Nguyen et al.
JOCFeatured Article
In summary, we developed a concise, nine-step enantiose-
lective route to (-)-salino A and derivatives from O-benzyl
serine. The key bis-cyclization of a β-ketoamide, amenable to
gram scale, constructs both the γ-lactam and the fused-β-
lactoneinone operationleading to the brevityof the synthesis.
The flexibility of the described strategy derives from the
versatility of the bicyclic core (-)-32 enabling attachment of
various C4-side chains, even in the presence of the β-lactone,
and the ability to use alternative ketene dimers 14 to vary the
C2side chain. Several derivativesincluding(-)-homosalino A
were synthesized, and their activity toward proteasome in-
hibition was measured. The ability of the described β-keto
tertiary amide substrates to maintain stereochemical integrity
by virtue of A1,3 strain raises interesting questions regarding
how such integrity is maintained with β-keto secondary
amides, known salino A precursors,4c or if a dynamic kinetic
resolution is operative during related biosynthetic aldol
lactonizations.31 The synthesis of additional hypothesis-dri-
ven salino derivatives by the described strategy, their bioac-
tivity, and crystallographic studiesof their interaction withthe
proteasome will be reported in due course.
129.2(2C), 128.6, 128.5(2C), 128.2, 128.0(2C), 113.9(2C), 83.4,
79.3, 73.5, 61.6, 55.2, 45.0, 44.3, 42.5, 28.4, 19.2; HRMS (ESI)
Calcd. For C24H27ClNO5 [M þ H] 444.1578, found 444.1590. The
enantiomeric excess of (-)-32 was determined to be 92% by chiral
HPLC (CHIRALPAK IA, 250 ꢀ 4.6 mm (L ꢀ I.D.), solvent
(isocratic) 87:13 hexanes/2-propanol, flow rate 1.0 mL/min, λ=
230 nm). Retention times: (-)-32, 13.68 min; (þ)-32, 16.12 min.
Hydroxy β-Lactone (-)-53. To a mixture of β-lactones (-)-32
and 320 (260 mg, 0.586 mmol, dr 7:1) in THF was added
palladium on carbon (52 mg, 20 wt %). After the flask was
evacuated twice by aspirator vacuum and refilled with H2, a
balloon of H2 was attached to the flask and the heterogeneous
solution was stirred vigorously at 23 °C for 12 h. The reaction
mixture was then diluted with Et2O and dried over MgSO4. The
organics were filtered through a pad of Celite, concentrated, and
purified by MPLC (SiO2, 5:95 EtOAc/CH2Cl2) to give the desired
alcohol(-)-53in75%yield (155 mg, dr>19:1, 92% ee) as a waxy
solid: Rf=0.29 (5:95 EtOAc/CH2Cl2); [R]23D=-67.0 (c=0.95,
1
CHCl3). Data for (-)-53: IR (neat) 3449, 1831, 1687 cm-1; H
NMR (500 MHz, CDCl3) δ 7.30 (d, J = 8.5 Hz, 2H), 6.89 (d, J =
8.5 Hz, 2H), 5.13 (d, J = 15.0 Hz, 1H), 4.06 (d, J = 15.5 Hz, 1H),
4.03 (ddd, J = 5.5, 7.5, 12.5 Hz, 1H), 3.92 (dd, J = 9.0, 13.5 Hz,
1H), 3.85 (dd, J = 4.5, 13.5 Hz, 1H), 3.80 (s, 3H), 3.78-3.82 (m,
1H), 2.94 (t, J=7.0 Hz, 1H), 2.32-2.38 (m, 1H), 2.01-2.18 (m,
1H), 1.77 (s, 3H), 0.86 (dd, J = 5.0, 9.5 Hz, 1H); 13C NMR (125
MHz, CDCl3) δ 174.2, 166.7, 159.6, 129.0(2C), 128.7, 114.7(2C),
83.6, 80.2, 55.3, 55.1, 44.9, 44.1, 42.4, 28.4, 19.1; HRMS (ESI)
Calcd. For C17H21ClNO5 [M þ H] 354.1108, found 354.1098.
Representative Procedure for Modified Moffatt Oxidation and
Zincate Formation/Addition As Described for N-PMB-salino A,
(-)-33. To a solution of alcohol (-)-53 (42 mg, 0.119 mmol, dr
>19:1) in DMSO/toluene (0.9 mL/0.9 mL) were added EDCI
(0.114 mg, 0.595 mmol) and dichloroacetic acid (5 μL, 0.060
mmol) at 23 °C. The reaction mixture was stirred for 6 h and then
diluted with EtOAc (100 mL). The reaction mixture was acidified
using 0.1 N HCl to pH 3. The organic layer was then washed with
brine (60 mL), dried over MgSO4, filtered, and concentrated. The
crude aldehyde was used for the next step without further puri-
fication due to some instability of the aldehyde observed upon
flash column chromatography.
Preparation of a stock solution of zincate 290: Zinc dust
(1.64 g, 25.0 mmol, Baker, 20 mesh) was activated by the addition
of an HCl solution (5 mL, 1.0 M) in a 25 mL round-bottomed flask.
After the mixture was stirred vigorously for 15 min at 23 °C, the acid
solution was decanted and the zinc was repeatedly washed with dry
THF (3 ꢀ 5 mL). The zinc was then dried under high vacuum for 2
h. Addition of dry THF (5 mL) was followed by slow addition of
3-bromocyclohexene (0.806 g, 5.0 mmol) over 15 min. The resulting
mixture was stirred at 23 °C for 18 h and then transferred via syringe
to a dry centrifuge tube to remove the excess zinc suspension (10
min, 80 rpm). The resulting clear solution was then utilized directly,
following titration as described below, being careful not to
disturb the pellet at the bottom during transfers. The concentra-
tion of the zincate 290 solution was determined as follows: A
stock solution of iodine in THF was prepared by addition of
iodine (254 mg, 1.0 mmol) in 10 mL of THF leading to a 100 μM
solution. A portion (500 μL) of this iodine solution was placed
into a dry 5 mL round-bottomed flask equipped with a magnetic
stir bar. The zincate solution, after being centrifuged, was added
dropwise into the iodine stock solution until the red color
disappeared. Using this procedure, the prepared zincate solu-
tion was determined to be ∼200 μM and was used (in excess)
directly in the following reaction.
Experimental Section
(2R,4R)-β-Keto Acid, 52. To a solution of ketoamide (2R,4R)-
31 (2.30 g, 4.55 mmol, ∼32:1 dr) in THF (91.0 mL) at -5 °C (ice
and saturated NaCl solution) was added Pd(PPh3)4 (526 mg,
0.455 mmol), followed by immediate addition of morpholine
(0.475 mL, 5.46 mmol). The reaction mixture was stirred at -5 °C
for 70 min and then diluted with ice-cold Et2O (800 mL). A 0.02 N
HCl solution was added until the pH was measured to be ∼3. The
layers were separated, and the organic layer was washed with brine
(400 mL), dried over MgSO4 ,and concentrated. The crude keto acid
(2R,4R)-52 (∼ 29:1 dr according to 500 MHz 1H NMR) was used
directly in the subsequent step without further purification. (Note:
longer reaction times led to epimerization).
Gram-Scale Bis-cyclization Leading to Benzyloxy β-Lactone
(-)-32/320. To a solution of 4-pyrrolidinopyridine (2.60 g, 18.2
mmol, 4.0 equiv) in toluene (84 mL) at -5 °C (ice and saturated
NaCl solution) was added MsCl (0.53 mL, 6.83 mmol, 1.5 equiv).
Immediately, a solution of the freshly prepared keto acid (R,R)-5a
(total amount from above, ∼4.55 mmol) in toluene (25 mL) was
added to the resulting suspension via syringe pump over 45 min,
and 5 mL of additional toluene was used to ensure complete
transfer. After 3 h, the reaction mixture was diluted with ice-cold
Et2O (700 mL) and washed with 20% CuSO4 solution (500 mL) to
remove most of the 4-pyrrolidinopyridine and saturated NH4Cl
(500 mL), and then washed with water (2ꢀ500 mL). The organic
layer was dried over MgSO4, filtered, and concentrated. The resi-
due was purified by flash chromatography (1:9 f 3:7 EtOAc/
hexanes) to give a mixture of two inseparable β-lactones (-)-32/
320 (1.05 g, 52%, dr 5:1, 500 MHz 1H NMR) as a yellow oil and
recovered keto acid (10%, 4:1 dr): Rf = 0.36 (20% EtOAc/
hexanes). The enantiomeric excess of β-lactone 32 was determined
to be 90%. The diasteromeric β-lactones were carried directly
forward to the deprotection at which point they could be sepa-
rated. Data for (-)-32: IR (neat) 1830, 1703 cm-1; 1H NMR (500
MHz, CDCl3) δ 7.32-7.36 (m, 3H), 7.13-7.15 (m, 4H), 6.80 (d,
J=8.5Hz, 2H), 4.73 (d, J=15.5 Hz, 1H), 4.31 (d, J=15.5 Hz, 1H),
4.17 (d, J = 12.0 Hz, 1H), 4.13 (d, J = 11.5 Hz, 1H), 4.01 (ddd,
J = 5.0, 7.5, 12.5 Hz, 1H), 3.77-3.81 (m, 1H), 3.77 (s, 3H), 3.73
(d, J = 11.5 Hz, 1H), 3.57 (d, J = 11.5 Hz, 1H), 2.91 (t, J =
7.5 Hz, 1H), 2.31-2.38 (m, 1H), 2.10-2.16 (m, 1H), 1.72 (s,
3H); 13C NMR (125 MHz, CDCl3) δ 173.8, 166.1, 159.2, 136.4,
To a solution of the crude aldehyde (∼0.119 mmol) in THF
(1.0 mL) at -78 °C was added ∼1.2 mL (0.238 mmol, ∼200 μM
in THF, ∼2.0 equiv) of zincate 290 via syringe pump over 30 min.
The resulting mixture was slowly warmed to -20 °C for 1 h,
quenched with water and diluted with Et2O (120 mL). Saturated
(31) The C2-epimer of salino A has been isolated; see ref 11.
J. Org. Chem. Vol. 76, No. 1, 2011 11