I. Erden et al. / Tetrahedron Letters 57 (2016) 2190–2193
2191
O
O
OH
H
CHO
OH
O
OH
SiO2
SiO2
22
23
OH
29
28
11
12
13
Scheme 7. SiO2 catalyzed isomerizations of 22 and 23.
Figure 1. Fulvenes 11, 12, and 13 used in this study carrying OH groups.
Whereas the formation of 14 was expected, furan 15 is an
unusual product, and its formation is unprecedented. A favorable
shift-often observed in oxygen centered radicals- is a 1,2-H shift
to the oxygen radical10 (transition state enthalpy = ꢀ11.8 kcal/mol
relative to 16), giving an allenyl enol, 19. Rapid tautomerization
would also lead to 20, and eventually to furan 15 via the
HO
O
OHC
OH
O
O
ν
O2, h , TPP
+
CH2Cl2, -78 o
C
RT
11
14
15
corresponding c-lactol followed by dehydration (Scheme 4).
Alternatively, a 1,2-hydrogen atom transfer (HC?CÅ) in the oxy-
gen-centered radical 18, ensuing a common b-fission of the initial
diradical might also account for the furan precursor (18?20?15),
however, we consider a 1,2-H transfer to the adjacent vinyl highly
unlikely due to a calculated activation enthalpy (M062x/6-311+G⁄⁄
with a PCM CH2Cl2 solvent model) that is 8.8 kcal/mol above the
energy of 16.11
Scheme 3. Photooxygenation of 11 leading to 14 and 15.
OH
OH
OH
.
.
.
O
O
O
H
O
O
O
18
~1,2-H
CH
17
16
H
In the second system we studied (12) the hydroxyl group is one
more carbon further down the alkyl chain than in 11, and this sub-
tle variation altered the course of the endoperoxide decomposition
dramatically. Upon singlet oxygen addition at 78 °C in CH2Cl2, and
subsequent warming the solution to room temperature, 12 gave a
2:1 mixture of 2212 and 23 in a combined yield of 68% (Scheme 5).
Again, the formation of the expected product 23 deserves no
further comment. Compound 23, on the other hand, apparently
-
2
,
C
1
~
O
CH
OH
O
-H2O
CHO
OH
O
CHO C
OHC
OH
15
20
19
Scheme 4. Mechanism for furan 15 formation.
stems from
a pathway wherein a fragmentation must have
occurred since 23 lacks a CH2O unit (formaldehyde) as compared
to the endoperoxide 21 derived from 12. A mechanism, consistent
with the results is outlined in Scheme 6.
HO
O
1O2
OH CH2Cl2
O
A
reasonable pathway rationalizing the formation of 23
OH
RT
O
OH
involves a rare 1,7-hydrogen atom transfer (HAT)13 from the
hydroxyl group to the proximal oxygen-centered radical in 25.
Though rarely observed, it has been found that if the proper spatial
orientation of the relevant hydrogen atom toward the oxygen- or
carbon-centered radical is provided (an eight-membered transition
state obviously suffers from a considerably unfavorable entropy of
activation), 1,7-hydrogen abstractions can effectively compete
with the more ubiquitous 1,5-H shifts. Molecular modeling at
B3LYP/6-31G⁄ level of theory indicates a favorable geometry for
the 1,7-H abstraction with a H—O distance of 1.83 Å. The ensuing
fragmentation in 26 leading to 23 and formaldehyde (27) is akin
to the retro-Paterno–Buchi reaction, though carbon-centered 1,4-
diradical intermediates have been implicated in the latter reac-
tions.14 Although 22 and 23 could be separated from one another
by flash chromatography, longer exposure of 23 to silica gel
resulted in acid-catalyzed epoxide ring opening followed by a
1,2-hydride shift to give cis-4-hydroxyl-5-isopropenylcyclopent-
2-enone (29). Moreover, compound 22 quantitatively isomerized
to 28 during SiO2 chromatography via acid-catalyzed translac-
tonization (Scheme 7).
+
-78 o
C
O
O
12
21
22
23
Scheme 5. Photooxygenation products from 12.
O
O
H
O
O
OH
O
~1,7-H
21
r.t.
O
25
26
24
O
O
H
H
OH
O
O
+
23
27
Scheme 6. Mechanism for the formation of 23 from 21.
Results and discussion
In the third system of our study, the hydroxyl group is placed in
the ortho position of a phenyl group at C6 in the fulvene derived
from salicyl aldehyde (13). Singlet oxygen addition was conducted
under the same conditions as before at ꢀ78 °C, however, CD2Cl2
was used as solvent in order to monitor the progress of the reaction
by 1H NMR and to avoid the loss of volatile products during the
solvent removal by rotary evaporation. Indeed, upon warming
the photolysate to room temperature, the 1H NMR of the crude
product mixture revealed that two products, furan (30) and
2-coumaranone (31, benzofuran-2(3H)-one) were formed in a 1:1
ratio (Scheme 8).
All three substrates are readily available in high yields by the
catalytic method we reported recently, or the stoichiometric
method by Stone and Little.9
Photooxygenation of 11 in CH2Cl2 at ꢀ78 °C, using TPP as
sensitizer and allowing the photolysate to warm up to room
temperature gave a mixture of two products, formed in a ratio of
3:1, respectively, which were separated from one another by flash
chromatography and identified as 14 and 15 (combined yield 72%,
Scheme 3).