Synthesis of covalent probes for the radiolabeling of the cannabinoid receptor

Article abstract of DOI:10.1021/jo0264978

The main psychoactive constituent of marijuana, (-)-Α⁹-tetrahydrocannabinol, produces most of its physiological effects by interacting with the CB1 cannabinoid receptor, a membrane protein belonging to the large superfamily of G-protein coupled receptors. The 3-D structure of the receptor binding site is of value in the design of novel medications for a variety of therapeutic indications. To obtain information on the amino acid residues associated with this binding site, we have designed and synthesized a cannabinergic CB1 ligand prototype carrying an electrophilic isothiocyanato group capable of reacting covalently with amino acid residues bearing thiol or unprotonated amino groups. The ligand also incorporates an iodide atom, which can serve as a high-activity radiolabel. The key step in our synthesis involves a rapid intramolecular Diels-Alder reaction of a transiently formed o-quinone methide, which proceeds stereospecifically with the formation of the tricyclic cannabinoid template. Introduction of the iodo group is the last step in the sequence and is compatible with the use of ¹²⁵I-radiolabel.

Full text of DOI:10.1021/jo0264978

Syn th esis of Cova len t P r obes for th e Ra d iola belin g of th e
Ca n n a bin oid Recep tor
Chester Chu,† Arun Ramamurthy,‡ Alexandros Makriyannis,*,‡ and Marcus A. Tius*,†
Chemistry Department, University of Hawaii, Honolulu, Hawaii 96822, and Center for Drug Discovery
and Departments of Pharmaceutical Sciences and Molecular and Cell Biology, University of Connecticut,
Storrs, Connecticut 06269
makriyan@uconnvm.uconn.edu
Received September 30, 2002
The main psychoactive constituent of marijuana, (-)-∆9-tetrahydrocannabinol, produces most of
its physiological effects by interacting with the CB1 cannabinoid receptor, a membrane protein
belonging to the large superfamily of G-protein coupled receptors. The 3-D structure of the receptor
binding site is of value in the design of novel medications for a variety of therapeutic indications.
To obtain information on the amino acid residues associated with this binding site, we have designed
and synthesized a cannabinergic CB1 ligand prototype carrying an electrophilic isothiocyanato group
capable of reacting covalently with amino acid residues bearing thiol or unprotonated amino groups.
The ligand also incorporates an iodide atom, which can serve as a high-activity radiolabel. The key
step in our synthesis involves a rapid intramolecular Diels-Alder reaction of a transiently formed
o-quinone methide, which proceeds stereospecifically with the formation of the tricyclic cannabinoid
template. Introduction of the iodo group is the last step in the sequence and is compatible with the
use of 125I-radiolabel.
In tr od u ction
is of great value in the design of novel, more selective
and more potent drug candidates. Unfortunately, the
structural analysis of GPCRs is severely limited by the
difficulties associated with their purification and crystal-
lization. A major development in this field has been the
recent publication of the three-dimensional crystallo-
graphic structure of bovine rhodopsin,5 a GPCR associ-
ated with visual signal transduction. The 2.8 Å resolution
crystal structure has provided a much-awaited basis for
the development of realistic computational models of
other GPCRs including CB1.6 However, there are very
few direct data regarding the CB1 active site. A method
currently pursued in our laboratories for obtaining such
information is based on the identification of key amino
acid residues associated with the receptor’s binding
domain. This experimental approach involves the devel-
opment of high-affinity selective ligands capable of form-
ing a covalent bond with an amino acid residue within
the receptor binding site or immediately adjacent to it.
The identity of the labeled amino acid residue is then
revealed through purification of the ligand-receptor
complex followed by digestion of the entire protein and
analysis of the individual peptide fragments using se-
quencing or mass spectral analysis. Overall, this is a
laborious process, which can be greatly enhanced by the
availability of an 125I-iodine radiolabeled covalent probe
with high specific radioactivity (over 2000 Ci/mmol) and
capable of serving as a highly sensitive marker.
The main psychoactive constituent of marijuana,
(-)-∆9-tetrahydrocannabinol, was shown to bind specif-
ically to the two known cannabinoid receptors, CB1 and
CB2. Both of these are membrane-bound proteins and
belong to the large superfamily of G-protein-coupled
receptors (GPCRs) characterized by seven transmem-
brane-spanning helical domains connected by intervening
intracellular and extracellular loops. CB1 has been
recognized as an important therapeutic target for pain,
appetite modulation, glaucoma, neurodegeneration, and
several other indications and is currently receiving
increasing attention by the pharmaceutical industry and
other drug discovery laboratories.1-4 Thus, understanding
the structural and functional properties of this receptor
† University of Hawaii.
‡ University of Connecticut.
(1) Goutopoulos, A.; Makriyannis, A. Pharmacol., Ther. 2002, 95,
103.
(2) Khanolkar, A. D.; Palmer, S. L.; Makriyannis, A. Chem. Phys.
Lipids 2000, 108, 37.
(3) Porter, A. C.; Felder, C. C. Pharmacol., Ther. 2001, 90, 45.
(4) Baker, D.; Pryce, G.; Croxford, J . L.; Brown, P.; Pertwee, R. G.;
Makriyannis, A.; Khanolkar, A. Layward, L.; Fezza, F.; Bisogno, T.;
DiMarzo, V. FASEB J . 2001, 15, 300.
(5) Palczewski, K.; Kumasaka, T.; Hori, T.; Behnke, C. A.; Mo-
toshima, H.; Fox, B. A.; Le Trong, I.; Teller, D. C.; Okada, T.; Stenkamp,
R. E.; Yamamoto, M.; Miyano, M. Science 2000, 289, 739.
(6) Barnett-Norris, J .; Hurst, D. P.; Lynch, D. L.; Guarnieri, F.;
Makriyannis, A.; Reggio, P. H. J . Med. Chem. 2002, 45, 3649.
(7) Guo, Y.; Abadji, V.; Morse, K. L.; Fournier, D. J .; Li, X.;
Makriyannis, A. J . Med. Chem. 1994, 37, 3867.
(8) Morse, K. L.; Fournier, D. J .; Li, X.; Grzybowska, J .; Makriyan-
nis, A. Life Sci. 1995, 56, 1957.
Receptor covalent probes can be designed by incorpo-
rating potentially reactive groups into a high-affinity
reversible prototypic ligand followed by suitable struc-
tural modifications, if necessary, to ensure that the novel
(9) Charalambous, A.; Yan, G.; Houston, D. B.; Howlett, A. C.;
Compton, D. R.; Martin, B. R.; Makriyannis, A. J . Med. Chem. 1992,
35, 3076.
10.1021/jo0264978 CCC: $25.00 © 2003 American Chemical Society
Published on Web 12/05/2002
J . Org. Chem. 2003, 68, 55-61
55

Chu et al.
SCHEME 1a
F IGURE 1. Bifunctional nonclassical cannabinoid.
ligand maintains its high affinity for the binding site.
Over the past decade our laboratory has been successful
in designing a number of CB1 receptor ligands incorpo-
rating an isothiocyanato group in a strategic position
within the ligand.7,8 This moderately reactive electrophile
is capable of reacting with amino acid residues bearing
either sulfhydryl or unprotonated amino side chains. The
isothiocyanate-bearing covalent probes, which have been
utilized in other receptor systems, offer the advantage
of high selectivity. Thus, they react very slowly with
water and other hydroxyl groups and form covalent bonds
only with more reactive nucleophiles that are suitably
positioned for a nucleophilic addition reaction within the
ligand binding site.
a
(a) 6-phenoxyhexyl bromide, Mg(0), THF, 0 °C, 6 h; 75%; (b)
TiCl4, Zn(CH3)2, CH2Cl2, -30 °C to rt, 6 h; 84%; (c) Ac2O, Yb(OTf)3,
CH3NO2, rt, 14 h; 76%; (d) BBr3, CH2Cl2, -78 °C to rt, 36 h; 100%;
(e) allyl bromide, n-Bu4NOH, CH2Cl2, H2O, rt, 24 h; 77%.
In earlier work, our laboratory had demonstrated the
usefulness of introducing either a photoactivatable group9
or an isothiocyanato at the end of the side chain at the
C-3 position of the aliphatic side chain. The design was
optimized with the development of 7′-isothiocyanato-11-
hydroxy-1′,1′-dimethylheptyl tetrahydrocannabinol7 with
a pseudo Ki value of 3.2 nm, which was shown to attach
covalently to a CB1 receptor preparation. Our present
task involved the introduction of an 125I-radiolabel while
maintaining at least part of the ligand’s reactivity at the
binding site. To optimize our radiochemical procedures,
the radiolabel should also be preferably introduced as the
last reaction step. This proved to be a difficult task
because earlier work on the tetrahydrocannabinols had
revealed that the introduction of bulky groups within the
tricyclic terpenoid structure of cannabinoids significantly
reduced their affinities for the receptor.
relative simplicity of their structures suggests. Few of
the compounds or of the intermediates are crystalline,
and the electron-rich phenolic portion of these molecules
is prone to oxidation by air. This process is especially
rapid in basic media. Unlike the natural tricyclic can-
nabinoids, C-6 is stereogenic in this series. This fact
increases the complexity of the problem, especially since
one is limited to kinetically controlled processes for the
control of C-6 stereochemistry, since there is an insig-
nificant energy difference between the two C-6 diastere-
oisomers. This fact precludes one from making use of the
methodologies that have been developed for the prepara-
tion of ∆8- and ∆9-THC. The task that we set before
ourselves in this work has an added complication,
namely, the need to introduce multiple reactive groups
within the molecule.10
We addressed this problem by designing a bifunctional
probe capable of bearing both an isothiocyanato group
and an iodo substituent using as a prototype a group of
classical/nonclassical hybrid cannabinoid ligands earlier
developed by our laboratories.10 This earlier work had
revealed a strong correlation between the stereochemis-
try at C-6 and chain length of the attached substituent
with the ligand’s affinity for the CB1 receptor. A par-
ticularly surprising result was the finding of a strong
increase in affinity for CB1 for the hybrid cannabinoid
bearing a â 1-iodopropyl group at C-6. Rather than
excluding the cannabinoid ligand from the binding site
for steric reasons, the large iodo substituent potentiated
its affinity for the receptor.10 To explore the above
findings, we decided to explore the possibility of prepar-
ing bifunctional cannabinoids bearing reactive groups at
C-7′ and C-1′′. Our first successful foray into this area is
described in what follows.
Discu ssion
The preparation of the aromatic portion of the molecule
is summarized in Scheme 1 and is a modified version of
a procedure we reported earlier.12 Weinreb amide 1 was
derived from commercially available 3,5-dimethoxyben-
zoic acid and was treated in THF with 6-phenoxyhexyl-
magnesium bromide to produce phenone 2 in 75% yield.
The conversion of the phenone carbonyl group to the
geminal dimethyl function took place in high yield in the
presence of TiCl4 and dimethyl zinc.13 Friedel-Crafts
acylation of 3 with acetic anhydride was catalyzed by Yb-
(OTf)3.14 It is noteworthy that the acylation is completely
regioselective, leading exclusively to 4. The presence of
the tert-alkyl group in 3 completely suppressed competi-
(11) For a review of cannabinoid synthesis, see: Tius, M. A. In
Studies in Natural Products Chemistry; Atta-ur-Rahman, Ed.; Elsevi-
er: Amsterdam, 1997; Vol. 19, p 185.
(12) Harrington, P. E.; Stergiades, I. A.; Erickson, J .; Makriyannis,
A.; Tius, M. A. J . Org. Chem. 2000, 65, 6576.
The hybrid tricyclic cannabinoids represent a much
greater challenge to the synthetic chemist than the
(13) Reetz, M. T.; Westermann, J .; Kyung, S.-H. Chem. Ber. 1985,
118, 1050.
(14) Kawada, A.; Mitamura, S.; Kobayashi, S. J . Chem. Soc., Chem.
Commun. 1993, 1157.
(10) Drake, D. J .; J ensen, R. S.; Busch-Petersen, J .; Kawakami, J .;
Fernandez-Garcia, M. C.; Fan, P.; Makriyannis, A.; Tius, M. A. J . Med.
Chem. 1998, 41, 3596.
56 J . Org. Chem., Vol. 68, No. 1, 2003

Radiolabeling of the Cannabinoid Receptor
SCHEME 2a
ficity of the process in which the stereochemistry at C-6,
C-6a, and C-10a are formed in a controlled fashion,
suggests a fast, possibly synchronous, process and does
not support the intermediacy of long-lived cationic in-
termediates. The cyclization reaction assembles the
tricyclic cannabinoid skeleton with control of the ring
junction stereochemistry and also the critical stereo-
chemistry at C-6. What remained to be accomplished was
the delicate task of introducing the sensitive functionality
at C-7′ and C-1′′. After several unsuccessful forays, we
settled on the approach that is summarized in Scheme
3.
Oxidation of the C-9 diastereomer of 10 with the Dess-
Martin reagent led to ketone 11 in 81% yield. It was now
necessary to remove the allyl protecting group. The
reductive protocol that makes use of borohydride and Pd-
(0) had served us well in the past, but the C-9 keto group
interfered by undergoing reduction back to the alcohol.17
Moreover, once the phenolic hydroxyl group is unmasked,
it is not possible to reoxidize C-9 for the reason indicated
in the Introduction. Consequently, C-9 was first protected
as dimethyl ketal 12, then the allyl protecting group was
removed to give 13. Although nonreductive methods are
known for removing the allyl protecting group, these
typically require activation of the ether through com-
plexation with a Lewis acid, followed by nucleophilic
cleavage of the allyl group.18 These conditions are incom-
patible with the sensitive tertiary propargyl ether func-
tion that is present in 11. The sequence in which the
subsequent reactions were carried out was critical.
Displacement of the bromide in 13 by tetramethylguani-
dinium azide led to C-7′ azide 14 in quantitative yield.19
Exposure of this material first to triphenyl phosphine,
then to carbon disulfide converted azide to isothiocyanate,
leading to 15 in 93% yield.20
Our next task was to remove both the ketal and silyl
ether protecting groups. We had surmised that once the
propargyl iodide was present, our options would be
limited by the lability of this group. Hydrolytic removal
of the dimethyl ketal would have to be carried out before
introduction of the iodide. Luckily, simultaneous removal
of both protecting groups could be accomplished in good
yield (76%) by exposure of 15 to oxalic acid in aqueous
THF at room temperature for 18 h. The two free hydroxyl
functions in ketodiol 16 presented us with a potential
problem, as the next step required us to selectively
functionalize the propargyl alcohol. We had hoped to
convert alcohol to iodide by exposure of 16 to the
combination of iodine, triphenylphosphine, and imida-
zole.21 After many failures to do so, we settled on a two-
step sequence, whereby the alcohol was first converted
to mesylate 17, and the mesylate displaced by iodide.
This led in excellent overall yield to our target cannab-
inoid.
a
(a) LDA, THF, -78 °C, 30 min; 77%; (b) NaBH4, MeOH, rt, 5
min; 98%; (c) TFA, CHCl3, 0 °C, 3 h; 88%.
tive acylation at C-4 and C-6. Exposure of 4 to an excess
of BBr3 led to cleavage of both methoxy groups as well
as the C-7′ phenoxy group, leading to 5 in quantitative
yield. Introduction of the reactive primary alkyl bromide
function at C-7′ at such an early stage carried the risk
of frustrating our plans by limiting our options. While
the presence of the bromide had to be taken into
consideration at each step, we were able to design
combinations of reagent, solvent, and reaction conditions
to suppress or limit any interference from this functional
group. This is reflected in the uniformly high yields for
each of the subsequent steps in the synthesis. Selective
protection of one of the phenolic hydroxyl groups in 5
under phase transfer conditions led to mono-allyl ether
6 in 77% yield.
The keto group in 6 provides the functionality to join
the two large fragments of the target (Scheme 2). The
synthesis of aldehyde 7 has been described. We had
predicted that the primary bromide function in 6 would
not interfere with the very fast aldol coupling reaction
with 7. This proved to be the case, since aldol adduct 8
was formed in 77% yield. It was, however, necessary to
quench the aldol reaction with aqueous acetic acid prior
to workup so as to suppress the base-catalyzed retro-aldol
fragmentation of 8 back to 6 and 7. Reduction of 8 with
sodium borohydride in methanol led to diol 9 as a mixture
of two diastereomers. No attempt was made to separate
these isomers. Rather, the mixture was treated at 0 °C
with trifluoroacetic acid in chloroform to produce a 1:1
mixture of C-9 diastereoisomers 10 in 88% yield. That
the products of the cyclization reaction differed only at
C-9 was demonstrated by oxidizing the C-9 equatorial
isomer of 10 to the C-9 ketone with the Dess-Martin
periodinane15 (81% yield), then reducing the ketone with
L-Selectride to the C-9 axial isomer of 10 (89% yield).16
The mechanism of the cyclization reaction leading to
10 can be thought of either as an intramolecular Diels-
Alder reaction of a transiently formed o-quinone methide
or as a cation-olefin cyclization process. The stereospeci-
(15) (a) Ireland, R. E.; Liu, L. J . Org. Chem. 1993, 58, 2899. (b) Dess,
D. B.; Martin, J . C. J . Org. Chem. 1983, 48, 4155.
(16) Brown, H. C.; Krishnamurthy, S. J . Am. Chem. Soc. 1972, 94,
7159.
(17) Beugelmans, R.; Bourdet, S.; Bigot, A.; Zhu, J . Tetrahedron Lett.
1994, 35, 4349.
(18) For example see: Thomas, R. M.; Reddy, G. S.; Iyengar, D. S.
Tetrahedron Lett. 1999, 40, 7293.
(19) Papa, A. J . J . Org. Chem. 1966, 31, 1426.
(20) Molina, P.; Alajarin, M.; Arques, A. Synthesis 1982, 596.
(21) Nicolaou, K. C.; Daines, R. A.; Ogawa, Y.; Chakraborty, T. K.
J . Am. Chem. Soc. 1988, 110, 4696.
J . Org. Chem, Vol. 68, No. 1, 2003 57

Chu et al.
SCHEME 3a
a
(a) Dess-Martin periodinane, CH2Cl2, 0 °C to rt, 3 h; 81%; (b) HC(OMe)3, Yb(OTf)3, MeOH, rt, 12 h; 73%; (c) Pd(PPh)4, NaBH4, THF,
rt, 14 h; 85%; (d) TMGN3, MeNO2, 50 °C, 3 h; 100%; (e) PPh3, PhH, 50 °C, 1 h; CS2, 50 °C, 2 h; 93%; (f) (COOH)2, THF, H2O, rt, 18 h; 76%;
(g) MsCl, 2,6-lutidine, CH2Cl2, 0 °C to rt, 14 h; 93%; (h) NaI, acetone, reflux, 12 h; 79%.
Earlier experiments have shown that our target com-
pound is indeed a successful covalent CB1 receptor probe.
When tested on a membrane receptor preparation ob-
tained from rat brains, our nonradiolabeled iodinated
bifunctional ligand at a concentration of 25 nM was
capable of irreversibly occupying approximately 50% of
the CB1 binding sites. Details of this work will be
reported elsewhere. We have also shown that the SN2
reaction of iodide with the mesylate precursor proceeds
at a satisfactory rate, thus ensuring that development
of the 125I-radiolabeled ligand is within reach.
Exp er im en ta l Section
Proton and 13C NMR spectra were recorded on a GE QE-
300 spectrometer operating at 300 MHz (1H) or 75 MHz (13C),
unless noted otherwise. Chemical shifts are reported in δ units
and are referenced to solvent (7.26/77.0 for CDCl3). Infrared
spectra were recorded on a Perkin-Elmer IR 1430 spectrom-
eter. EI mass spectra were obtained from a VG-70SE mass
spectrometer. Purity and homogeneity of all materials was
1
determined from TLC, H NMR, 13C NMR, and HPLC.
1-(3,5-Dim eth oxyp h en yl)-7-p h en oxyh ep ta n -1-on e, 2. To
a suspension of 885 mg (36.4 mmol) of magnesium turnings
in 22.5 mL of THF at room temperature was added dropwise
over 1 h a solution of 7.040 g (27.3 mmol) of 6-phenoxyhexyl
bromide in 18 mL of THF. The mixture was stirred for 30 min,
during which time heat was evolved and a gray suspension
was formed. The solution was cooled to 0 °C, and to the cooled
suspension was added neat Weinreb amide 1 (4.100 g, 18.2
mmol). The reaction mixture was stirred for 6 h at room
temperature and then quenched by addition of 1 M HCl. The
aqueous phase was separated and extracted with ether (3 ×
100 mL) followed by brine (3 × 100 mL), then dried over
MgSO4, filtered, and concentrated to give a pale yellow solid
(7.423 g). Flash column chromatography on silica gel afforded
pure phenone 2 (4.642 g, 75% yield) as a white solid: mp 51-
Con clu sion s
The availability of a bifunctional high-affinity covalent
probe for the CB1 receptor opens the door for a series of
radiolabeling experiments that can greatly assist in
identifying the binding site of cannabinoid ligands within
this receptor. This task is currently being pursued using
CB1 receptor preparations expressed in mammalian cell
cultures. Furthermore, the chemistry discussed here
demonstrates the versatility of the cyclization strategy
that we have developed for accessing the multifunctional
cannabinoid analogues. It is especially worthy of note
that multiple sites of high reactivity can be built into the
tricyclic cannabinoid scaffold.
1
52 °C; Rf ) 0.32 (15% EtOAc in hexanes); H NMR (CDCl3) δ
7.27 (t, J ) 8.0 Hz, 1H), 7.10 (d, J ) 2.4 Hz, 2H), 6.94-6.88
(m, 4H), 6.65 (t, J ) 2.4 Hz, 1H), 3.96 (t, J ) 7.0 Hz, 2H), 3.84
(s, 6H), 2.94 (t, J ) 7.0 Hz, 2H), 1.78 (quint, J ) 7.0 Hz, 4H),
1.56-1.45 (m, 4H), 1.18-1.12 (m, 2H); 13C NMR (CDCl3) δ
58 J . Org. Chem., Vol. 68, No. 1, 2003

Radiolabeling of the Cannabinoid Receptor
200.0, 160.8, 159.1, 139.0, 129.4, 120.5, 114.5, 105.9, 105.1,
67.7, 55.6 (2× OCH3), 38.6, 29.1, 29.0, 25.9, 24.3; IR (neat)
3932, 2857, 1734, 1684, 1600, 1464, 1420, 1307, 1249, 1152,
filtered, and concentrated to give the crude product as a deep
orange oil (1.073 g). Distillation of the volatiles (Kugelrohr
apparatus) yielded a residue of the product 5 (893 mg, 100%
yield). No further purification was performed, and the product
was carried to the next step. Phenone 5: Rf ) 0.13 (10% EtOAc
in hexanes); 1H NMR (CDCl3) δ 9.5-9.1 (br s, 2H), 6.35 (s,
2H), 3.37 (t, J ) 7.2 Hz, 2H), 2.71 (s, 3H), 1.79 (quint, J ) 7.2
Hz, 2H), 1.56-1.51 (m, 2H), 1.36 (quint, J ) 7.2 Hz, 2H), 1.27-
1.19 (m, 2H), 1.22 (s, 6H), 1.10-1.02 (m, 2H); 13C NMR (CDCl3)
δ 204.3, 160.9, 159.5, 108.1, 106.3, 43.8, 38.2, 34.0, 33.1, 32.7,
29.3, 28.4, 24.4; IR (neat) 3310, 2932, 2860, 1636, 1588, 1421,
1375, 1264, 1069 cm-1; mass spectrum (EI) m/z 358, 356 (M+,
3), 277 (M+ - Br, 7), 228 (5), 194 (93), 179 (25), 109 (6), 84
1065 cm-1; mass spectrum (EI) m/z 342 (M+, 14), 249 (M+
-
OPh, 11), 180 (47), 165 (100), 152 (23), 137 (33), 122 (24), 94
(34), 77 (24); exact mass calcd for C21H26O4 342.1831, found
342.1840.
[7-(3,5-D im e t h o x y p h e n y l)-7-m e t h y lo c t y lo x y ]b e n -
zen e, 3. To a solution of 0.66 mL (6.0 mmol) of TiCl4 in 12 mL
of CH2Cl2 at -30 °C was added dropwise 3.0 mL of a 2.0 M
solution of dimethylzinc in toluene. The solution was stirred
at -30 °C for 30 min, and to this mixture was added a cooled
(-30 °C) solution of phenone 2 (1.027 g, 3.0 mmol) in 12 mL
of CH2Cl2. The reaction mixture was warmed to room tem-
perature, stirred for 6 h, and quenched by slowly pouring into
ice/saturated aqueous NaHCO3 solution. Acidification with 1
M HCl solution was followed by extraction of the aqueous
phase with ether (3 × 50 mL). The combined organic extracts
were washed with brine (3 × 50 mL), dried (MgSO4), filtered,
and evaporated to produce 1.183 g of crude product. Purifica-
tion by flash column chromatography on silica gel afforded
pure product 3 as a pale yellow oil (902 mg, 84% yield): Rf )
0.54 (15% EtOAc in hexanes); 1H NMR (CDCl3) δ 7.30 (t, J )
8.0 Hz, 1H), 6.97-6.90 (m, 4H), 6.53 (d, J ) 2.2 Hz, 2H), 6.34
(t, J ) 2.2 Hz, 1H), 3.94 (t, J ) 7.0 Hz, 2H), 3.82 (s, 6H), 1.75
(quint, J ) 7.0 Hz, 2H), 1.64-1.59 (m, 2H), 1.43 (quint, J )
7.0 Hz, 2H), 1.34-1.26 (m, 2H), 1.30 (s, 6H), 1.18-1.12 (m,
2H); 13C NMR (CDCl3) δ 160.4, 159.1, 152.4, 129.3, 120.4,
114.4, 104.6, 96.6, 67.7, 55.1, 44.4, 37.9, 30.0, 29.2, 28.9, 25.9,
24.6; IR (neat) 2932, 2857, 1599, 1496, 1456, 1421, 1244, 1204,
1155, 1054 cm-1; mass spectrum (EI) m/z 356 (M+, 14), 263
(M+ - OPh, 23), 180 (100), 165 (17), 151 (22), 139 (20), 94 (20),
(100); exact mass calcd for
356.0984.
C17H25BrO3 356.0987, found
1-[2-Allyloxy-4-(7-br om o-1,1-dim eth ylh eptyl)-6-h ydr oxy-
p h en yl]eth a n on e, 6. To a solution of the resorcinol 5 from
the preceding reaction in 10 mL of CH2Cl2/water (10:7, v/v)
was added a large excess of allyl bromide at room temperature.
To this mixture was added 1.64 mL of a 40 wt % solution of
nBu4NOH in water, and the reaction mixture was stirred
vigorously for 24 h. Quenching with 1 M HCl was followed by
extraction of the aqueous phase with ether (3 × 25 mL). The
combined organic extracts were washed with brine (3 × 25
mL), dried (MgSO4), filtered, and concentrated to give the
crude product as a dark brown oil (961 mg). Purification by
flash column chromatography afforded pure 6 as a pale yellow
oil (761 mg, 77% yield): Rf ) 0.52 (10% EtOAc in hexanes);
1H NMR (CDCl3) δ 6.53 (d, J ) 1.5 Hz, 1H), 6.33 (d, J ) 1.5
Hz, 1H), 6.09 (ddt, J ) 17.4, 10.5, 5.4 Hz, 1H), 5.43 (dd, J )
17.4, 1.2 Hz, 1H), 5.34 (dd, J ) 10.5, 1.2 Hz, 1H), 4.63 (d, J )
5.4 Hz, 2H), 3.37 (t, J ) 7.1 Hz, 2H), 2.68 (s, 3H), 1.79 (quint,
J ) 7.1 Hz, 2H), 1.60-1.54 (m, 2H), 1.36 (quint, J ) 7.1 Hz,
2H), 1.26-1.19 (m, 2H), 1.20 (s, 6H), 1.10-1.04 (m, 2H); 13C
NMR (CDCl3) δ 204.4, 164.4, 160.1, 159.2, 132.7, 118.6, 109.4,
108.6, 100.5, 69.7, 43.9, 38.6, 33.9, 33.6, 32.7, 29.4, 28.4, 24.5;
IR (neat) 3214, 2933, 2860, 1634, 1564, 1463, 1417, 1372, 1307,
1222, 1102 cm-1; mass spectrum (EI) m/z 398, 396 (M+, 7),
317 (M+ - Br, 16), 275 (6), 234 (100), 219 (13), 193 (47), 165
75 (24); exact mass calcd for
356.2354.
C23H32O3 356.2351, found
1-[4-(1,1-Dim et h yl-7-p h en oxyh ep t yl)-2,6-d im et h oxy-
p h en yl]eth a n -1-on e, 4. To a solution of 1.780 g (5.0 mmol)
of dimethoxy aromatic compound 3 in 50 mL of nitromethane
at room temperature was added 620 mg (1.0 mmol) of ytter-
bium triflate in a single portion. To the resulting suspension
was added dropwise 2.36 mL (25.0 mmol) of acetic anhydride,
and the reaction mixture was stirred at room temperature for
14 h. The reaction was quenched by addition of saturated
aqueous NaHCO3 solution. The aqueous phase was extracted
with ether (3 × 100 mL), the combined organic extracts were
washed first with saturated aqueous NaHCO3 solution (3 ×
50 mL), followed by brine (3 × 50 mL), and then dried (MgSO4),
filtered, and concentrated to give the crude product as a dark
brown oil (2.166 g). Purification by flash column chromatog-
raphy on silica gel afforded pure 4 as a yellow oil (1.509 g,
76% yield): Rf ) 0.30 (10% EtOAc in hexanes); 1H NMR
(CDCl3) δ 7.29-7.25 (m, 2H), 6.92 (t, J ) 7.6 Hz, 1H), 6.88 (t,
J ) 7.6 Hz, 2H), 6.50 (s, 2H), 3.92 (t, J ) 7.0 Hz, 2H), 3.80 (s,
6H), 2.48 (s, 3H), 1.73 (quint, J ) 7.0 Hz, 2H), 1.61-1.57 (m,
2H), 1.40 (quint, J ) 7.0 Hz, 2H), 1.31-1.24 (m, 2H), 1.28 (s,
6H), 1.13-1.05 (m, 2H); 13C NMR (CDCl3) δ 202.96, 159.0,
156.5, 153.4, 129.4, 120.4, 118.0, 114.4, 101.9, 67.7, 55.8 (2x
OCH3), 44.4, 38.4, 32.4, 30.0, 29.2, 28.9, 25.9, 24.6; IR (neat)
2934, 2859, 1701, 1604, 1576, 1497, 1459, 1406, 1242, 1127,
(39), 149 (14), 121 (8), 82 (29); exact mass calcd for C20H29
BrO3 396.1300, found 396.1281.
-
P h en on e 8. A solution of LDA was prepared from 320 µL
of diisopropylamine (2.42 mmol) and 990 µL of a 2.38 M
solution of n-BuLi in 4.5 mL of THF at -78 °C. To the LDA
solution was added dropwise a solution of 480 mg (1.21 mmol)
of phenone 6 in 1.5 mL of THF, and the mixture was allowed
to react for 30 min. To the reaction mixture was added
dropwise a solution of 293 mg (1.10 mmol) of aldehyde 7 in
1.5 mL of THF. Stirring for another 30 min was followed by
warming of the mixture to 5 °C and quenching by addition of
280 µL of acetic acid. The reaction mixture was partitioned
between ether and water, and the aqueous layer was extracted
with ether (3 × 15 mL). The combined ether extracts were
washed with brine (3 × 15 mL), dried (MgSO4), filtered, and
concentrated to produce the crude product (895 mg). Purifica-
tion by flash column chromatography on silica gel led to 562
mg of aldol product 8 (77% yield) as a pale yellow oil: Rf )
0.24 (10% EtOAc in hexanes); 1H NMR (CDCl3) δ 6.54 (d, J )
1.5 Hz, 1H), 6.32 (d, J ) 1.5 Hz, 1H), 6.08 (ddt, J ) 17.2, 10.5,
5.5 Hz, 1H), 5.84 (t, J ) 7.5 Hz, 1H), 5.44 (dd, J ) 17.2, 1.2
Hz, 1H), 5.37 (dd, J ) 10.5, 1.2 Hz, 1H), 4.62 (d, J ) 5.5 Hz,
2H), 4.42 (s, 2H), 4.19-4.13 (m, 1H), 3.37 (t, J ) 6.7 Hz, 2H),
3.30 (dd, J ) 18.5, 2.3 Hz, 1H), 3.13 (dd, J ) 18.5, 9.3 Hz,
1H), 2.26-2.21(m, 2H), 1.83-1.52 (m, 6H), 1.80 (s, 3H), 1.36-
0.97 (m, 6H), 1.24 (s, 6H), 0.91 (s, 9H), 0.13 (s, 6H); 13C NMR
(CDCl3) δ 206.3, 164.7, 160.2, 159.9, 137.4, 132.5, 119.1, 118.1,
109.0, 100.8, 88.0, 84.6, 69.9, 67.4, 52.4, 51.8, 44.0, 43.8, 38.8,
35.9, 33.9, 32.9, 29.5, 28.6, 28.2, 26.1, 24.74, 24.69, 18.5, 17.2,
-4.8; IR (neat) 3422, 2929, 2857, 2248, 1626, 1600, 1563, 1462,
1412, 1371, 1256, 1222, 1096 cm-1; mass spectrum (EI) m/z
646, 644 (M+ - H2O, 1), 589, 587 (M+ - H2O, tBu, 11, 10),
1032 cm-1; mass spectrum (EI) m/z 398 (M+, 16), 383 (M+
-
CH3, 46), 305 (M+ - OPh, 16), 222 (100), 207 (18), 179 (14),
94 (27), 91 (11); exact mass calcd for C25H34O4 398.2457, found
398.2467.
1-[4-(7-Br om o-1,1-d im eth ylh ep tyl)-2,6-d ih yd r oxyp h en -
yl]eth a n -1-on e, 5. To a solution of 996 mg (2.5 mmol) of
phenone 4 in 5 mL of CH2Cl2 at -78 °C was added 12.5 mL of
a 1 M solution of BBr3 in CH2Cl2. The reaction mixture was
slowly warmed to room temperature and stirred for 26 h.
Quenching by careful addition of saturated aqueous NaHCO3
solution at 0 °C was followed by extraction of the aqueous
phase with ether (3 × 30 mL). The combined organic extracts
were washed with saturated aqueous NaHCO3 solution (3 ×
30 mL), followed by brine (3 × 30 mL), then dried (MgSO4),
J . Org. Chem, Vol. 68, No. 1, 2003 59

Chu et al.
396 (34), 341 (17), 317 (100), 275 (40); exact mass calcd for
C
added 79 mg (0.19 mmol) of the Dess-Martin periodinane in
a single portion. The reaction mixture turned orange, and a
white precipitate was formed. The reaction mixture was
allowed to slowly warm to room temperature, and stirring was
continued at room temperature for 3 h. The reaction was
quenched by the addition of saturated aqueous NaHCO3
solution and 90 mg of solid Na2S2O3. After 5 min the two-phase
mixture was diluted with CH2Cl2 and water. The aqueous layer
was extracted with CH2Cl2 (3 × 15 mL), and the combined
organic extracts were washed with brine (3 × 15 mL), dried
(MgSO4), and concentrated to give 61 mg of crude product.
Purification by flash column chromatography on silica gel gave
49 mg (81% yield) of pure 11 as a pale yellow oil: Rf ) 0.25
31H44BrO4Si (M+ - H2O, tBu) 587.2193, found 587.2206.
Tr iol 9. To a solution of 550 mg (0.83 mmol) of aldol product
8 in 11 mL of methanol was added 47 mg (1.24 mmol) of
NaBH4 in a single portion at room temperature. The reaction
mixture was stirred for 5 min at room temperature and then
quenched by the addition of 360 µL of acetic acid. The reaction
mixture was stirred at room temperature for 30 min, then
partitioned between ether and water. The aqueous layer was
extracted with ether (3 × 15 mL), and the combined organic
layers were washed with brine (3 × 20 mL), dried (MgSO4),
filtered, and concentrated to give the crude product (606 mg).
Purification by simple filtration through a short plug of silica
gel gave pure product 9 as a ca. 1:1 mixture of diastereoisomers
(540 mg, 98% yield, colorless oil). Diasteroisomeric mixture 9:
Rf ) 0.46 and 0.39 (20% EtOAc in hexanes); 1H NMR (CDCl3)
δ 6.48 (d, J ) 1.2 Hz, 2H), 6.33 (d, J ) 1.4 Hz, 2H), 6.08-5.94
(m, 2H), 5.81 (dd, J ) 17.2, 8.4 Hz, 2H), 5.71 (dd, J ) 10.0,
2.0 Hz, 1H), 5.56 (dd, J ) 9.0, 3.6 Hz, 1H), 5.41-4.34 (m, 2H),
5.29-5.25 (m, 2H), 4.51 (d, J ) 5.0 Hz, 4H), 4.42 (s, 2H), 4.41
(s, 2H), 4.08-3.90 (m, 4H), 3.36 (t, J ) 6.8 Hz, 4H), 2.20-2.12
(m, 4H), 1.83-1.48 (m, 12H), 1.78 (s, 3H), 1.77 (s, 3H), 1.40-
1.16 (m, 8H), 1.22 (s, 12H), 1.14-0.99 (m, 4H), 0.91 (s, 12H),
0.13 (s, 6H), 0.12 (s, 6H); IR (neat) 3397, 2929, 2857, 2109,
1626, 1579, 1419, 1365, 1253, 1221, 1082 cm-1; mass spectrum
(EI) m/z 648, 646 (M+ - H2O, 1), 407 (18), 405 (17), 217 (23),
1
(10% EtOAc in hexanes); H NMR (400 MHz, CDCl3) δ 6.51
(s, 1H), 6.38 (s, 1H), 6.06 (ddt, J ) 17.4, 10.5, 5.4 Hz, 1H),
5.39 (dd, J ) 17.4, 1.0 Hz, 1H), 5.29 (dd, J ) 10.5, 1.0 Hz,
1H), 4.53 (d, J ) 5.4 Hz, 2H), 4.43 (s, 2H), 3.82 (d, J ) 14.0
Hz, 1H), 3.36 (t, J ) 6.8 Hz, 2H), 2.88 (dt, J ) 12.0, 3.2 Hz,
1H), 2.62-2.40 (m, 2H), 2.28 (t, J ) 11.0 Hz, 1H), 2.13 (t, J )
14.0 Hz, 1H), 1.79 (quint, J ) 7.2 Hz, 2H), 1.60-1.49 (m, 2H),
1.40 (s, 3H), 1.38-1.32 (m, 2H), 1.30-1.17 (m, 2H), 1.22 (s,
6H), 1.10-1.04 (m, 2H), 0.92 (s, 9H), 0.90-0.86 (m, 2H), -0.14
(s, 6H); 13C NMR (CDCl3) δ 210.5, 157.0, 152.6, 150.5, 133.2,
117.6, 109.0, 108.6, 102.5, 85.5, 84.0, 74.3, 68.9, 51.7, 46.5, 45.6,
44.2, 40.8, 37.8, 33.9, 33.8, 32.8, 29.4, 28.8, 28.7, 28.0, 27.4,
25.8, 24.4, 19.3, 18.2, -5.1; IR (neat) 2932, 2857, 2109, 1715,
1649, 1614, 1571, 1461, 1414, 1361, 1252, 1096, 1059 cm-1
mass spectrum (EI) m/z 646, 644 (M+, 11), 587 (3), 565 (3),
;
192 (25), 165 (14), 91 (26), 75 (100); exact mass calcd for C35H55
-
BrO4Si (M+ - H2O) 646.3055, found 646.3050.
482 (11), 105 (16), 83 (16), 75 (100); exact mass calcd for C35H53
BrO4Si 644.2896, found 644.2896.
-
Cycliza tion to F or m 10. To a solution of 197 mg (0.30
mmol) of the diastereomers of 9 in 15 mL of chloroform at 0
°C was added 37 mg (0.33 mmol) of trifluoroacetic acid
dropwise. The reaction mixture was stirred at 0 °C for 3 h.
Quenching with saturated aqueous NaHCO3 solution was
followed by extraction of the aqueous phase with CH2Cl2 (3 ×
20 mL). The combined organic extracts were washed with
saturated aqueous NaHCO3 solution (1 × 20 mL) and brine
(3 × 20 mL), then dried (MgSO4) and evaporated to give 183
mg of crude product. Purification by flash column chromatog-
raphy on silica gel gave 86 mg of the R-C-9 diastereomer and
83 mg of the â-C-9 diastereomer, both as pale yellow oils (89%
combined yield. R-C-9 diastereomer: Rf ) 0.49 (20% EtOAc
in hexanes); 1H NMR (400 MHz, CDCl3) δ 6.48 (d, J ) 1.3 Hz,
1H), 6.36 (d, J ) 1.3 Hz, 1H), 6.07 (ddt, J ) 17.2, 11.5, 5.1 Hz,
1H), 5.45 (dd, J ) 17.2, 1.5 Hz, 1H), 5.28 (dd, J ) 11.5, 1.5
Hz, 1H), 4.52 (d, J ) 5.1 Hz, 2H), 4.42 (s, 2H), 4.23 (s, 1H),
3.36 (t, J ) 6.9 Hz, 2H), 3.25 (d, J ) 13.9 Hz, 1H), 2.99 (dt, J
) 11.5, 7.8 Hz, 1H), 1.99 (t, J ) 14.7 Hz, 2H), 1.87-1.48 (m,
10 H), 1.39 (s, 3H), 1.37-1.31 (m, 2H), 1.21 (s, 6H), 1.10-1.04
(m, 2H), 0.92 (s, 9H), 0.13 (s, 6H); 13C NMR (CDCl3) δ 215.8,
157.4, 153.3, 149.7, 133.8, 117.2, 114.4, 108.9, 102.8, 86.3, 83.8,
74.8, 69.1, 67.1, 52.0, 48.5, 44.5, 27.9, 37.6, 34.2, 33.4, 29.6,
29.0, 28.8, 28.3, 26.1, 26.0, 24.7, 23.6, 19.8, -4.8; IR (neat)
3433, 2930, 2857, 2109, 1654, 1568, 1462, 1412, 1365, 1328,
1259, 1120, 1089 cm-1. â-C-9 diastereomer: Rf ) 0.39 (20%
Dim eth yl Aceta l 12. To a solution of 55 mg (85 µmol) of
ketone 11 in 3.5 mL of methanol was added 350 µL of trimethyl
orthoformate dropwise at room temperature. To this mixture
was added 5 mg (ca. 8.5 µmol) of ytterbium triflate in a single
portion, and stirring was continued at room temperature for
12 h. The reaction was quenched by addition of saturated
aqueous NaHCO3 solution, and the mixture was diluted with
water and ether. The aqueous layer was extracted with ether
(3 × 5 mL), and the combined organic phase was washed with
saturated aqueous NaHCO3 solution (1 × 5 mL) followed by
brine (3 × 5 mL). Drying (K2CO3), filtration, and evaporation
of the solvent gave the crude product. Purification by flash
column chromatography on silica gel led to 43 mg (73% yield)
of pure acetal 12 as a pale yellow oil: Rf ) 0.48 (10% EtOAc
in hexanes); 1H NMR (CDCl3) δ 6.51 (d, J ) 1.6 Hz, 1H), 6.38
(d, J ) 1.6 Hz, 1H), 6.09 (ddt, J ) 17.0, 10.8, 5.6 Hz, 1H), 5.45
(dd, J ) 17.0, 1.3 Hz, 1H), 5.28 (dd, J ) 10.8, 1.3 Hz, 1H),
4.53 (d, J ) 5.6 Hz, 2H), 4.44 (s, 2H), 3.54-3.47 (m, 1H), 3.39
(t, J ) 6.8 Hz, 2H), 3.29 (s, 3H), 3.26 (s, 3H), 2.64 (dt, J )
11.6, 2.3 Hz, 1H), 2.25-2.10 (m, 2H), 1.92-1.78 (m, 1H), 1.69-
1.64 (m, 2H), 1.58-1.21 (m, 7H), 1.39 (s, 3H), 1.24 (s, 6H),
1.16-1.05 (m, 2H), 0.95-0.88 (m, 2H), 0.94 (s, 9H), 0.09 (s,
6H); 13C NMR (CDCl3) δ 157.5, 152.8, 149.8, 133.7, 118.0,
110.2, 108.9, 102.5, 100.7, 86.4, 83.8, 74.7, 69.4, 52.0, 48.2, 47.9,
44.5, 37.9, 36.5, 33.9, 33.6, 30.6, 29.9, 29.6, 29.0, 28.3, 25.5,
24.7, 19.8, 18.4, -4.8; IR (neat) 2930, 2855, 2109, 1613, 1569,
1459, 1412, 1362, 1250, 1099, 1050 cm-1; mass spectrum (EI)
m/z 692, 690 (M+, 2,3), 660 (3), 619 (9), 496 (7), 262 (10), 183
(16), 149 (30), 69 (100); exact mass calcd for C37H59BrO5Si
690.3315, found 690.3317.
1
EtOAc in hexanes); H NMR (CDCl3) δ 6.48 (d, J ) 1.5 Hz,
1H), 6.37 (d, J ) 1.5 Hz, 1H), 6.07 (ddt, J ) 16.9, 10.8, 5.3 Hz,
1H), 5.40 (dd, J ) 16.9, 1.5 Hz, 1H), 5.28 (dd, J ) 10.8, 1.5
Hz, 1H), 4.48 (d, J ) 5.3 Hz, 2H), 4.41 (s, 2H), 4.12-4.04 (m,
1H), 3.43-3.37 (m, 1H), 3.33 (t, J ) 7.0 Hz, 2H), 2.51 (dt, J )
11.2, 2.4 Hz, 1H), 2.21-2.16 (m, 2H), 1.84-1.47 (m, 10H), 1.34
(s, 3H), 1.40-1.04 (m, 4H), 1.21 (s, 6H), 0.91 (s, 9H), 0.13 (s,
6H); 13C NMR (CDCl3) δ 223.2, 157.5, 153.4, 150.0, 133.8,
117.3, 108.8, 102.7, 86.4, 83.9, 74.7, 71.0, 69.1, 52.0, 47.7, 44.5,
39.6, 37.9, 35.9, 34.2, 33.1, 33.0, 29.6, 29.0, 28.3, 28.1, 27.1,
26.0, 24.7, 19.7, -4.8; IR (neat) 3433, 2930, 2857, 2109, 1654,
1568, 1462, 1412, 1365, 1328, 1259, 1120, 1089 cm-1; mass
spectrum (EI) m/z 648, 646 (M+, 1), 630, 628 (M+ - H2O, 1),
548 (1), 484 (2), 445 (5), 407 (17), 405 (17), 217 (23), 192 (25),
75 (100); exact mass calcd for C35H55BrO4Si 646.3053, found
646.3050.
P h en ol 13. To a solution of 200 mg (0.31 mmol) of dimethyl
acetal 12 in 20 mL of THF at room temperature was added
36 mg (ca. 0.03 mmol) of Pd(PPh3)4 and 117 mg (3.1 mmol) of
sodium borohydride. The reaction mixture was stirred at room
temperature for 14 h and was then quenched by addition of
saturated aqueous KH2PO4 solution. The mixture was neutral-
ized with saturated aqueous NaHCO3 solution, and the aque-
ous layer was extracted with ether (3 × 20 mL). The combined
organic extracts were washed with brine (3 × 20 mL), dried
(K2CO3), filtered, and concentrated to give the crude product.
Purification by flash column chromatography on silica gel gave
158 mg (79% yield) of pure 13 as a pale yellow oil: Rf ) 0.15
C-9 Keton e 11. To a solution of 60 mg (0.09 mmol) of the
R-C-9 diastereoisomer of 10 at 0 °C in 3.6 mL of CH2Cl2 was
60 J . Org. Chem., Vol. 68, No. 1, 2003

Radiolabeling of the Cannabinoid Receptor
1
(10% EtOAc in hexanes); H NMR (CDCl3) δ 6.44 (d, J ) 1.7
(3 × 10 mL), and the combined organic extracts were washed
with saturated aqueous NaHCO3 solution (3 × 15 mL) and
brine (3 × 15 mL) and were then dried (MgSO4). Filtration
and solvent evaporation gave the crude product, which was
purified by flash column chromatography on silica gel to give
34 mg (76% yield) of pure 16 as a colorless oil: Rf ) 0.15 (30%
Hz, 1H), 6.20 (d, J ) 1.5 Hz, 1H), 5.16 (br s, 1H), 4.41 (s, 2H),
3.54 (d, J ) 13.7 Hz, 1H), 3.37 (t, J ) 6.8 Hz, 2H), 3.29 (s,
3H), 3.28 (s, 3H), 2.63 (dt, J ) 10.3, 3.8 Hz, 1H), 2.24-2.07
(m, 2H), 1.90-1.73 (m, 1H), 1.67-1.43 (m, 7H), 1.38 (s, 3H),
1.35-1.04 (m, 6H), 1.19 (s, 6H), 0.91 (s, 9H), 0.13 (s, 6H); 13
C
1
NMR (CDCl3) δ 154.6, 153.4, 150.0, 108.2, 107.9, 105.8, 100.9,
86.1, 83.6, 74.5, 51.8, 47.7, 47.6, 44.2, 37.3, 36.3, 35.4, 34.0,
33.3, 32.8, 30.7, 29.4, 28.7, 28.6, 28.0, 25.8, 25.2, 24.4, 19.6,
18.2, -5.1; IR (neat) 3324, 2931, 2856, 1622, 1576, 1459, 1414,
1363, 1254, 1092, 1054 cm-1; mass spectrum (EI) m/z 620, 618
(M+ - MeOH, 14, 13), 563 (12), 561 (12), 479 (12), 477 (12),
456 (28), 383 (13), 381 (10), 323 (18), 159 (38), 73 (100); exact
mass calcd for C33H51BrO4Si (M+ - MeOH) 618.2741, found
618.2782.
EtOAc in hexanes); H NMR (CDCl3) δ 6.48 (d, J ) 1.7 Hz,
1H), 6.39 (d, J ) 1.7 Hz, 1H), 4.42 (s, 2H), 4.07 (d, J ) 15.1
Hz, 1H), 3.49 (t, J ) 13.2 Hz, 2H), 2.94 (dt, J ) 12.0, 3.0 Hz,
1H), 2.71-2.48 (m, 2H), 2.34 (dt, J ) 11.8, 1.8 Hz, 1H), 2.21
(dt, J ) 13.9, 1.7 Hz, 1H), 1.76-1.50 (m, 6H), 1.45 (s, 3H),
1.39-1.02 (m, 6H), 1.23 (s, 6H); 13C NMR (CDCl3) δ 212.7,
154.8, 152.8, 150.9, 129.5, 107.6, 107.2, 106.2, 86.6, 83.5, 74.3,
51.1, 46.2, 45.0, 44.8, 44.1, 40.8, 37.4, 33.8, 29.8, 29.3, 28.8,
28.6, 27.5, 26.4, 24.3, 19.6; IR (neat) 2933, 2857, 2184, 2104,
Azid e 14. To a solution of 152 mg (0.23 mmol) of bromide
13 in 12.5 mL of CH2Cl2 was added 55 mg (0.35 mmol) of
tetramethylguanidinium azide in a single portion at room
temperature. The reaction mixture was stirred at 50 °C for 3
h and was subsequently cooled to room temperature. The
workup consisted of solvent evaporation followed by filtration
of the residue through a short plug of silica gel eluting with
ether. This removed the tetramethylguanidinium salt and
afforded pure azide 14 (156 mg, 100% yield) as a colorless oil
following solvent evaporation. Azide 14: Rf ) 0.18 (10% EtOAc
in hexanes); 1H NMR (CDCl3) δ 6.46 (d, J ) 1.7 Hz, 1H), 6.23
(d, J ) 1.5 Hz, 1H), 5.29 (br s, 1H), 4.43 (s, 2H), 3.58-3.50
(m, 1H), 3.31 (s, 3H), 3.29 (s, 3H), 3.24 (t, J ) 7.2 Hz, 2H),
2.65 (dt, J ) 11.3, 2.1 Hz, 1H), 2.27-2.09 (m, 2H), 1.89 (dt, J
) 11.7, 2.2 Hz, 1H), 1.66-1.38 (m, 7H), 1.45 (s, 3H), 1.32-
1.06 (m, 6H), 1.27 (s, 6H), 0.94 (s, 9H), 0.15 (s, 6H); 13C NMR
(CDCl3) δ 154.6, 153.4, 150.0, 108.1, 107.9, 105.7, 101.1, 86.0,
83.5, 74.5, 51.8, 47.7, 47.5, 44.2, 37.3, 35.3, 33.3, 30.6, 29.7,
28.7, 28.6, 26.5, 25.8, 25.2, 24.4, 19.6, 18.2, -5.1; IR (neat)
3324, 2932, 2861, 2095, 1623, 1577, 1458, 1414, 1364, 1258,
1702, 1615, 1570, 1462, 1414, 1360, 1259, 1098, 1050 cm-1
;
mass spectrum (EI) m/z 468 (M+ - H, 1), 412 (M+ - SCN, 5),
328 (10), 289 (7), 178 (11), 148 (89), 69 (100); exact mass calcd
for C27H34NO4S (M+ - H) 468.2209, found 468.2564.
Mesyla te 17. To a solution of 7.5 mg (14.5 µmol) of ketodiol
16 at 0 °C in 0.5 mL of CH2Cl2 was added 3.5 µL (2.1 equiv) of
2,6-lutidine followed by methanesulfonyl chloride (0.5 mL of
a 0.35 µM solution in CH2Cl2, 1.2 equiv). The reaction mixture
was stirred at 0 °C for 3 h and then at room temperature for
12 h. Solvent evaporation was followed by flash column
chromatography on silica gel to give ca. 8 mg (93% yield) of
pure mesylate 17 as an oil: Rf ) 0.32 (20% EtOAc in hexanes);
1H NMR (CDCl3) δ 6.45 (d, J ) 1.7 Hz, 1H), 6.38 (d, J ) 1.7
Hz, 1H), 4.98 (s, 2H), 4.06 (d, J ) 14.9 Hz, 1H), 3.49 (t, J )
7.0 Hz, 2H), 3.18 (s, 3H), 2.94 (dt, J ) 12.1, 3.0 Hz, 1H), 2.73-
2.43 (m, 2H), 2.39-2.16 (m, 2H), 1.67-1.51 (m, 6H), 1.46 (s,
3H), 1.39-1.04 (m, 6H), 1.23 (s, 6H); 13C NMR (CDCl3) δ 212.1,
154.8, 152.5, 151.0, 129.5, 107.5, 107.1, 106.4, 90.5, 74.2, 65.8,
60.4, 57.2, 46.0, 45.0, 44.8, 44.1, 40.7, 37.4, 33.7, 29.8, 29.3,
28.7, 28.6, 27.4, 26.3, 19.3; IR (neat) 3279, 2933, 2857, 2184,
2102, 1700, 1684, 1653, 1623, 1576, 1506, 1457, 1414, 1369,
1138, 1093, 1043 cm-1; mass spectrum (EI) m/z 581 (M+
-
1256, 1174, 1095, 1033 cm-1
.
OMe, 1), 553 (2), 540 (7), 494 (8), 456 (8), 393 (11), 303 (15),
277 (15), 239 (10), 159 (30), 75 (100); exact mass calcd for
Bifu n ction a l Ca n n a bin oid . To a solution of 11 mg (ca.
19 µmol) of mesylate 17 in 0.8 mL of anhydrous acetone was
added 12 mg (4 equiv) of NaI in a single portion. The reaction
mixture was heated to reflux for 12 h, then the solvents were
evaporated. The crude product was dissolved in CH2Cl2, and
the inorganic precipitate was filtered off. Concentration of the
filtrate followed by flash column chromatography on silica gel
gave ca. 9.5 mg of the desired product as an oil (79% yield):
C
33H51N3O4Si (M+ - OMe) 581.3651, found 581.3539.
Isoth iocya n a te 15. To a solution of 147 mg (0.24 mmol) of
azide 14 in 10.5 mL of benzene was added 69 mg (0.26 mmol)
of PPh3 in a single portion at room temperature. The reaction
mixture was stirred at 50 °C for 1 h and was subsequently
cooled to room temperature. To the mixture was added
dropwise 37 mg (0.48 mmol) of carbon disulfide and the
mixture stirred at 50 °C for 2 h. The solvent was evaporated
and the crude product purified by flash column chromatogra-
phy on silica gel to provide 140 mg (93% yield) of pure
isothiocyanate 15 as a colorless oil: Rf ) 0.34 (10% EtOAc in
hexanes); 1H NMR (CDCl3) δ 6.41 (d, J ) 1.7 Hz, 1H), 6.20 (d,
J ) 1.7 Hz, 1H), 6.00 (br s, 1H), 4.41 (s, 2H), 3.65 (d, J ) 13.7
Hz, 1H), 3.45 (t, J ) 6.6 Hz, 2H), 3.31 (s, 3H), 3.30 (s, 3H),
2.61 (dt, J ) 11.7, 2.2 Hz, 1H), 2.25-2.09 (m, 2H), 1.85 (dt, J
) 11.5, 2.0 Hz, 1H), 1.65-1.45 (m, 7H), 1.38 (s, 3H), 1.35-
1.04 (m, 6H), 1.18 (s, 6H), 0.91 (s, 9H), 0.13 (s, 6H); 13C NMR
(CDCl3) δ 154.6, 153.4, 149.9, 129.6, 108.3, 108.0, 105.8, 100.8,
86.1, 83.7, 74.6, 51.8, 45.0, 47.74, 47.70, 47.6, 45.0, 44.2, 37.3,
35.5, 33.3, 30.7, 29.8, 29.3, 28.7, 28.6, 26.4, 25.8, 25.2, 24.3,
19.6, 18.2, -5.1; IR (neat) 3324, 2931, 2859, 2184, 2101, 1624,
1578, 1456, 1414, 1360, 1258, 1138, 1093, 1043 cm-1; mass
spectrum (EI) m/z 526 (M+ - OMe, CH2NCS, 8), 412 (3), 378
(4), 184 (20), 105 (22), 75 (100); exact mass calcd for C32H50O4-
Si (M+ - OMe, CH2NCS) 526.2355, found 526.2355.
1
Rf ) 0.28 (10% EtOAc in hexanes); H NMR (CDCl3) δ 6.48
(d, J ) 1.7 Hz, 1H), 6.38 (d, J ) 1.7 Hz, 1H), 4.42 (s, 2H), 4.05
(dd, J ) 13.7, 1.2 Hz, 1H), 3.49 (t, J ) 6.6 Hz, 2H), 2.93 (dt, J
) 11.7, 3.4 Hz, 1H), 2.71-2.47 (m, 2H), 2.37-2.16 (m, 2H),
1.76-1.49 (m, 6H), 1.45 (s, 3H), 1.30-1.03 (m, 6H), 1.23 (s,
6H); 13C NMR (CDCl3) δ 212.2, 154.6, 152.8, 150.9, 129.5,
107.8, 107.3, 106.2, 86.4, 83.5, 74.3, 51.1, 46.2, 45.0, 44.9, 44.1,
40.8, 37.4, 33.8, 29.8, 29.3, 28.7, 28.6, 27.5, 26.3, 24.3, 19.5;
IR (neat) 3354, 2932, 2857, 2184, 2101, 1702, 1621, 1579, 1462,
1415, 1360, 1333, 1259, 1178, 1096, 1034 cm-1; mass spectrum
(EI) m/z 452 (M+ - I, 1), 412 (M+ - I, SCN, 4), 328 (8), 289
(7), 148 (89), 69 (100).
Ack n ow led gm en t. This work was supported by
grants from the National Institutes of Health (DA 3801;
DA7215; DA9158)
Ketod iol 16. To a solution of 55 mg (87 µmol) of 15 in 10.4
mL of THF and 2.0 mL of water was added 110 mg (87 µmol)
of oxalic acid in a single portion at room temperature. The
reaction mixture was stirred at room temperature for 18 h and
was subsequently quenched by addition of saturated aqueous
NaHCO3 solution. The aqueous phase was extracted with ether
Su p p or tin g In for m a tion Ava ila ble: Spectroscopic data
(13C NMR) for 2-6, 8-11, 13-17, and the bifunctional
cannabinoid. This material is available free of charge via the
Internet at http://pubs.acs.org.
J O0264978
J . Org. Chem, Vol. 68, No. 1, 2003 61