 1.78 – 1.73 (m, 2 H); 1.21 (s, 18 H). 13C-NMR (CDCl3): 209.4 (s); 98.0 (s);
76.5 (s); 53.6 (d); 46.7 (s); 42.6 (t); 36.5 (t); 31.4 (q); 27.5 (s); 26.2 (t); 23.1 (t). EI-MS: 380 (100, Mþ).
X-Ray Crystal-Structure Determination of 91). Pale colorless blocks (0.50 ꢀ 0.29 ꢀ 0.10 mm) from
hexane, C26H36O2, Mr 380.55; monoclinic, space group P2(1)/n; Z ¼ 4, a ¼ 11.078(4), b ¼ 11.586(4), c ¼
17.747(7) ꢄ, b ¼ 93.055(5)8; V¼ 22.74.8(15) ꢄ3, Dx ¼ 1.111 g cmꢁ3
.
3.1.2. Photodimerization of 2. Quant. conversion afforded a 3 :2 mixture of 14 and 15 (monitoring by
1H-NMR). CC (SiO2; hexane/AcOEt 2 :1) afforded first 74 mg (50%) of (7aa,7bb,14bb,14ca)-14b,14c-
bis(3,3-dimethylbut-1-yn-1-yl)-5,6,7a,7b,9,10,14b,14c-octahydro-2,3,12,13-tetramethoxy-dibenzo[c,c]cy-
clobuta[1,4-a :2,3-a’]dicyclohept-5-ene-7,8-dione (14; Rf 0.45). Light yellow solid. M.p. 85 – 868. 1H-NMR
(CDCl3): 7.18 (s, 2 H); 6.67 (s, 2 H); 4.49 (s, 2 H); 3.90 (s, 6 H); 3.81 (s, 6 H); 3.71 (ddd, J ¼ 4.0, 9.0, 10.0,
2 H); 2.80 – 2.62 (m, 4 H); 2.58 (ddd, J ¼ 4.0, 13.0, 17.0, 2 H); 0.93 (s, 18 H). 13C-NMR (CDCl3): 205.8 (s);
147.5 (s); 146.3 (s); 132.0 (s); 131.8 (s); 113.0 (d); 112.1 (d); 97.3 (s); 81.7 (s); 55.8 (q); 55.6 (q); 53.9 (d);
51.1 (s); 44.1 (t); 30.4 (q); 29.7 (t); 28.1 (s). FAB-MS: 597.4 ([M þ H]þ).
X-Ray Crystal-Structure Determination of 141). Pale colorless blocks (0.50 ꢀ 0.17 ꢀ 0.03 mm) from
hexane, C38H44O6, Mr 596.73; monoclinic, space group P2(1)/n; Z ¼ 8, a ¼ 17.401(4), b ¼ 23.086(5), c ¼
20.478(3) ꢄ, b ¼ 124.261(12)8; V¼ 680.1 ꢄ3, Dx ¼ 1.166 g cmꢁ3
.
This is followed by 42 mg (29%) of (7aa,7bb,14ba,14cb)-14b,14c-bis(3,3-dimethylbut-1-yn-1-
yl)-5,6,7a,7b,9,10,14b,14c-octahydro-2,3,12,13-tetramethoxy-dibenzo[c,c’]cyclobuta[1,4-a;2,3-a’]dicy-
clohept-5-ene-7,8-dione (15; Rf 0.27). Light yellow solid. M.p. 188 – 1898. 1H-NMR (CDCl3): 7.25 (s, 2 H);
6.77 (s, 2 H); 3.90 (s, 6 H); 3.87 (s, 2 H); 3.84 (s, 6 H); 3.26 (ddd, J ¼ 2.0, 11.0, 13.0, 2 H); 3.02 (ddd, J ¼ 2.0,
7.0, 15.0, 2 H); 2.78 (ddd, J ¼ 2.0, 7.0, 15.0, 2 H); 2.65 (ddd, J ¼ 2.0, 11.0, 13.0, 2 H); 0.96 (s, 18 H).
1
)
CCDC-734158 – 734162 contain the supplementary crystallographic data for 2, 9, 14, 15, and 17,
resp. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif.

Helvetica Chimica Acta – Vol. 93 (2010)
23
13C-NMR (CDCl3): 206.3 (s); 147.3 (s); 146.6 (s); 132.8 (s); 131.0 (s); 114.8 (d); 111.0 (d); 98.2 (s); 77.3
(s); 55.9 (q); 55.8 (q); 54.7 (d); 51.8 (s); 41.9 (t); 30.9 (t); 30.5 (q); 27.5 (s). FAB-MS: 597.4 ([M þ H]þ).
X-Ray Crystal-Structure Determination of 151). Pale colorless blocks (0.50 ꢀ 0.17 ꢀ 0.03 mm) from
hexane, C38H44O6, Mr 596.73; monoclinic, space group P2(1)/c; Z ¼ 4, a ¼ 12.348(3), b ¼ 10.460(3), c ¼
25.793(7) ꢄ, b ¼ 101.647(52)8; V¼ 3262.8(15) ꢄ3, Dx ¼ 1.215 g cmꢁ3
.
3.2. Photocycloadditions to 2,3-Dimethylbuta-1,3-diene. Solns. of either 1 or 2 (0.5 mmol) and 2,3-
dimethylbuta-1,3-diene (0.41 g, 5 mmol) in benzene (5 ml) were irradiated for 6 – 7 h.
3.2.1. Photocycloaddition of 1. After total conversion, monitoring by 1H-NMR indicates the
formation of a 1:2 :2 :2 mixture of 1:1 photoproducts 10 – 13 (numbering by CC-elution order). CC
(SiO2; CH2Cl2) afforded first 23 mg (11%) of cis-3-(3,3-dimethylbut-1-ynyl)-2-(3-methyl-2-methylidene-
but-3-en-1-yl)cycloheptan-1-one (10; Rf 0.54). Light yellow oil. 1H-NMR (CDCl3): 5.17 (s); 5.06 (s); 5.01
(s); 4.97 (s); 2.94 (ddd, J ¼ 1.0, 3.5, 14.5); 2.85 (ddd, J ¼ 3.0, 3.0, 6.0); 2.74 (ddd, J ¼ 3.0, 3.0, 10.2); 2.58 –
2.54 (m, 2 H); 2.32 (dd, J ¼ 10.2, 14.5); 2.10 – 2.02 (m, 2 H); 1.89 (s, 3 H); 1.84 – 1.73 (m, 2 H); 1.55 – 1.47
(m, 2 H); 1.19 (s, 9 H). 13C-NMR (CDCl3): 212.8 (s); 145.8 (s); 142.3 (s); 114.7 (t); 113.0 (t); 94.0 (s); 77.4
(s); 54.4 (d); 43.8 (t); 35.3 (t); 34.8 (t); 32.0 (q); 30.5 (d); 27.4 (s); 26.1 (t); 25.4 (t); 21.3 (q). EI-MS: 272 (6,
Mþ), 178 (100).
The second fraction consists of 35 mg (25%) of 7-(3,3-dimethylbut-1-yn-1-yl)-9,10-dimethylbicy-
1
clo[5.4.0]undec-9-en-2-one (11; Rf 0.49). Light yellow oil. H-NMR (CDCl3): 3.12 (ddd, J ¼ 2.8, 12.0,
12.0); 2.38 – 2.35 (m); 2.34 – 2.32 (m); 2.27 (dd, J ¼ 0.5, 12.0); 2.10 – 2.05 (m, 2 H); 2.05 – 2.02 (m, 2 H);
1.95 (d, J ¼ 15.4); 1.86 – 1.83 (m); 1.81 – 1.75 (m, 2 H); 1.68 (s, 3 H); 1.58 (s, 3 H); 1.39 – 1.33 (m); 1.17 (s,
9 H). 13C-NMR (CDCl3): 215.5 (s); 124.4 (s); 123.2 (s); 93.8 (s); 80.1 (s); 57.0 (d); 48.6 (s); 47.7 (t); 42.3
(t); 34.5 (t); 34.3 (t); 31.4 (s); 27.5 (q); 25.0 (t); 24.5 (t); 18.1 (q); 17.7 (q). EI-MS: 272 (89, Mþ), 215 (100).
The third fraction (70 mg, 50%) is a 5 :4 mixture of diastereoisomeric (1a,7b)-7-(3,3-dimethylbut-1-
ynyl)-8-methyl-8-(1-methylethenyl)bicyclo[5.2.0]nonan-2-ones 12 and 13 (Rf 0.32), which can be
differentiated by NOE. The major component is the 8b-diastereoisomer 12. 1H-NMR (CDCl3): 4.73
(s); 4.61 (s); 3.25 (dd, J ¼ 7.3, 10.4); 2.51 (dd, J ¼ 10.4, 10.4); 2.45 (ddd, J ¼ 2.8, 12.0, 12.1); 1.89 – 1.88 (m);
1.81 – 1.75 (m, 4 H); 1.72 – 1.69 (m, 2 H); 1.66 (s, 3 H); 1.63 (dd, J ¼ 7.3, 10.4); 1.26 (s, 3 H); 1.20 (s, 9 H).
13C-NMR (CDCl3): 212.1 (s); 151.3 (s); 108.8 (t); 91.9 (s); 79.0 (s); 50.4 (d); 47.9 (s); 46.7 (s); 43.3 (t); 36.7
(t); 30.5 (t); 30.4 (s); 27.4 (q); 26.5 (t); 24.5 (t); 21.8 (q); 18.4 (q).
Data of the 8a-Diastereoisomer 13. 1H-NMR (CDCl3): 5.02 (s); 4.96 (s); 3.28 (dd, J ¼ 7.5, 10.8); 2.28
(ddd, J ¼ 2.8, 12.0, 12.1); 2.15 (dd, J ¼ 7.5, 12.5); 2.11 (dd, J ¼ 10.9, 12.5); 1.81 – 1.75 (m, 5 H); 1.74 (s, 3 H);
1.72 – 1.69 (m, 2 H); 1.27 (s, 3 H); 1.21 (s, 9 H). 13C-NMR (CDCl3): 212.1 (s); 147.1 (s); 110.7 (t); 98.6 (s);
80.1 (s); 50.9 (d); 47.7 (s); 47.5 (s); 43.2 (t); 36.5 (t); 31.4 (s); 29.1 (t); 27.7 (q); 26.7 (t); 23.9 (q); 21.8 (q).
3.2.2. Photocycloaddition of 2. After total conversion, monitoring by 1H-NMR in benzene indicated
the formation of a 2 :1 mixture of 1:1 photoproducts 16 an 17. On contact with SiO2 or on standing in
CDCl3, 16 cleanly isomerized to 18. The spectroscopic data of 12-(1,1-dimethylethyl)-5,6,7a,8-tetrahydro-
2,3-dimethoxy-9,10-dimethyl-7H,11H-benzo[3,4]cyclohepta[1,2]cycloocten-7-one (16), therefore, stem
1
directly from the reaction mixture. H-NMR (C6D6): 6.78 (s); 6.65 (s); 3.86 (s, 3 H); 3.85 (s, 3 H); 3.49
(dd, J ¼ 3.8, 12.0); 3.10, 2.91 (AB, J ¼ 15.5); 2.90 – 2.88 (m, 3 H); 2.57 – 2.55 (m, 2 H); 2.45 (dd, J ¼ 3.8,
12.0); 1.88 (dd, J ¼ 12.0, 12.0); 1.73 (s, 3 H); 1.57 (s, 3 H); 1.07 (s, 9 H). 13C-NMR ((D6)benzene; allenic
C-atoms indicated): 209.7 (s); 201.0 (s, C(13)); 147.7 (s); 147.5 (s); 129.4 (s); 129.3 (s); 127.4 (s); 127.0 (s);
126.2 (s, C(12)); 113.5 (d); 111.3 (d); 105.5 (s, C(13a)); 55.9 (q); 55.8 (q); 47.5 (d); 41.3 (t); 33.1 (t); 32.1
(s); 30.9 (t); 30.8 (t); 29.2 (q); 21.8 (q); 20.3 (q). CC of the crude reaction mixture (SiO2; hexane/AcOEt
4 :1) afforded first 80 mg (42%) of 12-(1,1-dimethylethyl)-5,6,7,7a-tetrahydro-2,3-dimethoxy-9,10-
dimethyl-7H-benzo[3,4]cyclohepta[1,2]cycloocten-7-one (18; Rf 0.30). Colorless solid. M.p. 71 – 728.
1H-NMR (CDCl3): 6.67 (s); 6.62 (s); 6.09 (s); 5.67 (s); 3.88 (s, 3 H); 3.87 (s, 3 H); 3.64 (dd, J ¼ 5.6, 12.5);
3.25 (dd, J ¼ 5.6, 12.5); 3.06 (ddd, J ¼ 2.2, 10.3, 16.0); 2.96 – 2.94 (m); 2.91 – 2.89 (m); 2.60 – 2.58 (m); 1.90
(dd, J ¼ 12.5, 12.5); 1.70 (s, 3 H); 1.69 (s, 3 H); 1.09 (s, 9 H). 13C-NMR (CDCl3): 209.3 (s); 148.1 (s); 147.4
(s); 146.6 (s); 140.0 (s); 138.4 (s); 130.4 (d, C(13)); 129.8 (s); 129.4 (s); 125.1 (d, C(11)); 112.7 (d); 112.6
(d); 56.3 (q); 56.0 (q); 54.1 (d); 42.0 (t); 36.3 (s); 35.2 (t); 30.9 (t); 30.0 (q); 18.4 (q); 17.6 (q). FAB-MS:
381.3 ([M þ H]þ).
The second fraction consisted of 42 mg (24%) of (1a,2aa,9bb)-9b-(3,3-dimethylbut-1-yn-1-yl)-
1,2,2a,4,5,9b-hexahydro-7,8-dimethoxy-1-methyl-1-(1-methylethenyl)-3H-benzo[a]cyclobuta[c]cyclohept-

24
Helvetica Chimica Acta – Vol. 93 (2010)
en-3-one (17; Rf 0.24). Colorless solid. M.p. 128 – 1298. 1H-NMR (CDCl3): 6.59 (s); 6.58 (s); 4.97 (s); 4.90
(d); 4.12 (dd, J ¼ 7.7, 11.1); 3.85 (s, 3 H); 3.80 (s, 3 H); 3.29 (ddd, J ¼ 2.0, 12.6, 16.7); 3.00 (ddd, J ¼ 2.2,
12.6, 17.4); 2.90 (ddd, J ¼ 2.0, 6.5, 16.7); 2.57 (ddd, J ¼ 2.0, 6.5, 17.5); 2.32 (dd, J ¼ 11.1, 11.2); 2.05 (dd, J ¼
7.7, 11.2); 1.66 (s, 3 H); 1.65 (s, 3 H); 1.14 (s, 9 H). 13C-NMR (CDCl3): 210.3 (s); 148.8 (s); 148.7 (s); 147.4
(s); 134.3 (s); 128.5 (s); 114.4 (t); 114.1 (d); 109.9 (d); 96.8 (s); 81.1 (s); 55.8 (q); 55.7 (q); 51.7 (s); 51.3
(s); 50.1 (d); 41.4 (t); 31.5 (t); 31.1 (t); 30.9 (q); 27.6 (s); 22.7 (q); 22.6 (q). FAB-MS: 381.3 ([M þ H]þ).
X-Ray Crystal-Structure Determination of 171). Pale colorless blocks (0.50 ꢀ 0.21 ꢀ 0.24 mm) from
hexane, C25H32O3, Mr 380.51; monoclinic, space group P2(1)/c; Z ¼ 4, a ¼ 14.3801(14), b ¼ 10.8618(10),
c ¼ 14.3929(14) ꢄ, b ¼ 108.031(10)8; V¼ 2137.7(4) ꢄ3, Dx ¼ 1.182 g cmꢁ3
.
We gratefully acknowledge the assistance of Mrs. Isabelle Nevoigt in performing the X-ray crystal-
structure determinations.
REFERENCES
[1] N. Hoffmann, Chem. Rev. 2008, 108, 1052.
[2] P. Margaretha, in ꢀMolecular and Supramolecular Photochemistryꢁ, Eds. A. G. Griesbeck, J. Mattay,
Marcel Dekker, New York, 2005, Vol. 12, pp. 211 – 238, and refs. cited therein.
[3] E. J. Corey, M. Tada, R. LaMahieu, L. Libit, J. Am. Chem. Soc. 1965, 87, 2051.
[4] P. E. Eaton, K. Lin, J. Am. Chem. Soc. 1965, 87, 2052.
[5] H. Hart, E. Dunkelblum, J. Org. Chem. 1979, 44, 4752.
[6] R. A. Bunce, V. L. Taylor, E. M. Holt, J. Photochem. Photobiol., A 1991, 57, 317.
[7] A. B. Smith III, J. L. Wood, T. P. Keenan, N. Liverton, M. Visnick, J. Org. Chem. 1994, 59, 6652.
[8] W. C. Agosta, P. Margaretha, Acc. Chem. Res. 1996, 29, 179.
[9] B. Kisilowski, W. C. Agosta, P. Margaretha, Chem. Commun. 1996, 2065.
[10] H.-J. Rathjen, W. C. Agosta, P. Margaretha, A.-M. Chiu, L. Todaro, S. Wolff, Acta Crystallogr., Sect.
C 1991, 47, 2176.
[11] P. Margaretha, K. Schmidt, J. Kopf, V. Sinnwell, Synthesis 2007, 1426.
[12] W. R. Roth, T. Bastigkeit, Liebigs Ann. Chem. 1996, 2171.
[13] J. D. Price, R. B. Johnson, J. Org. Chem. 1991, 56, 6372.
[14] B. Witte, P. Margaretha, Org. Lett. 1999, 1, 173.
[15] B. Witte, L. Meyer, P. Margaretha, Helv. Chim. Acta 2000, 83, 554.
[16] D. Venugopal, L. D. Todaro, P. Margaretha, W. C. Agosta, J. Chem. Soc., Chem. Commun. 1993,
1014.
[17] E. B. Jochnowitz, X. Zhang, M. R. Nimlos, M. E. Varner, J. F. Stanton, G. B. Ellison, J. Phys. Chem.
A 2005, 109, 3812.
[18] M. Zora, J. Org. Chem. 2002, 69, 857.
[19] J. A. Ragan, T. W. Makowski, D. J. am Ende, P. J. Clifford, G. R. Young, A. K. Conrad, S. A.
Eisenbeis, Org. Process Res. Dev. 1998, 2, 379.
[20] W. F. Gannon, H. O. House, in ꢀOrganic Synthesisꢁ, Coll. Vol. V, Ed. H. Baumgarten, J. Wiley &
Sons, New York, 1973, p. 294.
[21] G. S. Thompson, J. A. Hirsch, J. Org. Chem. 1998, 63, 1098.
[22] H. J. Rathjen, P. Margaretha, S. Wolff, W. C. Agosta, J. Am. Chem. Soc. 1991, 113, 3904.
[23] A. Facchetti, A. Streitwieser, J. Org. Chem. 2004, 69, 8345.
Received May 28, 2009