A convenient and versatile route to hydroquinolines by inter- and intramolecular Aza-Diels-Alder pathways

Article abstract of DOI:10.1002/(sici)1521-3773(19990712)38:13/14<1928::aid-anie1928>3.0.co;2-1

Surprisingly, the simplest method possible for o-azaxylylene production, base-induced elimination of hydrogen chloride from amide or sulfonamide derivatives of o-(chloromethyl)aniline, has never been reported. This powerful approach provides easy and stereoselective access to polycyclic hydroquinolines, as shown for an example in Equation (1).

Full text of DOI:10.1002/(sici)1521-3773(19990712)38:13/14<1928::aid-anie1928>3.0.co;2-1

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reflections for 2.18 < q < 25.558, 2813 reflections with I > 2s(I); R ˆ 0.0446,
R' ˆ 0.0482, S ˆ 0.933.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge Crystallo-
graphic Data Center as supplementary publication no. CCDC-114199 (3)
and CCDC 114200 (4). Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge CB21EZ, UK (fax:
(‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
Received: February 11, 1999 [Z13025IE]
German version: Angew. Chem. 1999, 111, 2051 ± 2054
Keywords: ansa compounds ´ C C coupling ´ hafnium ´
sandwich complexes ´ zirconium
[1] E. D. Bergmann, Chem. Rev. 1968, 68, 41 ± 84.
[2] P. Yates, Adv. Alicyclic Chem. 1968, 2, 59 ± 184.
[3] H. Schwemlein, H. H. Brintzinger, J. Organomet. Chem. 1983, 254,
69 ± 73.
[4] L. Djakovitch, W. A. Herrmann, J. Organomet. Chem. 1997, 545 ± 546,
399 ± 405.
[5] V. I. Koshutin, V. M. Levchenko, L. L. Koshchutina, J. Org. Chem.
(USSR) 1980, 16(1), 49 ± 56 [Chem. Abstr. 1980, 92, 214933].
[6] I. F. Urazovskii, I. E. Nifantꢁev, K. A. Butakov, V. I. Ponomarev,
Metalloorg. Khim. 1991, 4(4), 745 ± 751.
[7] A. Bertuleit, C. Fritze, G. Erker, R. Fröhlich, Organometallics 1997,
16, 2891 ± 2899.
Figure 2. The molecular structure of 4. Selected bond lengths [Š] and
angles [8]: Hf1 C1 2.509(4), Hf1 C2 2.486(4), Hf1 C3 2.495(5), Hf1 C4
2.513(4), Hf1 C5 2.479(4), C1 C11 1.476(6), C11 C12 1.363(6), C12 C13
1.459(6), C6 C13 1.492(6), C13 C14 1.354(6), N1 C11 1.389(6), N1 C16
1.414(7); Cl2-Hf1-Cl1 96.31(5), C1-Hf1-C6 73.77(14), C12-C11-C1
123.0(4), C11-C12-C13 124.6(4), C6-C13-C12 117.9(4), C14-C13-C6
119.4(4).
and 2.163 Š, and the corresponding Cp(centroid)-Zr-
Cp'(centroid) angle is 128.48. The single and double bonds
of the bridging allyl group are 1.476(6) and 1.363(6) Š long,
respectively, and the N1 C11 bond length is 1.389(6) Š.
[8] L. Duda, G. Erker, R. Frönlich, F. Zippel, Eur. J. Inorg. Chem. 1998,
1153 ± 1162.
[9] K. Hafner, K. H. Vöpel, G. Ploss, C. König, Organic Synthesis. Coll.
Vol. 5, Wiley, New York, 1973, pp. 431 ± 433.
[10] D. W. Macomber, W. P. Hart, M. D. Rausch, J. Am. Chem. Soc. 1982,
104, 884 ± 886.
Experimental Section
[11] T. Kawase, N. Nisato, M. Oda, J. Chem. Soc. Chem. Commun. 1989,
1145 ± 1146.
[12] C. H. Saldarriaga-Molina, A. Clearfield, I. Bernal, J. Organomet.
Chem. 1974, 80, 79 ± 90.
2: In an typical run a solution of LiCH(SiMe3)2 (7.4 mmol, 1.23 g) in THF
(ca. 10 mL) was added dropwise to a solution of 1 (1 g, 7.4 mmol) in THF
(ca. 20 mL) at 08C with stirring. The mixture was allowed to warm to room
temperature for 2 h. The solvent was removed in vacuo to give 2 as a light
1
brown powder (1.02 g, 98%). H NMR (300 MHz, [D6]benzene): d ˆ 6.76,
6.54 (d, 4H; Cp), 4.48, 4.05 (d, 2H; CH2), 2.94 (s, 6H; NMe2).
3: ZrCl4 (0.85 g, 3.6 mmol) was added in several portions with stirring to a
solution of 2 (1.02 g, 7.2 mmol) in THF (ca. 20 mL) at 788C. The mixture
was allowed to warm to room temperature and stirred overnight. The
solvent was removed in vacuo. The residue was extracted with dichloro-
methane, and the extract filtered. The solvent was slowly removed from the
filtrate in vacuo to give yellow crystalline 3 (1.30 g, 91% ). Elemental
analysis calcd for C16H17Cl2NZr (385.43): C 49.81, H 4.41, N 3.63; found: C
48.71, H 4.28, N 3.41; 1H NMR (300 MHz, CDCl3): d ˆ 6.74, 6.14, 6.08 (m,
8H; Cp), 5.62 (s, 1H; CH), 5.03, 4.93 (d, 2H; CH2), 2.62 (s, 6H; NMe2);
13C NMR (75 MHz, CDCl3): d ˆ 148.7, 140.2, 138.0, 126.9 (ipso-C), 125.0,
124.5, 114.9, 112.0 (CH of Cp), 111.6 (CH2), 105.8 (CH), 42.7 (NMe2).
A Convenient and Versatile Route to
Hydroquinolines by Inter- and Intramolecular
Aza-Diels ± Alder Pathways**
Henning Steinhagen and E. J. Corey*
Described herein is a practical method for the generation of
o-azaxylylenes and their application to the synthesis of a
variety of hydroquinoline derivatives. The formation of o-
azaxylylenes has previously been reported by photochemical
fragmentation,[1, 2] fluoride induced elimination of o-trime-
4: The procedure is similar to that of 3, except that Et2O was used as the
solvent instead of THF. The reaction of HfCl4 (1.20 g, 3.74 mmol) and 2
(1.05 g, 7.44 mmol) resulted in yellow crystalline 4 (1.10 g, 77%). Elemental
analysis calcd for C16H17Cl2NHf (472.70): C 40.65, H 3.62, N 2.96; found: C
40.64, H 3.39, N 3.00; 1H NMR (300 MHz, CDCl3): d ˆ 6.65, 6.07, 5.99 (m,
8H; Cp), 5.62 (s, 1H; CH), 4.99, 4.95 (d, 2H; CH2), 2.64 (s, 6H; NMe2);
13C NMR (75 MHz, CDCl3): d ˆ 147.3, 138.8, 134.5, 124.6 (ipso-C), 122.1,
121.7, 111.9, 108.9 (CH of Cp), 110.7 (CH2), 104.3 (CH), 41.4 (NMe2).
[*] Prof. Dr. E. J. Corey, Dr. H. Steinhagen
Department of Chemistry and Chemical Biology
Harvard University
Crystal data for 3: C16H17Cl2NZr, Mr ˆ 385.43, monoclinic, space group P21/
12 Oxford Street, Cambridge, MA 02138 (USA)
Fax: (‡1)617-495-0376
n, a ˆ 10.8208(9), b ˆ 10.5382(12), c ˆ 13.9891(10) Š, b ˆ 101.380(5)8, Vˆ
3
1563.8(2) Š3, F(000) ˆ 776; Z ˆ 4, 1calcd ˆ 1.637 gcm
,
m(MoKa) ˆ
10.33 cm 1, Tˆ 294(2) K, crystal size 0.38  0.36  0.08 mm, 3504 unique
reflections for 2.18 < q < 25.008, 2722 reflections with I > 2s(I); R ˆ 0.0269,
R' ˆ 0.0380, S ˆ 0.917.
E-mail: corey@chemistry.harvard.edu
[**] This research was supported by a postdoctoral fellowship to H.S. from
the Deutscher Akademischer Austauschdienst (DAAD) and a grant
from the National Institute of Health. We thank Mr. Michael Grogan
and Dr. Richard Staples for X-ray crystallographic analyses.
Crystal data for 4: C16H17Cl2NHf, Mr ˆ 472.70, monoclinic, space group P21/
n, a ˆ 10.830(2), b ˆ 10.516(2), c ˆ 13.965(3) Š, b ˆ 101.51(3)8, Vˆ
3
1558.5(2) Š3, F(000) ˆ 904; Z ˆ 4, 1calcd ˆ 2.015 gcm
,
m(MoKa) ˆ
Supporting information for this article is available on the WWW
under http://www.wiley-vch.de/home/angewandte/ or from the author.
10.33 cm 1, Tˆ 294(2) K, crystal size 0.40  0.38  0.20 mm, 2827 unique
1928
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Angew. Chem. Int. Ed. 1999, 38, No. 13/14

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thylsilylaminobenzyltrimethylammonium salts,[3] and by py-
rolytic elimination of o-hydroxymethylanilines at 140 ±
1808C.[4] Surprisingly, the simplest method possible for o-
azaxylylene production, base-induced elimination of hydro-
gen chloride from amide or sulfonamide derivatives of o-
chloromethylaniline, has never been reported. We have found
that this process is highly effective for the generation of o-
azaxylylenes and that these intermediates are readily trapped
by olefins, especially p-electron-rich olefins, to form hydro-
quinoline derivatives as exemplified in Scheme 1.[5] The
suprafacial (cis) cycloaddition. The structures of the products
were unambiguously determined in each case by X-ray
crystallographic analysis.[7] In each case NMR analysis of the
crude reaction mixture indicated the absence of diastereo-
meric product.
The formation of the 3-chlorotetrahydroquinoline deriva-
tive 6 is especially interesting because this structure repre-
sents the core of the potent antiviral agent virantmycin,[8]
previously accessible only by lengthy multistep routes.[9] The
enantiospecific synthesis of the chiral tetracyclic tetrahydro-
quinoline 8 ([a]2D5 ˆ ‡82, c ˆ 0.95 in CHCl3, 99% ee by chiral
HPLC analysis)[10] from the readily available precursor 7[11] is
also noteworthy and illustrative of the potential of this
method for the production of enantiomerically pure
(ꢀ99% ee) products. X-ray crystallographic analysis of 8
provided proof of the stereochemistry and demonstrated a
molecular conformation in which the chair-form cyclohexane
ring is approximately orthogonal to the planar tricyclic core
(Figure 1).
Cl
Cs2CO3
, CH2Cl2
NH
Y
N
Y
OEt
OEt
1a, Y = SO2Me
1b, Y = SO2pTol
1c, Y = CO2tBu
2a, Y = SO2Me (78%)
2b, Y = SO2pTol (74%)
2c, Y = CO2t Bu (83%)
Scheme 1. Generation and trapping by ethyl vinyl ether of o-azaxylylenes.
synthesis of hydroquinolines 2a and 2b was conducted at
788C with addition of the chloromethylaniline derivative to
the reaction mixture containing ethyl vinyl ether (2 equiv) and
cesium carbonate (2.5 equiv) in dichloromethane over 4 h by
syringe pump.[6] The hydroquinoline 2c was prepared in a
similar manner but with addition by syringe pump at 238C
over 24 h.
This new approach to hydroquinolines is especially power-
ful in the intramolecular version. As shown in Scheme 2, these
intramolecular reactions proceed under mild conditions and
provide hydroquinolines 4, 6, and 8 stereospecifically by a
Figure 1. ORTEP plot of 8 as determined by X-ray crystallography.[7c]
A variety of other trapping experiments with in situ
generated o-azaxylylenes are summarized in Table 1. Vinyl
ether substrates generally gave the products of [2 ‡ 4] cyclo-
addition (9, 10, 11). On the other hand, ketene acetals
afforded the conjugate adduct (12 with 1b) and the cyclo-
adduct (13 with 1c). The p-electron-rich substituted acety-
lenes N,N-diethyl-1-propynylamine and 1-ethoxy-1-propyne
furnished the dihydroquinoline derivatives 14 and 15. In
contrast, the p-electron-deficient dienophile ethylpropiolate
did not afford Diels ± Alder products with the o-azaxylylenes
from 1b or 1c.
Cl
2.5 equiv Cs2CO3
CH2Cl2, rt, 20 h
NH
N
O
O
O
O
3
rac-4 (85%)
Cl
H
H
Cl
2.5 equiv Cs2CO3
CH2Cl2, rt, 48 h
R
R
NaOH, EtOH
reflux
S
S
R
R
NH
Cl
N
N
N
H
H
O
O
H
H
H
90 %
O
OH
O
O
O
8
16
17
rac-6 (91%)
5
Et
O
CO2Me
Cl
HBr, phenol
reflux
H
Et
N
H
NH
R
2.5 equiv Cs2CO3
CH2Cl2, rt, 40 h
95%
S
R
SO2pTol
12
NH
N
H
O
H
R
O
O
The various products described above can be transformed
O
H
8 (76%, 99% ee)
into a variety of compounds. For example, the tetracyclic
carbamate 8 is easily cleaved upon treatment with ethanolic
NaOH to the corresponding chiral tricyclic aminoalcohol 16
7
Scheme 2. Intramolecular [2‡4] cycloaddition reactions of o-azaxylylenes
to form tri- and tetracyclic hydroquinolines.
Angew. Chem. Int. Ed. 1999, 38, No. 13/14
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Table 1. Trapping reactions of o-azaxylylenes with p-electron-rich dienophiles.
an additional 30 min. This solution of chloroformate was
transferred by cannula to a solution of o-aminobenzyl
alcohol (Alfa, 185 mg, 1.5 mmol) and pyridine (0.14 mL,
1.7 mmol) in dichloromethane (5 mL) over 15 min at 08C.
The reaction mixture was warmed to 238C and stirred for
an additional 2 h. Aqueous workup and chromatography on
silica gel with hexane/ethyl acetate (6/1) afforded the
hydroxy carbamate coupling product as a colorless crystal-
line solid (282 mg, 76%). M.p. 91 ± 928C; [a]2D5 ˆ ‡150 (c ˆ
0.77 in CHCl3); IR (film): nÄ ˆ 3348, 3344, 2939, 1731, 1702,
Substrate
Dienophile Conditions[a]
Product
Yield [%][b]
C
95
1593, 1529, 1454, 1305, 1231, 1219, 1192, 1040, 1008,
1
938 cm
;
1H NMR (400 MHz, CDCl3, 258C): d ˆ 7.94 (d,
A
B
91
90
J ˆ 8 Hz, 1H), 7.82 (brs, 1H, NH), 7.32 (t, J ˆ 7.4 Hz, 1H),
7.17 (d, J ˆ 7.4 Hz, 1H), 7.03 (t, J ˆ 7.4 Hz, 1H), 5.95 ± 6.00
(m, 1H), 5.78 ± 5.83 (m, 1H), 5.26 ± 5.29 (m, 1H), 4.69 (d,
J ˆ 5.2 Hz, 2H), 2.16 (t, J ˆ 5.6 Hz, 1H, OH), 1.60 ± 2.14 (m,
6H); 13C NMR (100 MHz, CDCl3, 258C): d ˆ 153.98,
137.82, 132.70, 129.25, 129.00, 128.96, 126.00, 123.38,
121.12, 69.04, 64.28, 28.57, 24.94, 18.94; HR-MS (EI): calcd
‡
(found) for C14H17NO3 [M ]: 247.1208 (247.1218). A solu-
tion of thionyl chloride (102 mL, 1.40 mmol) in dichloro-
methane (4 mL) was added over 15 min to a solution
containing the hydroxy carbamate (240 mg, 0.97 mmol) and
triethylamine (195 mL, 1.40 mmol) in dichloromethane
(4 mL). The solution was stirred for 2 h and the solvent
was removed in vacuo. Chromatography of the crude
product on silica gel with hexane/ethyl acetate (6/1)
afforded 7 as a colorless crystalline solid (230 mg, 89%).
M.p. 120 ± 1218C; [a]D25 ˆ ‡141 (c ˆ 0.95 in CHCl3); IR
A
B
90
40
(film): nÄ ˆ 3285, 3037, 3029, 2945, 2913, 1687, 1590, 1527,
1
1456, 1296, 1248, 1049 cm
;
1H NMR (400 MHz, CDCl3,
258C): d ˆ 7.87 (d, J ˆ 7.8 Hz, 1H), 7.37 (t, J ˆ 1.4, 7.8 Hz,
1H), 7.28 (d, J ˆ 1.4, 7.6 Hz, 1H), 7.09 (t, J ˆ 7.5 Hz, 1H),
6.87 (brs, 1H, NH), 5.98 ± 6.02 (m, 1H), 5.79 ± 5.83 (m, 1H),
5.29 ± 5.31 (m, 1H), 4.62 (s, 2H), 1.65 ± 2.14 (m, 6H);
13C NMR (100 MHz, CDCl3, 258C): d ˆ 153.66, 136.89,
132.94, 130.14, 130.08, 127.19, 125.83, 124.30, 122.82, 69.40,
44.03, 28.56, 24.94, 18.92; HR-MS (EI): calcd (found) for
B
B
60
95
‡
C14H16NO2Cl [M ]: 265.0869 (265.0878).
8:
A suspension of 7 (53 mg, 0.2 mmol) and cesium
carbonate (163 mg, 0.5 mmol) in dichloromethane (5 mL)
was stirred for 40 h at 238C. The reaction mixture was
filtered through
a pad of celite and the solvent was
evaporated in vacuo. Chromatography of the crude product
on silica gel with hexane/ethyl acetate (6/1) gave 8 as a
colorless crystalline solid (35 mg, 76%). M.p. 149 ± 1508C;
[a]2D5 ˆ ‡82 (c ˆ 0.95 in CHCl3); IR (film): nÄ ˆ 2929, 1744,
1494, 1459, 1390, 1364, 1349, 1339, 1324, 1293, 1227, 1209,
[a] A: 2 h, 788C, 10 equiv dienophile, addition of substrate over 15 min; B: 24 ± 48 h, rt,
10 equiv dienophile, addition of substrate over 1 min; C: 24 h, rt, 2 equiv dienophile,
addition of substrate by syringe pump over 24 h; [b] yields of isolated products after
chromatography on silica gel with mixtures of hexane/EtOAc (‡1% NEt3 for 14 and 15)
for elution.
1
1185, 1135, 1104, 1060, 994, 946 cm 1; H NMR (400 MHz,
CDCl3, 258C): d ˆ 8.40 (d, J ˆ 8.4 Hz, 1H), 7.21 (t, J ˆ
7.8 Hz, 1H), 7.10 (d, J ˆ 7.5 Hz, 1H), 6.98 (t, J ˆ 7.4 Hz,
1H), 4.64 (dt, J ˆ 6.6, 8.6 Hz, 1H), 4.26 (dd, J ˆ 3.8, 6.6 Hz,
1H), 3.12 (dd, J ˆ 5.9, 16.7 Hz, 1H), 2.65 (d, J ˆ 16.4 Hz,
1H), 2.08 ± 2.22 (m, 2H), 1.71 ± 1.77 (m, 1H), 1.44 ± 1.61 (m,
2H), 1.31 ± 1.36 (m, 1H), 1.15 ± 1.22 (m, 1H); 13C NMR (100 MHz, CDCl3,
258C): d ˆ 154.28, 134.50, 129.47, 127.06, 122.74, 122.09, 116.31, 73.06, 56.03,
32.55, 29.19, 27.14, 24.44, 19.66; HR-MS (FAB): calcd (found) for
([a]2D5 ˆ ‡53, c ˆ 0.9 in CHCl3), and the sulfonamide 12
underwent cyclization with HBr/phenol to the bicyclic lactam
17 in 95% yield.
The reactions described herein define a practical and
versatile method for the generation and use of o-azaxylylenes
in aza-Diels ± Alder reactions which proceed from readily
available precursors and under mild conditions.
‡
C14H15NO2Na [M ‡ Na ]: 252.1000 (252.1008).
16: A solution of 8 (31.5 mg, 0.137 mmol) in 10% ethanolic NaOH (2 mL)
was heated at reflux for 1 h. After evaporation of the solvent in vacuo the
mixture was suspended in water (5 mL) and extracted with chloroform
(10 mL Â 3). The combined organic layers were dried over sodium sulfate,
filtered, and the solvent was removed in vacuo. Chromatography of the
crude mixture on silica gel with hexane/ethyl acetate (6/1) afforded the
chiral aminoalcohol 16 as a colorless crystalline solid (25 mg, 90%).
M.p. 134 ± 1358C; [a]2D5 ˆ ‡53 (c ˆ 0.9 in CHCl3); IR (film): nÄ ˆ 3419, 3405,
Experimental Section
All reactions were carried out under an inert atmosphere with anhydrous
solvents.
3378, 3371, 3300, 3235, 3225, 3201, 3017, 2936, 2920, 2895, 2855, 1606, 1504,
1
1496, 1486, 1444, 1374, 1289, 1104, 1054, 1043, 1008 cm
;
1H NMR
7: A solution of (R)-2-cyclohexen-1-ol[10] (147 mg, 1.5 mmol) in diethyl
ether (1 mL) was added to a solution of phosgene (3 mmol) in toluene
(1.6 mL) at 788C. The mixture was stirred at 158C for 3 h and at 08C for
(500 MHz, CDCl3, 258C): d ˆ 6.97 (t, J ˆ 7.6 Hz, 1H), 6.93 (d, J ˆ 7.4 Hz,
1H), 6.57 (t, J ˆ 7.3 Hz, 1H), 6.49 (d, J ˆ 8 Hz, 1H), 4.21 (brs, 1H, NH),
3.81 ± 3.85 (m, 1H), 3.64 (t, J ˆ 3 Hz, 1H), 3.03 (dd, J ˆ 5.6, 16.2 Hz, 1H),
1930
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Angew. Chem. Int. Ed. 1999, 38, No. 13/14

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2.50 (dd, J ˆ 1.6, 16.2 Hz, 1H), 1.89 ± 1.93 (m, 1H), 1.73 ± 1.81 (m, 2H),
1.60 ± 1.63 (m, 1H), 1.55 (brs, 1H, OH), 1.28 ± 1.40 (m, 3H); 13C NMR
(100 MHz, CDCl3, 258C): d ˆ 143.47, 129.73, 126.68, 119.18, 116.49, 113.43,
72.67, 53.93, 33.16, 32.96, 27.80, 25.31, 23.35; HR-MS (CI): calcd (found) for
re- and si-Face-Selective Nitroaldol Reactions
Catalyzed by a Rigid Chiral Quaternary
Ammonium Salt: A Highly Stereoselective
Synthesis of the HIV Protease Inhibitor
Amprenavir (Vertex 478)**
‡
C13H17NO [M ]: 204.1388 (204.1397).
Received: February 3, 1999 [Z12991IE]
German version: Angew. Chem. 1999, 111, 2054 ± 2056
E. J. Corey* and Fu-Yao Zhang
Keywords: antiviral agents ´ azaxylylenes ´ cycloadditions ´
heterocycles ´ stereocontrol
The development of therapeutically useful HIV protease
inhibitors has been one of the major contributions of synthetic
and medicinal chemistry to human well-being during this
decade.[1, 2] Since several of these agents possess chiral
substituted 1,3-diamino-2-hydroxypropyl segments as a cen-
tral structural subunit, for example, the phenylalanine-related
subunit 1, considerable research effort has been directed at
methods for their synthesis.[3, 4] Described herein is a new
strategy for stereocontrol in the synthesis of subunits of type 1
and a specific application with a practical synthesis of
amprenavir (2),[5, 6] an important second generation HIV
[1] E. M. Burgess, L. McCullagh, J. Am. Chem. Soc. 1966, 88, 1580 ± 1581.
[2] a) M. Lancaster, D. J. H. Smith, J. Chem. Soc. Chem. Commun. 1980,
471 ± 472; b) K. Wojciechowski, Tetrahedron 1993, 49, 7277 ± 7286.
[3] Y. Ito, S. Miyata, M. Nakatsuka, T. Saegusa, J. Am. Chem. Soc. 1981,
103, 5250 ± 5251.
[4] a) R. D. Bowen, D. E. Davies, C. W. G. Fishwick, T. O. Glasbey, S. J.
Noyce, R. C. Storr, Tetrahedron Lett. 1982, 23, 4501 ± 4504; b) J. M.
Â
Wiebe, A. S. Caille, L. Trimble, C. K. Lau, Tetrahedron 1996, 52,
11705 ± 11724; see also c) E. Foresti, P. Spagnolo, P. Zanirato, J. Chem.
Soc. Perkin Trans. 1, 1989, 1354 ± 1356.
[5] For a general review of Diels ± Alder additions with other types of
azadienes, see L. Tietze in Top. Curr. Chem. 1997, 189, 1 ± 120.
[6] The substrates 1a ± 1c of Scheme 1 were prepared from o-amino-
benzyl alcohol by the sequence: 1) N-acylation or N-sulfonylation,
and 2) conversion of the resulting benzyl alcohol into the correspond-
ing chloride by reaction with thionyl chloride in dichloromethane.
[7] a) Crystal structure data for 4: C12H13NO2; monoclinic; P21c; a ˆ
14.972(7), b ˆ 5.599(3), c ˆ 13.403(5) Š; a ˆ 90, b ˆ 114.93(3), g ˆ
908; Z ˆ 4; R1 [I > 2s(I)] ˆ 0.0386; b) crystal structure data for 8:
C14H15NO2; monoclinic; P21c; a ˆ 17.7653(11), b ˆ 9.1414(6), c ˆ
15.4356(10) Š; a ˆ 90, b ˆ 115.5670(10), g ˆ 908; Z ˆ 8; R1 [I >
2s(I)] ˆ 0.0481; c) crystal structure data for 6: C12H12NO2Cl; mono-
clinic; P21n; a ˆ 6.2128(5), b ˆ 12.1974(12), c ˆ 14.4788(13) Š; a ˆ 90,
b ˆ 98.188(3), g ˆ 908; Z ˆ 4; R1 [I > 2s(I)] ˆ 0.0601. Crystallographic
data (excluding structure factors) for the structures reported in this
paper have been deposited with the Cambridge Crystallographic Data
Centre as supplementary publication nos. CCDC-113239/113240/
113241. Copies of the data can be obtained free of charge on
application to CCDC, 12 Union Road, Cambridge, CB2 1EZ, UK
(fax: (‡44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
H
N
OH
2
R2
OH
2
R1NH
N
O
N
3
R3
SO2
NH2
O
(2R or 2S)
1
2
protease inhibitor with a number of clinical advantages over
first generation agents. The method currently being used to
produce 2 involves the activation of the carboxyl group of N-
tert-butoxycarbonylphenylalanine and transformation to the
corresponding diazomethyl ketone with diazomethane as a
key element.[7] Because of the intrinsic hazards of this step a
safer alternative seems preferable.
The sequence utilized for the synthesis of amprenavir is
summarized in Scheme 1. N,N-Dibenzyl-(S)-phenylalaninal
(3)[8] in tetrahydrofuran (THF) was added to a mixture of
quaternary ammonium salt 4,[9, 10] nitromethane, and finely
divided potassium fluoride in THF with stirring. After 6 h the
nitro alcohol 5 was isolated in 86% yield by flash chromatog-
raphy on silica gel.[10] The more polar C(2) diastereomer of 5
was isolated in 5% yield, which indicated a 17:1 diastereo-
selectivity for the nitroaldol reaction.[11] In contrast, the
nitroaldol reaction of 3 in the presence of tetra-n-butylam-
monium fluoride[8c] afforded a mixture of 5 and the C(2)
diastereomer with only 4:1 diastereoselectivity under the
same reaction conditions. Treatment of nitro alcohol 5 with
2.5 equivalents of NiCl2 and 25 equivalents of NaBH4 in
methanol at 08C for 10 min with vigorous agitation provided
Å
[8] S. Omura, A. Nakagawa, Tetrahedron Lett. 1981, 22, 2199 ± 2202.
[9] a) M. L. Hill, R. A. Raphael, Tetrahedron 1990, 46, 4587 ± 4594; b) Y.
Morimoto, H. Shirahama, Tetrahedron 1996, 52, 10609 ± 10630.
[10] Chiralpak AD column (Daicel Chemical Industries); flow rate:
1
0.7 mL min
; eluent: 6% iPrOH in hexane; UV detection at
254 nm; tR[(‡)-8] ˆ 26.4 min, tR[( )-8] ˆ 30.0 min.
[11] The substrates 3, 5, and 7 were prepared starting from the corre-
sponding allylic alcohols by the sequence: 1) conversion into chlor-
oformates by reaction with phosgene, 2) treatment of the chlorofor-
mates with o-aminobenzyl alcohol, and 3) conversion of the resulting
hydroxy carbamates into the corresponding chlorides with thionyl
chloride (see experimental section for details). 3-Chloro-2-methyl-2-
propen-1-ol was prepared according to A. Mooradian, J. B. Cloke, J.
Am. Chem. Soc. 1946, 68, 785 ± 789; L. F. Hatch, J. J. Russ, L. B.
Gordon, J. Am. Chem. Soc. 1947, 69, 2614 ± 2616. Enantiomerically
pure (R)-2-cyclohexen-1-ol was prepared by kinetic resolution with
lipase as described by T. Fukazawa, T. Hashimoto, Tetrahedron:
Asymmetry 1993, 4, 2323 ± 2326.
[*] Professor E. J. Corey, Dr. F.-Y. Zhang
Department of Chemistry and Chemical Biology
Harvard University
12 Oxford Street, Cambridge, Massachusetts, 02138 (USA)
Fax: (‡1)617-495-0376
E-mail: corey@chemistry.harvard.edu
[**] This research was supported by grants from the National Science
Foundation and the National Institutes of Health. We thank Mr.
David Barnes-Seeman for the graphic illustrations in Figures 1 and 2.
Angew. Chem. Int. Ed. 1999, 38, No. 13/14
ꢀ WILEY-VCH Verlag GmbH, D-69451 Weinheim, 1999
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