Cholesterol surrogates incorporating a benzophenone as part of the sterol tetracycle

Article abstract of DOI:10.1021/jo060480y

Photoactivatable analogues 4-6 of cholesterol (1), having their cross-linking site in the ring D sterol region, have been synthesized starting from bromotetralone 14 via enantioselective Robinson annulation to enone 13 and Suzuki carbonylative coupling to

Full text of DOI:10.1021/jo060480y

Cholesterol Surrogates Incorporating a Benzophenone as Part of the
Sterol Tetracycle
Yonghong Gan and Thomas A. Spencer*
Department of Chemistry, Dartmouth College, HanoVer, New Hampshire 03755
taspen@dartmouth.edu
ReceiVed March 6, 2006
Photoactivatable analogues 4-6 of cholesterol (1), having their cross-linking site in the ring D sterol
region, have been synthesized starting from bromotetralone 14 via enantioselective Robinson annulation
to enone 13 and Suzuki carbonylative coupling to the appropriate phenylboronic acid. Each of 4-6 was
shown to substitute successfully for 1 in an assay of apo A-I-induced cellular cholesterol efflux, indicating
that these analogues equilibrated with 1 in all major cellular pools.
Introduction
For use in studies of cellular cholesterol efflux and HDL
formation,1-3 we have been synthesizing photoactivatable
analogues of cholesterol (1) having benzophenone groups at
different defined positions relative to the sterol structure.
Preparation of eight analogues with the potentially cross-linking
benzophenone carbonyl group carbon atom at either end of the
cholesterol structure in the positions indicated in Figure 1 has
previously been described.4 All of these analogues substituted
successfully for cholesterol in an isotope dilution assay measur-
ing apolipoprotein A-I-induced cholesterol efflux,4 suggesting
an unexpected tolerance of biological membranes regarding the
incorporation of surrogates of 1 having differing structure. In
the preceding paper,5 the synthesis and preliminary biochemical
evaluation of additional analogues lacking the steroidal tetra-
cyclic nucleus is reported. Compounds 2 and 3, exemplifying
this type of analogue with the photo-cross-linking site in the
FIGURE 1. Structure of cholesterol (1) showing the approximate
location of the photo-cross-linking carbonyl carbon atom when each
of the analogues prepared to date is superimposed upon it. The black
circles refer to compounds described in ref 4, the blue circles to
compounds in ref 5, and the green circle to compounds described in
this paper.
ring B region (Figure 1), have also been shown to substitute
successfully for 1 in the isotope dilution assay of cholesterol
efflux.5 These results again suggest that rather large structural
variations can be tolerated in the major cellular cholesterol pools.
In this paper are described the preparation and preliminary
biochemical evaluation of a third type of benzophenone-
containing analogue of 1, having the photophore located in the
ring D region as shown in Figure 1, specifically compounds 4,
5, and 6. Examination of Dreiding molecular models and use
of the Spartan molecular modeling program indicated that both
5 and 6, although having the surrogate ring D alkyl side chain
in different positions on the benzophenone, can approximate
the size and shape of cholesterol. The Spartan molecular
* To whom correspondence should be addressed: Phone: 603-646-2805.
Fax: 603-646-3946. E-mail: taspen@dartmouth.edu.
(1) Fielding, P. E.; Russel, J. S.; Spencer, T. A.; Hakamata, H.; Nagao,
K.; Fielding, C. J. Biochemistry 2002, 41, 4929-4937.
(2) Fielding, P. E.; Chau, P.; Liu, D.; Spencer, T. A.; Fielding, C. J.
Biochemistry 2004, 43, 2578-2586.
(3) Nakamura, Y.; Kotite, L.; Gan, Y.; Spencer, T. A.; Fielding, C. J.;
Fielding, P. E. Biochemistry 2004, 43, 14811-14820.
(4) Spencer, T. A.; Wang, P.; Li, D.; Russel, J. S.; Blank, D. H.;
Huuskonen, J.; Fielding, P. E.; Fielding, C. J. J. Lipid Res. 2004, 45, 1510-
1518.
(5) Gan, Y.; Blank, D. H.; Ney, J. E.; Spencer, T. A. J. Org. Chem.
2006, 8, 5864-5869.
10.1021/jo060480y CCC: $33.50 © 2006 American Chemical Society
Published on Web 07/08/2006
5870
J. Org. Chem. 2006, 71, 5870-5875

Cholesterol Surrogates Incorporating a Benzophenone
FIGURE 2. Spartan molecular modeling structures of cholesterol (1), analogue 6, and 1 superimposed on 6 in top and side views. The structure
of 1 was energy minimized and that of 6 was conformationally manipulated for most effective overlap using Dreiding models as a guide.
SCHEME 1
modeling structures of 1 and 6 shown in Figure 2 demonstrate
that 6 can overlap 1 very well, with the sole exception of a
protruding portion of the distal benzene ring of 6.
In the present work, these compounds were prepared much more
expeditiously, as shown in Scheme 2, by Negishi coupling10,11
of iodoesters 15 and 16 with the commercially available
organozinc reagents 17 and 18 to obtain esters 19 and 20, which
were saponified to afford 10 and 11 in 89 and 90% overall yield,
respectively.
The starting material for synthesis of the tricyclic boronic
acid 9, to be coupled with suitable derivatives of benzoic acid
or substituted benzoic acids 10 or 11, was bromotetralone 14,6
which was prepared in 71% yield from p-bromophenylacetic
Results and Discussion
Synthesis. The strategy adopted for preparation of 4-6
envisioned coupling of a chiral tricyclic intermediate 7 with an
appropriately substituted benzene to forge the benzophenone
moiety near the end of the synthesis (Scheme 1). The first
approach undertaken experimentally toward realization of this
strategy involved Suzuki coupling of boronic acid 9 (7, X )
B(OH)2) with anhydride derivatives of benzoic acid (8) or
substituted benzoic acids 10 or 11. The key tricyclic intermediate
9 would be secured via protected allylic alcohol 12, obtained
by standard procedures from chiral enone 13, in turn prepared
from known bromotetralone 146 by methylation and enantiose-
lective Robinson annulation.
(6) Tschaen, D. M.; Abramson, L.; Cai, D.; Desmond, R.; Dolling, U.-
H.; Frey, L.; Karady, S.; Shi, Y.-J.; Verhoeven, T. R. J. Org. Chem. 1995,
60, 4324-4330.
(7) Agoulnik, S.; Akasaka, K.; Fang, F.; Harmange, J.-C.; Hawkins, L.;
Jiang, Y.; Johannes, C.; Li, X.-Y.; Mcguinness, P.; Murphy, E.; Schiller,
S.; Vermeulen, M.; Wu, J. Patent Appl. WO 2003-US390 20030108, 2003.
(8) Harvey, W. E. J. Chem. Soc. 1958, 2060-2062.
(9) Andrews, E. D.; Harvey, W. E. J. Chem. Soc. 1961, 4687-4689.
(10) Baba, S.; Negishi, E. J. Am. Chem. Soc. 1976, 98, 6729-6731.
(11) Negishi, E. Acc. Chem. Res. 1982, 15, 340-348.
The requisite substituted benzoic acids 107 and 118,9 have
each been prepared before by routes requiring at least five steps.
J. Org. Chem, Vol. 71, No. 16, 2006 5871

Gan and Spencer
SCHEME 2
anhydride, followed by deprotection, afforded target analogue
4 in a modest but acceptable 41% yield. However, when 25
was coupled with m-isobutylbenzoic acid (10) by the mixed
anhydride method employing pivaloyl anhydride,21 only 18%
of 5 could be obtained after deprotection. With o-isopentylben-
zoic acid (11), only a trace of 6 could be obtained by this
approach, presumably as a result of increased steric hindrance
to coupling across the series benzoic acid, 10, and 11.
To overcome this problem, attention was turned to reversing
the functional groups of the Suzuki coupling partners by using
boronic acids 2622 and 27 (Scheme 4) to attach the second
aromatic ring bearing an alkyl side chain. Compounds 2622 and
27 were synthesized from 3-bromo- and 2-bromobenzaldehyde,
respectively, via Wittig reactions to afford alkenes 2823 (78%
yield) and 2924 (90% yield of a 3:4 E:Z mixture), which, after
nearly quantitative diimide reduction using tosylhydrazide25 to
afford 3022,26 and 31,27 were converted20 to 26 and 27 in 82
and 53% yield, respectively. For coupling with 26 and 27,
tricyclic bromide was converted in 92% yield by the procedure
of Klapars and Buchwald28 to the more reactive iodide 32, which
was combined with 26 and 27 by the carbonylative Suzuki
coupling procedure29,30 to give 31% of 5 and 57% of 6,
respectively, after fluoride ion deprotection. Compound 4 was
also prepared from 32 and phenylboronic acid by this procedure
in the same 41% yield obtained via 25. Although these yields
of the target analogues remain relatively modest, more than
ample material is readily available through this approach.
acid by the interesting Friedel-Crafts method of Burckhalter
and Campbell12 (Scheme 3). Methylation of 14 by the Stork
enamine method13 gave 74% of 21, which, it was hoped, could
be converted to chiral enone 13 with high enantioselectivity by
use of an appropriate chiral amine catalyst to effect Robinson
annulation. The simple enantiomeric phenylethylamines, used
by d’Angelo and co-workers14 to effect enantioselective Michael
additions, seemed an excellent choice.
When 21 was treated with R-(+)-1-phenylethylamine, the
enantiomer predicted to give 13 selectively, then with methyl
vinyl ketone, the major product obtained was crystalline bridged
ketol 22 in 57% yield, analogous to results of d’Angelo15,16 and
of Morgan et al.17 The stereochemistry at the hydroxyl-bearing
carbon atom of 22 was assigned by NOE experiments, particu-
larly observation of an interaction between the methyl group at
that site and the benzylic proton shown in 22. Isolation of the
readily purified 22 ensured that, after conversion to 13 in 94%
Preliminary Biochemical Evaluation. The synthesized
analogues 4, 5, and 6 were evaluated as cholesterol surrogates
in the same isotope dilution assay used previously4 and in the
preceding paper.5 The results shown in Figure 3 indicate that
this third type of analogue also can replace 1 in all major cellular
pools. The tolerance for structural variation in successful
cholesterol surrogates discussed in the preceding paper5 thus
extends to compounds 4-6 with the photo-cross-linking site in
the ring D region. The ability of 4 to replace 1 is perhaps
unexpected in view of reports31,32 that sterols with severely
truncated side chains do not promote lipid raft formation in
model membranes. Analogues of all the types represented in
Figure 1, possessing photophoric sites within and beyond each
end of the cholesterol structure, have now been prepared and
have met a demanding initial criterion as cholesterol surrogates.
In future studies, we hope to be able to utilize the range of
yield by treatment with acid,15-17 we had essentially enantiopure
25
tricyclic enone. The positive optical rotation of 13 ([R]D
)
+117°) is consistent with those of similar tricyclic enones having
angular methyl groups.15,16,18
Completion of the synthesis of a tricyclic boronic acid 9 ready
for formation of the benzophenone moiety (Scheme 3) began
with a standard deconjugation-reduction sequence consisting
in conversion of 13 to its dienol acetate derivative, followed
by treatment with ethanolic NaBH4 to give 23 in 88% overall
yield, essentially according to the procedures of Joannou and
Reeder.19 After protection of 23 as its tert-butyldiphenylsilyl
ether 24 in 82% yield, successive lithiation and treatment with
triisopropoxyborane by the procedure of Brown and Cole20
afforded the desired 25. Suzuki coupling21 of 25 with benzoic
(22) Previously prepared by a different route: Murugesan, N.; Hunt, J.
T. Eur. Pat. Appl. EP 569193 A1 19931110, 1993; Murugesan, N.; Hunt,
J. T.; Stein, P. D. U.S. Patent 5514696 A 19960507, 1996.
(23) Biller, S. A.; Magnin, D. R. U.S. Patent 5157027 A 19921020, 1992.
(24) What appeaers to be the E isomer of 29 has been previously prepared
by a different route: Atmaram, S.; Forrester, A. R.; Gill, M.; Napier, R. J.;
Thomson, R. H. Acta Chem. Scand. B 1982, 36, 641-647.
(25) Harrowven, D. C.; Sutton, B. J.; Coulton, S. Tetrahedron 2002, 58,
3387-3400.
(26) Previously prepared by a different route: Okada, S.; Sawada, K.;
Kayakiri, N.; Saitoh, Y.; Tanaka, H.; Hashimoto, M. Eur. Pat. Appl. EP
458207 A2 19911127, 1991.
(27) Previously prepared by a different route: Reich, H. J.; Goldenberg,
W. S.; Gudmundsson, B. O.; Sanders, A. W.; Kulicke, K. J.; Simon, K.;
Guzei, I. A. J. Am. Chem. Soc. 2001, 123, 8067-8079.
(28) Klapars, A.; Buchwald, S. L. J. Am. Chem. Soc. 2002, 124, 14844-
14845.
(12) Burckhalter, J. H.; Campbell, J. R. J. Org. Chem. 1961, 26, 4232-
4235.
(13) Stork, G.; Brrizzolara, A.; Landesman, H.; Szmuszkovicz, J.; Terrell,
R. J. Am. Chem. Soc. 1963, 85, 207-222.
(14) Pfau, M.; Revial, G.; Guingant, A.; d’Angelo, J. J. Am. Chem. Soc.
1985, 107, 273-274.
(15) Volpe, T.; Revial, G.; Pfau, M.; d’Angelo, J. Tetrahedron Lett. 1987,
28, 2367-2370.
(16) d’Angelo, J.; Revial, G.; Volpe, T.; Pfau, M. Tetrahedron Lett. 1988,
29, 4427-4430.
(17) Morgan, B. P.; Swick, A. G.; Hargrove, D. M.; LaFlamme, J. A.;
Moynihan, M. S.; Carroll, R. S.; Martin, K. A.; Lee, E.; Decosta, D.;
Bordner, J. J. Med. Chem. 2002, 45, 2417-2424.
(18) Nerinckx, W.; Vandewalle, M. Tetrahedron: Asymmetry 1990, 1,
265-274.
(29) Ishiyama, T.; Kizaki, H.; Hayashi, T.; Suzuki, A.; Miyaura, N. J.
Org. Chem. 1998, 63, 4726-4731.
(19) Joannou, G. E.; Reeder, A. Y. Steroids 1996, 61, 18-21.
(20) Brown, H. C.; Cole, T. E. Organometallics 1983, 2, 1316-1319.
(21) Goossen, L. J.; Ghosh, K. Angew. Chem., Int. Ed. 2001, 40, 3458-
3460.
(30) Miyaura, N.; Suzuki, A. Chem. ReV. 1995, 95, 2457-2483.
(31) Xu, X.; Bittman, R.; Duportail, G.; Heissler, D.; Vilcheze, C.;
London, E. J. Biol. Chem. 2001, 276, 33540-33546.
(32) Wenz, J. J.; Barrantes, F. J. Biochemistry 2003, 42, 14267-14276.
5872 J. Org. Chem., Vol. 71, No. 16, 2006

Cholesterol Surrogates Incorporating a Benzophenone
SCHEME 3 a
a Reagents and conditions: (a) pyrrolidine, PhH, ∆; CH3I, dioxane, ∆; (b) R-(+)-1-phenylethylamine, PhCH3, ∆; H2CCHCOCH3, THF, rt, 2 days; (c)
p-TsOH, PhCH3, ∆, 4 h; (d) Ac2O, EtOAc, 70% HClO4, rt, 10 min; (e) NaBH4, EtOH, 0 °C, overnight; (f) TBDPSCl, imidazole, DMF, 80 °C, 4 h; (g)
n-BuLi, THF, -78 °C, 30 min; B(OiPr)3, to rt, 3 h; (h) Pd(OAc)2, PPh3, THF; (i) (PhCO)2, ∼2 equiv of H2O, ∆, overnight; (j) 10, [(CH3)3CO]2O, ∼2 equiv
of H2O, ∆, overnight; (k) 11, [(CH3)3CO]2O, ∼2 equiv of H2O, ∆, overnight; (l) TBAF, THF, rt, 4 h.
SCHEME 4 a
a Reagents and conditions: (a) TsNHNH2, NaOAc, THF, ∆, overnight; (b) n-BuLi, THF, -78 °C, 40 min; B(OiPr)3, -78 °C, 2 h, rt, overnight; (c) CuI,
NaI, D,L-dimethylcyclohexane-1,2-diamine, dioxane, 100 °C, 24 h; (d) Pd(dppf)Cl2, CO, K2CO3, 80 °C, overnight, anisole; (e) TBAF, THF, rt, 4 h.
Experimental Section
3-Isobutylbenzoic Acid (10). To a mixture of 1.50 g (5.43 mmol)
of ethyl 3-iodobenzoate (15), prepared by Fischer esterification of
3-iodobenzoic acid, and 130 mg (0.16 mmol) of Pd(dppf)Cl2 in 5
mL of THF was added 25 mL of 0.5 M isobutylzinc bromide (17)
in THF via syringe. This mixture was stirred at rt for 6 h, diluted
with 5 mL of NH4Cl solution, and extracted with ethyl acetate.
The combined organic layers were washed with brine, dried, filtered,
and evaporated to give 1.21 g of crude 19 as a brown oil which
was dissolved in 5 mL of ethanol and treated with a solution of 2
g of NaOH in 5 mL of water. The resulting mixture was stirred
overnight, acidified by concentrated HCl to pH ) 1, and extracted
with ethyl acetate. The combined organic layers were washed with
brine, dried, filtered, and evaporated to give 1.09 g of brown oil
which was chromatographed with 30:1 hexane:ethyl acetate to give
857 mg (89%) of colorless oily 10: 1H NMR (300 MHz) δ 8.02-
7.96 (m, 2H), 7.45-7.42 (m, 2H), 2.59 (d, J ) 7.2 Hz, 2H), 2.02-
1.88 (m, 1H), 0.96 (d, J ) 6.6 Hz, 6H); 13C NMR (75 MHz) δ
FIGURE 3. The dilution of [3H]cholesterol ([3H]1) label in fibroblast
173.0, 142.4, 135.0, 131.0, 129.4, 128.5, 128.0, 45.4, 30.5, 22.5.
monolayers by 4, 5, or 6. The dilution ratio is the reduction in [3H]1
Anal. Calcd for C11H14O2: C, 74.13; H, 7.92. Found: C, 74.28; H,
efflux, induced by apolipoprotein A-I, to the cellular medium after
8.08.
fibroblast monolayers were equilibrated (48 h, 37 °C) with 10 µCi [3H]1
2-Isopentylbenzoic Acid (11). As in the preparation of 10, 1.46
plus unlabeled 1 or analogue equal to the total sterol content of cells
g (5.29 mmol) of ethyl 2-iodobenzoate (16) prepared by Fischer
and medium compared with cells labeled with the same level of tracer
esterification of 2-iodobenzoic acid was treated with 3-methylbu-
[3H]1 only. Complete equilibration between sterol pools is indicated
by a dilution ratio of 0.5, which is shown as the calculated dilution
ratio for 1. Values shown represent means (1 standard deviation of
three independent experiments, each including triplicate dishes of
fibroblasts incubated as described in detail in ref 4 and in the Supporting
Information for ref 5.
tylzinc bromide (18) to give 1.49 g of crude 20 which in turn gave
1
943 mg (93%) of 11: mp 47-48 °C (lit.8 45.5-46 °C); H NMR
(300 MHz) δ 8.09-8.06 (m, 1H), 7.53-7.48 (m, 1H), 7.34-7.28
(m, 2H), 3.09-3.03 (m, 2H), 1.77-1.64 (m, 1H), 1.58-1.50 (m,
2H), 1.00 (d, J ) 6.3 Hz, 6H); 13C NMR (75 MHz) δ 174.2, 146.7,
133.2, 132.1, 131.6, 128.4, 126.1, 41.6, 33.1, 28.7, 22.8. Anal. Calcd
for C12H16O2: C, 74.97; H, 8.39. Found: C, 74.83; H, 8.43.
10-Hydroxy-5-bromo-1,10-dimethyltricyclo[7.3.1.0.2,7]trideca-
2(7),3,5-trien-13-one (22). To a solution of 11.02 g (46.3 mmol)
cross-linking locations that these analogues present to elucidate
lipid-lipid and lipid-protein interactions in membranes of
living cells.
J. Org. Chem, Vol. 71, No. 16, 2006 5873

Gan and Spencer
of 21 in 100 mL of toluene was added 5.60 g (5.8 mL, 46.3 mmol)
of (R)-(+)-phenylethylamine. The mixture was attached to a Dean-
Stark trap and heated at reflux until no further separation of H2O
was detected. The solvent was evaporated, and the residue was
dissolved in 100 mL of THF and treated with 3.53 g (4.2 mL, 50.4
mmol) of freshly distilled methyl vinyl ketone. The resulting
mixture was stirred at rt for 2 days, diluted with 30 mL of H2O
and 5 mL of HOAc, and extracted with ethyl acetate. The combined
organic layers were washed with brine, dried, filtered, and
evaporated to give 12.83 g of brown oil which was chromatographed
with 10:1 to 2:1 hexane:ethyl acetate to give 8.13 g (57%) of 22:
mp 171-173 °C. In another preparation, the crude product was
washed with 3:1:1 hexane:ether:CH2Cl2 to give 50% of 22 without
chromatography. Recrystallization from ethyl acetate gave 22: mp
179-181 °C; 1H NMR δ 7.40-7.38 (m, 1H), 7.28 (br s, 1H), 7.16-
7.15 (m, 1H), 3.32 (dd, J ) 18.0, 7.5 Hz, 1H), 3.18 (d, J ) 18.0
Hz, 1H), 2.59 (d, J ) 7.5 Hz, 1H), 2.20-2.14 (m, 1H), 1.85 (br s,
1H), 1.64-1.60 (m, 1H), 1.51-1.48 (m, 2H), 1.47 (s, 3H), 1.39
(s, 3H); 13C NMR δ 213.7, 141.9, 136.3, 130.6, 130.3, 127.4, 120.8,
78.9, 57.7, 48.7, 40.7, 34.7, 32.5, 28.3, 19.9. Anal. Calcd for C15H17-
BrO2: C, 58.27; H, 5.54. Found: C, 58.27; H, 5.45.
in 25 mL of THF were added 7 mg (0.03 mmol) of Pd(OAc)2, 18
mg (0.068 mmol) of PPh3, 44 µL (2.4 mmol) of H2O, and 443 mg
(1.17 mmol) of benzoic anhydride. This mixture was heated at reflux
overnight, then passed through a pad of Celite, and washed with
ether. The filtrate was collected and concentrated to give 313 mg
of crude product, which was dissolved in 10 mL of THF. Then 2.5
mL of 1.0 M TBAF in THF was added, and the resulting mixture
was stirred at rt for 4 h, quenched by adding 20 mL of saturated
NH4Cl solution, and extracted with ether. The combined organic
layers were dried, filtered, and evaporated to give 312 mg crude
product which was chromatographed with 1:1 hexane:ether to give
125 mg (41%) of 4 as a colorless oil: 1H NMR δ 7.83-7.81 (m,
2H), 7.65-7.58 (m, 3H), 7.51-7.43 (m, 3H), 5.65 (br s, 1H), 3.66-
3.60 (m, 1H), 3.51-3.38 (m, 2H), 2.54-2.50 (m, 1H), 2.46-2.41
(m, 1H), 2.26-2.22 (m, 1H), 2.18 (br s, 1H), 2.06-2.02 (m, 1H),
1.85-1.76 (m, 1H), 1.65-1.59 (m, 1H), 1.44 (s, 3H); 13C NMR δ
196.9, 149.2, 138.1, 138.1, 135.2, 132.9, 132.6, 130.5, 130.3, 128.5,
128.4, 126.4, 119.2, 71.2, 42.3, 38.1, 37.9, 32.3, 30.1, 27.0. Anal.
Calcd for C22H22O2: C, 82.99; H, 6.96. Found: C, 82.88, H, 6.90.
((4bS,7S)-7-Hydroxy-4b-methyl-4b,5,6,7,8,10-hexahydrophenan-
thren-2-yl)(3-isobutylphenyl)methanone (5) via 25. As in the
preparation of 4 via 25, 447 mg (0.84 mmol) of 24 was converted
to 501 mg of crude 25. According to the method of Goossen and
Ghosh,21 to this crude 25 were added a solution of 150 mg (0.84
mmol) of 10 in 3 mL of THF, 10 mg (0.04 mmol) of Pd(OAc)2,
25 mg (0.09 mmol) of PPh3, 234 mg (1.26 mmol) of trimethyl acetic
anhydride, and 37 µL (2.1 mmol) of H2O. This mixture was heated
at reflux overnight, then passed through a pad of Celite, and washed
with ether. The filtrate was collected and concentrated to give 251
mg of crude product, which was dissolved in 10 mL of THF and
deprotected as in the preparation of 4 from 25 to give 224 mg of
crude product which was purified by preparative TLC on a 20 cm
× 20 cm Analtech TLC plate with 1:1 hexane:ether to give 57 mg
(18%) of colorless oily 5: 1H NMR δ 7.66-7.62 (m, 3H), 7.60
(br s, 1H), 7.57-7.38 (m, 3H), 5.66 (br s, 1H), 3.67-3.61 (m, 1H),
3.50-3.38 (m, 2H), 2.57-2.51 (m, 3H), 2.46-2.41 (m, 1H), 2.27-
2.23 (m, 1H), 2.06-2.02 (m, 1H), 1.95-1.87 (m, 2H), 1.85-1.77
(m, 1H), 1.67-1.60 (m, 1H), 1.45 (s, 3H), 0.94 (d, J ) 6.5 Hz,
6H); 13C NMR δ 197.1, 149.0, 142.0, 138.1, 137.9, 135.4, 133.4,
132.8, 130.8, 130.5, 128.3, 128.2, 127.8, 126.3, 119.1, 71.2, 45.4,
42.2, 38.0, 37.8, 32.3, 30.5, 30.1, 26.9, 22.5. HRMS m/z: calcd
for C26H30O2, 374.2246; found 374.2239.
(E,Z)-1-Bromo-2-(3-methyl-1-butenyl)benzene (29). To a mix-
ture of 14.5 g (36.7 mmol) of isobutyltriphenylphosphonium
bromide in 50 mL of THF was added 25 mL of 1.6 M n-BuLi in
hexane. This mixture was stirred at rt for 2 h, and a solution of
5.66 g (30.6 mmol) of 2-bromobenzaldehyde in 20 mL of THF
was added dropwise via syringe. The resulting mixture was stirred
overnight, quenched with 50 mL of saturated NH4Cl solution and
extracted with hexane. The combined organic layers were washed
with brine, dried, filtered, and evaporated to a volume of ca. 5 mL.
The precipitate of Ph3PO was removed by filtration and washed
with hexane. The filtrates were evaporated, and the 7.14 g of crude
product was chromatographed with hexane to give 6.14 g (90%)
of colorless oily 29 as a 3:4 mixture of E and Z isomers: 1H NMR
δ 7.63-7.61 (m, 1H), 7.58-7.53 (m, 2H), 7.33-7.26 (m, 3H),
7.16-7.08 (m, 2H), 6.74 (d, J ) 16.5 Hz, 1H, E), 6.37 (d, J )
11.0 Hz, 1H, Z), 6.21-6.17 (m, 1H, E), 5.65-5.60 (m, 1H, Z),
2.74-2.67 (m, 1H, Z), 2.61-2.54 (m, 1H, E), 1.17 (d, J ) 6.5 Hz,
6H, E), 1.07 (d, J ) 7.5 Hz, 6H, Z); (lit.24 1H NMR δ 7.5 (dd, J )
2 Hz, 2H), 7.23 (t, J ) 2 Hz, 1H), 7.04 (t, J ) 2 Hz, 1H), 6.68 (d,
J ) 5 Hz, 1H), 6.12 (dd, J ) 2, 5 Hz, 1H), 2.5 (oct, J ) 2 Hz,
1H), 1.08 (d, J ) 2 Hz, 6H)); 13C NMR δ 141.4, 141.2, 138.3,
138.0, 133.1, 132.8, 130.7, 128.4, 128.4, 127.6, 127.2, 127.1, 126.4,
126.2, 124.3, 123.6, 32.0, 27.5, 23.3, 22.6. Anal. Calcd for C11H13-
Br: C, 58.69; H, 5.82. Found: C, 58.40; H, 5.82.
7-Bromo-4-methyl-4,4,9,10-tetrahydro-3H-phenanthren-2-
one (13). To a solution of 113 mg (0.37 mol) of 22 in 5 mL of
toluene was added 4 mg of p-toluenesulfonic acid. The mixture
was heated at reflux for 4 h, diluted with 3 mL of saturated NaHCO3
solution, and extracted with ether. The combined organic layers
were washed with brine, dried, filtered, and evaporated to give 121
mg of yellow oil which was chromatographed with 4:1 hexane:
ether to give 101 mg (94%) of 13 as a colorless oil: 1H NMR δ
7.37-7.35 (m, 1H), 7.28-7.27 (m, 1H), 7.19-7.17 (m, 1H), 5.91
(br s, 1H), 3.02-2.97 (m, 1H), 2.91-2.85 (m, 1H), 2.76-2.65 (m,
2H), 2.57-2.50 (m, 2H), 2.38-2.33 (m, 1H), 2.09-2.02 (m, 1H),
1.56 (s, 3H); 13C NMR δ 198.8, 168.8, 143.0, 137.3, 131.5, 130.3,
25
128.2, 124.7, 120.1, 39.2, 37.0, 34.9, 31.0, 31.0, 27.8; [R]D
)
117° (c 2.95, CHCl3). Anal. Calcd for C15H15BrO: C, 61.87; H,
5.19. Found: C, 62.09; H, 5.37.
7-Bromo-4a-methyl-1,2,3,4,4a,9-hexahydrophenanthren-2-
ol (23). To a solution of 6.01 g (20.7 mmol) of 13 in 300 mL of
EtOAc and 40 mL of Ac2O was added a solution of 0.2 mL of
70% HClO4 in 250 mL of EtOAc. The mixture was stirred at rt for
10 min and diluted with 200 mL of saturated NaHCO3 water
solution. The organic layer was washed repeatedly with NaHCO3
solution and then once with brine, dried, filtered, and evaporated
to give crude yellow oily dienol acetate which was dissolved in
100 mL of ethanol, treated with 6.24 g (160 mmol) of NaBH4,
stirred at 0 °C overnight, and diluted with 10 mL of HOAc. The
ethanol was evaporated, and the residue was partitioned between
50 mL of H2O and 50 mL of ethyl acetate. The organic layers were
washed with brine, dried, filtered, and evaporated to give 7.6 g of
yellow oil which was chromatographed with 3:1 hexane:ethyl
acetate to give 5.3 g (88%) of colorless oily 23: 1H NMR δ 7.33-
7.19 (m, 3H), 5.61-5.60 (m, 1H), 3.65-3.58 (m, 1H), 3.44-3.30
(m, 2H), 2.52-2.48 (m, 1H), 2.43-2.38 (m, 1H), 2.18-2.14 (m,
1H), 2.03-1.97 (m, 2H), 1.81-1.73 (m, 1H), 1.59-1.53 (m, 1H),
1.38 (s, 3H); 13C NMR δ 143.2, 138.2, 135.0, 131.0, 129.6, 128.2,
119.5, 118.8, 71.2, 42.1, 38.1, 37.3, 32.2, 29.8, 26.9. Anal. Calcd
for C15H17BrO: C, 61.45; H, 5.84. Found: C, 61.19; H, 5.98.
((4bS,7S)-7-Hydroxy-4b-methyl-4b,5,6,7,8,10-hexahydrophenan-
thren-2-yl)(phenyl)methanone) (4) via Boronic Acid 25. To a
solution of 1.17 g (2.2 mmol) of compound 24 in 10 mL of THF
was added 2.7 mL of 1.0 M n-BuLi in hexane at -78 °C. After 30
min, 413 mg (2.2 mmol) of B(O-i-Pr)3 was added dropwise. The
resulting mixture was stirred for 1 h at -78 °C, stirred at rt for
another 2 h, hydrolyzed by adding 25 mL of H2O containing 0.1
mL of concentrated H2SO4, and quickly extracted twice with ether.
The combined organic layers were washed with brine, dried, filtered,
and evaporated to give 1.00 g of crude 25 as a colorless oil which
was used without purification. According to the method of Goossen
and Ghosh,21 to a solution of 484 mg (0.98 mmol) of this crude 25
t-Butyl((2S,4aS)-7-iodo-4a-methyl-1,2,3,4,4a,9-hexahydro-
phenanthren-2-yloxy)diphenylsilane (32). According to a proce-
dure of Klapars and Buchwald,28 to a solution of 480 mg (0.90
5874 J. Org. Chem., Vol. 71, No. 16, 2006

Cholesterol Surrogates Incorporating a Benzophenone
mmol) of 24 in 1.5 mL of dioxane were added 85 mg (0.45 mmol)
of CuI, 337 mg (2.25 mmol) of NaI, and 0.14 mL (0.9 mmol) of
D,L-dimethylcyclohexane-1,2-diamine. The flask was purged with
N2, sealed with a septum, and heated at 100 °C for 24 h. The cooled
reaction mixture was treated with 4 mL of concentrated NH4OH
solution and extracted with ether. The organic layer was washed
with brine, dried, filtered, and evaporated to give 843 mg of a brown
oil which was chromatographed to give 482 mg (92%) of colorless
solid 32. An analytical sample was obtained by recrystallization
from hexane: mp 131-133.5 °C; 1H NMR δ 7.72-7.69 (m, 4H),
7.47-7.37 (m, 8H), 7.02-7.00 (m, 1H), 5.34 (br s, 1H), 3.62-
3.56 (m, 1H), 3.34-3.17 (m, 2H), 2.50-2.45 (m, 1H), 2.32-2.28
(m, 1H), 2.04-2.00 (m, 1H), 1.87-1.83 (m, 2H), 1.36 (m, 1H),
1.36 (s, 3H), 1.10 (m, 9H); 13C NMR δ 144.2, 138.6, 137.1, 136.0,
136.0, 135.4, 135.4, 134.8, 134.8, 129.8, 129.8, 128.4, 127.8, 127.8,
118.3, 91.0, 72.8, 42.5, 38.2, 37.4, 32.6, 29.6, 27.2, 27.0, 19.4. Anal.
Calcd for C31H35ISiO: C, 64.35; H, 6.1 Found: C, 64.31; H, 6.11.
Compound 4 via 32. According to the procedure of Miyaura et
al.,29 to a solution of 603 mg (1.13 mmol) of 32 in 6 mL of anisole
were added 55 mg (0.07 mmol) of Pd(dppf)Cl2, 152 mg (1.24
mmol) of phenylboronic acid, 467 mg (3.39 mmol) of K2CO3, and
850 mg (5.65 mmol) of NaI. The mixture was heated under CO at
80 °C overnight, then passed through a pad of Celite, which was
washed with 50 mL of ether. The filtrate was evaporated to give
6.02 g of brown oil, which was dissolved in 10 mL of THF, and
2.5 mL of TBAF 1.0 M solution in THF was added. The resulting
mixture was stirred at rt for 4 h, quenched with 20 mL of saturated
NH4Cl solution, and extracted with ether. The combined organic
layers were washed with brine, dried, filtered, and evaporated to
give 312 mg of oil which was chromatographed with 1:1 hexane:
mmol) of B(O-iPr)3 dropwise via syringe. The resulting mixture
was stirred at -78 °C for 2 h, allowed to stand at rt overnight,
quenched with 20 mL of 1 N HCl, and extracted with ether. The
combined organic layers were washed twice with brine, dried,
filtered, and evaporated to give 901 mg of yellow oil which was
chromatographed with 3:2 hexane:ether to give 420 mg (53%) of
white solid 27: 1H NMR δ 8.20-8.19 (m, 1H), 7.51-7.48 (m,
1H), 7.32-7.28 (m, 2H), 3.22-3.18 (t, J ) 8.0 Hz, 2H), 1.71-
1.65 (m, 1H), 1.62-1.57 (m, 2H), 0.95 (d, J ) 7.0 Hz, 6H); 13C
NMR δ 151.8, 137.5, 132.3, 129.9, 125.3, 42.8, 33.9, 28.4, 23.0.
Upon prolonged standing or drying in vacuo, 27 underwent
transformation to 2,4,6-tri-2-isopentylphenylboroxin: mp 87.5-
89 °C. HRMS m/z: calcd for C33H45O3B3, 522.3648; found,
522.3641. As in the preparation of 4 via 32, 208 mg (0.36 mmol)
of 32 and 76 mg (0.40 mol) of freshly prepared 27 gave 231 mg of
dark oil which was chromatographed with 1:1 hexane:ether to give
81 mg (57%) of 6 as a colorless oil: 1H NMR δ 7.63-7.61 (m,
1H), 7.55 (m, 1H), 7.44-7.40 (m, 2H), 7.34-7.23 (m, 3H), 5.65
(br s, 1H), 3.67-3.60 (m, 1H), 3.48-3.36 (m, 2H), 2.66-2.63 (m,
2H), 2.54-2.50 (m, 1H), 2.46-2.40 (m, 1H), 2.25-2.21 (m, 1H),
2.05-2.00 (m, 1H), 1.84-1.76 (m, 1H), 1.68 (br s, 1H), 1.65-
1.59 (m, 1H), 1.53-1.40 (m, 3H), 1.43 (s, 3H), 0.82 (d, J ) 6.5
Hz, 6H); 13C NMR δ 198.8, 149.8, 142.1, 138.9, 137.9, 135.7,
133.0, 130.5, 130.3, 130.2, 128.6, 128.3, 126.5, 125.3, 119.2, 71.2,
42.2, 41.2, 38.0, 37.9, 32.3, 31.4, 30.1, 28.2, 26.9, 22.6, 22.6. Anal.
Calcd for C27H32O2: C, 83.46; H, 8.3 Found: C, 83.38; H, 8.37.
Acknowledgment. We thank Drs. C. J. Fielding and P. E.
Fielding of the University of California, San Francisco, for
generously performing the biochemical assays described herein.
Dr. Pingzhen Wang kindly provided some final experimental
details. This research was supported by National Institutes of
Health Grant HL-67294.
ether to give 125 mg (41%) of 4 as a colorless oil with 1H and 13
NMR spectra identical to those of 4 prepared via 25.
C
Compound 5 via 32. As in the preparation of 4 via 32, 454 mg
(0.85 mmol) of 32 and 152 mg (0.85 mmol) of freshly prepared 26
gave 312 mg of oil which was chromatographed with 1:1 hexane:
Supporting Information Available: General experimental
methods, preparations of compounds 14, 21, 24, 28, 30, 26, and
31, 1H and 13C NMR spectra for all compounds, NOE experiments
on compound 22, atom coordinates for molecular modeling of
compounds 1, 6, and 22. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
ether to give 91 mg (31%) of 5 as a colorless oil with H and 13C
NMR spectra identical to those of 5 prepared via 25.
((4bS,7S)-7-Hydroxy-4b-methyl-4b,5,6,7,8,10-hexahydrophenan-
thren-2-yl)(2-isopentylphenyl)methanone (6). 2-Isopentylphenyl-
boronic acid (27) was prepared by adding 3 mL of 1.6 M n-BuLi
to a solution of 934 mg (4.13 mol) of 31 in 5 mL of THF at -78
°C and, after stirring for 40 min at -78 °C, adding 178 mg (4.13
JO060480Y
J. Org. Chem, Vol. 71, No. 16, 2006 5875