Heterocyclic photochemistry in contrast with carbon behavior. Regioselective photochemical rearrangement of an azacyclohexadienone: Mechanistic and exploratory organic photochemistry

Article abstract of DOI:10.1021/jo900901g

(Chemical Equation Presented) The Type-A photochemistry of cyclohexadienones is well-studied and follows a well-established mechanistic pathway. One early example is the rearrangement of santonin to lumisantonin. Another example is the rearrangement of 4,4-diphenylcyclohexa-1,5-dienone. Remarkably, replacement of one carbon by nitrogen alters the reaction course to give a regioselective phenyl migration.

Full text of DOI:10.1021/jo900901g

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Heterocyclic Photochemistry in Contrast with Carbon Behavior.
Regioselective Photochemical Rearrangement of an Azacyclohexadienone:
Mechanistic and Exploratory Organic Photochemistry1
Howard E. Zimmerman* and Sergey Shorunov
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
zimmerman@chem.wisc.edu
Received April 30, 2009
The Type-A photochemistry of cyclohexadienones is well-studied and follows a well-established
mechanistic pathway. One early example is the rearrangement of santonin to lumisantonin. Another
example is the rearrangement of 4,4-diphenylcyclohexa-1,5-dienone. Remarkably, replacement of
one carbon by nitrogen alters the reaction course to give a regioselective phenyl migration.
Introduction
(i.e., 1,5 in santonin and 3,5 in the diphenyldienone) and are
reactions of the n-π* triplets. It was of basic interest to
ascertain the effect of replacing one ring carbon by nitrogen
as in the azacyclohexadienone (5).
5,5-Disubstituted 2,5-cyclohexadienones undergo a facile
rearrangement from the triplet. A very early example was the
rearrangement of santonin (1) to lumisantonin (2) (note
eq 1).2 In 1961, the mechanism was established,3 and the
example of the 4,4-diphenyl-2,5-cyclohexadienone (3) rear-
rangement to bicyclo[3.1.0]hexenone (4) was presented3
(note eq 2). These reactions proceed by initial β,β-bonding
Results
Synthesis of Photochemical Reactants and Potential Pro-
ducts. The synthesis of 5,5-diphenylpyridin-2(5H)-one (5)
was carried out as described in Scheme 1. A starting point
was the known4 cyano-aldehyde (6). The latter had been
reported to be converted to the diphenylazacyclohexe-
none (10) by Cragoe4a in 1958 by heating with sulfuric
acid in acetic acid. However, the repetition of the proce-
dure led to a compound with inconsistent proton and
carbon NMR spectra. Instead, the compound proved to
be pyridone (12) resulting from a rearrangement discussed
below.
(1) (a) Publication 287. (b) For Publication 286, see: Zimmerman, H. E.,
J. Org. Chem., 2008, 73, 1247-1251. (c) For Publication 285, see: Zimmerman,
H. E.; Suryanarayan, V., Eur. J. Org. Chem., 2007, 72, 4091-4102.
Utilizing the same cyanoaldehyde (6) and the sequence in
Scheme 1, the desired aza-enone (10) was obtained and with a
melting point not too far (170 vs 174 °C) from that of 12. The
€
(2) (a) Arigoni, D.; Bosshard, H.; Bruderer, H.; Buchi, G.; Jeger, O.;
Krebaum, L. J. Helv. Chim. Acta 1957, 40, 1733–1748. (b) Barton, D. H. R.;
DeMayo, P.; Sahfiq, M. J. Chem. Soc. 1958, 140–145.
(3) (a) Zimmerman, H. E. 17th National Organic Symposium of the
American Chemical Society, Abstracts, Bloomington, IN, 1961, pp 31-41.
(b) Zimmerman, H. E.; Schuster, D. I. J. Am. Chem. Soc. 1962, 84, 4527–
4540.
(4) (a) Cragoe, E. J.; Pietruszkiewicz, A. M.; Robb, C. M. J. Org. Chem.
1958, 23, 971–980. (b) Beard, C.; Burger, A. J. Org. Chem. 1962, 27, 1647–
1650.
DOI: 10.1021/jo900901g
r
Published on Web 06/23/2009
J. Org. Chem. 2009, 74, 5411–5416 5411
2009 American Chemical Society

Zimmerman and Shorunov
JOCArticle
SCHEME 1. Synthesis of Heterocycles
SCHEME 2. Photochemical Results: Type-B Heterocyclic
Products, Regioselectivity, Stereochemistry, And Lack of the
Common Type-A Photochemistry So Characteristic of Dienone
Reactivity
structures of compounds 10 and 12 were established from the
NMR (1H, 13C, DEPT, C,H-COSY, GHMBC, and homo-
nuclear decoupling); note the Experimental Section.
Aza-cyclohexenone (10) was then converted to the desired
aza-cyclohexadienone (5).
Details are given in the Experimental Section for the
sequences 9 to 5 and 9 to 12. The remaining conversions
are given in the Supporting Information.
Photochemical Results. The photochemistry proved to be
quite unusual and reminiscent of carbocyclic enone re-
activity (note eq 3).5 The aza-cyclohexadienone behavior
is outlined in Scheme 2. The Type-A photoproducts,
common in a myriad of all-carbon examples,3 went
unobserved. Instead, two bicyclic Type-B5 rearrangement
products were formed. The structures of compounds 15
and 16 were established from the NMR (1H, 13C, DEPT,
COSY, and NOESY) with consideration of the AB
methine coupling and also the downfield imine hydrogen
signal; note the Experimental Section and Supporting
Information again.
was a preference for the endophenyl stereochemistry
(i.e., 15 vs 16). And, it is seen from a theoretical stand
point that the Type-B preference is understandable (vide
infra).
Theoretical and Computational Results. As noted, the
curious aspect of the photochemistry was the preference for
the Type-B reaction course rather than the ubiquitous Type-A.
Thus, we carried out computations with Gaussian 03, using
density functional theory, namely b3lyp/6-31g*. The triplets
were geometry optimized since they were the initially formed
species. The singlets (i.e., S0) were given the geometry of the
triplets from whence they derive. Interestingly in the case of the
Type-A species 23 the energy with an S0 configuration proved
Beyond this, there was encountered regioselectivity
in which phenyl migration was to the terminus of the
carbon to carbon π-system rather than the imine one.
However, as in all-carbon Type-B photochemistry5 there
(5) (a) Zimmerman, H. E.; Wilson, J. W. J. Am. Chem. Soc. 1964, 86,
4036–4042. (b) Zimmerman, H. E.; Rieke, R. D.; Scheffer, J. R. J. Am. Chem.
Soc. 1967, 89, 2033–2047. (c) Zimmerman, H. E.; Kutateladze, A. G. J. Org.
Chem. 1995, 60, 6008–6009.
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Zimmerman and Shorunov
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carbocyclic dienone rearrangement3 illustrated in eq 4.
higher than that of the corresponding triplet. However, this
species had the geometry of the triplet and was unrelaxed.
Geometry optimization led to an energy close to that of the
triplet (-785.479) but the process also led further to three-ring-
opening without a local minimum being obvious. The experi-
mental evidence suggests that this zwitterionic species is
not reached in the photochemistry. This differs from the
TABLE 1. Energies of Reactants, Intermediate Species, and Products
singlets
triplets
singlets
Type-B (M)
triplets
Type-B (N)
dienone 5
-785.5737
-785.4912
-785.5539
-785.4661
-785.4571
-785.4266
-785.5737
-785.4706
-785.5399
-785.4661
-785.4706
-785.4622
Spiroa 21, 22
endo bicyclic 15, 19
Type-Ab (N)
dienone 5
Type-A 23
Type-A (M)
-785.5737
-785.4584
-785.5539
-785.4661
-785.4789
-785.4430
-785.5737
-785.4584
-785.5625
-785.4661
-785.4789
-785.4607
DiPhCc 17, 18
a Spiro is the phenyl-bridged diradicals. b Type-A is the 3,5-bridged species. c DiPhC is the Type-A rearranged bicyclic. Energies in hartrees (627.5
kcal/mol per hartree).
FIGURE 1. Type-A and Type-B rearrangement energy diagrams.
J. Org. Chem. Vol. 74, No. 15, 2009 5413

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SCHEME 3. Possible Rearrangements: Type-A and Type-B
the N migration, i.e., on the nitrogen side of the heterocycle,
the T1-S0 intersection is lacking and intersystem crossing
is much less likely. Thus, the experimentally observed
Type-B rearrangement with a preference for the M migration
(i.e., distant from nitrogen) is in accord with the computa-
tional prediction.
It is clear that the reaction course is not controlled by
relative excited-state energies for alternative pathways.
Thus, three of the reactions begin as triplets reacting exother-
mically while the nonobserved Type-B (N-migration) path-
way begins on a upward energy path, and we see that there is
no correlation with energetics. The reaction course is con-
trolled by a requirement for a T1-S0 intersection which does
not occur on the portion of the S0 surface leading back
to reactant. This requirement is fulfilled only by the Type-B
“M-migration” (i.e., the top right scheme in Figure 1).
Interestingly, the heterocyclic photochemistry of the aza-
dienone parallels that of the all-carbon cyclohexenone as
depicted in eq 3.
Conclusion
The surprising result in this study, which a priori would
not have been predicted, is the lack of Type-A rearrangement
so characteristic of 4,4-disubstituted cyclohexadienones. It is
thus seen that the photochemistry of nitrogen heterocycles
cannot be assumed to parallel that of the corresponding
all carbon reactants and promises to afford new reactivity.
The naive assumption that organic photochemistry is a
mature field would then lead to loss of exciting new results.
Discussion
In a brief diversion we note that the rearrangement
encountered by Cragoe proved not to be a reaction of aza
enone 10 but, rather, a reaction of its precursor 26. See eq 5.
Experimental Section
5-Hydroxy-4,4-diphenylvaleramide (9). To a stirred solution
of 14.0 g (55.5 mmol) of 5,5-diphenylvalerolactone (8) in 150 mL
of methanol and 50 mL of dichloromethane was added dropwise
ca. 25 mL of liquid ammonia. The reaction mixture was stirred
for 24 h at room temperature. Solvent was then removed
in vacuo and the residue recrystallized from chloroform: yield
In the aza-dienone photochemistry the singlet and triplet
energies of one reactant, four a priori photoproducts, and
three intermediates were considered computationally (see
Table 1). This is shown graphically in Figure 1. It then
becomes possible to compare theory with reality.
The reactions for which the computations done for the
Type-A and for the Type-B rearrangements, as described
above, are now outlined in Scheme 3. In both instances,
rearrangements involving the nitrogen side of the heterocycle
(Type N) and the side distant from nitrogen (Type M) were a
priori possible. The energies obtained, and listed in Table 1,
are now utilized in considering the relative preferences of the
four reaction pathways as discussed below. Throughout the
correlation assumes the unrelaxed S0 species.
Turning to consideration of Figure 1, we note that for the
Type-A rearrangement, the triplets (blue) intersect the
ground state (i.e., S0) surfaces before the S0 singlets (red)
reach a maximum. Hence, one can anticipate intersystem
crossing and reversion to reactants. In contrast, in the all-
carbon Type-A rearrangement intersystem crossing of the
3,5-bridged triplet leads to a zwitterion (note eq 4). In the
present case this species is clearly not reached (vide infra).
In the case of the aza-Type-B process, with migration to
the distant (i.e., M) side of the heterocycle there is an essential
degeneracy between T1 and S0 and anticipated intersystem
crossing (T1 to S0) with product formation. In contrast, for
1
12.6 g (84%); mp 136-138 °C; HNMR (300 MHz, CDCl3)
δ 2.00 (t, 2H, J = 7.5 Hz), 2.47 (br s, 1H), 2.55 (t, 2H, J =
7.5 Hz), 4.11 (s, 2H), 5.50 (br s, 1H), 5.63 (br s, 1H), 7.16-7.32
(m, 10H); 13CNMR (75 MHz, DMSO-d6) δ 31.1; 31.5; 51.1;
67.0; 126.3; 128.4; 128.5; 147.3; 175.2; HRMS (EI) Calcd for
C17H19NO2+ ([M+]) 269.1411, found 269.1410.
5,5-Diphenyl-3,4-dihydro-2(5)-pyridone (10). (Caution! Due
care should be exercized when working with IBX, since the
compound is shock- and heat-sensitive!). To solution of 30.0 g
(0.11 mol) of 2-iodoxybenzoic acid6 (IBX) in 400 mL of DMSO
was added 10.92 g (40.5 mmol) of 5-hydroxy-4,4-diphenylvaler-
amide (9). The reaction mixture was stirred for 6 days at room
temperature, poured into 2000 mL of water, and extracted with
ethyl acetate (5 Â 200 mL). The combined organic phase was
washed with water (3 Â 400 mL) and dried over sodium sulfate
and the solvent removed in vacuo. The residue was recrystallized
from 2-propanol: yield 6.75 g; mp 169-170 °C. The mother
liquor was concentrated in vacuo and the residue purified by
column chromatography (15 Â 4 cm) with hexane-ethyl acetate
(2/1) as eluent. Fractions containing product were combined
and concentrated: yield 1.7 g; mp 168-170 °C; total yield 8.45 g
(84%); 1HNMR (300 MHz, DMSO-d6) δ 2.31 (t, 2H, J =
6.3 Hz), 2.76 (t, 2H, J = 6.3 Hz), 7.12-7.16 (m, 4H), 7.26-7.38
(m, 6H), 11.09 (s, 1H); 13CNMR (75 MHz, DMSO-d6) δ 30.2,
(6) Frigerio, M.; Santagostino, M.; Sputore, S. J. Org. Chem 1999, 64,
4537–4538.
5414 J. Org. Chem. Vol. 74, No. 15, 2009

Zimmerman and Shorunov
JOCArticle
30.3, 56.7, 127.9, 128.7, 129.0, 141.8, 173.2, 175.5; HRMS (EI)
calcd for C17H15NO+ ([M+]) 249.1149, found 249.1160.
6-Hydroxy-5,5-diphenyl-2-piperidone (26). To a suspension of
600 mg (2.2 mmol) of 5-hydroxy-4,4-diphenylvaleramide (9) in
25 mL of dichloromethane was added 1.25 g (3.3 mmol) of
pyridinium dichromate. The reaction mixture was stirred for 3 h
at room temperature and filtered through a short column (1Â
4 cm), and the silica gel was washed with 100 mL of ethyl acetate.
The filtrate was concentrated in vacuo and the dark residue
weighing 430 mg was purified by column chromatography (1Â
25 cm). Initially an 8/1 mixture of hexane and ethyl acetate was
used, and 10 mg of benzophenone was eluted. Then 3.5/1
hexane-ethyl acetate eluted 35 mg of 5,5-diphenylvalerolactone
(8) followed by 86 mg of 5,5-diphenyl-3,4-dihydro-2(5)-pyri-
done (10). Then pure ethyl acetate was employed, and 6-hydro-
xy-5,5-diphenyl-2-piperidone (26) was eluted: yield 230 mg
(39%); mp 193-195 °C dec (from chloroform); 1HNMR
(300 MHz, DMSO-d6) δ 1.64 (m, 1H), 2.19 (dd, 1H, J = 4.8 Hz
and 18 Hz), 2.42-2.50 (m, 1H), 2.84 (m, 1H), 5.49 (t, 1H, J=
4.8 Hz), 5.76 (d, 1H, J= 6.0 Hz), 7.07-7.29 (m, 10H), 8.27 (d, 1H,
J=4.2 Hz); 13CNMR (75 MHz, DMSO-d6)δ24.7, 29.4, 49.6, 78.7,
126.1, 126.6, 127.8, 128.0, 128.5, 128.9, 144.8, 147.0, 170.4; HRMS
(EI) calcd for C17H17NO2+ ([M+]) 267.1254, found 267.1260.
Rearrangement of 6-Hydroxy-5,5-diphenyl-2-piperidone (26)
into 5,6-Diphenyl-4,5-dehydro-2-piperidone (12). A mixture of
500 mg (1.8 mmol) of 6-hydroxy-5,5-diphenyl-2-piperidone
(26), 1.0 mL of concd sulfuric acid, 4.0 mL of glacial acetic acid,
and 1.0 mL of water was refluxed for 2 h. The mixture was then
poured, with stirring, into 100 mL of cold water and extracted
with dichloromethane (4Â25 mL). The combined organic phase
was washed with water (3Â15 mL), dried with sodium sulfate,
and concentrated in vacuo. The residue was purified by column
chromatography (10Â1 cm) with ethyl acetate as eluent: yield
220 mg (47%); mp 172-174 °C (from ethyl acetate).
Photolysis of 5,5-Diphenylpyridin-2(5H)-one. A solution of
1.3 g (5.2 mmol) of 5,5-diphenylpyridin-2(5H)-one (5) in 300 mL
of dry benzene was purged with oxygen free nitrogen for 1 h.
Then the reaction mixture was irradiated with a 400 W medium
pressure mercury lamp. After 45 min, the 1HNMR showed
complete consumption of starting material. Concentration in
vacuo and chromatography (17Â2 cm) with hexane-ethyl acetate
(5/1) as an eluent afforded two compounds. Compound 15: yield
0.71 g (55%); mp 165-167 °C (from 2-propanol); 1HNMR (300
MHz, DMSO-d6) δ 3.32 (d, 1H, J= 9.0 Hz), 3.55 (d, 1H, J= 9.0
Hz), 7.32-7.42 (m, 8H), 7.60-7.63 (m, 2H), 10.55 (br s, 1H);
13CNMR (75 MHz, CD3OD) δ 33.6, 41.7, 43.2, 127.9, 128.1,
128.6, 128.7, 129.1, 129.2, 133.6, 174.6, 175.5; HRMS (EI) calcd
for C17H13NO+ ([M+]) 247.0992, found 247.0988. Compound
16: yield 0.36 g (27%); mp 222-225 °C (from 2-propanol);
1HNMR (300 MHz, DMSO-d6) δ 3.49 (d, 1H, J = 3.6 Hz), 3.79
(d, 1H, J=3.6 Hz), 6.98-7.01 (m, 2H), 7.07-7.11 (m, 3H), 7.18-
7.28 (m, 5H), 10.91 (br s, 1H); 13CNMR (75 MHz, CD3OD) δ
31.1, 43.2, 44.2, 127.2, 127.9, 128.0, 128.8, 131.1, 133.8, 176.0,
176.6; HRMS (EI) calcd for C17H13NO+ ([M+]) 247.0992,
found 247.0988.
3-Phenylselenyl-5,5-diphenyl-3,4-dihydro-2(5)pyridone (11).
A solution of 2.33 g (23 mmol) of diisopropylamine into 15 mL
of anhydrous THF was cooled to -70 °C in an atmosphere of
argon. Then 15.0 mL (23 mmol) of 1.55 M n-butyllithium in
hexane was added via syringe over 1 min. The reaction mixture
allowed to warm to 0 °C and then cooled back to -70 °C.
Thereafter, a solution of 2.3 g (9.2 mmol) of 5,5-diphenyl-3,4-
dihydro-2(5)-pyridone (10) in 15 mL of anhydrous THF
was added dropwise and the reaction mixture stirred for 1.5 h
at -70 °C; a yellow stirrable suspension was formed. A solution
of phenylselenium bromide, prepared by addition of 1.84 g
(11.5 mmol) of bromine to a stirred solution of 3.6 g
(11.5 mmol) of diphenyldiselenide to 15 mL of anhydrous
THF, was rapidly added via syringe. The solution rapidly
became colorless. The reaction mixture was allowed to warm
to room temperature, poured into mixture of 700 mL of water
and 10 mL of concd HCl, and extracted with ethyl acetate (4 Â
250 mL). The combined organic phase was washed with water
(3Â150 mL) and brine (150 mL) and dried over sodium sulfate
and the solvent removed on a rotary evaporator. The residue
was purified by column chromatography (45Â4 cm) using
hexane-ethyl acetate (3/1) as eluent. Fractions containing
product were combined and concentrated in vacuo, and the
product was recrystallized from 2-propanol: yield 3.66 g (98%);
1
mp 165-167 °C; HNMR (300 MHz, CDCl3) δ 2.83 (dd, 1H,
J=5.1 and 14.1 Hz), 2.96 (tapp, 1H, J=14.4 Hz), 3.98 (dd, 1H, J=
5.1 and 12.3 Hz), 6.91-6.94 (m, 2H), 7.09-7.13 (m, 2H), 7.22-
7.40 (m, 9H), 7.55-7.58 (m, 2H), 8.07 (s, 1H). Partial homo-
nuclear decoupling with saturating spin states of the PhSeCH
proton leads to simplification of spectra and doublet of doublets
at δ 2.83 (one proton of the adjacent diastereotopic CH2 group
which is cis to the PhSeCH proton) along with a triplet at
δ 2.96 (the other proton from the adjacent diastereotopic
CH2 group which is trans to the PhSeCH proton) both
collapse to a doublet of doublets at δ 2.83 and 2.96 with true
J values of -14.1 Hz (geminal) and 1.8 Hz (probably long-range
coupling to imine CH proton): 13CNMR (75 MHz, CDCl3)
δ 39.3, 39.6, 58.1, 126.7, 127.7, 127.8, 128.4, 128.5, 128.6, 129.1,
129.5, 129.6, 136.0, 137.7, 141.8, 170.9, 173.9.
5,5-Diphenylpyridin-2(5H)-one (5). To a suspension, cooled to
0 °C, of 6.6 g (16.3 mmol) of 3-phenylselenyl-5,5-diphenyl-3,4-
dihydro-2(5)pyridone (11) in 850 mL of ethanol was added to a
solution of 1.71 g (20.3 mmol) of sodium bicarbonate in 170 mL
of water followed by 8.72 g (40.7 mmol) of solid sodium
periodate. The cooling bath was removed, and the reaction
mixture, a milky suspension, was stirred for 48 h at room
temperature, poured into 3500 mL of water, and extracted with
dichloromethane (5 Â 250 mL). The combined organic phase
was washed with saturated sodium bicarbonate solution (1 Â
1000 mL) and water (2Â1000 mL), dried with sodium sulfate,
and concentrated in vacuo. The residue was purified by column
chromatography (16 Â 5.5 cm) using hexane-ethyl acetate (9/1)
to elute the diphenyl diselenide (from disproportionation of
phenylseleneninic acid) and then hexane-ethyl acetate (1.5/1)
mixture to elute the crude product (3.65 g), which was recrys-
tallized from heptane-toluene (1/1): yield 3.20 g; mp 174-
177 °C. Concentration of the mother liquor from crystallization
followed by column chromatography (3 Â 15 cm) with hexane-
ethyl acetate (3/1) afforded an additional 0.4 g of product which
was recrystallized from heptane-toluene (1/1): yield 0.34 g; mp
NMR Evidence for Structure and Stereochemistry Elucidation
for Photoproducts (15) and (16). The structures of compounds 15
and 16 were assigned by NMR spectroscopy. 13C NMR of both
compounds along with DEPT (90) and DEPT (135) data
indicate presence of two CdX groups (signals around δ 175,
carbonyl and imine carbons), two aromatic quaternary and six
aromatic CH carbons, two CH methine carbons, and one
aliphatic quaternary carbon. This does not fit structures of
compounds 17, 19, and 20 since each of them must have only
one downfield signal from a CdX group, eight aromatic signals,
one signal from aliphatic CH carbon, one signal from quatern-
ary aliphatic carbon, and two downfield signals from vinylic CH
carbons from the CHdCH- double bond conjugated with the
carbonyl group. The 1H NMR of compound 15 has two doub-
lets at δ 3.32 and 3.55 with vicinal coupling constant 3J = 9 Hz,
1
174-177 °C; overall yield 3.54 g (88%); HNMR (300 MHz,
CDCl3) δ 6.29 (dd, 1H, J=2.1 and 10.2 Hz), 7.05 (d, 1H, J= 10.2
Hz), 7.23-7.39 (m, 10H), 8.48 (s, 1H); 13CNMR (75 MHz,
CDCl3) δ 59.9, 118.9, 128.4, 128.8, 129.1, 140.6, 150.7, 164.8,
173.3; HRMS (EI) calcd for C17H13NO+ ([M+]) 247.0992,
found 247.0986.
J. Org. Chem. Vol. 74, No. 15, 2009 5415

Zimmerman and Shorunov
JOCArticle
1
geometry only fits compound 16. In both of the H spectra of
which correspond to ca. 30° dihedral angle between two cis
methine protons; this fits well for both compounds 15 and 18. If
compound 18 were involved the proton from the CH group
bonded to the imine CHdN group should be split and appear as
a doublet of doublets due to additional coupling to imine CH
proton (ca. J = 3 Hz). This is not observed (absence of coupling
was also confirmed by COSY). Further proof of structure of 15
derived from a NOESY experiment which did not indicate
proximity of any of the aliphatic protons to imine CH proton.
The 1H NMR of compound 16 has two doublets at δ 3.49 and
3.79 with vicinal coupling constant 3J = 3.6 Hz, corresponding7
to ca. 110° dihedral angle between two aliphatic protons. That
compounds 15 and 16 there are downfield singlets around δ 10.9
from imine CH protons, which would not be observed in case of
compounds 17, 19, and20. Inaddition, the1H NMR of compounds
15 and 16 lacks signals from vinylic protons as would be the case of
compounds 17, 19, and 20. Finally the HRMS of both compounds
15 and 16 were in agreement with their molecular formulas.
Acknowledgment. Partial support of this research by the
National Science Foundation is gratefully acknowledged
with appreciation for its interest in basic research.
Supporting Information Available: Experimental details.
This material is available free of charge via the Internet at http://
pubs.acs.org.
(7) (a) MestRec-J Programming. (b) Haasnot, C. A. G.; de Leeuw, F. A.
A. M.; Altona, C. Tetrahedron 1980, 36, 2792. (c) Smith, W. B.; Barfield, M.
Magn. Reson. Chem. 1993, 31, 696.
5416 J. Org. Chem. Vol. 74, No. 15, 2009