Syntheses of 13C2-labelled 11Z-retinals

Article abstract of DOI:10.1016/j.tet.2011.07.092

To enable solid-state NMR investigations of the rhodopsin chromophore and its photointermediates, a series of 11Z-retinal isotopomers have been synthesised containing pairs of adjacent ¹³C labels at C9/C10, C10/C11 or C11/C12, respectively. The C9 labelled carbon atom was introduced through the Heck reaction of a ¹³C-labelled Weinreb acrylamide derivative, and the label at the C12 position derived from a ¹³C-containing ethoxy Bestmann-Ohira reagent. The ¹³C labels at C10 and C11 were introduced through the reaction of β-ionone with labelled triethyl phosphonoacetate.

Full text of DOI:10.1016/j.tet.2011.07.092

Tetrahedron 67 (2011) 8404e8410
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13
Syntheses of C2-labelled 11Z-retinals
Neville J. McLean, Axel Gansmuller, Maria Concistre, Lynda J. Brown, Malcom H. Levitt,
*
Richard C.D. Brown
School of Chemistry, University of Southampton, Highfield, Southampton, SO17 1BJ, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
To enable solid-state NMR investigations of the rhodopsin chromophore and its photointermediates,
a series of 11Z-retinal isotopomers have been synthesised containing pairs of adjacent 13C labels at C9/
C10, C10/C11 or C11/C12, respectively. The C9 labelled carbon atom was introduced through the Heck
reaction of a 13C-labelled Weinreb acrylamide derivative, and the label at the C12 position derived from
Received 10 February 2011
Received in revised form 30 June 2011
Accepted 18 July 2011
Available online 22 August 2011
a
13C-containing ethoxy BestmanneOhira reagent. The 13C labels at C10 and C11 were introduced
through the reaction of
b
-ionone with labelled triethyl phosphonoacetate.
Ó 2011 Elsevier Ltd. All rights reserved.
Keywords:
Retinal
Rhodopsin
Polyene
Labelled compounds
1. Introduction
11Z-retinals containing two adjacent 13C labels in defined positions
(Fig. 1).
Located in the rod cells of the retina, rhodopsin is responsible for
dim light vision in vertebrates. It is a 40-kd G-protein coupled re-
ceptor (GPCR) consisting of an 11Z-retinylidene chromophore
bound to the apoprotein opsin by a protonated Schiff base linkage
to the -amino group of lysine-296. Upon absorption of a photon,
3
the chromophore photoisomerises to all E-retinylidene in approx-
imately 200 fs leading to conformational changes within the pro-
tein producing the active form of rhodpsin, metarhodpsin II.1
Members of the GPCR super-family of receptors are frequently
identified as targets for the development of therapies to treat a di-
versity of diseases.2 Detailed structural knowledge of GPCRs and
their bound ligands is therefore of great significance, and rhodop-
sins have been one of the most widely studied GPCRs due to the
availability of crystal structural data and adequate amounts of
material.3 Understanding in detail how the protein environment in
rhodopsin affects and accelerates isomerisation of the retinylidene
chromophore, and how isomerisation of the chromophore in-
fluences conformational changes in the protein, is important for
GPCR research. Recently, solid-state NMR has been used as a pow-
erful tool to study the conformation of the retylidene chromophore
in rhodopsin and its photointermediates with a level of resolution
Fig. 1. Structures of 11Z-retinal isotopomers 1aed and all E-retinal (2a).
unmatched using other techniques.4,5 Double-quantum filtered 13
C
magic angle spinning structural studies,6 however, require access to
Previously, 13C-labelled11Z-retinal isotopomers have been
obtained by photoisomerisation of all E-retinals already containing
appropriately positioned 13C labels introduced by total syn-
thesis.7e9 This approach requires separation of the desired 11Z-
retinal from a complex mixture of retinal stereoisomers and
* Corresponding author. Tel.: þ44 (0) 23 8059 4108; fax: þ44 (0) 23 8058 6805;
e-mail address: rcb1@soton.ac.uk (R.C.D. Brown).
0040-4020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2011.07.092

N.J. McLean et al. / Tetrahedron 67 (2011) 8404e8410
8405
degradation products by means of preparative normal phase HPLC.
2.1. [9,10-13C2]-11Z-Retinal
Stereocontrolled syntheses of unlabelled 11Z-retinals have been
reported,10,11 although we are not aware of reports detailing the
application of these synthetic routes to obtain 13C-labelled com-
pounds. The stereocontrolled synthesis of 11Z-retinal is compli-
cated by facile chemical and photochemical isomerisation of the
polyene system. In the present work, our objective was to develop
stereocontrolled syntheses of 11Z-retinals containing adjacent 13C
labels at positions close to the site of isomerisation in the rhodopsin
retylidene chromophore. Here we describe syntheses of three 11Z-
retinal isotopomers 1bed.
Incorporation of the 13C label at the C9 position in retinal re-
quired its introduction during the synthesis of 13C-
-ionone 7b
b
(Scheme 2). The direct Heck reaction between labelled methyl vinyl
ketone and the cyclohexenyl triflate 10 was considered to be un-
attractive due to the practical difficulties in manipulating relative
small quantities of volatile and expensive labelled building
blocks.15 We therefore targeted a novel Weinreb amide derivative
11b, which would serve as a precursor to the methyl ketone once
coupled to vinyl triflate 10. Thus, bromination of commercial
[1-13C]-acetic acid (12b) and subsequent treatment with N-ben-
zylmethoxyamine afforded bromoacetamide derivative 13 in 56%
yield over the two steps. Arbuzov reaction of 13 with triethyl
phosphite followed by HornereEmmons olefination with formal-
dehyde provided acrylamide 11b. Triflate 10 was prepared from
2,6-dimethylcyclohexanone as described in good yield,15 then
coupled with acrylamide 11b to afford the Weinreb amide de-
rivative 16. Finally, addition of methyllithium to the Weinreb amide
For the solid-state NMR studies, three different doubly label-
led11Z-retinals 1bed were required, containing pairs of adjacent
13C labels at C9/C10, C10/C11 and C11/C12, respectively. Our syn-
thetic approach was based on the previously demonstrated cou-
pling of the alkyne and iodoalkene fragments 4a and 5 to afford
dehydroretinol (3a), followed by zinc-mediated semi-hydrogena-
tion of the alkyne 3a, and subsequent oxidation of the allylic al-
cohol to afford 11Z-retinal (Scheme 1).10b,12 Alkyne fragment 4a was
to be derived from aldehyde 6a, which would in turn come from E-
analogue 16 provided the desired
b-ionone isotopomer 7b con-
selective olefination of
b
-ionone (7a).13,14 Synthesis of
b
-ionone
taining the required 13C label at C9.
would proceed by Heck coupling of cyclohexenyl triflate 10 and the
acrylamide derivative 11a.15 With the synthetic plan in place, ap-
propriate commercially available 13C-labelled starting materials
were identified to be: [1-13C]-acetic acid, [2-13C]-triethyl phos-
phonoacetate, [1,2-13C2]-bromoethyl acetate and 13CH3I.
Scheme 2. Reagents and conditions: (a) (i) [1-13C]-acetic acid, PBr3, Br2, reflux, (ii)
BnNH(OMe), Et3N, CH2Cl2, 0 ꢀC; (b) P(OEt)3, 180 ꢀC; (c) CH2O, K2CO3, H2O, 40 ꢀC; (d) 10,
Pd(PPh3)2Cl2, Et3N, DMF, 75 ꢀC; (e) MeLi, THF, ꢁ78 ꢀC/rt.
Olefination of the
b-ionone isotopomer 7b with commercial
[2-13C]-triethyl phosphonoacetate proceeded with good E/Z selec-
tivity (E/Z¼7:1) and excellent yield (Scheme 3).13 However, the
two-step ester reductionealcohol oxidation sequence returned an
unexpectedly poor yield of aldehyde 6b due to decomposition and
isomerisation at the C9 double bond during silica gel purification.
The same chemistry, previously carried out on the unlabelled
material, had progressed smoothly in 71% yield and without
substantial isomerisation, although the sensitivity of the aldehyde
6 has been noted by others.14 Reaction of the mixture of stereo-
isomers 6b (E/Z¼3:1) with TMSCHN2 afforded alkyne 4b in 57%
yield, with enrichment of the E-isomer to 12:1 after chromatogra-
phy. Palladium-catalysed cross-coupling of alkyne 4b with the vinyl
iodide fragment 5 returned the desired enyne 18b as a mixture with
unreacted vinyl iodide 5.10b This mixture was subjected to silyl
deprotection to secure [9,10-13C2]-11Z-dehydroretinol (3b) in 44%
isolated yield. Hydrogenation with activated zinc provided
[9,10-13C2]-11Z-retinol with Z/E ratio at C11 of 3:1.10b Oxidation of
the crude unseparated retinols with TPAP and NMO occurred rap-
idly to provide pure samples of [9,10-13C2]-11Z-retinal (1b) and
[9,10-13C2]-11E-retinal (2b) after preparative HPLC separation on
a silica column. The isomers were identified on the basis of HPLC
retention times, and through successful incorporation of
[9,10-13C2]-11Z-retinal into rhodopsin.16
Scheme 1. Overview of the synthesis of 11Z-retinals suitable for introducing pairs of
13C labels at the C9/C10, C10/C11 or C11/C12 positions.
2. Results and discussion
For clarity, the syntheses of the three different doubly 13C2-la-
belled isotopomers 1bed are described separately. Some of the
reactions discussed below were initially performed using the
unlabelled material, and any significant differences in yields or
stereoselectivities, where observed, are discussed.

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N.J. McLean et al. / Tetrahedron 67 (2011) 8404e8410
direct exposure of the reaction mixture to silica gel column chro-
matography. As a precaution in subsequent reactions, an aqueous
extraction was conducted and reaction mixtures were filtered
through basic alumina prior to purification on silica gel. Separation
of the desired 13E-isomer from the 13Z-isomer was possible fol-
lowing silyl deprotection. Pleasingly, selective hydrogenation pro-
vided the [10,11-13C2]-11Z-retinol as the only observable isomer by
NMR, a considerable improvement on the 3:1 previously attained.
This result maybe attributed to the exclusion of light and the use of
HPLC grade solvents (including water). Final oxidation proceeded
smoothly and [10,11-13C2]-11Z-retinal was isolated by preparative
HPLC in good yield along with smaller amounts of the 11E-isomer
and mixed isomers, which were formed during manipulation of the
material rather than as a result of the reactions themselves.
Scheme 3. Synthesis of [9,10-13C2]-11Z-retinal (1b). Reagents and conditions: (a) tri-
ethyl phosphonoacetate or [2-13C]-triethyl phosphonoacetate, NaH, Et2O; (b) (i) LiAlH4,
Et2O, ꢁ78 ꢀC/rt, (ii) TPAP, NMO, 4 A sieves, CH2Cl2; (c) TMSCHN2, LDA, THF, ꢁ78 ꢀC;
ꢀ
(d) 5, Pd(PPh3)4, CuI, i-PrNH2; (e) TBAF, THF, 0 ꢀC/rt; (f) (i) Zn, Cu(OAc)2, AgNO3, H2O/
Scheme 4. Synthesis of [10,11-13C2]-11Z-retinal (1c). Reagents and conditions: (a)
MeOH, 40 ꢀC, (ii) TPAP, NMO, 4 A sieves, CH2Cl2.
ꢀ
P(OEt)3, 180 ꢀC; (b)
b
-ionone (7a), NaH, Et2O; (c) (i) LiAlH4, Et2O, ꢁ78 ꢀC/rt, (ii) TPAP,
NMO, 4 A sieves, CH2Cl2; (d) TMSCHN2, LDA, THF, ꢁ78 ꢀC; (e) 5, Pd(PPh3)4, CuI, i-
ꢀ
The final steps of the syntheses had been conducted in the dark,
or in dim red light when necessary. Despite these precautions,
some isomerisation of 11Z-retinal occurred during manipulation of
material. It is evident that several of the steps involving the labelled
intermediates were accomplished with significantly reduced yields
and selectivities in comparison to the same reactions using unla-
belled material. The cause was later traced to a poor quality batch of
silica gel, and although similar problems were largely avoided in
subsequent syntheses, these results do highlight the well-known
sensitivity of the polyene intermediates towards thermal, acid
catalysed and photochemical isomerisation.8d,17 However, once
purified, the samples of 11Z-retinals have been stored at ꢁ78 ꢀC in
the dark, and have been used for incorporation into rhodopsin
without problem over a number of years.
PrNH2; (f) TBAF, THF, 0 ꢀC/rt; (g) (i) Zn, Cu(OAc)2, AgNO3, H2O/i-PrOH, 40 ꢀC, (ii) TPAP,
NMO, 4 A sieves, CH2Cl2.
ꢀ
2.3. [11,12-13C2]-11Z-Retinal
Incorporation of the 13C label at C11 was performed as pre-
viously described (10,11-13C2-11Z-retinal) utilising a 13C-labelled
triethyl phosphonate in the Horner Emmons reaction with b-ion-
one (7a) in high yield and E/Z selectivity (Scheme 5). Ester re-
duction and immediate oxidation yielded the labelled aldehyde 6d.
The previous isotopomer syntheses had made use of TMSCHN2 to
2.2. [10,11-13C2]-11Z-Retinal
[10,11-13C2]-11Z-Retinal (1c) was the most conveniently acces-
sible of the three isotopomers due to the availability of both 13C
labels in commercially available [1,2-13C2]-bromoethyl acetate,
which was readily converted to the phosphonate 8c using the
Arbuzov reaction (Scheme 4).9 Sequential HornereEmmons re-
action with b-ionone (7a), ester reduction then oxidation gave al-
dehyde 6c in excellent yield and E/Z ratio of 11:1. Subsequent
reaction with TMSCHN2 provided the alkyne 4c, which was then
taken through the sequence described above to afford [10,11-13C2]-
11Z-retinal (1c). Unfortunately, we were once again inconvenienced
byan unexpected isomerisation, this time at C13 (E/Z¼1.5:2.0) in the
enyne 18c during column chromatography. This isomerisation had
not been observed in previous syntheses, and was attributed to
Scheme 5. Synthesis of [11,12-13C2]-11Z-Retinal. Reagents and conditions: (a) [1-13C]-
triethyl phosphonoacetate, NaH, Et2O; (b) (i) LiAlH4, Et2O, ꢁ78 ꢀC/rt, (ii) TPAP, NMO,
4 A sieves, CH2Cl2; (c) 9d, NaOMe, MeOH (1 equiv), THF, ꢁ78 ꢀC/rt; (d) (i) 5,
ꢀ
Pd(PPh3)4, CuI, i-PrNH2, (ii) TBAF, THF, 0 ꢀC/rt; (e) (i) Zn, Cu(OAc)2, AgNO3, H2O/
MeOH, 40 ꢀC, (ii) TPAP, NMO, 4 A sieves, CH2Cl2.
ꢀ

N.J. McLean et al. / Tetrahedron 67 (2011) 8404e8410
8407
introduce C12 into 4aec. However, a 13C-labelled BestmanneOhira
reagent was preferred to TMS 13CHN2on the basis of synthetic ac-
cessibility.18 Initial investigations began with the preparation of
a labelled methoxy reagent 19 using a modified MichaeliseBecker
reaction,19 however isotopic dilution was observed (Scheme 6). This
problem was resolved by starting from diethyl phosphite, resulting
in the formation of the desired phosphonate 20 in high yield.20
Acetylation of phosphonate 20 followed by diazo transfer fur-
nished the [1,2-13C3]-BestmanneOhira reagent 9d. Deacylative ac-
tivation of 9d was carried out by pre-treatment with 1 equiv of
NaOMe before adding aldehyde 6d, which was smoothly converted
to the alkyne 4d. More conventional reaction conditions (K2CO3,
MeOH) resulted in the formation of the desired product 4d in 74%
yield,18 but with isomerisation (6:1) resulting from methoxide
additioneelimination to the unsaturated aldehyde 6d. Sonogashira
reaction of the alkyne 4d with iodide 5 proceeded efficiently, and
the crude coupling reaction mixture was subjected to silyl depro-
tection. Careful filtration of crude mixture through neutral alumina
followed by silica gel chromatography helped to minimise iso-
merisation. Subsequent alkyne reduction, final alcohol oxidation
and purification by preparative HPLC afforded [11,12-13C2]-11Z-
retinal (1d).
were dried by distillation from CaH2. 1H NMR and 13C NMR spectra
were recorded in CDCl3 or C6D6 solutions using Bruker AC300,
AV300 (300 and 75 MHz, respectively) or Bruker DPX400 (400 and
100 MHz, respectively) spectrometers. 19F and 31P NMR spectra
were recorded in solution using a Bruker AV300 (282 and 121 MHz,
respectively). Chemical shifts are reported in
C6H6 as an internal standard (
7.27 ppm 1H,
7.15 ppm 1H, 128.62 ppm 13C, respectively). Infrared spectra
d
units using CHCl3 or
d
d
77.36 ppm 13C,
d
d
were recorded on a Nicolet 380 fitted with a Diamond platform, as
solids or neat liquids. Melting points were obtained using a Gal-
lenkamp Electrothermal apparatus and are uncorrected. Electron
impact and chemical ionisation mass spectra were obtained using
a Fisons VG platform single quadropole mass spectrometer. Elec-
trospray mass spectra were obtained using a Micromass platform
mass analyser with an electrospray ion source. Reactions in-
troducing and containing the 11Zisomers were carried out in dim
red light conditions using base washed glassware. HPLC purifica-
tion of retinal was performed with a Shimadzu VP series HPLC and
Phenomenex silica column, eluting with Et2O/hexane. The posi-
tions of 13C labels are indicated using the retinal numbering system,
except for compounds 9d and 20.
4.2. Preparation of compounds
4.2.1. [9-13C]-N-Benzyl-2-bromo-N-methoxyacetamide (13). To [1-13C]-
acetic acid (1.00 g, 16.4 mmol) and PBr3 (1.95 mL, 16.4 mmol) was
added bromine (2.10 mL, 41.0 mmol) dropwise. The mixture was
heated to 75 ꢀC and stirred for 3 h. Distillation at atmospheric
pressure removed excess bromine and then [1-13C]-bromoacetyl
bromide (155e158 ꢀC). The resulting [1-13C]-bromoacetyl bromide
was dissolved in CH2Cl2 (45 mL), then cooled to 0 ꢀC. A solution of
N-benzyl-O-methylhydroxylamine21 (2.25 g, 16.4 mmol) in CH2Cl2
(5 mL) was added dropwise. After 10 min, Et3N (2.28 mL,
16.4 mmol) was added dropwise and the reaction stirred at 0 ꢀC.
After 1 h the reaction was quenched with H2O and extracted into
CH2Cl2 and the combined organics were washed with brine, dried
(MgSO4) and concentrated in vacuo. Purification by silica gel col-
umn chromatography eluting with 1% Et2O/CH2Cl2 gave the amide
13 as a colourless oil (2.39 g, 9.21 mmol, 56%). FT-IR (neat) nmax1620
Scheme 6. Synthesis of a 13C-labelled BestmanneOhira Reagent. Reagents and con-
ditions: (a) 13CH3I, K2CO3, 35 ꢀC then
m
W, 110 ꢀC; (b) n-BuLi, CuI, THF, ꢁ60 ꢀC/ꢁ30 ꢀC,
then AcCl, THF, ꢁ40 ꢀC/rt; (c) TsN3, NaH, benzene/THF, 0 ꢀC/rt.
(
13C]O)cmꢁ1; 1H NMR (300 MHz, CDCl3)
d 7.45e7.30 (5H, m), 4.83
3. Conclusions
(2H, d, JCH¼1.8 Hz), 4.05 (2H, d, JCH¼3.8 Hz), 3.74 (3H, s) ppm; 13C
The stereoselective synthesis of three isotopomers, [9,10-13C2]-
11Z-retinal (1b), [10,11-13C2]-11Z-retinal (1c), [11,12-13C2]-11Z-ret-
inal (1c), has been achieved. The syntheses utilised relatively in-
expensive 13C enriched building blocks, lead to the development of
a Weinreb acrylamide and incorporates the use of a 13C-labelled
NMR (75 MHz, CDCl3) d
168.1 (13C), 135.9 (C), 128.9 (CH), 128.7 (CH),
128.3 (CH), 62.8 (CH3), 49.7 (CH2), 25.8 (CH2, d, JCC¼57.7 Hz) ppm;
LRMS (CI, NH3) m/z 259 [M (79Br)þH]þ, 261 [M (81Br)þH]þ;HRMS
(ESþ) C913CH1379BrNO2, calculated 259.0159, found 259.0160.
4.2.2. [9-13C]-Diethyl(N-benzyl-N-methoxycarbamoyl)-methyl phos-
phonate (14). A mixture of amide 13 (2.37 g, 9.16 mmol) and tri-
ethyl phosphite (1.57 mL, 9.16 mmol) was heated to 180 ꢀC for 1 h
giving a colourless oil. Purification by silica gel column chroma-
tography eluting with EtOAc then 10% MeOH/CH2Cl2 gave the title
compound14 as a colourless oil (2.51 g, 7.95 mmol, 87%). FT-IR
BestmanneOhira reagent on an
a,b-unsaturated aldehyde, using
the pre-deacetylation of the reagents to minimise isomerisation.
Samples of the doubly 13C-labelled retinal isotopomers have been
successful combined with the apoprotein opsin to generate rho-
dopsin with a labelled chromophore, and these specifically labelled
proteins have been used in solid-state NMR experiments to study
conformational changes in the rhodopsin chromophore and its
photointermediates.5c Some difficulties were encountered due to
undesired isomerisation of certain polyunsaturated intermediates,
highlighting their sensitive nature. However, these technical chal-
lenges were largely overcome during the synthesis of the final
isotopomer 1d.
(neat) nmax 1615 (13C]O), 1250 cmꢁ1 1H NMR (300 MHz, CDCl3)
;
d
7.40e7.20 (5H, m), 4.83 (2H, d, JCH¼2.2 Hz), 4.17 (4H, qd,
JHH¼7.0 Hz, JHP¼8.2 Hz), 3.72 (3H, s), 3.20 (2H, dd, JCH¼6.8 Hz,
JHP¼22.0 Hz), 1.33 (6H, td, JHH¼7.0 Hz, JHP¼0.6 Hz) ppm; 13C NMR
(75 MHz, CDCl3) d
166.5 (13C),136.3 (C),128.9 (CH),128.8 (CH),128.1
(CH), 62.9 (CH2, d, JCP¼6.1 Hz), 62.6 (CH3), 49.1 (CH2), 32.0 (CH2, dd,
JCP¼135.5 Hz, JCC¼53.1 Hz), 16.6 (CH3, d, JCP¼6.0 Hz) ppm; 31P NMR
4. Experimental
(121 MHz, CDCl3)
d
21.6 (s) ppm; LRMS (ESþ) m/z 339 [MþNa]þ;
HRMS (ESþ) C1313CH23NO5P, calculated 317.1342, found 317.1340.
4.1. General methods
4.2.3. [9-13C]-N-Benzyl-N-methoxyacrylamide (11b). Phosphonate
14 (2.45 g, 7.73 mmol) and K2CO3 (3.21 g, 23.2 mmol) were sus-
pended in H2O (5 mL) and stirred for 15 min. CH2O(aq) (1.15 mL,
15.5 mmol) was added dropwise and the reaction was warmed to
All reactions were carried out under an inert atmosphere in
oven dried glassware. THF and Et2O were distilled from sodium and
benzophenone prior to use. Triethylamine and dichloromethane

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N.J. McLean et al. / Tetrahedron 67 (2011) 8404e8410
40 ꢀC and stirred for 30 min. To the reaction 6 aliquots of CH2O(aq)
(0.58 mL, 7.73 mmol) were added at 30 min intervals. The reaction
was diluted with Et2O and H2O then separated and the aqueous
phase extracted with Et2O (ꢂ3). The combined organics were
washed with brine, dried (MgSO4) and concentrated in vacuo. Pu-
rification silica gel column chromatography eluting with 30%
EtOAc/hexane gave the title compound 11b as a colourless oil
2.96 mmol) in Et2O (2 mL) dropwise, the yellow solution was stir-
red for 62 h forming a white suspension. H2O was added and the
mixture was extracted with hexane (ꢂ4). The combined organics
were washed with brine, dried (MgSO4) and concentrated in vacuo.
Purification by silica gel column chromatography eluting with 2%/
3% EtOAc/hexane afforded the title ester 17d as a pale yellow oil
(713 mg, 2.71 mmol, 91%, E/Z¼13:1). Spectroscopic and physical
data were consistent with reported values for the unlabelled
(965 mg, 5.02 mmol, 65%). FT-IR (neat) nmax1634
(
13C]O),
1595 cmꢁ1 1H NMR (300 MHz, CDCl3)
;
d
7.45e7.22 (5H, m), 6.77
compound.23 FT-IR (neat) nmax 1669 (13C]O), 1606 cmꢁ1
(400 MHz, CDCl3)
;
1H NMR
(1H, ddd, JHH¼17.0, 10.2 Hz, JCH¼4.4 Hz), 6.50 (1H, ddd, JHH¼17.0,
2.0 Hz, JCH¼6.8 Hz), 5.81 (1H, ddd, JHH¼10.2, 2.0 Hz, JCH¼12.4 Hz),
4.87 (2H, d, JCH¼2.0 Hz), 3.65 (3H, s) ppm; 13C NMR (75 MHz, CDCl3)
d
6.60 (1H, d, J¼16.1 Hz), 6.10 (1H, d, J¼16.1 Hz),
5.75 (1H, s), 4.18 (2H, qd, JHH¼7.1 Hz, JCH¼3.0 Hz), 2.34 (3H, t,
J¼1.3 Hz), 2.03 (2H, t, J¼6.0 Hz), 1.70 (3H, s), 1.67e1.59 (2H, m),
1.51e1.45 (2H, m), 1.30 (3H, t, J¼7.1 Hz), 1.03 (6H, s) ppm; 13C NMR
d
166.8 (13C), 136.7 (C), 130.0 (CH2), 128.9 (CH) 128.8 (CH), 128.1
(CH), 126.4 (CH, JCC¼65.8 Hz), 63.1 (CH3), 49.7 (CH2) ppm; LRMS
(ESþ) m/z 215 [MþNaþ]; HRMS (ESþ) C1013CH14NO2, calculated
193.1053, found 193.1050.
(100 MHz, CDCl3) d
167.6 (13C), 153.1 (C), 137.6 (C), 136.6 (CH, d,
JCC¼8.7 Hz), 134.0 (CH), 131.4 (C), 118.4 (CH, d, JCC¼77.8 Hz), 59.9
(CH2, d, JCC¼1.9 Hz), 39.9 (CH2), 34.6 (C), 33.4 (CH2), 29.2 (CH3), 22.0
(CH3), 19.5 (CH2), 14.7 (CH2, d, JCC¼1.4 Hz), 14.0 (CH3) ppm; LRMS
(ESþ) m/z 264 [MþH]þ; HRMS (ESþ) for C1613CH27O2, calculated
264.2040, found 264.2039.
4.2.4. [9-13C]-(2E)-N-Benzyl-N-methoxy-3-(2,6,6-trimethylcyclo-
hex)-1-enyl acrylamide (16). To Pd(PPh3)2Cl2 (56 mg, 0.08 mmol)
suspended in DMF (5 mL) was added a solution of amide 11b
(548 mg, 2.85 mmol), triflate 10 (981 mg, 3.60 mmol) and Et3N
(1.76 mL, 12.6 mmol) in DMF (5 mL) the reaction was warmed to
75 ꢀC and stirred for 22 h. The mixture was diluted with Et2O and
H2O then separated. The aqueous phase was extracted with Et2O
(ꢂ4) and the combined organics washed with brine (ꢂ3), dried
(MgSO4) and concentrated in vacuo. Purification by silica gel col-
umn chromatography eluting with 15% EtOAc/hexane gave the title
compound 16 as an amber oil (708 mg, 2.25 mmol, 79%). FT-IR
4.2.7. [11-13C]-(2E,4E)-3-Methyl-5-(2,6,6-trimethylcyclohex-1-enyl)
penta-2,4-dienal (6d). To a slurry of LiAlH4 (162 mg, 4.28 mmol) in
Et2O (8 mL) at ꢁ78 ꢀC was added ester 17d (705 mg, 2.68 mmol, 9-
E/Z¼13:1) in Et2O (27 mL) dropwise, and the reaction stirred for 1 h
at ꢁ78 ꢀC. The reaction was warmed to rt and stirred for 2.5 h. The
reaction was quenched with H2O (0.2 mL), 15% NaOH (0.2 mL) and
H2O (0.6 mL) sequentially and stirred for 20 min producing a white
precipitate. The heterogeneous mixture was dried (MgSO4) and the
precipitate removed by filtration, the solution was concentrated in
vacuo giving a colourless oil. The oil was taken up in CH2Cl2 (28 mL)
and crushed molecular sieves (1.6 g), NMO (628 mg, 5.36 mmol)
and TPAP (94 mg, 0.27 mmol) added. After stirring at rt for 30 min
the black suspension was filtered through Celite and concentrated
in vacuo. Purification by silica gel column chromatography eluting
with 4% EtOAc/hexane afforded the title aldehyde 6d as a pale
yellow oil (431 mg, 1.97 mmol, 73%) and 9Z-aldehyde (44 mg,
0.20 mmol, 7%). 1H NMR data were consistent with reported values
for the unlabelled compound.14,24 FT-IR (neat) nmax1629 (13C]O)
(neat) nmax 1631 (13C]O), 1596 cmꢁ1 1H NMR (400 MHz, CDCl3)
;
d
7.52 (1H, dd, JHH¼16.1 Hz, JCH¼6.5 Hz), 7.40e7.25 (5H, m), 6.43
(1H, dd, JHH¼16.1 Hz, JCH¼4.3 Hz), 4.89 (2H, d, JCH¼1.8 Hz), 3.64 (3H,
s), 2.07 (2H, t, J¼6.3 Hz), 1.79 (3H, d, J¼0.5 Hz), 1.70e1.60 (2H, m),
1.55e1.45 (2H, m), 1.09 (6H, s) ppm; 13C NMR (100 MHz, CDCl3)
d
167.7 (13C), 143.8 (CH), 137.1 (C), 136.9 (C, JCC¼5.8 Hz), 135.0 (C),
128.9 (CH), 128.8 (CH), 127.9 (CH), 119.9 (CH, JCC¼66.6 Hz), 63.2
(CH3), 49.8 (CH2), 40.1 (CH2), 34.4 (C), 33.8 (CH2), 29.2 (CH3), 22.1
(CH3), 19.3 (CH2) ppm; LRMS (ESþ) m/z 315 [MþH]þ; HRMS (ESþ)
for C1913CH28NO2, calculated 315.2148, found 315.2150.
cmꢁ1
;
1H NMR (400 MHz, CDCl3)
d
10.14 (1H, dd, JHH¼8.0 Hz,
4.2.5. [9-13C]-
b-Ionone (7b). To acrylamide 16 (702 mg, 2.23 mmol)
JCH¼169.8 Hz), 6.75 (1H, d, J¼16.1 Hz), 6.22 (1H, d, J¼16.1 Hz), 5.94
(1H, d, J¼8.0 Hz), 2.32 (3H, s), 2.06 (2H, br t, J¼6.3 Hz), 1.73 (3H, s),
1.68e1.59 (2H, m), 1.52e1.45 (2H, m), 1.05 (6H, s) ppm; 13C NMR
in THF (40 mL) at ꢁ78 ꢀC was added MeLi (2.23 mL of 1.6 M in Et2O,
3.57 mmol) dropwise. The reaction was stirred at ꢁ78 ꢀC for 45 min
then warmed to ꢁ20 ꢀC and stirred for 20 min. The reaction was
quenched with a saturated NH4Cl(aq), diluted with Et2O and sep-
arated. The organics were washed with a saturated NH4Cl(aq) and
the aqueous phase extracted with Et2O (ꢂ3). The combined or-
ganics were washed with brine, dried (MgSO4) and concentrated in
vacuo. Purification by silica gel column chromatography eluting
with 10% Et2O/hexane gave the title compound 7b as a pale yellow
oil (370 mg, 1.91 mmol, 86%). Spectroscopic data were consistent
(100 MHz, CDCl3)
d
191.6 (13CH), 155.3 (C, d, JCC¼2.9 Hz), 137.4 (C),
136.0 (CH, d, JCC¼6.8 Hz), 135.9 (C), 133.1 (CH), 129.1 (CH, d,
JCC¼56.4 Hz), 39.9 (CH2), 34.6 (C), 33.6 (CH2), 29.3 (CH3), 22.1 (CH3),
19.4 (CH2), 13.3 (CH3, d, JCC¼4.9 Hz) ppm; LRMS (ESþ) m/z 220
[MþH]þ; HRMS (ESþ) for C1413CH23O, calculated 220.1777, found
220.1780.
4.2.8. [11,12-13C2]-1,3,3-Trimethyl-2-((1E,3E)-3-methylhexa-1,3-
dien-5-ynyl)cyclohex-1-ene (4d). To 13C-labelled diazophosphonate
9d (391 mg, 1.77 mmol) in THF (20 mL) at ꢁ78 ꢀC was added
NaOMe (0.5 M in MeOH, 3.54 mL, 1.77 mmol) dropwise over
15 min. After stirring for 1 h at ꢁ78 ꢀC aldehyde 6d (259 mg,
1.18 mmol) in THF (8 mL) was added dropwise over 10 min. The
yellow solution was stirred at ꢁ78 ꢀC for 30 min, warmed to rt over
30 min and stirred for 7.5 h. The reaction was quenched with H2O,
dried (MgSO4) and concentrated in vacuo. Purification by silica gel
column chromatography eluting with 5% EtOAc/hexane gave the
desired alkyne 4d as a pale yellow oil (162 mg, 0.75 mmol, 63%) and
starting aldehyde 6d (39 mg, 0.18 mmol, 15%).Spectroscopic data
were consistent with selected reported values for the unlabelled
with selected reported values for the labelled and unlabelled
ionone.22 FT-IR (neat) nmax 1665 (13C]O), 1647 cmꢁ1 1H NMR
(300 MHz, CDCl3)
b-
;
d
7.28 (1H, dd, JHH¼16.5 Hz, JCH¼6.8 Hz), 6.12 (1H,
dd, JHH¼16.5 Hz, JCH¼3.1 Hz), 2.30 (3H, d, JCH¼5.7 Hz), 2.08 (2H, t,
J¼6.1 Hz), 1.77 (3H, s), 1.70e1.55 (2H, m), 1.53e1.40 (2H, m), 1.08
(6H, s) ppm; 13C NMR (75 MHz, CDCl3)
d
199.1 (13C), 143.5 (CH, d,
JCC¼1.7 Hz), 136.4 (2ꢂ C), 131.9 (CH, d, JCC¼52.7 Hz), 40.1 (CH2), 34.4
(C), 33.9 (CH2), 29.1 (2ꢂ CH3), 27.5 (CH3, d, JCC¼42.0 Hz), 22.1 (CH3),
19.2 (CH2) ppm; LRMS (CI, NH3) m/z 194 [MþH]þ; HRMS (EI) for
C1213CH20O, calculated 193.1548, found 193.1549.
4.2.6. [11-13C]-(2E,4E)-Ethyl3-methyl-5-(2,6,6-trimethylcyclo hex-1-
enyl)penta-2,4-dienoate (17d). To
a
slurry of NaH (178 mg,
;
compound.25 FT-IR (neat) nmax2010 cmꢁ1 1H NMR (400 MHz,
4.44 mmol) in Et2O (5 mL) was added [1-13C]-triethyl phospho-
noacetate (1.00 g, 4.44 mmol) dropwise. This mixture was stirred
CDCl3)
d
6.29 (1H, d, J¼16.1 Hz), 6.10 (1H, d, J¼16.1 Hz), 5.41 (1H, br
s), 3.29(1H, m), 2.09 (3H, s), 2.02 (2H, t, J¼6.0 Hz), 1.70 (3H, br d,
J¼1.0 Hz), 1.66e1.58 (2H, m), 1.50e1.45 (2H, m), 1.02 (6H, s) ppm;
for 2 h. To the reaction was added
b-ionone (7a) (569 mg,

N.J. McLean et al. / Tetrahedron 67 (2011) 8404e8410
8409
13C NMR (100 MHz, CDCl3)
d
149.6 (C), 137.7 (C), 135.4 (CH, d,
column of neutral alumina topped with Celite, flushing with Et2O
and concentrated in vacuo. Purification by HPLC eluting with Et2O
(2.00 mL/min) and hexane (7.99 mL/min) gave [11,12-13C2]-11Z-
retinal (1d) as a yellow oil (69 mg, 0.24 mmol, 51% over two steps),
[11,12-13C2]-all-E-retinal as a yellow oil (5 mg, 0.02 mmol, 4%) and
[11,12-13C2]-retinal as a mixture of isomers (22 mg, 0.08 mmol,16%).
Spectroscopic data for 1d were consistent with those reported for
the unlabelled compound.27 FT-IR (neat) nmax1657 (s), 1566 (m)
JCC¼8.8 Hz), 130.6 (CH), 130.5 (C), 107.6 (CH, dd, JCC¼70.0, 30.1 Hz),
84.1 (13CH, d, JCC¼200.2 Hz), 82.4 (13C, d, JCC¼200.2 Hz), 39.9 (CH2),
34.6 (C), 33.4 (CH2), 29.2 (CH3), 22.0 (CH3), 19.6 (CH2), 15.4 (CH3)
ppm; LRMS (CI, NH3) m/z 217 [MþHþ].
4.2.9. [11,12-13C2]-3,7-Dimethyl-9-(2,6,6-trimethylcyclohex-1-enyl)-
nona-2,6,8-trien-4-yn-1-ol (3d). Following the method of Borhan
et al.,10b to a solution of iodide 5(405 mg, 1.30 mmol) in i-PrNH2
(3 mL) was added Pd(PPh3)4 (13 mg, 10.82
was stirred for 5 min. CuI (2 mg, 10.82 mol) was added and the
cmꢁ1; 1H NMR (400 MHz, C6D6)
d
9.91 (1H, d, J¼7.8 Hz), 6.58 (1H, br
mmol) and the reaction
d, J¼11.1 Hz), 6.38 (1H, dq, JHH¼12.1 Hz, JCH¼148.3 Hz), 6.34 (1H, d,
J¼16.1 Hz), 6.22 (1H, d, J¼16.1 Hz), 6.10 (1H, br t, JHH¼7.8 Hz,
JCH¼7.8 Hz), 5.58 (1H, dd, JHH¼11.8 Hz, JCH¼154.7 Hz), 1.91 (2H, t,
J¼6.4 Hz),1.77 (3H, dd, JHH¼1.4 Hz, JCH¼4.1 Hz),1.74 (3H, s),1.68 (3H,
s), 1.60e1.52 (2H, m), 1.46e1.42 (2H, m), 1.07 (6H, s) ppm; 13C NMR
m
reaction was stirred for a further 5 min. Alkyne 4d (234 mg,
1.08 mmol) in i-PrNH2 (1.6 mL) was added dropwise and the re-
action stirred for 3.5 h. The reaction was concentrated in vacuo,
redissolved in Et2O, washed with H2O (ꢂ3) and brine, and the or-
ganics concentrated in vacuo. The residue as a mixture of the de-
sired alkyne 18d26 and iodide 5 (4:1, 465 mg, w1.21 mmol) was
dissolved in THF (6 mL) and cooled to 0 ꢀC. TBAF (1.0 M in THF,
1.34 mL, 1.34 mmol) was added dropwise and the reaction warmed
to rt and stirred for 1 h. The reaction was quenched with H2O and
the extracted with Et2O (ꢂ3). The combined organics were washed
with brine, dried (MgSO4) and concentrated in vacuo. The crude
material was filtered through a plug of neutral alumina eluting with
20% EtOAc/hexane and concentrated in vacuo. Purification by silica
gel column chromatography eluting with 15% EtOAc/hexane gave
the desired dehydroretinol 3d (191 mg, 0.67 mmol, 69% over two
steps) and the 13Z-isomer (23 mg, 0.08 mmol, 8%). Data for 3d: FT-
(100 MHz, C6D6)
d
190.5 (CH), 154.6 (C, dd, JCC¼39.4, 13.1 Hz), 141.4
(C, d, JCC¼5.8 Hz), 138.8 (CH, d, JCC¼4.9 Hz), 138.6 (C), 131.9 (13CH, d,
JCC¼70.0 Hz), 131.1 (13CH, d, JCC¼70.0 Hz), 131.1 (CH, dd, JCC¼3.9,
2.0 Hz), 130.7 (C), 130.1 (CH), 127.0 (CH, dd, JCC¼41.8, 13.6 Hz), 40.4
(CH2), 35.1 (C), 33.8 (CH2), 29.7 (CH3), 22.4 (CH3), 20.2 (CH2), 18.0
(CH3), 12.8 (CH3, d, JCC¼3.4 Hz) ppm; LRMS (ESþ) m/z 287 [MþH]þ;
HRMS (ESþ) for C1813C2H29O, calculated 287.2280, found 287.2279.
4.2.11. [1-13C]-Diethyl methyl phosphonate (20). To a mixture of
diethyl phosphite (0.70 mL, 5.38 mmol) and K2CO3 (1.49 g,
10.8 mmol) was added 13CH3I (1.00 g, 6.99 mmol) dropwise and the
vessel was sealed. The reaction was stirred at 35 ꢀC for24 h. After
which crushed molecular sieves and K2CO3 (740 mg, 5.38 mmol)
were added. The reaction was stirred for a further 24 h at 35 ꢀC.
After this time the mixture was transferred to a microwave tube
washing with CHCl3 (2 mL), before the mixture was irradiated
(110 ꢀC, 300 W), the reaction was stopped at 9 min. The suspension
was filtered washing with CHCl3 and CH2Cl2 and the solution was
concentrated in vacuo giving a yellow oil. Purification by vacuum
transfer (0.4 mbar, rt) yielded the desired phosphonate 20as a col-
ourless oil (663 mg, 4.33 mmol, 80%). Spectroscopic data were
consistent with those reported for the unlabelled compound.28 FT-
IR (neat) nmax3313 cmꢁ1 1H NMR (400 MHz, CDCl3)
; d 6.27 (1H, d,
J¼16.1 Hz), 6.12 (1H, d, J¼16.1 Hz), 6.01 (1H, br t d, JHH¼7.5 Hz,
JCH¼7.5 Hz), 5.53 (1H, d, JCH¼4.8 Hz), 4.27 (2H, br t, J¼5.4 Hz), 2.07
(3H, s), 2.02 (2H, t, J¼6.3 Hz), 1.90 (3H, m), 1.70 (3H, s), 1.66e1.58
(2H, m), 1.51e1.44 (2H, m), 1.03 (6H, s) ppm; 13C NMR (100 MHz,
CDCl3)
d
147.8 (C), 137.8 (C), 135.8 (CH, d, JCC¼9.3 Hz), 134.7 (CH, d,
JCC¼3.9 Hz), 130.4 (C), 129.9 (CH), 108.8 (CH, dd, JCC¼91.4, 11.7 Hz),
98.8 (13C, d, JCC¼177.9 Hz), 87.2 (13C, d, JCC¼177.9 Hz), 59.6 (CH2, d,
JCC¼6.8 Hz), 39.9 (CH2), 34.6 (C), 33.4 (CH2), 29.3 (CH3), 22.0 (CH3),
19.6 (CH3), 18.0 (CH2), 15.4 (CH3, d, JCC¼3.9 Hz) ppm; LRMS (ESþ) m/
z 269 [MꢁH2OþH]þ. Data for 13Z-Isomer: 1H NMR (300 MHz,
IR (neat) nmax 1305, 1227, 1026 cmꢁ1 1H NMR (300 MHz, CDCl3)
;
d
4.10 (4H, qdd, JHH¼7.1 Hz, JCH¼4.6 Hz, JHP¼8.2 Hz), 1.47 (3H, dd,
CDCl3)
d
6.29 (1H, d, J¼16.1 Hz), 6.12 (1H, d, J¼16.1 Hz), 5.86 (1H,
JCH¼128.2 Hz, JHP¼17.5 Hz), 1.33 (6H, t, J¼7.1 Hz) ppm; 13C NMR
(75 MHz, CDCl3)Ô 61.8 (CH2, d, JCP¼6.1 Hz), 16.7(CH3, d, JCP¼6.1 Hz),
11.6 (13CH3, d, JCP¼144.8 Hz) ppm; 31P NMR (121 MHz, CDCl3)Ô 31.1
(d, JCP¼144.8 Hz) ppm; LRMS (ESþ) m/z 154 [MþH]þ.
tqd, JHH¼6.8, 1.5 Hz, JCH¼13.8 Hz,), 5.57 (1H, br s), 4.36 (2H, d,
J¼6.8 Hz), 2.08 (3H, dd, JHH¼0.8 Hz, JCC¼0.8 Hz), 2.02 (2H, t,
J¼5.9 Hz), 1.95 (3H, m), 1.70 (3H, br s), 1.67e1.55 (2H, m), 1.52e1.43
(2H, m), 1.03 (6H, s) ppm; 13C NMR (75 MHz, CDCl3)
d 147.9 (C),
137.8 (C), 135.7 (CH, dd, JCC¼5.5, 3.3 Hz), 134.8 (CH), 130.5 (C), 130.2
(CH), 121.9 (C, dd, JCC¼58.1, 38.7 Hz), 108.6 (CH, dd, JCC¼61.1,
41.2 Hz), 95.4 (13C, d, JCC¼176.9 Hz), 93.0 (13C, d, JCC¼176.9 Hz), 61.9
(CH2), 39.9 (CH2), 34.6 (C), 33.4 (CH2), 29.2 (CH3), 23.6 (CH3), 22.0
(CH3), 19.5 (CH2), 15.5 (CH3) ppm.
4.2.12. [1-13C]-Diethyl (1-diazo-2-oxopropyl)phosphonate (9d).
Following the procedure of Mathey and Savignac,29 to a solution of
phosphonate 20 (303 mg, 1.98 mmol) in THF (2.5 mL) at ꢁ60 ꢀC was
added n-BuLi (2.34 M in hexane, 0.93 mL, 2.18 mmol) dropwise,
after 5 min CuI (415 mg, 2.18 mmol) was added. The reaction was
slowly warmed to ꢁ30 ꢀC and stirred for 1.5 h, then cooled to
ꢁ40 ꢀC. Acetyl chloride (0.15 mL, 2.08 mmol) in Et2O (1.5 mL) was
added slowly and the reaction stirred at ꢁ35 ꢀC for 2.5 h. The re-
action was warmed to rt and stirred for 17 h. The reaction was
quenched with H2O producing a white suspension, which was fil-
tered through Celite, flushing with THF then CH2Cl2. The phases
were separated and the aqueous extracted with CH2Cl2 (ꢂ3), and
the combined organics were dried (MgSO4) and concentrated in
vacuo. Purification by distillation under reduced pressure
(0.4 mbar, 60 ꢀC) gave [1-13C]-diethyl (2-oxopropyl)phosphonate as
a colourless oil (323 mg, 1.66 mmol, 84%). Spectroscopic data were
consistent with reported values for the unlabelled compound.30 FT-
4.2.10. [11,12-13C2]-11Z-Retinal (1d). Following the method of
Borhan et al.,10b argon was bubbled through a suspension of zinc
dust (11.42 g, 0.175 mol) in H2O (68 mL) for 15 min. Cu(OAc)2 (1.14 g,
6.28 mmol) was then added and after 15 min stirring AgNO3 (1.14 g,
6.72 mmol) added (Care!-exothermic). After stirring for 30 min the
activated zinc was filtered and washed with H2O, MeOH, acetone
and Et2O sequentially. The moist zinc catalyst was suspended in H2O
(19 mL) and i-PrOH (19 mL) and the labelled dehydroretinol 3d
(134 mg, 0.47 mmol) in i-PrOH (19 mL) added. The reaction was
heated at 40 ꢀC for 26 h. The reaction was filtered through Celite
flushing with H2O and Et2O and the phases were separated. The
aqueous was extracted with Et2O (ꢂ4) and the combined organics
washed with brine, dried (Na2SO4) and concentrated in vacuo. The
crude labelled retinol (136 mg, w0.47 mmol) was dissolved in
CH2Cl2 (5 mL) and crushed molecular sieves (280 mg), NMO
(110 mg, 0.94 mmol) and TPAP (49 mg, 0.14 mmol) added sequen-
tially. After 30 min stirring the reaction was passed through a short
IR (neat) nmax1714, 1246 cmꢁ1 1H NMR (300 MHz, CDCl3)
; d 4.16
(4H, qdd, JHH¼7.0 Hz, JCH¼1.3 Hz, JHP¼8.2 Hz), 3.09 (2H, dd,
JCH¼128.8 Hz, JHP¼22.9 Hz), 2.33 (3H, d, JCH¼1.5 Hz), 1.35 (6H, td,
JHH¼7.0 Hz, JHP¼0.6 Hz) ppm; 13C NMR (75 MHz, CDCl3)
d 62.9 (CH2,
d, JCP¼6.6 Hz), 43.8 (CH2, d, JCP¼127.2 Hz), 31.7 (CH3, d,
JCC¼14.9 Hz), 16.6 (CH3, d, JCP¼6.1 Hz) ppm; 31P NMR (121 MHz,

8410
N.J. McLean et al. / Tetrahedron 67 (2011) 8404e8410
8. For reviews of retinoid syntheses: (a) Valla, A. R.; Cartier, D. L.; Labia, R. Curr.
CDCl3)
d
20.3 (d, J¼127.0 Hz) ppm; LRMS (ESþ) m/z 196 [MþH]þ. To
Org. Synth. 2004, 1, 167e209; (b) Valla, A. R.; Cartier, D. L.; Labia, R. In Studies in
Natural Products Chemistry; Atta-ur, R., Ed.; Elsevier: 2003; pp 69e107; (c)
Dominguez, B.; Alvarez, R.; de Lera, A. R. Org. Prep. Proced. Int. 2003, 35,
239e306; (d) For a review of the photochemistry and synthesis of the ste-
reoisomers of vitamin A: Liu, R. S. H.; Asato, A. E. Tetrahedron 1984, 40,
1931e1969.
a slurry of NaH (42 mg, 1.06 mmol) in benzene (1.5 mL) and THF
(2 mL) at 0 ꢀC was added [1-13C]-diethyl (2-oxopropyl)phosphonate
(188 mg, 0.96 mmol) in THF (2 mL). After stirring for 1 h at 0 ꢀC,
TsN3 (209 mg, 1.06 mmol) in THF (1 mL) was added slowly drop-
wise. The reaction was warmed to rt and stirred for 16 h. The re-
sultant red suspension was filtered through Celite flushing with
benzene (ꢂ3) followed by EtOAc (ꢂ3), the filtrate was concentrated
in vacuo. Purification by silica gel column chromatography eluting
with EtOAc afforded the title diazo compound 9d as an oil (156 mg,
0.71 mmol, 73%). Spectroscopic data were consistent with reported
values for the unlabelled compound.31 FT-IR (neat) nmax 2113, 1655,
9. For a synthesis of all E-retinal 13C-labelled at every position: Creemers, A. F. L.;
Lugtenburg, J. J. Am. Chem. Soc. 2002, 124, 6324e6334.
10. (a) Uenishi, J.; Kawahama, R.; Yonemitsu, O.; Wada, A.; Ito, M. Angew. Chem., Int.
Ed. 1998, 37, 320e323; (b) Borhan, B.; Souto, M. L.; Um, J. M.; Zhou, B. S.; Na-
kanishi, K. Chem.dEur. J. 1999, 5, 1172e1175; (c) Wada, A.; Fujioka, N.; Tanaka,
Y.; Ito, M. J. Org. Chem. 2000, 65, 2438e2443; (d) Lopez, S.; Montenegro, J.; Saa,
C. J. Org. Chem. 2007, 72, 9572e9581; (e) Montenegro, J.; Bergueiro, J.; Saa, C.;
Lopez, S. Org. Lett. 2009, 11, 141e144.
11. For syntheses of 11Z-retinal where mixtures of stereoisomers were obtained:
(a) Knudsen, C. G.; Chandraratna, R. A. S.; Walkeapaa, L. P.; Chauhan, Y. S.; Carey,
S. C.; Cooper, T. M.; Birge, R. R.; Okamura, W. H. J. Am. Chem. Soc. 1983, 105,
1626e1631; (b) Hosoda, A.; Taguchi, T.; Kobayashi, Y. Tetrahedron Lett. 1987, 28,
65e68; (c) Oroshnik, W. J. Am. Chem. Soc. 1956, 78, 2651e2652.
12. For an example of Cu promoted cross coupling: Hopf, H.; Krause, N. Tetrahedron
Lett. 1985, 26, 3323e3326.
1260 cmꢁ1 1H NMR (300 MHz, CDCl3)
; d 4.21 (4H, m), 2.29 (3H, d,
JCH¼2.2 Hz), 1.40 (6H, td, JHH¼7.0 Hz, JHP¼0.7 Hz) ppm; 13C NMR
(75 MHz, CDCl3)
d
64.6 (13C, d, JCP¼218.4 Hz), 53.7 (CH2, d,
JCP¼5.5 Hz), 27.6 (CH3, d, JCC¼21.6 Hz),16.5 (CH3, d, JCP¼6.6 Hz) ppm
(carbonyl carbon not observed); 31P NMR (121 MHz, CDCl3)
d 11.7
13. (a) Bergen, H. R.; Furr, H. C.; Olson, J. A. J. Labelled Compd. Radiopharm. 1988, 25,
11e21; (b) Tanumihardjo, S. A. J. Labelled Compd. Radiopharm. 2001, 44,
365e372.
(d, JCP¼218.4 Hz) ppm; LRMS (ESþ) m/z 222 [MþH]þ; HRMS (ESþ)
for C613CH14N2O4P, calculated 222.0719, found 222.0718.
14. Dugger, R. W.; Heathcock, C. H. Synth. Commun. 1980, 10, 509e515.
15. Breining, T.; Schmidt, C.; Polos, K. Synth. Commun. 1987, 17, 85e88.
16. 13C NMR Spectra for the labelled chromophores are provided in the Supporting
data.
Supplementary data
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isomerisation, and in particular to blue, violet, or ultraviolet light: Hubbard, R.;
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Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.tet.2011.07.092.
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