Synthesis of 5-(2-methoxy-1-naphthyl)- and 5-[2- (methoxymethyl)-1- naphthyl]-11H-benzo[b]fluorene as 2,2'-disubstituted 1,1'-binaphthyls via benzannulated enyne-allenes

Article abstract of DOI:10.3762/bjoc.7.58

5-(2-Methoxy-1-naphthyl)- and 5-[2-(methoxymethyl)-1-naphthyl]-11H-benzo[b] fluorene were synthesized by treatment of the corresponding benzannulated enediynes with potassium tert-butoxide in refluxing toluene to give benzannulated enyne-allenes for the subsequent Schmittel cascade cyclization reactions. The structures of these two 5-(1-naphthyl)-11H-benzo[b]fluorenes could be regarded as 2,2′-disubstituted 1,1′-binaphthyls with the newly constructed benzofluorenyl group serving as a naphthyl moiety.

Full text of DOI:10.3762/bjoc.7.58

Synthesis of 5-(2-methoxy-1-naphthyl)- and 5-[2-
(methoxymethyl)-1-naphthyl]-11H-benzo[b]fluorene
as 2,2'-disubstituted 1,1'-binaphthyls via
benzannulated enyne–allenes
Yu-Hsuan Wang, Joshua F. Bailey, Jeffrey L. Petersen and Kung K. Wang*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2011, 7, 496–502.
C. Eugene Bennett Department of Chemistry, West Virginia
University, Morgantown, West Virginia 26506-6045, USA
doi:10.3762/bjoc.7.58
Received: 27 January 2011
Accepted: 25 March 2011
Published: 19 April 2011
Email:
Kung K. Wang* - kung.wang@mail.wvu.edu
* Corresponding author
This article is part of the Thematic Series "Allene chemistry" and
dedicated to Professor Anthony Winston on the occasion of his 85th
birthday.
Keywords:
benzannulated enediynyl alcohols; benzannulated enyne–allenes;
2,2'-disubstituted 1,1'-binaphthyls; 5-(1-naphthyl)-11H-benzo[b]fluo-
renes; Schmittel cascade cyclizations
Guest Editor: K. M. Brummond
© 2011 Wang et al; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
5-(2-Methoxy-1-naphthyl)- and 5-[2-(methoxymethyl)-1-naphthyl]-11H-benzo[b]fluorene were synthesized by treatment of the
corresponding benzannulated enediynes with potassium tert-butoxide in refluxing toluene to give benzannulated enyne–allenes for
the subsequent Schmittel cascade cyclization reactions. The structures of these two 5-(1-naphthyl)-11H-benzo[b]fluorenes could be
regarded as 2,2'-disubstituted 1,1'-binaphthyls with the newly constructed benzofluorenyl group serving as a naphthyl moiety.
Introduction
Benzannulated enyne–allenes bearing an aryl substituent at the turn underwent a sequence of Schmittel cascade cyclization
alkynyl terminus are excellent precursors of 5-aryl-11H- reactions to form 5-phenyl-11H-benzo[b]fluorene (3a) in a
benzo[b]fluorenes [1-5]. Several synthetic pathways to the single operation (Scheme 1) [5]. It is interesting to note that the
benzannulated enyne–allene systems have been reported, newly formed benzo[b]fluorenyl moiety in 3a could also be
including generation in situ from the corresponding benzannu- regarded as a 1-arylnaphthyl derivative with three additional
lated enediynes. Specifically, treatment of the benzannulated substituents at the 2, 3, and 4 positions.
enediyne 1a with potassium tert-butoxide in refluxing toluene
for six hours promoted a 1,3-prototropic rearrangement to The reaction is not sensitive to the steric requirement of the
produce, in situ, the benzannulated enyne–allene 2a, which in substituent at the alkynyl terminus. The benzannulated enediyne
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Beilstein J. Org. Chem. 2011, 7, 496–502.
methods involved coupling of two properly substituted 1-naph-
thyl derivatives. Construction of a new 1-naphthyl ring as an
essential step toward 1,1'-binaphthyls is rare [17-19].
Results and Discussion
The Sonogashira reaction between 1-ethynyl-2-methoxyben-
zene (4a) and 1-iodo-2-[2-(trimethylsilyl)ethynyl]benzene
produced 5a, which was desilylated to give 6a (Scheme 3).
Condensation between 6a and pivalophenone (7) then furnished
the benzannulated enediynyl alcohol 8a. Subsequent reduction
with triethylsilane in the presence of trifluoroacetic acid
afforded the benzannulated enediyne 9a. Similarly, the benzan-
nulated enediyne 9b was synthesized from 1-ethynyl-2-
(methoxymethyl)benzene (4b). On exposure to potassium tert-
butoxide in refluxing toluene for five hours, 9a was trans-
formed to 5-(2-methoxyphenyl)-11H-benzo[b]fluorene 13a
along with a small amount (ca. 2%) of 14a in a single operation.
Presumably, the cascade sequence involved an initial 1,3-
protropic rearrangement to form the corresponding benzannu-
lated enyne–allene 10a. A Schmittel cyclization reaction [1-4]
Scheme 1: Synthesis of 5-aryl-11H-benzo[b]fluorenes via benzannu-
lated enyne–allenes.
1b with a sterically demanding 2,6-dibromophenyl substituent generates biradical 11a, which then undergoes an intramolec-
was also smoothly converted to 3b [6]. With 1c having a 1,1'- ular radical–radical coupling to afford 12a. This is followed by
binaphthyl substituent, a 1:1 mixture of the syn and the anti a second prototropic rearrangement to restore the aromaticity to
atropisomers of 3c was likewise obtained (Scheme 2) [7].
furnish 13a as proposed previously [5]. An intramolecular [2 +
2] cycloaddition reaction of 10a or a direct radical–radical
coupling of 11a could account for the formation of 14a [5].
From 9b, 5-[2-(methoxymethyl)phenyl]-11H-benzo[b]fluorene
13b and the [2 + 2] cycloaddition adduct 14b were produced in
a 5:1 ratio. The presence of the carbon–carbon double bonds in
14b allows easy removal of 14b by treatment of the resulting
mixture with BH3·THF followed by silica gel column chroma-
tography. The presence of a benzofluorenyl moiety and a
methoxy or a methoxymethyl group in 13a and 13b could allow
these compounds to serve as hetero-bidentate ligands for com-
plex formation with transition metals [20].
Scheme 2: Synthesis of 1,1'-binaphthyl-substituted 11H-benzo[b]fluo-
rene 3c.
We also investigated the possibility of using the benzannulated
enediynes bearing a 1-naphthyl, a 2-methoxy-1-naphthyl, or a
We now have successfully extended the cascade sequence to the 2-(methoxymethyl)-1-naphthyl substituent at one of the alkynyl
synthesis of other sterically congested analogues with an ortho- termini for the cascade cyclization reaction. Using a slightly
methoxy or an ortho-methoxymethyl group on the phenyl different synthetic sequence from that shown in Scheme 3 but
substituent. In addition, by placing a 2-methoxy-1-naphthyl or a reminiscent of a sequence developed for the synthesis of 4,5-
2-(methoxymethyl)-1-naphthyl group at one of the alkynyl diheteroarylphenanthrenes [21], the benzannulated enediynes
termini of the benzannulated enediyne system, the resulting 19a and 19b were obtained (Scheme 4). It was gratifying to
naphthyl-substituted benzo[b]fluorenes could be regarded as observe that on treatment with potassium tert-butoxide, 19a and
2,2'-disubstituted 1,1'-binaphthyls with two additional 19b were smoothly converted to 20a and 20b, respectively.
substituents at the 3 and 4 positions. The versatility of 1,1'- Similarly 20c was obtained via the synthetic sequence outlined
binaphthyl-2,2'-diol (BINOL) and BINOL derivatives as in Scheme 5. It is worth noting that the synthetic sequences
reagents in asymmetric synthesis has stimulated the develop- outlined in Scheme 4 and Scheme 5 represent new routes to
ment of new synthetic methods for 2,2'-disubstituted 1,1'- 2,2'-disubstituted 1,1'-binaphthyls with the newly constructed
binaphthyls [8-16]. However, the great majority of the reported benzofluorenyl serving as one of the naphthyl groups.
497

Beilstein J. Org. Chem. 2011, 7, 496–502.
Scheme 3: Synthesis of 5-(2-methoxyphenyl)- and 5-[2-(methoxymethyl)phenyl]-11H-benzo[b]fluorene 13a and 13b.
Scheme 4: Synthesis of 5-(1-naphthyl)- and 5-(2-methoxy-1-naphthyl)-
11H-benzo[b]fluorene 20a and 20b.
Scheme 5: Synthesis of 5-[2-(methoxymethyl)-1-naphthyl]-11H-
benzo[b]fluorene 20c.
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Beilstein J. Org. Chem. 2011, 7, 496–502.
The 1H NMR spectrum of 13a in C6D6 recorded on a 600 MHz in a variety of synthetic applications. The configurational
NMR spectrometer exhibited an AB system with signals at δ stability of 20b and 20c, which could be regarded as 2,2'-disub-
4.21 (J = 21.0 Hz) and 4.13 (J = 21.0 Hz), attributable to the stituted 1,1'-binaphthyls with two additional substituents at the
methylene hydrogens on the five-membered ring. AB systems 3 and 4 positions, could also be expected to be high. AB
from the methylene hydrogens were also observed in other patterns were observed for the methylene hydrogens on the
similar 11H-benzo[b]fluorenyl structures [7,22-24]. The AB five-membered ring of 20b and on the carbon bearing the
pattern remained unchanged at 70 °C, suggesting a relatively methoxy group of 20c.
slow rate of rotation, on the NMR time scale, around the
carbon–carbon single bond connecting the 2-methoxyphenyl Treatment of 20b with boron tribromide converted the methoxy
substituent to the C5 of the benzofluorenyl moiety. The rota- group to the hydroxy group, providing a handle for resolution of
tional barrier was calculated to be at least 16.7 kcal/mol at 24 with (1S)-(−)-camphanoyl chloride (Scheme 6) [29]. It was
70 °C on the basis of the lack of coalescence of signals at this possible to achieve partial separation of a small fraction of the
temperature. This lowest possible rotational barrier is signifi- two diastereomeric (1S)-camphanates in a 5:1 ratio by silica gel
cantly higher than that of 1-phenylnaphthalene, which was column chromatography.
calculated by MM2' to be 12.4 kcal/mol [25,26]. Similarly, the
1H NMR spectrum of 13b taken in CDCl3 showed a clear AB Conclusion
system at δ 4.07 (J = 13.8 Hz) and 4.04 (J = 13.8 Hz), attribut- In conclusion, the use of benzannulated enediynes as precur-
able to the methylene hydrogens on the carbon attached to the sors to 2,2'-disubstituted 1,1'-binaphthyls represents a new syn-
methoxy group. The signals of the methylene hydrogens on the thetic approach to these sterically hindered molecules. The
five-membered ring could barely be discerned as an AB system assembly of the enediynyl precursors from three separate
with the two inner signals overlapped at δ 4.51 and two small aromatic fragments allows the possibility of placing a variety of
outer signals at δ 4.55 and 4.47.
functional groups at various positions of the 1,1'-binaphthyl
system. Transformation of the methoxy group in 20b to a
The rotational barrier of the parent 1,1'-binaphthyl in N,N- hydroxy group provides a handle for resolution with optically
dimethylformamide was determined to be 23.5 kcal/mol at active reagents.
50 °C, corresponding to a half-life of 14.5 minutes for racemi-
Experimental
zation [27,28]. Because the structure of 20a could be regarded
as a 2,3,4-trisubstituted 1,1'-binaphthyl, the rate of rotation can General information
be expected to be even slower. Again, in C6D6 recorded on a All organometallic reactions were conducted in oven-dried
600 MHz NMR spectrometer, the signals of the methylene (120 °C) glassware under a nitrogen atmosphere. Diethyl ether
hydrogens on the five-membered ring could be discerned as an and tetrahydrofuran (THF) were distilled from benzophenone
AB system at δ 4.27 (J = 21 Hz) and 4.25 (J = 21 Hz).
ketyl prior to use. 1-Ethynyl-2-methoxybenzene (4a), pivalo-
phenone (7), n-butyllithium (1.6 M) in hexanes, 1-bromo-2-[2-
The rotational barrier of BINOL as a member of the 2,2'-disub- (trimethylsilyl)ethynyl]benzene, triethylsilane, trifluoroacetic
stituted 1,1'-binaphthyls was determined to be 37.2 kcal/mol at acid, potassium tert-butoxide (1.0 M) in 2-methyl-2-propanol,
195 °C in naphthalene, corresponding to a half-life of 4.5 hours lithium diisopropylamide (LDA, 2.0 M) in heptane/THF/ethyl-
for racemization [28]. The high stability of the configuration benzene, Pd(PPh3)2Cl2, Pd(PPh3)4, copper(I) iodide, triethyl-
even at such an elevated temperature allows BINOL to be used amine, 1.0 M BH3·THF solution in THF, 1-ethynylnaphthalene
(18a), 2-methoxy-1-naphthaldehyde, 1-bromo-2-(methoxy-
methyl)naphthalene, (trimethylsilyl)acetylene, boron tribro-
mide, and (1S)-(−)-camphanoyl chloride were purchased from
chemical suppliers and were used as received. 1-Iodo-2-[2-
(trimethylsilyl)ethynyl]benzene was prepared by treatment of
1-bromo-2-[2-(trimethylsilyl)ethynyl]benzene in THF with
n-butyllithium at −78 °C followed by treatment with iodine [7].
1-Ethynyl-2-iodo-benzene (15) [30] was prepared in quantitat-
ive yield by desilylation of 1-iodo-2-[2-(trimethylsi-
lyl)ethynyl]benzene with sodium hydroxide in methanol.
1-Ethynyl-2-(methoxymethyl)benzene (4b) [31] and dimethyl
Scheme 6: Demethylation of 22b to form 5-(2-hydroxy-1-naphthyl)-
11H-benzo[b]fluorene 24.
(1-diazo-2-oxopropyl)phosphonate [32] were prepared
according to the reported procedures.
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Beilstein J. Org. Chem. 2011, 7, 496–502.
1-Iodo-2-(4,4-dimethyl-3-phenyl-1-pentynyl)benzene (17). 15 mL of diethyl ether were added. The organic layer was sep-
To 1.032 g (4.528 mmol) of 15 in 10 mL THF under a nitrogen arated and the aqueous layer back extracted with diethyl ether.
atmosphere at 0 °C, was added 3.77 mL of a 2.0 M solution of The combined organic layers were washed successively with
LDA (7.55 mmol) in THF. After stirring for 30 min, a solution brine and water, dried over sodium sulfate, and concentrated.
of 0.616 g of 7 (3.774 mmol) in 10 mL of THF was introduced Purification of the residue by flash column chromatography
via cannula, and the reaction mixture allowed to warm to room (silica gel/5% methylene chloride in hexanes) afforded 0.255 g
temperature. After an additional 3 h, 20 mL of water was intro- of 19c (0.577 mmol, 71%) as a colorless liquid: 1H NMR
duced, and the reaction mixture extracted with diethyl ether. (CDCl3, 270 MHz) δ 8.51 (1H, d, J = 7.7 Hz), 7.88–7.83 (2H,
The combined organic extracts were washed successively with m), 7.67–7.57 (3H, m), 7.53–7.42 (2H, m), 7.38–7.32 (4H, m),
brine and water, dried over sodium sulfate, and concentrated. 7.12–7.05 (3H, m), 4.92 (1H, d, J = 13.1 Hz), 4.83 (1H, d, J =
The residue was purified by flash chromatography (silica gel/ 12.9 Hz), 3.69 (2H, s), 3.41 (3H, s), 0.97 (9H, s); 13C NMR
10% diethyl ether in hexanes) to afford 1.476 g of crude 16 as a (CDCl3, 67.9 MHz) δ 139.1, 138.9, 133.2, 132.5, 132.1, 129.6,
light yellow liquid. Crude 16 without any further purification 128.7, 128.1, 128.0, 127.4, 126.8, 126.6, 126.3, 126.2, 125.5,
was treated with 0.810 g of triethylsilane (6.983 mmol) and 125.0, 119.0, 98.3, 96.0, 88.4, 82.7, 72.8, 58.3, 50.5, 35.5, 27.7.
2.1 g of trifluoroacetic acid (18.4 mmol) to afford 1.382 g
(3.699 mmol, 86% for 2 steps) of 17 as a colorless liquid: IR 5-(2-Methoxy-1-naphthyl)-10-(1,1-dimethylethyl)-11H-
2966, 1463 cm−1; 1H NMR (CDCl3, 270 MHz) δ 7.83 (1H, dd, benzo[b]fluorene (20b). To 0.295 g of 19b (0.689 mmol) in
J = 7.9, 1.2 Hz), 7.47–7.41 (3H, m), 7.36–7.23 (4H, m), 6.96 10 mL of anhydrous toluene under a nitrogen atmosphere, was
(1H, td, J = 7.7, 1.7 Hz), 3.69 (1H, s), 1.09 (9H, s); 13C NMR added 0.77 mL of a 1.0 M solution of potassium tert-butoxide
(CDCl3, 67.9 MHz) δ 138.9, 138.6, 132.9, 130.5, 129.9, 128.8, (0.77 mmol) in 2-methyl-2-propanol. The reaction mixture was
127.6, 126.7, 100.5, 95.5, 85.5, 50.6, 35.7, 27.9.
then heated under reflux for 6 h. After the reaction mixture was
allowed to cool to room temperature, 10 mL of water and
Benzannulated enediyne 19b. To a mixture of 0.307 g of 17 40 mL of methylene chloride were introduced. The organic
(0.822 mmol), Pd(PPh3)4 (0.040 g, 0.035 mmol), and copper(I) layer was separated, dried over sodium sulfate and concen-
iodide (0.015 g, 0.080 mmol) in 10 mL of toluene, was added trated. The residue was purified by flash column chromatog-
via cannula a solution of 0.150 g of 18b (0.824 mmol) in 5 mL raphy (silica gel/5% methylene chloride in hexanes) to provide
of triethylamine. After stirring at 120 °C for 12 h, 15 mL of a 0.263 g of 20b (0.614 mmol, 89%) as a light yellow liquid: IR
saturated ammonium chloride solution and 15 mL of diethyl 1267, 1250, 766 cm−1; 1H NMR (CDCl3, 600 MHz) δ 8.66 (1H,
ether were added. The organic layer was separated and the d, J = 9.0 Hz), 8.11 (1H, d, J = 9.6 Hz), 7.92 (1H, d, J =
aqueous layer back extracted with diethyl ether. The combined 8.4 Hz), 7.53 (1H, d, J = 9.0 Hz), 7.44 (1H, d, J = 7.2 Hz), 7.40
organic layers were washed successively with brine and water, (1H, ddd, J = 8.4, 6.6, 1.8 Hz), 7.35 (1H, d, J = 9.0 Hz), 7.31
dried over sodium sulfate, and concentrated. Purification of the (1H, td, J = 6.6, 1.2 Hz), 7.18 (1H, t, J = 7.8 Hz), 7.13–7.05
residue by flash column chromatography (silica gel/30% meth- (3H, m), 6.77 (1H, t, J = 7.8 Hz), 6.08 (1H, d, J = 7.8 Hz), 4.55
ylene chloride in hexanes) afforded 0.331 g of 19b (2H, s), 3.68 (3H, s), 1.97 (9H, s); 1H NMR (C6D6, 600 MHz) δ
(0.773 mmol, 94%) as a colorless liquid: IR 2207, 1271, 8.67 (1H, d, J = 9.0 Hz), 7.90 (1H, d, J = 9.6 Hz), 7.79 (1H, d,
1078 cm−1; 1H NMR (CDCl3, 270 MHz) δ 8.43–8.39 (1H, m), J = 8.4 Hz), 7.70 (1H, d, J = 8.4 Hz), 7.42 (1H, d, J = 8.4 Hz),
7.85 (1H, d, J = 9.2 Hz), 7.82–7.77 (1H, m), 7.70–7.65 (1H, m), 7.29 (1H, t, J = 7.8 Hz), 7.23 (1H, d, J = 7.2 Hz), 7.17 (1H, d,
7.56–7.52 (1H, m), 7.43–7.25 (7H, m), 7.10–7.00 (3H, m), 4.00 J = 6.6 Hz), 7.12 (1H, t, J = 7.5 Hz), 7.06 (1H, t, J = 7.5 Hz),
(3H, s), 3.66 (1H, s), 0.95 (9H, s); 13C NMR (CDCl3, 6.98 (1H, t, J = 7.2 Hz), 6.89 (1H, t, J = 7.8 Hz), 6.72 (1H, t, J =
67.9 MHz) δ 158.9, 139.0, 134.5, 132.3, 130.2, 129.7, 128.4, 7.5 Hz), 6.53 (1H, d, J = 7.8 Hz), 4.25 (1H, d, J = 21.6 Hz),
127.9, 127.8, 127.3, 127.2, 126.4, 126.3, 126.0, 125.7, 124.1, 4.19 (1H, d, J = 21.6 Hz), 3.13 (3H, s), 1.77 (9H, s); 13C NMR
112.6, 106.5, 97.8, 95.5, 87.3, 82.6, 56.6, 50.5, 35.5, 27.7; (CDCl3, 150 MHz) δ 154.8, 144.2, 141.0, 140.3, 139.3, 137.9,
HRMS m/z [M + H]+ calcd for C32H29O, 429.2218; found, 134.7, 133.9, 131.8, 129.8, 129.5, 128.1, 127.8, 127.2, 127.0,
429.2217.
126.7, 126.6, 126.4, 125.2, 124.1, 123.9, 123.7, 123.3, 122.8,
122.2, 114.4, 56.9, 40.3, 38.9, 34.5; MS m/z 428 (M+), 413,
Benzannulated enediyne 19c. To a mixture of 0.242 g of 23 400, 371; HRMS m/z calcd for C32H28O, 428.2140; found,
(0.812 mmol), Pd(PPh3)2Cl2 (0.030 g, 0.043 mmol), and 428.2126.
copper(I) iodide (0.015 g, 0.080 mmol) in 6 mL of triethyl-
amine, was added via cannula a solution of 0.265 g of 22 Recrystallization from a mixture of isopropyl alcohol and meth-
(0.974 mmol) in 2 mL of triethylamine. After stirring at 60 °C ylene chloride produced a crystal for X-ray structure analysis.
for 12 h, 15 mL of a saturated ammonium chloride solution and Although the weakly diffracting crystal limited the amount of
500

Beilstein J. Org. Chem. 2011, 7, 496–502.
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5-[2-(Methoxymethyl)-1-naphthyl]-10-(1,1-dimethylethyl)-
11H-benzo[b]fluorene (20c). The same procedure as described
for 20b was repeated except that 0.142 g of 19c (0.321 mmol)
was treated with 0.48 mL of a 1.0 M solution of potassium tert-
butoxide (0.48 mmol) in 2-methyl-2-propanol to afford 0.102 g
of 20c (0.231 mmol, 72%) as a light yellow liquid: IR 2943,
1273, 1248, 774 cm−1; 1H NMR (CDCl3, 270 MHz) δ 8.67 (1H,
d, J = 8.9 Hz), 8.12 (1H, d, J = 8.7 Hz), 7.97 (1H, d, J =
8.2 Hz), 7.91 (1H, d, J = 8.6 Hz), 7.47–7.39 (3H, m), 7.28–7.08
(5H, m), 6.75 (1H, t, J = 7.7 Hz), 5.89 (1H, d, J = 7.9 Hz), 4.56
(2H, s), 4.16 (1H, d, J = 13.4 Hz), 4.10 (1H, d, J = 13.3 Hz),
3.09 (3H, s), 1.98 (9H, s); 13C NMR (CDCl3, 67.9 MHz) δ
144.1, 141.5, 139.7, 139.2, 137.7, 135.3, 134.3, 134.0, 133.2,
132.7, 131.6, 128.4, 128.1, 127.9, 126.95, 126.91, 126.6, 126.4,
126.0, 125.9, 124.9, 124.5, 123.8, 123.6, 122.9, 71.9, 58.4, 40.3,
39.0, 34.5; MS m/z 442 (M+), 427, 395; HRMS m/z calcd for
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Supporting Information File 1
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19a–c, 20a–c, 21–24, and the (1S)-camphanates of 24.
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