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M. Gutierrez-Rodrıguez et al. / Tetrahedron 60 (2004) 5177–5183
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[m, 5H, 3-H (Leu), 2-H (Leu), 4-H (Pro)], 2.00 [m, 2H, 3-H
(Pro)], 2.64 [m, 1H, 1-H (allyl)], 2.86 [m, 1H, 1-H (allyl)],
3.15 [m, 1H, 5-H (Pro)], 3.55 [m, 1H, 5-H (Pro)], 4.41 [m,
1H, 2-H (Leu)], 5.05 [s, 2H, CH2 (Bn)], 5.10 [dd, 2H, J¼14,
6 Hz, 3-H (allyl)], 5.63–5.72 [m, 1H, 2-H (allyl)], 7.28–
7.35 (m, 5H, aromatics), 7.46 (bs, 1H, NH). 13C NMR
(75 MHz, CDCl3): d 21.65, 22.30, 22.87, 24.67, 28.32,
34.71, 38.26, 41.15, 49.41, 50.98, 66.76, 69.73, 80.14,
119.37, 128.34, 128.54, 132.77, 135.63, 153.90, 172.79,
173.92. ESI-MS m/e 459.3 [MþH]þ. Anal. Calcd for
C26H38N2O5: C, 68.10; H, 8.35; N, 6.11. Found: C, 68.02;
H, 8.47; N, 6.08.
Me (Leu)], 1.41 [s, 9H, Me (Boc)], 1.45–1.78 [m, 5H, 3-H
(Leu), 4-H (Leu), 4-H, 3-H), 1.84–1.91 (m, 2H, 9-H), 2.10
(m, 1H, 3-H), 2.20 (m, 1H, 4-H), 2.80 (m, 1H, 9-H), 3.38–
3.61 (m, 2H, 2-H), 3.92 (m, 1H, 8-H), 4.23 [m, 1H, 2-H
(Leu)], 4.41 (m, 1H, 8-H), 5.10 [m, 2H, CH2 (Bn)], 7.34 (m,
5H, aromatics). 13C NMR (75 MHz, CDCl3): d 21.15, 23.32,
23.42, 24.20, 28.40, 34.31, 39.42, 41.01, 42.39, 47.75,
57.58, 65.96, 66.34, 67.08, 79.32, 127.92, 128.00, 128.22,
128.35, 136.41, 153.14, 165.99, 173.80. ESI-MS m/e 445.3
[MþH]þ. Anal. Calcd for C25H36N2O5: C, 67.54; H, 8.16;
N, 6.30. Found: C, 67.40; H, 8.36; N, 6.27.
4.1.6. Synthesis of (5R,8RS)-7-[(1R)-1-benzyloxy-car-
bonyl-3-methylbutyl]-1-(tert-butoxycarbonyl)-8-
hydroxy-6-oxo-1,7-diazaspiro-[4.4]nonane (12a). OsO4
(2.5%, w/w solution in tert-butanol, 2.9 mL, 0.23 mmol)
was added to a solution of the dipeptide 6a (1.567 g,
3.42 mmol) in (2:1) MeOH/H2O (108 mL), under argon,
which was stirred at room temperature for 10 min. NaIO4
(2.195 g, 10.3 mmol) was slowly added. After 24 h of
stirring at room temperature, H2O (40 mL) was added, and
the mixture was extracted with EtOAc (3£60 mL). The
combined organic extracts were washed with brine (50 mL),
dried over Na2SO4, and evaporated to dryness. The residue
was purified by flash chromatography, using a 20–100%
gradient of EtOAc in hexane as eluant, to give the two
epimeric hemiaminals 12a, as white solids, whose absolute
configuration at C8 could not be assigned. Epimer A.
(584 mg, 38%). Mp 140–141 8C. [a]2D0 248 (c, 1 in MeOH).
HPLC [Novapack C18] (A/B, 40:60) tR 14.45 min. 1H NMR
(300 MHz, acetone-d6): d 0.88 and 0.91 [2d, 6H, J¼7, 6 Hz,
Me (Leu)], 1.29 [s, 9H, Me (Boc)], 1.64–2.30 [m, 9H, 3-H
(Leu), 4-H (Leu), 3-H, 4-H, 9-H], 2.68 (dd, 1H, J¼6, 13 Hz,
OH), 3.37 (m, 2H, 2-H), 4.51 [dd, 1H, J¼5, 11 Hz, 2-H
(Leu)], 5.13 [d, 2H, J¼15 Hz, CH2 (Bn)], 5.79 (t, 2H,
J¼5 Hz, 8-H), 7.40 (m, 5H, aromatics). 13C NMR (75 MHz,
acetone-d6): d 21.80, 23.10, 24.58, 24.87, 28.59, 39.57,
40.79, 41.65, 48.48, 53.42, 66.89, 67.06, 79.41, 81.24,
128.64, 128.99, 129.08, 129.29, 135.26, 153.74, 171.67,
172.01. ESI-MS m/e 483.4 [MþNa]þ. Anal. Calcd for
C25H36N2O6: C, 65.20; H: 7.88; N, 6.08. Found: C, 65.18;
H, 7.92; N, 6.06. Epimer B. (584 mg, 38%). Mp 134–
135 8C. [a]2D0 þ268 (c, 1.2 in MeOH). HPLC [Novapack
C18] (A/B, 40:60) tR 15.97 and 18.75 min. 1H NMR
(300 MHz, CDCl3): d 0.87 and 0.89 [2d, 6H, J¼7, 6 Hz,
Me (Leu)], 1.40 [s, 9H, Me (Boc)], 1.50–2.60 [m, 9H, 3-H
(Leu), 4-H (Leu), 3-H, 4-H, 9-H], 3.40 (m, 2H, 2-H), 4.20–
5.40 [m, 5H, 2-H (Leu), CH2 (Bn), 8-H], 7.40 (m, 5H,
aromatics). 13C NMR (75 MHz, acetone-d6): d 21.30, 23.47,
24.14, 24.93, 28.28, 38.93, 40.25, 42.55, 47.73, 53.21,
66.82, 67.74, 77.58, 80.66, 128.11, 128.38, 128.54, 128.64,
135.83, 154.14, 171.24, 174.16. ESI-MS m/e483.4
[MþNa]þ. Anal. Calcd for C25H36N2O6: C, 65.20; H,
7.88; N, 6.08.
4.1.4. Synthesis of N-[N-(tert-butoxycarbonyl)-2-(2-
hydroxyethyl)-L-prolyl]-D-leucine benzyl ester (10a).
OsO4 (2.5%, w/w solution in tert-butanol, 4.81 mL,
0.38 mmol) was added to a solution of the dipeptide 6a
(2.603 g, 5.68 mmol) in (2:1) MeOH/H2O (180 mL), under
argon, which was stirred at room temperature for 10 min.
NaIO4 (3.645 g, 17.11 mmol) was slowly added. After 2 h
of stirring at room temperature, H2O (40 mL) was added,
and the mixture was extracted with EtOAc (3£80 mL). The
combined organic extracts were washed with brine (50 mL),
dried over Na2SO4, and evaporated to dryness. The residue
was dissolved into dry EtOAc (180 mL), and the solution
was cooled to 278 8C. To this solution, NaBH4 (215 mg,
5.68 mmol) dissolved into MeOH (13 mL) was added, the
mixture was stirred at 278 8C for 1 h, and then 30 min at
room temperature. H2O (50 mL) was added, and the organic
phase was washed with brine (50 mL), dried over Na2SO4,
and evaporated to dryness. The residue was purified by flash
chromatography, using a 20–100% gradient of EtOAc in
hexane as eluant, to give the alcohol 10a as a syrup (1.200 g,
50%). [a]2D0 21.88 (c, 1.5 in MeOH). HPLC [mBondapack
C18] (A/B, 37:63) tR 9.98 min. 1H NMR (300 MHz, DMSO-
d6, 90 8C): d 0.89 and 0.86 [2d, 6H, J¼7, 6 Hz, Me (Leu)],
1.38 [s, 9H, Me (Boc)], 1.55–1.73 [m, 5H, 3-H (Leu), 4-H
(Leu), 4-H (Pro)], 1.90–2.08 [m, 4H, 3-H (Pro), 1-H
(hydroxyethyl)], 3.13–3.63 [m, 4H, 5-H (Pro), 2-H (hydro-
xyethyl)], 4.35 [m, 1H, 2-H (Leu)], 5.11 [s, 2H, CH2 (Bn)],
7.34 (m, 5H, aromatics), 7.80 (bs, 1H, NH). 13C NMR
(75 MHz, CDCl3): d 21.58, 22.77, 24.59, 28.27, 36.44,
38.22, 41.13, 48.57, 51.01, 58.78, 60.29, 66.87, 80.10,
128.21, 128.32, 128.51, 135.01, 153.51, 172.51, 174.54.
ESI-MS m/e 463.3 [MþH]þ. Anal. Calcd for C25H38N2O6:
C, 64.91; H, 8.28; N, 6.06. Found: C, 64.79; H, 8.45; N,
6.03.
4.1.5. Synthesis of (5R)-6-[(1R)-1-benzyloxycarbonyl-3-
methylbutyl]imino-1-(tert-butoxycarbonyl)-7-oxa-1-
azaspiro-[4.4]nonane (11a). Triphenylphosphine (1.348 g,
5.35 mmol) and DEAD (588 mg, 3.38 mmol) were added to
a solution of the alcohol 10a (1.200 g, 2.6 mmol) in dry THF
(20 mL), and the mixture was stirred at room temperature
for 14 h. The solvent was evaporated, and the residue was
treated with ethyl ether (20 mL), to precipitate the formed
triphenylphosphine oxide, which was filtered off. The
solution was evaporated to dryness, and the residue was
purified by flash chromatography, using a 15–50% gradient
of EtOAc in hexane as eluant, to give the cyclic imidate 11a
as a syrup (700 mg, 61%). [a]2D0 238.38 (c, 1.4 in MeOH).
HPLC [mBondapack C18] (A/B, 37:63) tR 9.51 min. 1H
NMR (300 MHz, CDCl3): d 0.84 and 0.87 [2d, 6H, J¼6 Hz,
4.1.7. Synthesis of (5R)-7-[(1R)-1-benzyloxycarbonyl-3-
methylbutyl]-6-oxo-1,7-diaza-spiro[4.4]nonane tri-
fluoroacetate (3b). NaBH4 (106 mg, 2.8 mmol) was
added to a solution of the epimeric mixture of hemiaminals
12a (430 mg, 0.93 mmol) in neat TFA (10 mL), and the
mixture was stirred at room temperature for 2 h. The solvent
was evaporated to dryness, and the residue was dissolved in
CH2Cl2 (20 mL). This solution was washed with H2O