Photocycloisomerization of Boc-protected 5-alkenyl-2,5-dihydro-1H-pyrrol-2-ones

Article abstract of DOI:10.1002/hlca.200390051

On irradiation (254 nm), the newly synthesized Boc-protected 5-alkenyl-2,5-dihydro-1H-pyrrol-2-ones 13 undergo regioselective intramolecular [2 + 2] photocycloadditions. While the allyl derivatives 13a-13c afford mainly azatricyclo[3.3.0.0^2.7]octanones, i.e., crossed cycloadducts, the butenyl- and pentenyl-substituted compounds 13d and 13e isomerize preferentially to straight cycloadducts.

Full text of DOI:10.1002/hlca.200390051

Helvetica Chimica Acta ± Vol. 86 (2003)
515
Photocycloisomerization of Boc-Protected 5-Alkenyl-2,5-dihydro-1H-
pyrrol-2-ones
1
by Matthias N. Wrobel ) and Paul Margaretha*
Institut f¸r Organische Chemie, Universit‰t Hamburg, M.L. King Platz 6, D-20146 Hamburg
(
Phone: ‡4940-428384316; fax: ‡4940-428385592; e-mail: Paul Margaretha@chemie.uni-hamburg.de)
Dedicated to Professor William C. Agosta on the occasion of his 70th birthday
On irradiation (254 nm), the newly synthesized Boc-protected 5-alkenyl-2,5-dihydro-1H-pyrrol-2-ones 13
undergo regioselective intramolecular [2 ‡ 2] photocycloadditions. While the allyl derivatives 13a ± 13c afford
2
,7
mainly azatricyclo[3.3.0.0 ]octanones, i.e., crossed cycloadducts, the butenyl- and pentenyl-substituted
compounds 13d and 13e isomerize preferentially to straight cycloadducts.
Introduction. ± Intramolecular enone ‡ alkene [2 ‡ 2] photocycloadditions, i.e.,
light-induced cycloisomerizations of a,b-unsaturated carbonyl compounds bearing an
additional CˆC bond [1][2], have attracted the attention of organic chemists as a
powerful method for assembling polycyclic molecules. Well-known examples are the
carvone ! carvonecamphor conversion [3] and a key step in the synthesis of cubane
[
4]. Regarding the regioselectivity of such reactions, the number of atoms between the
two reactive CˆC bonds has turned out as being a decisive factor: many examples
wherein the two CˆC bonds are separated by one, two, or three additional atoms, are
governed by the so-called −rule of five×, which states that −if triplet cyclizations can lead
to rings of different size, the one formed by 1,5-cyclization is preferred kinetically× [5][6].
While the behavior of five- and six-membered cyclic enones bearing an alkenyl side
chain either at C(2) or C(3) has been extensively investigated, detailed results on 4-
alkenyl-substituted cyclopent-2-enones and related heterocyclic enones are scarce. As
expected, irradiation of cyclopentenone 1 affords the tricyclic ketone 2 in a −crossed×
addition mode [7], while, under similar conditions, enone 3 affords ketone 4, now in a
−
straight× addition mode [8]. Similarly, on irradiation, the 5-allyl-2(5H)-furan-2-one (5)
isomerizes predominantly to the crossed tricyclic lactone 6 [9], while the corresponding
5
-but-4-enyl derivative 7 gives mainly the straight tricycle 8 [10] (Scheme 1).
We have recently reported [11] that the diastereoisomeric u- and l-dihydropyrroles
and 10 photoisomerize selectively to azatetracyclodecanones 11 and 12, respectively;
9
these conversions represent the first example of a photochemical reaction wherein two
diastereoisomeric hexa-1,5-dienes undergo photoisomerization to a crossed and a
straight cycloadduct, respectively (Scheme 2). Here, we report the synthesis of 13a ±
13e and the photochemical behavior of these five novel (open-chain) alkenyl
derivatives containing the same dihydropyrrole moiety as 9 and 10.
1)
Part of the Ph.D. thesis of M. N. W., Universit‰t Hamburg, 2002.

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16
Helvetica Chimica Acta ± Vol. 86 (2003)
Scheme 1
Scheme 2
Results. ± Compounds 13 were obtained by alkylation of tert-butyl 2-[(tert-
butyl)dimethylsilyloxy]-1H-pyrrole-1-carboxylate (14) [12] with the appropriate
alkenyl halide 15 (X ˆ Br for 15a ± 15c and X ˆ I for 15d,e) in the presence of
CF COOAg (AgTFA) [13] (Scheme 3). Irradiations (254 nm) of 13 were performed in
3
MeCN as solvent, and the separation/isolation of products (Scheme 4) were achieved
by chromatography. The yields given are those of isolated products 16 ± 21. The
structural assignment of the methylene-bridged tricycles 16a and 19 stems from their
1
H-NMR spectra wherein the H-atoms of this methylene bridge resonate as an AB
2
system with J ˆ À7.1 and À10.2 Hz, respectively. This finding is in agreement with the
general knowledge [14] that the (absolute) value of a geminal coupling constant

Helvetica Chimica Acta ± Vol. 86 (2003)
517
Scheme 3
Scheme 4
decreases with increasing sp character of the corresponding C-atom. In the 13C-NMR
2
spectra, the ring carbonyl C-atoms of the straight cycloadducts 17, 18, and 20 (d ꢀ 177 ±
179 ppm) always resonate at lower field than those of the crossed cycloadducts 16 and
1
9 (d ꢀ 171 ± 173 ppm), reflecting the higher strain in a bicyclo[2.2.0]hexane as
compared to a bicyclo[2.1.1]hexane unit [15]. Finally, the structure of 17a was
established by X-ray analysis.

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Helvetica Chimica Acta ± Vol. 86 (2003)
Discussion. ± Reactions at C(5) of silyloxy-pyrrole 14 with formation of a new CÀC
bond [16] continue to be of interest, as illustrated in a recent enantioselective synthesis
of the antiinfluenza compound A-315675 [17]. Moreover the sequence a) intermo-
lecular photocycloaddition of a Boc-protected pyrrolone to ethene, and b) methanol-
ysis of the lactam ring has been used in the synthesis of a g-amino acid derivative
containing a cyclobutane skeleton [18]. In this connection, pyrrolones 13a ± 13c
represent convenient model compounds for studying the regiochemical control in
intramolecular photochemical reactions of 1-acylhexa-1,5-dienes, as they also undergo
preferential cyclization by way of 1,5 (crossed) closure. The relatively high amount of
17c formed on irradiation of 13c, i.e., a shift to 1,6 (straight) closure, is of interest, as, for
open-chain model compounds, alkyl substitution at C(6) normally does not modify the
regiochemistry [19]. The preferential formation of 18 from 13d proceeds in analogy to
the selective conversion of furanone 7 to tricycle 8 [10]. In this context, it should be
mentioned that the corresponding S-heterocycles, i.e., 5-alkenyl-2(5H)-thiophen-2-
ones, undergo efficient SÀC(O) homolysis from their excited singlet state and,
therefore, do not afford any products resulting from (triplet) interaction between the
two CˆC bonds [20].
The selective conversion of 13e to 20, analogous to that of 13d to 18, seems to
correspond to the assumption [21] that straight cycloadducts are formed in high
regioselectivity, whenever the −enone× is tethered to the −olefin× by three or four atoms,
i.e., for both 1-acylhepta-1,6-dienes and 1-acylocta-1,7-dienes. Nevertheless, such
general inferences have to be applied with caution, as quite often subtile conforma-
tional factors govern the outcome of such reactions. This has been shown very recently
for the allyl- and butenyl-substituted 2H-furan-3-ones 22 (Scheme 5): while 22a
photoisomerizes selectively to the straight cycloadduct 23 [22], a complete reversal in
the regioselectivity is observed for 22b, which affords the crossed cycloadduct 24 in 90%
yield [23].
Scheme 5
Experimental Part
1
. General. tert-Butyl 2-[(tert-butyl)dimethylsilyloxy]-1H-pyrrole-1-carboxylate (14) was prepared accord-
ing to the procedure in [12]. 3-Bromoprop-1-ene (15a), (E)-1-bromobut-2-ene (15b), and 1-bromo-3-methylbut-
-ene (15c) were commercially available. 4-Iodobut-1-ene (15d) and 5-iodopent-1-ene (15e) were obtained by
2

Helvetica Chimica Acta ± Vol. 86 (2003)
519
reaction of the (commercially available) bromoalkenes with NaI in acetone. H- and 13C-NMR Spectra: 400 and
1
1
00.6 MHz, resp.; chemical shifts d in ppm rel. to Me
equipped with 254-nm lamps. X-Ray crystal-structure analysis: Enraf-Nonius CAD-4 four-circle diffractometer
at 293 K with CuK
radiation (l ˆ 1.54178 ä).
. Synthesis of Unsaturated Lactams 13a ± 13e. To a soln. of 1.488 g (5 mmol) 14 and the haloalkene
10 mmol) in 15 ml of CH Cl at 08, 2.21 g (10 mmol) CF COOAg was added, and the mixture was stirred for 1 h
and allowed to warm up to r.t. After addition of 25 ml of sat. aq. NaHCO , the org. phase was separated, and the
aq. phase was extracted 5 Â with Et O (15 ml). The combined org. phases were washed with sat. aq. NaCl soln.,
dried (MgSO ), the solvent was evaporated, and the residue was purified by column chromatography (CC) on
4
Si (ˆ0 ppm). Photolyses: Rayonet RPR-100 photoreactor
a
2
(
2
2
3
3
2
4
SiO
2
.
2
.1. tert-Butyl 2,5-Dihydro-2-oxo-5-(prop-2-enyl)-1H-pyrrole-1-carboxylate (13a). With 15a. 606 mg
1
(
6
8
54.3%). Colorless oil. R
f
(Et
2
O/hexane 5 :3) 0.43. H-NMR (CDCl
3
): 7.14 (dd, J ˆ 2.0, 6.1); 6.11 (dd, J ˆ 1.5,
.1); 5.64 (dddd, J ˆ 6.3, 8.1, 10.4, 16.8); 5.13 (m, 2 H); 4.64 (dddd, J ˆ 1.5, 2.0, 3.6, 8.1); 2.80 (m); 2.50 (ddd, J ˆ
13
3
.1, 8.1, 13.7); 1.57 (s, 9 H). C-NMR (CDCl ): 169.2 (s); 149.9 (d); 149.4 (s); 131.1 (d); 126.9 (d); 119.6 (t); 83.0
(
s); 61.7 (d); 35.8 (t); 28.1 (q).
2
.2. tert-Butyl 2-[(E)-But-2-enyl]-2,5-dihydro-5-oxo-1H-pyrrole-1-carboxylate (13b). With 15b. 688 mg
1
(
58%). Colorless oil. R
f
(AcOEt/hexane 1:1) 0.58. H-NMR (CDCl
3
): 7.13 (dd, J ˆ 2.0, 6.1); 6.09 (dd, J ˆ 1.5,
6
.1); 5.54 (m); 5.26 (m); 4.57 (m); 2.72 (m); 2.41 (ddd, J ˆ 8.1, 8.1, 13.7); 1.65 (d, J ˆ 6.4, 3 H); 1.57 (s, 9 H).
1
3
3
C-NMR (CDCl ): 169.4 (s); 150.3 (d); 149.4 (s); 130.4 (d); 126.7 (d); 123.5 (d); 82.9 (s); 62.2 (d); 34.8 (t); 28.1
(
q); 18.0 (q).
2
.3. tert-Butyl 2,5-Dihydro-2-(3-methylbut-2-enyl)-5-oxo-1H-pyrrole-1-carboxylate (13c). With 15c. 543 mg
1
(
6
(
43.2%). Colorless oil. R
f
(AcOEt/hexane 1 :2) 0.37. H-NMR (CDCl
3
): 7.12 (dd, J ˆ 2.0, 6.1); 6.09 (dd, J ˆ 1.5,
): 169.4
s); 150.6 (d); 149.5 (s); 136.5 (s); 126.6 (d); 116.8 (d); 82.9 (s); 62.4 (d); 30.5 (t); 28.2 (q); 25.8 (q); 17.8 (q).
.1); 4.99 (m); 4.58 (m); 2.76 (m); 2.39 (m); 1.70 (s, 3 H); 1.62 (s, 3 H); 1.57 (s, 9 H). 13C-NMR (CDCl
3
2
.4. tert-Butyl 2-(But-3-enyl)-2,5-dihydro-5-oxo-1H-pyrrole-1-carboxylate (13d). With 15d. 347 mg
1
(
6
(
(
29.2%). Colorless oil. R
f
(Et
2
O/hexane 5 :2) 0.47. H-NMR (CDCl
3
): 7.17 (dd, J ˆ 2.0, 6.1); 6.10 (dd, J ˆ 1.5,
.1); 5.78 (dddd, J ˆ 6.6, 6.6, 10.2, 16.8); 5.08 ± 4.99 (m, 2 H); 4.63 (m); 2.20 ± 1.94 (m, 3 H); 1.84 (m); 1.56
13
s, 9 H). C-NMR (CDCl
t); 28.4 (t); 28.2 (q).
3
): 169.2 (s); 150.0 (d); 149.4 (s); 137.0 (d); 126.8 (d); 115.8 (t); 83.0 (s); 61.9 (d); 30.6
2
.5. tert-Butyl 2,5-Dihydro-5-oxo-2-(pent-4-enyl)-1H-pyrrole-1-carboxylate (13e). With 15e. 637 mg
1
(
6
(
(
50.7%). Colorless oil. R
f
(Et
2
O/hexane 3 :1) 0.54. H-NMR (CDCl
3
): 7.15 (dd, J ˆ 2.0, 6.1); 6.10 (dd, J ˆ 1.5,
.1); 5.75 (dddd, J ˆ 6.6, 6.6, 10.2, 17.0); 5.01 (m); 4.98 (m); 4.62 (m); 2.04 (m, 3 H); 1.78 (m); 1.56 (s, 9 H); 1.35
13
m, 2 H). C-NMR (CDCl
t); 30.8 (t); 28.2 (q); 23.2 (t).
. Photolyses. Ar-degassed solns. of 13 (1 mmol) in 75 ml of MeCN were irradiated in a quartz vessel for 6 h.
After evaporation of the solvent, the residue was separated and purified by CC on SiO
.1. Irradiation of 13a. Elution with AcOEt/hexane 1:2 afforded first (R 0.41) 116 mg (52%) of tert-butyl
-oxo-4-azatricyclo[3.3.0.0 ]octane-4-carboxylate (16a). M.p. 1128. H-NMR (C
): 4.38 (m, HÀC(5)); 2.33
m, HÀC(2)); 2.16 (m, HÀC(7)); 2.06 (m, HÀC(1)); 1.48 (s, 9 H); 1.37 (dd, J ˆ 4.6, 11.2, HexoÀC(6)); 1.19
3
): 169.4 (s); 150.3 (d); 149.4 (s); 137.8 (d); 126.7 (d); 115.3 (t); 82.9 (s); 62.3 (d); 33.4
3
2
.
3
f
2
,7
1
3
(
(
6 6
D
1
3
m, HendoÀC(6), HsynÀC(8)); 0.39 (d, J ˆ 7.1, HantiÀC(8)). C-NMR (C
6 6
D ): 170.4 (s, C(3)); 150.7 (s); 81.2 (s);
5
7.1 (d, C(5)); 55.5 (d, C(2)); 45.8 (d, C(1)); 39.6 (d, C(7)); 36.3 (t, C(6)); 35.0 (t, C(8)); 28.2 (q).
The second fraction (R
-carboxylate (17a). M.p. 838. H-NMR (CDCl
f
0.34) consisted of 25 mg (10.2%) of tert-butyl 7-oxo-6-azatricyclo[3.2.1.03,8]octane-
1
6
3
1
3
): 4.69 (m, HÀC(5)); 3.28 (m, HÀC(8)); 3.20 (m, HÀC(1));
.07 (m, HexoÀC(4)); 2.95 (m, HexoÀC(2)); 2.81 (m, HÀC(3)); 2.31 (m, HendoÀC(2)); 2.12 (m, HendoÀC(4));
13
.54 (s, 9 H). C-NMR (C
6 6
D ): 178.8 (s, C(7)); 150.2 (s); 82.7 (s); 63.2 (d, C(5)); 40.7 (d, C(1)); 37.3 (t, C(4));
3
6.7 (d, C(8)); 32.9 (t, C(2)); 32.0 (d, C(3)); 28.1 (q).
X-Ray Crystal-Structure Determination of 17a. Pale colorless transparent blocks (1.2 Â 0.5 Â 0.5 mm) from
≈
3
hexane, C12
7
H
17NO
3
, M
r
223.27, triclinic, space group P1, Z ˆ 4, a ˆ 5.878(1), b ˆ 14.148(1), c ˆ 14.422(1) ä, a ˆ
À3
6.31(1)8, b ˆ 84.89(1)8, g ˆ 87.80(1)8, Vˆ 1160.5(2) ä , D
x
ˆ 1.278(1) g ¥ cm
.
3
.2. Irradiation of 13b. Elution with Et O/hexane 2 :1 afforded as main product (R
2
f
0.49) 128 mg (53.9%) of
2
,7
1
tert-butyl syn-8-methyl-3-oxo-4-azatricyclo[3.3.0.0 ]octane-4-carboxylate (16b). M.p. 528. H-NMR (C
6
D
6
):
4
.40 (m, HÀC(5)); 2.86 (m, 2 H); 1.89 (m, HÀC(1), HÀC(7)); 1.49 (s, 9 H); 1.46 (dd, J ˆ 4.6, 11.2, HexoÀC(6));
13
1
.22 (dd, J ˆ 2.0, 11.2, HendoÀC(6)); 0.93 (q, J ˆ 6.6, HantiÀC(8)); 0.68 (d, J ˆ 6.6, 3 H). C-NMR (C
s, C(3)); 150.6 (s); 82.0 (s); 57.7 (d, C(5)); 52.9 (d, C(2)); 50.4 (d, C(1)); 43.6 (d, C(7)); 43.4 (d, C(8)); 37.5
t, C(6)); 28.2 (q); 12.6 (q, Me).
6 6
D ): 171.1
(
(

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20
Helvetica Chimica Acta ± Vol. 86 (2003)
.3. Irradiation of 13c. Elution with Et O/hexane 3 :2 afforded first (R
3
2
f
0.40) 86 mg (34.2%) of tert-butyl
2
,7
1
8
,8-dimethyl-3-oxo-4-azatricyclo[3.3.0.0 ]octane-4-carboxylate (16c). M.p. 818. H-NMR (CDCl
3
): 4.52
(
1
1
m, HÀC(5)); 3.12 (m, HÀC(2)); 2.61 (m, HÀC(7)); 2.56 (m, HÀC(1)); 2.40 (dd, J ˆ 4.4, 12.0, HexoÀC(6));
13
.54 (s, 9 H); 1.367 (dd, J ˆ 1.9, 12.0, HendoÀC(6)); 1.29 (s, 3 H); 0.84 (s, 3 H). C-NMR (CDCl
3
): 173.1 (s, C(3));
49.6 (s); 82.7 (s); 58.5 (d, C(5)); 54.6 (d, C(1)); 51.9 (d, C(2)); 47.5 (d, C(7)); 44.1 (s, C(8)); 33.7 (t, C(6)); 28.2
(
q); 21.5, 21.0 (q, Me).
The second fraction (R
f
0.33) consisted of 53 mg (21%) of tert-butyl 2,2-dimethyl-7-oxo-6-azatri-
cyclo[3.2.1.03 ]octane-6-carboxylate (17c). M.p. 538. H-NMR (CDCl
,8
1
): 4.63 (m, HÀC(5)); 3.23
3
(
1
(
m, HÀC(8)); 2.75 (m, HÀC(1), HexoÀC(4)); 2.42 (m, HÀC(3)); 2.17 (ddd, J ˆ 1.5, 1.5, 13.9, HendoÀC(4));
13
.53 (s, 9 H); 1.31, 1.13 (s, 3 H). C-NMR (CDCl
d, C(1)); 42.6 (d, C(3)); 41.8 (s, C(2)); 32.5 (d, C(8)); 31.6 (q, Me); 28.1 (q); 20.8 (q, Me).
.4. Irradiation of 13d. Elution with AcOEt/hexane 1:2 afforded first (R 0.43) 20 mg (8%) of tert-butyl 8-
oxo-7-azatricyclo[4.3.0.0 ]nonane-7-carboxylate (19). M.p. ca. 308. H-NMR (C
): 4.32 (m, HÀC(6)); 2.49
m, HÀC(9)); 2.14 (m, HÀC(3)); 2.06 (m, HÀC(1)); 1.84 (m, HendoÀC(5)); 1.73 (m, HendoÀC(4)); 1.54
3
): 176.9 (s, C(7)); 150.0 (s); 82.6 (s); 55.6 (d, C(5)); 52.3
3
f
3
,9
1
6
D
6
(
(
(
1
3
s, 9 H); 1.49 (m, HexoÀC(5)); 1.39 ± 1.28 (m, HexoÀC(4), HantiÀC(2)); 1.06 (d, J ˆ 10.2, HsynÀC(2)). C-NMR
6 6
C D
): 172.5 (s, C(8)); 151.2 (s); 81.8 (s); 58.5 (d, C(6)); 49.8 (d, C(9)); 36.6 (d, C(1)); 28.2 (q); 23.6 (t, C(2));
2
2.9 (t, C(4)); 22.4 (t, C(5)).
The second fraction (R
0.37) consisted of 147 mg (61.7%) of tert-butyl 4-oxo-5-azatricyclo[4.2.1.03,9]no-
f
1
nane-5-carboxylate (18). Colorless oil. H-NMR (CD
3
COCD
3
): 4.52 (m, HÀC(6)); 3.09 (m, HÀC(9)); 2.91
(
m, HÀC(3)); 2.77 (m, HÀC(1), HexoÀC(2)); 2.16 (m, HendoÀC(7)); 2.04 (m, HendoÀC(8)); 1.86 (dddd, J ˆ 5.1,
8
.1, 10.2, 13.5, HexoÀC(7)); 1.64 (ddd, J ˆ 3.0, 3.0, 11.6, HendoÀC(2)); 1.55 (m, HexoÀC(8)); 1.51 (s, 9 H).
1
3
C-NMR (CD
3 3
COCD ): 176.6 (s, C(4)); 151.1 (s); 82.1 (s); 64.3 (d, C(6)); 39.9 (d, C(9)); 39.2 (d, C(3)); 37.0
(
d, C(1)); 36.3 (t, C(7)); 32.2 (t, C(8)); 31.6 (t, C(2)); 28.2 (q).
3
.5. Irradiation of 13e. Elution with Et
2
O/hexane 3 :2 afforded as main product (R
f
0.38) 153 mg (60.8%) of
3
,10
1
tert-butyl 4-oxo-5-azatricyclo[4.3.1.0 ]decane-5-carboxylate (20). M.p. 698. H-NMR (CDCl
3
): 4.36
(
1
1
1
m, HÀC(6)); 3.13 (ddd, J ˆ 6.1, 8.2, 10.2, HÀC(3)); 2.86 ± 2.71 (m, HÀC(1), HexoÀC(2)); 2.56 (dddd, J ˆ 2.2,
0.2, 10.3, 12.7, HÀC(10)); 2.26 (m, HendoÀC(7)); 2.00 (ddd, J ˆ 5.9, 7.6, 12.7, HendoÀC(2)); 1.54 (s, 9 H)); 1.70 ±
13
.30 (m, HexoÀC(7), HexoÀC(8), HendoÀC(8), HexoÀC(9), HendoÀC(9)). C-NMR (CDCl
50.4 (s); 82.7 (s); 55.6 (d, C(6)); 38.3 (d, C(3)); 28.2 (t, C(2)); 28.1 (q); 28.0 (d, C(10)); 27.8 (d, C(1)); 27.3
t, C(7)); 26.9 (t, C(9)); 14.3 (t, C(8)).
3
): 177.7 (s, C(4));
(
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Received December 9, 2002