Synthesis of the pentacylic core of (+)-salvileucalin B

Article abstract of DOI:10.1021/jo500164x

A concise preparation of the prochiral pentacyclic core of (+)-salvileucalin B is presented. The key feature in the synthesis is the Cu-catalyzed intramolecular cyclopropanation of a symmetrical indane-derived α-diazo β-keto ester. This symmetry is carrie

Full text of DOI:10.1021/jo500164x

Article
pubs.acs.org/joc
Synthesis of the Pentacylic Core of (+)-Salvileucalin B
Douglass F. Taber* and Craig M. Paquette
Department of Chemistry and Biochemistry, University of Delaware, Newark, Delaware 19716, United States
S
* Supporting Information
ABSTRACT: A concise preparation of the prochiral pentacyclic
core of (+)-salvileucalin B is presented. The key feature in the
synthesis is the Cu-catalyzed intramolecular cyclopropanation of
a symmetrical indane-derived α-diazo β-keto ester. This
symmetry is carried through the remainder of the synthesis.
This practical approach could allow the ready preparation of
derivatives for further chemical and biological studies of this
class of natural products.
Scheme 1. Retrosynthetic Analysis of (+)-Salvileucalin B
INTRODUCTION
■
Salvileucalins A and B (1) were isolated from the aerial parts of
Salvia leucantha Cav. by Takeya and co-workers1 in 2008.
Salvileucalins C and D were also isolated from Salvia leucantha
Cav. by Takeya and co-workers2 in 2011 (Figure 1). The
revealed an element of symmetry in the core structure. We
envisioned using this to our advantage. A copper-mediated
cyclopropanation of compound 5 could lead to the meso-
norcaradiene core 4. Further manipulation would deliver the
lactone, completing the southern portion of 1. Rearrangement
of the epoxide 3 could lead to the allylic alcohol 2, either in its
racemic form or as a single enantiomer. Banwell7 recognized
this same advantage, effecting intramolecular cyclopropanation
of a symmetrical arene. Reisman5 also effected intramolecular
cyclopropanation of an arene, but further along in the synthesis.
Accessing the cyclopropane 4 required the construction of
the diazo ketone 5 (Scheme 2). Our starting material was the 2-
indanylacetic acid 7. This commercially available acid was
expensive, so we prepared it on a 20 g scale from 1-indanone
following the Hay8 protocol. The acid 6 was reduced with Pd/
C under 300 psi H2 for 24 h to give the acid 7. Birch reduction
with lithium metal and NH3 gave the diene 8 that was shelf-
stable. The crude derived acid chloride was treated with the
Figure 1. Salvileucalins A−D.
bislactone 1 presented an interesting puzzle for total synthesis.
The unusually stable norcaradiene3 core includes seven rings.
The molecule also contains three heterocycles, two lactones,
and one furan. It also features a hexasubstituted cyclopropane, a
significantly challenging structure. Of interest, 1 showed
promising cytotoxicity against A549 (human lung adeno-
carcinoma) and HT-29 (human colon adenocarcinoma). To
date, only one total synthesis of this architecturally complex
rearranged neoclerodane diterpene4 by Reisman and co-
workers5 has been reported. In addition, a modular synthesis
of the structural domains by Chen and co-workers6 and a
synthesis of the norcaradiene core by Banwell and co-workers7
have been described.
RESULTS AND DISCUSSION
When we began our work, no efforts toward the synthesis of 1
had yet been reported. A retrosynthetic analysis (Scheme 1)
■
Received: January 23, 2014
Published: March 24, 2014
© 2014 American Chemical Society
3410
dx.doi.org/10.1021/jo500164x | J. Org. Chem. 2014, 79, 3410−3413

The Journal of Organic Chemistry
Article
Scheme 2. Route to the Neoclerodane Core
Scheme 4. Route to Racemic Enone
was secured by X-ray crystallography, was prepared by exposure
of 13 to mCPBA. Treatment of the epoxide 3 with diphenyl
diselenide and sodium borohydride gave the phenylselenyl
alcohol 14. Oxidation of 14 with sodium periodate led to the
selenoxide, which upon heating underwent elimination to give
the allylic alcohol 2. Dess-Martin periodinane oxidation
delivered the required enone 15.
CONCLUSION
■
The work presented here provides an alternative approach to
the prochiral pentacylic core of (+)-salvileucalin B 1. The route
to the alkene 13 from the commercial acid 7 is eight steps with
26% overall yield. The highlights of this route include the short
practical preparation of the neoclerodane core 4. The enone 15
could potentially be converted into a variety of derivatives for
biological screening.
enolate of ethyl acetate9 to give the β-keto ester 9. Diazo
transfer using mesyl azide10 set the stage for the cyclization.
Treatment of the diazo ester 9 with catalytic copper gave the
cyclopropane 4. With this method, gram quantities of 4 were
easily prepared with minimal need for purification. A brief
screen of copper catalysts was performed. We found that a
soluble Cu(II) catalyst11 gave a slightly better yield of the
cyclopropane than did Cu bronze powder.
EXPERIMENTAL SECTION
■
With a scalable route to the neoclerodane core in hand, we
turned our attention to installing the southern lactone (Scheme
3). This was readily accomplished by first forming the vinyl
General Experimental: 1H NMR and 13C NMR spectra were
recorded as solutions in deuteriochloroform (CDCl3) at 400 and 100
MHz, respectively. 13C multiplicities were determined with the aid of a
JVERT pulse sequence, differentiating the signals for methyl and
methine carbons as “d” from methylene and quaternary carbons as “u”.
The infrared (IR) spectra were determined as neat oils. Rf values
indicated refer to thin layer chromatography (TLC) on 2.5 × 10 cm,
250 μm analytical plates coated with silica gel GF, unless otherwise
noted, and developed in the solvent system indicated. All glassware
was oven-dried and rinsed with dry solvent before use. THF and
diethyl ether were distilled from sodium metal/benzophenone ketyl
under dry nitrogen. MTBE is methyl tert-butyl ether, and PE is 30−60
petroleum ether. All reactions were conducted under N2 and stirred
magnetically. HRMS data were obtained by a EI method of ionization
on a sector instrument.
Scheme 3. Formation of the Southern Lactone
2-(2,3,4,7-Tetrahydro-1H-inden-2-yl)acetic acid (8): In a 500
mL three-neck round-bottom flask, 2-indanone acetic acid (3.5 g,
19.88 mmol) and anhydrous EtOH (100 mL) were combined. The
reaction was cooled to −78 °C by a dry ice acetone bath. Anhydrous
ammonia (100 mL) was condensed into the flask via a coldfinger
condenser. Pieces of lithium metal were added portionwise until a blue
color persisted. The reaction was allowed to stir at rt overnight. Ice
water was added, and the mixture was acidified with concentrated
aqueous HCl. The mixture was extracted with Et2O, and the organic
extract was dried (Na2SO4) and concentrated. The resulting oil was
diluted with 50 mL of PE and placed in the freezer overnight. The
resulting off white crystals were filtered and washed with cold PE to
afford 8 (2.45 g, 70% yield): mp = 68−72 °C; TLC Rf (Et2O) = 0.46;
1H NMR δ 1.98 (m, 2 H), 2.47 (d, J = 7.6 Hz, 2 H), 2.56 (m, 2 H),
2.64 (s, 4 H), 2.70 (m, 1 H), 5.76 (s, 2 H); 13C NMR δ (u) 179.7,
130.6, 41.8, 40.9, 27.4; (d) 124.6, 32.0; IR (film, cm−1) 3438, 2841,
1698, 1439; HRMS calcd for C11H14O2 178.0989, observed 178.1001.
triflate of the β-keto ester 4 by treatment with LDA followed by
exposure to the McMurry−Hendrickson reagent.12 The ester
11 was then reduced to the primary alcohol 12. Exposure to
CO in the presence of Pd then delivered the lactone 13,
previously reported7 by Banwell.
We next addressed the northern functionality of salvileucalin
B 1 (Scheme 4). The meso-epoxide 3, the structure of which
3411
dx.doi.org/10.1021/jo500164x | J. Org. Chem. 2014, 79, 3410−3413

The Journal of Organic Chemistry
Article
Ethyl 3-Oxo-4-(2,3,4,7-tetrahydro-1H-inden-2-yl)butanoate
(9): In a 100 mL round-bottom flask 8 (1.89 g, 10.62 mmol),
CH2Cl2 (30 mL), and 10 drops of DMF were combined at 0 °C.
Oxalyl chloride (4.00 g, 31.85 mmol) was added dropwise over 10 min
to maintain control of foaming. Once the foaming ended, the reaction
mixture was concentrated and then diluted with THF (30 mL). In a
separate 250 mL round-bottom flask, diisopropylamine (6.69 g, 66.9
mmol) and THF (60 mL) were combined at −78 °C. nBuLi (2.0 M in
hexanes, 30.0 mL, 60.0 mmol) was added dropwise over 5 min. The
solution was allowed to warm to −30 °C. The solution was then
cooled to −78 °C. EtOAc (4.40 g, 50.0 mmol) was added all at once.
The solution was held for 15 min, then the acid chloride solution was
added dropwise over 5 min. The mixture was allowed to stir for 2 h at
−78 °C. The reaction was quenched with 100 mL of 1 M aqueous
HCl. The mixture was extracted with Et2O. The organic extract was
dried (Na2SO4) and concentrated. The residual oil was chromato-
graphed to obtain 9 (2.36 g, 90% yield) as a clear oil: TLC Rf (30%
MTBE/PE) = 0.43; 1H NMR δ 1.29 (t, J = 11.6 Hz, 3 H), 1.92 (d, J =
14.8 Hz, 2 H), 2.52 (m, 2 H), 2.55 (s, 4 H), 2.57 (d, J = 6.8 Hz, 2 H),
3.45 (d, J = 11.6 Hz, 2 H), 4.22 (q, J = 11.6 Hz, 2 H), 5.75 (s, 2 H);
13C NMR δ (u) 202.9, 167.2, 130.8, 61.9, 50.5, 42.1, 39.0, 27.3; (d)
then Dibal-H (1.2 M in toluene, 1.75 mL, 2.1 mmol) was added
dropwise over 5 min. The solution was held at −30 °C for 30 min. The
reaction was quenched slowly with saturated aqueous sodium fluoride
and stirred until solids formed. The solution was filtered, and the filter
cake was washed with EtOAc. The filtrate was concentrated then taken
up in PE and cooled to −78 °C. The crystals formed were filtered to
yield 12 (0.242 g 90% yield from 4) as white crystals: mp = 60−63 °C;
1
TLC Rf (30% MTBE/PE) = 0.63; H NMR δ 1.37 (d, J = 11.9 Hz, 2
H), 1.57 (m, 4 H), 2.50 (s, 4 H), 2.60 (m, 1 H), 3.95 (d, J = 5.5 Hz, 2
H), 5.78 (s, 2 H), 5.94 (d, J = 8.1 Hz, 1 H); 13C NMR δ (u) 146.7,
57.7, 36.6, 34.0, 26.4, 23.3; (d) 125.6, 116.1, 30.2; IR (film, cm−1)
3336 (br), 2915, 1446, 1651, 1421; HRMS [M − H2O] calcd for
C14H13O3F3S 318.0533, observed 318.0522.
4-Oxapentacyclo[6.6.1.01,10.02,6.02,10]pentadeca-6,12-dien-5-
one (13): In a 25 mL round-bottom flask, the alcohol 12 (3.4 g, 10.17
mmol) was added to DMF (8 mL), EtOH (3 mL), and triethylamine
(3 mL). The solution was sparged with CO via a balloon for 10 min.
Under the CO balloon, Pd(OAc)2 (0.068 g, 0.305 mmol) and
triphenylphosphine (0.160 g, 0.610 mmol) were added. The solution
was stirred for 2 h at rt. The reaction was quenched with 1 M aqueous
HCl and extracted with Et2O. The organic extract was dried (Na2SO4)
and concentrated. The residue was chromatographed to yield 13 (2.06
g, 95% yield) as a white powder: mp = 140−143 °C; TLC Rf (60%
MTBE/PE) = 0.66 ; 1H NMR δ 0.98 (d, J = 11.9 Hz, 2 H), 1.69 (dd, J
= 11.9, 4.8 Hz, 2 H), 2.3−2.6 (m, 4H) 2.90 (dt, J = 6.8, 4.8 Hz, 1 H),
4.11 (s, 2 H), 5.74 (s, 2 H), 6.96 (d, J = 7.1 Hz, 1 H); 13C NMR δ (u)
169.8, 126.4, 67.2, 32.9, 31.6, 25.3, 23.6; (d) 133.8, 124.6, 32.7; IR
(film, cm−1) 3479, 2918, 2840, 1751, 1643; HRMS calcd for C14H14O2
214.0989, observed 214.0998.
124.9, 31.1, 14.0; IR (film, cm−1) 3439, 2880, 1740, 1643, 1403;
HRMS calcd for C15H20O3 248.1408, observed 248.1415.
Ethyl 2-Diazo-3-oxo-4-(2,3,4,7-tetrahydro-1H-inden-2-yl)-
butanoate (5): In a 250 mL round-bottom flask, β-keto ester 9
(2.0 g, 8.06 mmol), CH2Cl2 (40 mL), and triethylamine (1.63 g, 16.12
mmol) were combined at 0 °C. Mesyl azide (1.46 g, 12.09 mmol) was
added all at once. The reaction was stirred for 4 h in the dark. The
reaction was quenched with 1 M aqueous HCl and extracted with
CH2Cl2. The organic extract was dried (Na2SO4) and concentrated.
The residual oil was chromatographed to obtain 5 (1.89 g, 85% yield)
4,13-Dioxahexacyclo[7.6.1.01,7.03,5.07,15.011,15]hexadec-10-
en-12-one (3): In a 25 mL round-bottom flask, lactone 13 (0.025 g,
0.114 mmol), mCPBA (0.025 g 0.114 mmol), and CH2Cl2 (1 mL)
were combined at 0 °C. The reaction was stirred overnight at 0 °C.
The residue was chromatographed directly to yield 3 (0.012 g, 41%
yield, 80% yield based on starting material not recovered) as white
1
as clear oil: TLC Rf (30% MTBE:PE) = 0.75; H NMR δ 1.35 (t, J =
7.2 Hz, 3 H), 1.98 (d, J = 5.5 Hz, 2 H), 2.56 (m, 2 H), 2.63 (s, 4 H),
2.78 (m, 1 H), 2.99 (d, J = 7.3 Hz, 2 H), 4.31 (q, J = 7.2 Hz, 2 H), 5.76
(s, 2 H); 13C NMR δ (u) 192.7, 130.7, 61.4, 47.2, 41.9, 27.4; (d)
124.6, 31.7, 14.4; IR (film, cm−1) 2837, 2511, 1851, 1753, 1441;
HRMS calcd for C15H19O3N2 275.1391, observed 275.1392.
1
crystals: mp = 180−185 °C; TLC Rf (60% MTBE/PE) = 0.33; H
NMR δ 0.92 (d, J = 12.0 Hz, 2 H), 1.66 (d, J = 16.7 Hz, 2 H), 1.95 (d,
J = 16.7 Hz, 2 H), 2.46 (d, J = 16.7 Hz, 2 H), 2.85 (dt, J = 6.9, 4.8 Hz,
1 H), 3.15 (s, 2 H), 4.23 (s, 2 H), 6.96 (d, J = 7.1 Hz, 1 H); 13C NMR
δ (u) 169.2, 125.7, 65.8, 33.1, 21.6, 20.2; (d) 134.3, 50.4, 33.1; IR
(film, cm−1) 3488.9, 2919.9, 2850.0, 1788.0, 1666.4; HRMS calcd for
C14H14O3 230.0938, observed 230.0952.
Ethyl 10-Oxotetracyclo[6.3.1.01,6.06,11]dodec-3-ene-11-car-
boxylate (4): In a 100 mL round-bottom, flask diazo ester 5 (1.09
g, 3.98 mmol), toluene (40 mL), and copper(II) salicylideneimine
(0.166 g, 0.40 mmol) were combined. The reaction was stirred at
reflux for 1 h. The mixture was chromatographed directly to obtain 4
(0.587 g, 60% yield) as off white crystals: mp = 105−108 °C; TLC Rf
(60% MTBE/PE) = 0.41; 1H NMR δ 1.26 (t, J = 7.2 Hz, 3 H), 1.96 (s,
4 H), 2.16 (d, J = 3.2 Hz, 2 H), 2.34 (d, J = 15.6 Hz, 3 H), 2.89 (m, 2
H), 4.15 (q, J = 7.2 Hz, 2 H), 5.49 (s, 2 H); 13C NMR δ (u) 204.0,
167.3, 130.7, 61.0, 43.7, 38.7, 36.0, 26.5; (d) 123.8, 27.3, 14.0; IR (film,
cm−1) 3421 (br), 2931, 2091, 1728, 1644; HRMS calcd for C15H18O3
246.1251, observed 246.1252.
4-Hydroxy-12-oxapentacyclo[6.6.1.01,6.06,14.010,14]penta-
deca-2,9-dien-11-one (2): In a 100 mL round-bottom flask,
diphenyl diselenide (2.17 g, 6.96 mmol) and EtOH (40 mL) were
combined. Sodium borohydride (0.514 g, 13.90 mmol) was added.
The solution was held at rt until the foaming stopped, and a clear
solution was obtained. In a separate 250 mL round-bottom flask, the
epoxide 3 (0.326 g, 1.39 mmol) and EtOH (40 mL) were combined.
The two solutions were combined all at once. The solution was heated
to reflux for 2 h. The solution was quenched with 1 M aqueous HCl
and extracted with Et2O. The organic extract was dried (Na2SO4) and
concentrated. The residue was chromatographed to yield 14 (0.323 g,
60% yield) as white crystals: mp = 160−165 °C; TLC Rf (80%
MTBE/PE) = 0.54; 1H NMR δ 0.87 (d, J = 11.9 Hz, 1 H), 1.00 (d, J =
11.9 Hz, 1 H), 1.63 (m, 3 H), 1.80 (dd, J = 14.2, 10.1 Hz, 1 H), 1.94
(dd, J = 14.2, 10.1 Hz, 1 H), 2.44 (dd, J = 13.9, 5.1 Hz, 1 H), 2.60 (dd,
J = 15.4, 5.3 Hz, 1 H), 2.90 (m, 3 H), 3.28 (s, 1 H), 3.78 (d, J = 9.6 Hz,
1 H), 4.15 (d, J = 9.6 Hz, 1 H), 6.92 (d, J = 7.1 Hz, 1 H), 7.38 (m, 3
H), 7.60 (d, J = 8.1 Hz, 2 H); 13C NMR δ (u) 169.1, 125.6, 124.6,
65.9, 34.2, 33.1, 30.6, 28.2, 27.2, 26.2; (d) 136.8, 134.3, 129.4, 129.2,
68.3, 49.6, 33.7.
{10-(Trifluoromethylsulfonyloxy)tetracyclo[6.3.1.01,6.06,11]-
dodeca-3,9-dien-11-yl}methanol (12): In a 25 mL round-bottom
flask, diisopropylamine (0.250 g, 2.03 mmol) and THF (4 mL) were
cooled to −78 °C, then nBuLi (2.5 M in hexanes, 0.8 mL, 2.03 mmol)
was added dropwise over 5 min. The solution was allowed to warm to
−35 °C, then recooled to −78 °C, and β-keto ester 4 (0.201 g 0.81
mmol) in THF (4 mL) was added dropwise over 5 min. The solution
was stirred for 30 min; N-phenylbis(trifluoromethanesulfonimide)
(0.348 g 0.975 mmol) was added, and the solution was stirred
overnight. The reaction was quenched with water and extracted with
Et2O. The organic extract was dried (Na2SO4) and concentrated to
yield 11 (0.292 g, crude 95% yield) as a clear oil: TLC Rf (40%
MTBE/PE) = 0.69; 1H NMR δ 1.27 (t, J = 7.2 Hz, 3 H), 1.55 (d, J =
11.9 Hz, 2 H), 1.60 (m, 2 H), 2.50 (d, J = 16.0 Hz, 2 H), 2.68 (m, 1
H), 2.80 (m, 2 H), 4.13 (q, J = 7.3 Hz, 2 H), 5.57 (s, 2 H), 5.96 (d, J =
8.3 Hz, 1 H); 13C NMR δ (u) 166.0, 143.2, 129.8, 127.4, 120.0, 61.4,
35.0, 28.4, 24.4; (d) 124.5, 115.2, 29.4, 13.8; IR (film, cm−1) 3436
(br), 2925, 1732, 1651, 1421; HRMS calcd for C16H18O5F3S 379.0822,
observed 379.0821.
In a 50 mL round-bottom flask, selenide 14 (0.323 g, 0.836 mmol),
sodium periodate (2.50 g, 13.60 mmol), saturated aqueous sodium
bicarbonate (15 mL), and THF (10 mL) were combined. The solution
was stirred at rt for 1 h. The solution was diluted with brine and
extracted with EtOAc. The organic extract was dried (Na2SO4) and
then heated to reflux for 20 min until a yellow color developed. The
solution was concentrated and chromatographed to yield 2 (0.096 g,
30% yield from 3) as a white powder: mp = 163−165 °C; TLC Rf
In a 250 mL round-bottom flask, the crude ester 11 (0.30 g, 0.770
mmol) and CH2Cl2 (4 mL) were cooled to −30 °C via a dry ice bath,
3412
dx.doi.org/10.1021/jo500164x | J. Org. Chem. 2014, 79, 3410−3413

The Journal of Organic Chemistry
Article
(80% MTBE/PE) = 0.30; 1H NMR δ 1.00 (d, J = 11.9 Hz, 1 H), 1.10
(d, J = 11.9 Hz, 1 H), 1.59 (dd, J = 12.0, 4.7 Hz, 1 H), 1.73 (dd, J =
13.8, 9.5 Hz, 1 H), 1.83 (m, 1 H), 1.95 (s, 1 H), 2.48 (dd, J = 13.5, 8.0
Hz, 1 H), 3.04 (dt, J = 6.8, 4.8 Hz, 1 H), 3.93 (t, J = 8.5 Hz, 1 H), 4.11
(d, J = 9.6 Hz, 1 H), 4.32 (d, J = 9.6 Hz, 1 H), 5.84 (dd, J = 10.1, 1.8
Hz, 1 H), 6.01 (dd, J = 10.1, 1.8 Hz, 1 H), 7.05 (d, J = 7.1 Hz, 1 H);
13C NMR δ (u) 169.3, 125.5, 67.3, 31.1, 29.8, 29.5; (d) 135.3, 131.4,
For its use as a cyclopropanation catalyst, see: (b) Corey, E. J.; Myers,
A. G. Tetrahedron Lett. 1984, 25, 3559.
(12) Hendrickson, J. B.; Bergeron, R. Tetrahedron Lett. 1973, 46,
4607.
125.5, 65.0, 34.6; IR (film, cm−1) 3338 (br s), 2861, 1770, 1169, 1019;
HRMS calcd for C14H14O3 230.0938, observed 230.0958.
12-Oxapentacyclo[6.6.1.01,6.06,14.010,14]pentadeca-4,9-diene-
3,11-dione (15). In a 25 mL round-bottom flask, the allylic alcohol 3
(0.153 g, 0.660 mmol) and CH2Cl2 (6 mL) were combined. Dess-
Martin periodinane (15% in CH2Cl2, 2.25 mL, 0.79 mmol) was added
all at once. The solution was held at rt for 20 min, then
chromatographed directly to yield 15 (0.135 g, 90% yield) as a
white powder: mp = 148−150 °C; TLC Rf (80% MTBE/PE
1
developed twice) = 0.40; H NMR δ 1.15 (m, 2 H), 1.75 (dd, J =
12.0, 4.7 Hz, 1 H), 2.02 (dd, J = 12.1, 4.8 Hz, 1 H), 2.80 (m, 1 H),
3.11 (m, 1 H), 4.21 (d, J = 4.3 Hz, 2 H), 6.12 (d, J = 10.1 Hz, 1 H),
7.05 (d, J = 12.0 Hz, 1 H), 7.15 (d, J = 8.0 Hz, 1 H); 13C NMR δ (u)
194.6, 168.5, 125.4, 65.7, 39.9, 34.8, 33.5, 29.3, 29.1, 27.6; (d) 146.7,
135.5, 128.4, 33.9; IR (film, cm−1); 3341 (br s), 2951, 1775, 1171,
1022; HRMS calcd for C14H12O3 228.0782, observed 228.0797.
ASSOCIATED CONTENT
■
S
* Supporting Information
1H and 13C NMR spectra for all new compounds. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail: taberdf@udel.edu.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank John Dykins and Jesse McAtee for high-resolution
mass spectrometry financially supported by NSF 054117, Dr.
Shi Bai for NMR spectra financially supported by NSF
CRIF:MU, CHE 080401, and Glenn Yap for X-ray
crystallography. We thank Peter W. DeMatteo for the
retrosynthetic analysis that inspired this work.
REFERENCES
■
(1) Aoyagi, Y.; Yamazaki, A.; Nakatsugawa, C.; Fukaya, H.; Takeya,
K.; Kawauchi, S.; Izumi, H. Org. Lett. 2008, 10, 4429.
(2) Aoyagi, Y.; Yamazaki, A.; Kato, R.; Tobe, F.; Fukaya, H.;
Nishikawa, T.; Nakahashi, A.; Miura, N.; Monde, K.; Takeya, K.
Tetrahedron Lett. 2011, 52, 1851.
(3) Buchner, E.; Curtius, T. Ber. Dtsch. Chem. Ges. 1885, 18, 2377.
(4) Esquivel, B.; Dominguez, R. M.; Hernandez-Ortega, S.; Toscano,
R. A.; Rodriguez-Hahn, L. Tetrahedron 1994, 50, 11593.
(5) Levin, S.; Nani, R. R.; Reisman, S. E. J. Am. Chem. Soc. 2011, 133,
774.
(6) Tseng, C.; Ding, H.; Li, A.; Guan, Y.; Chen, D. Y. Org. Lett. 2011,
13, 4410.
(7) Heinrich, N.; Banwell, M. G.; Willis, A. C.; Cade, A. I.; Capon, R.
J.; Huang, X.-C. Aust. J. Chem. 2012, 65, 1679.
(8) Hay, M. P.; Hicks, K. O.; Pchalek, K.; Lee, H. H.; Blaser, A.;
Pruijn, F. B.; Anderson, R. F.; Shinde, S. S.; Wilson, W. R.; Denny, W.
A. J. Med. Chem. 2008, 51, 6853.
(9) Taber, D. F.; Deker, P. B.; Fales, H. M.; Jones, T. H.; Lloyd, H. A.
J. Org. Chem. 1988, 53, 2968.
(10) Taber, D. F.; Ruckle, R. E., Jr.; Hennessey, M. J. J. Org. Chem.
1986, 51, 4077.
(11) For the preparation of the Cu(II) complex from salicylaldehyde
and t-butylamine, see: (a) Charles, R. G. J. Org. Chem. 1957, 22, 677.
3413
dx.doi.org/10.1021/jo500164x | J. Org. Chem. 2014, 79, 3410−3413