Photochemistry of spiro[6H-[1,3]oxathiin-2,2′-tricyclo[3.3.1.1 3.7]decan]-6-one

Article abstract of DOI:10.1002/hlca.200490169

On irradiation (300 nm) in the solid state, the title compound 8 affords tricyclo[3.3.1.1^3.7]decan-2-one (=adamantan-2-one; 9) selectively via [4+2] cycloreversion. A similar result is obtained on photolysis in solution (MeCN or acetone), also in the presence of added alkenes. On irradiation in MeOH, a solvent adduct 11 is isolated in addition to 9. From experiments in CD₃OD, it can be inferred that 11 is formed via syn-addition of MeOH to the ground-state (E)-heterocycle 16.

Full text of DOI:10.1002/hlca.200490169

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Helvetica Chimica Acta Vol. 87 (2004)
Photochemistry of Spiro[6H-[1,3]Oxathiin-2,2'-tricyclo[3.3.1.13,7]decan]-
6-one
by Kerstin Schmidt and Paul Margaretha*
Institut f¸r Organische Chemie, Universit‰t Hamburg, M.L.King Platz6, D-20146 Hamburg
(phone: ‡ 4940-428384316; fax: ‡ 4940-428385592; e-mail: Paul.Margaretha@chemie.uni-hamburg.de)
Dedicated to Professor Amos B. Smith III on the occasion of his 60th birthday
On irradiation (300 nm) in the solid state, the title compound 8 affords tricyclo[3.3.1.13,7]decan-2-one
(ˆadamantan-2-one; 9) selectively via [4 ‡ 2] cycloreversion. A similar result is obtained on photolysis in
solution (MeCN or acetone), also in the presence of added alkenes. On irradiation in MeOH, a solvent adduct
11 is isolated in addition to 9. From experiments in CD3OD, it can be inferred that 11 is formed via syn-addition
of MeOH to the ground-state (E)-heterocycle 16.
Introduction. In the course of our investigations on the photochemistry of (S-
hetero)cyclic unsaturated carbonyl compounds [1], we had observed that the photo-
chemical behavior of 3,4-dihydro-2H-thiin-4-ones, e.g., 1a, resembles strongly that of
cyclohept-2-enones: due to the longer bond length of the CÀS bond, these rings
become less rigid, and, therefore, the twisting around the CˆC bond in the excited state
is facilitated (Scheme 1). This is the reason why photodimerization and photo-
cycloaddition to simple alkenes occurs with very low efficiency [2], whereas the so
formed ground-state (E)-diastereoisomer 2 is efficiently trapped by MeOH to afford a
3 :2 mixture of 3 and 4, respectively [3]. This behavior is indeed typical for 3,4-dihydro-
2H-thiin-4-one, as the corresponding oxa-enones, e.g., 1b, exhibit typical enone
behavior by undergoing efficient [2 ‡ 2] photocycloaddition to alkenes to afford
oxabicyclooctanones 5 [4]. The transition from a cyclic oxa-enone to an unsaturated
−oxa-lactone×, e.g., 6, does not alter this typical enone behavior, as this latter dioxinone
is cleanly converted to bicycles 7 on long-wavelength irradiation in the presence of 2,3-
dimethylbut-2-ene [5]. A recently reported easy synthetic access to 2,2-dialkyl-6H-1,3-
oxathiin-6-ones [6] prompted us to investigate the photochemical behavior of such
analogous thiacycles. Here, we report the photochemical behavior of the title
compound 8 (Scheme 2).
Results. Irradiation (300 nm) of 8 (0.1 1m) in MeCN led to the exclusive
formation of adamantan-2-one (9) besides some polymeric precipitate. Changing the
solvent to acetone hindered this precipitation but, again, 9 was the only detectable (GC,
NMR) product, which was isolated by chromatography in yields up to 90%. Iradiation
of 8 in MeCN in the presence of a 20-fold molar excess of either 2,3-dimethylbut-2-ene
or furan led to results identical to those mentioned above, i.e., exclusive formation of 9
as detectable product. Interestingly, this same reactive behavior was also observed on
irradiation of 8 as solid film, the selective conversion to 9 occuring without any

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Scheme 1
liquefaction. In all these experiments, no evidence for any low-molecular-weight
derivative of thioacylketene 10 was obtained. Irradiation of 8 in MeOH afforded a 3 :2
mixture (1H-NMR) of 9 and the MeOH adduct 11, whose separation/isolation was
achieved by column chromatography. The assignment of the MeO group location stems
1
from the H-NMR spectrum, wherein the geminal coupling constant of the new CH2
group (J ˆ 9.8 Hz) evidences a CH2 group adjacent to a S-atom (Scheme 2).
To obtain additional information on the mode of the MeOH addition and also to
eventually trap the postulated thioacylketene 10, 8 was irradiated in CD3OD in an
1
NMR tube, and the reaction was monitored by H-NMR (without removal of the
solvent). In addition to the signals corresponding to 9 and to the trans-4-deuterio-5-
(trideuteriomethoxy) adduct 12 (vicinal coupling constant J(eq,eq) ˆ 3.5 Hz), two new
sets of AB systems, one resonating at 7.77 and 5.80 ppm with J ˆ 15 Hzand the other
one resonating at 7.27 and 5.96 ppm with J ˆ 10 Hz, were observed. Upon allowing the
mixture to stand at room temperature, these latter signals gradually disappeared. The
original (molar) composition of the mixture corresponds to 9 (1.5), 12 (1.0), (Z)-13
(1.1), and (E)-13 (0.4). The 1H-NMR data for (Z)-13 in CD3OD are in agreement with
the chemical shifts reported for the corresponding non-deuterated ester in CDCl3 (7.10
and 5.98 ppm, J ˆ 10.4) [7]. Finally, irradiation of 8 in i-PrOH afforded a 9 :1 mixture
(1H-NMR) of 9 and alcohol adduct 14. Due to its very low yield of formation, 14 was
1
not isolated but characterized by H-NMR and MS in the mixture.

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Helvetica Chimica Acta Vol. 87 (2004)
Scheme 2
Discussion. The clean conversion of 8 to 9 is, to our knowledge, one of the very
first examples of a formal DielsÀAlder-type cycloreversion occuring in the solid-state.
Fragmentation reactions are relatively rare in the solid state because the crystal lattice
holds the fragments together, and they just recombine. This changes when one of the
products formed is a gas, which escapes, exemplified by recently reported solid-state
photodecarbonylation reactions [8]. In solution, retro-DielsÀAlder reactions are well-
known [9], but most of the reactions in which a CˆO compound is formed as
−heterodienophile× are thermal processes, whereby the presence of a second hetero-
atom in the ring usually allows lowering of the cleavage temperature [10]. Only very
few such light-induced reactions have been reported, i.e., the cycloreversion of 2,2,6,6-
tetramethyl-2H,6H-pyran-3-one to 2-methylprop-2-enylketene and acetone, which
occurs in dilute solution in competition with photodimerization [11], and the short-
wavelength (254 nm) photocleavage of 2,2-dialkyl- and 2,2,6-trialkyl-1,3-dioxin-4-ones
to acylketenes and carbonyl compounds, respectively [12][13].
Very little indeed is known about thioacylketenes, e.g., 10. Flash vacuum pyrolysis
studies of either 1,3-dioxin-4-thiones [14] or of 1,6-dioxa-6a-l4-thiapentalenes [15]
indicate these species to be slightly lower in energy than the isomeric acylthioketenes
but nothing is known about their behavior in solution. The alcohol adducts of 10, i.e., 3-
sulfanylacrylates 13, and the corresponding thiolate ions are known to be stable only at

Helvetica Chimica Acta Vol. 87 (2004)
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temperatures below 08 [16], as they undergo both rapid oligomerization as well as
oxidation to disulfanes [7].
The above mentioned solid-state photodecarbonylations are known to proceed in
two steps, i.e., primary a-cleavage, followed by CO elimination from the intermediate
acyl-alkyl biradical [8]. It is, therefore, reasonable to assume that the light-induced
cycloreversion of 8 proceeds in a stepwise sequence, in which the cleavage of the
(weakest) SÀC(2) bond occurs first, followed by cleavage of the OÀC(O) bond.
Nevertheless, a synchronous process cannot be excluded at the moment, and it is also
not evident whether the twisting around the CˆC bond in excited 8 facilitates the
elimination of 9.
A final comment regards the regiochemistry of the MeOH addition to highly
strained (E)-cycloalkenones, e.g., (E)-cyclohept-2-enone (15), 2, and 16, to afford 17
[17, 18], a 3 :2 mixture of 3 and 4 [3], and 11, respectively (Scheme 3).
Scheme 3
The exclusive formation of 3-methoxycycloheptanone (17) from 15 has been
explained by assuming a Michael-type addition of the alcohol. It is doubtful that this
interpretation is correct as, in highly twisted (E)-cycloalkenones, conjugation between
the CˆO and the CˆC bond should become negligible. If this conjugative effect would
be important, 2 and 16 should also undergo selective MeOH addition at the
(vinylogous) C(b)-atom.
The reversal of regioselectivity observed in the reactions above indicates several
combined effects, i.e., a) higher stabilization of the negative charge at C(a) (resulting
from the MeOH addition at C(b)) rather by the ketone than by the carboxy CO group,
b) hindrance of the approach of MeOH to C(b) by the lone pairs of the S-atom, and c)
stabilization of the negative charge at C(b) by this same heteroatom. The common
feature in all these reactions is the fact that they proceed stereospecifically as syn-
additions, affording products wherein the MeOH group and the (new) H-atom are
trans-located.
Experimental Part
1
1. General. H- and 13C-NMR Spectra in (CD3)2CO (unless otherwise stated): 500 and 125.8 MHz, resp.;
chemical shifts in d in ppm rel. to Me4Si (ˆ0 ppm). MS: at 70 eV; in m/z (rel. intensity in %). Photolyses in a
RPR-100 Rayonet photochemical reactor equipped with 300-nm lamps.

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2. Starting Materials. The title compound 8 was synthesized according to [6]. UV (MeCN): lmax 300.5 nm,
log e 4.081. Adamantan-2-one (ˆtricyclo[3.3.1.13,7]decan-2-one; 9) is commercially available.
3. Photolyses. 3.1 Irradiation of 8 in Soln. Ar-Degassed solns. containing 118 mg (0.5 mmol) of 8 in 5 ml of
MeCN or acetone were irradiated for 12 h, until monitoring by TLC indicated total consumption of starting
material. In MeCN, the formation of a (polymeric) precipitate was observed. Both GC analysis of the crude
photolysate and NMR analysis of the residue after evaporation of the solvent indicated the exclusive formation
of 9, isolated in 87% yield by purification (CC) on a short column (SiO2; CH2Cl2).
3.2 Solid-State Irradiation of 8. An Ar-degassed soln. of 118 mg (0.5 mmol) of 8 in CH2Cl2 (5 ml) in a 25-ml
tapered flask was slowly evaporated to produce a homogeneous solid film. After irradiation for 21 h, the
(originally colorless) solid residue had turned yellowish. According to both GC and NMR analysis, the residue
consisted of > 90% 9.
3.3 Irradiation of 8 in the Presence of Alkenes. Ar-Degassed solns. containing 118 mg (0.5 mmol) of 8 and a
20-fold molar excess of either 2,3-dimethylbut-2-ene or furan in 5 ml of MeCN were irradiated for 12 h, until
monitoring by TLC indicated total consumption of starting material. After filtration of the polymeric precipitate
and evaporation of the solvent, both GC and NMR analysis of the residue indicated the exclusive formation of
9.
3.4 Irradiation of 8 in MeOH. An Ar-degassed soln of 590 mg (2.5 mmol) of 8 in 10 ml of MeOH was
irradiated for 4 h. 1H-NMR Analysis of the crude photolysate indicated the presence of traces of 8 and a mixture
of 9 and a novel compound 11 in a 3 :2 molar ratio. CC (SiO2; CH2Cl2) afforded first 8 mg (1.5%) of 8 (Rf 0.70)
then 130 mg (20%) of pure 5-methoxyspiro[[1,3]oxathiane-2,2'-tricyclo[3.3.1.13,7]decan]-6-one (11; Rf 0.68).
Colorless viscous oil. 1H-NMR: 4.40 (dd, J ˆ 3.5, 7.0); 3.78 (dd, J ˆ 3.5, 9.8); 3.68 (dd, J ˆ 7.0, 9.8); 3.34 (s, 3 H);
2.19 2.01 (m, 5 H); 1.88 1.73 (m, 9 H). 13C-NMR: 172.48 (s); 96.27 (s); 73.90 (t); 59.13 (q); 48.23 (d); 41.46
.
‡
(d); 41.42 (d); 37.80 (t); 35.94 (t); 35.39 (t); 34.84 (t); 34.61 (t); 27.15 (d); 27.04 (d). MS: 268 (20, M ), 151.
The next fraction (Rf 0.66; 205 mg; representing an additional 20% yield of 11) consisted of a 1 :1 mixture
11/9, which was then finally obtained pure (150 mg) in the last fraction (Rf 0.64).
3.5 Irradiation of 8 in CD3OD. An Ar-degassed soln. of 23.6 mg (0.1 mmol) of 8 in 1 ml of CD3OD was
irradiated for 1 h. 1H-NMR Analysis indicated on the one hand the expected 3 :2 mixture of 9 and trans-5-
([2H3]methoxy)spiro[[4-2H1][1,3]oxathiane-2,2'-tricyclo[3.3.1.13,7]decan]-6-one (12): 1H-NMR (CD3OD): 4.36
.
‡
(d, J ˆ 3.5); 3.74 (d, J ˆ 3.5); 2.19 2.01 (m, 5 H); 1.88 1.73 (m, 9 H). MS: 272 (20, M ), 151. In addition, the
equivalent amount of a 1:3 mixture of (E)- and (Z)-sulfanylacrylates 13 was observed by their two AB systems,
resonating at 7.77 and 5.89 (J ˆ 15) and 7.27 and 5.96 (J ˆ 10), respectively.
3.6 Irradiation of 8 in i-PrOH. An Ar-degassed soln. of 47.2 mg (0.2 mmol) of 8 in 2 ml of i-PrOH was
irradiated for 3 h. After evaporation of the solvent, 1H-NMR analysis indicated a 9 :1 molar ratio of 9 and 5-(1-
Methylethoxy)spiro[[1,3]oxathiane-2,2'-tricyclo[3.3.1.13,7]decan]-6-one (14): 1H-NMR: 4.35 (dd, J ˆ 3.5, 6.7);
3.83 (dd, J ˆ 3.5, 9.8); 3.74 (dd, J ˆ 6.7, 9.8); 3.65 (sept., J ˆ 6.0) ; 2.19 2.01 (m, 5 H); 1.88 1.73 (m, 9 H); 1.13
.
‡
(d, J ˆ 6.0, 3 H); 1.12 (d, J ˆ 6.0, 3 H). MS: 296 (5, M ), 224.
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Received February 25, 2004