Design of β-Secretase Inhibitors by Introduction of a Mandelyl Moiety in DAPT Analogues
591
H 4.4, N 6.6%. C24H18F2N2O4 requires C 66.1, H 4.2, N 6.4%.) Rf
0.38 (toluene/EtOAc, 9 : 1). δH (250 MHz, CDCl3) 1.48 (3 H, d, CH3 β
Ala, 3J 6.9), 3.62 (2 H, s, –CH2–C(O)–), 4.12 (1 H, br s, NHAla), 4.62–
4.74 (1 H, m, CH α Ala), 6.60–6.92 (3 H, m, ArH), 7.20 (1 H, d, H2,
3J 8.8), 7.29 (1 H, dd, H1, 3J 9.0, 4J 2.2), 7.55 (1 H, td, Hꢀ3, 3J 8.0, 4J
1.1), 7.74 (1 H, td, Hꢀ4, J 7.7, J 1.2), 7.93 (1 H, d, Hꢀ5, J 8.2), 8.32
(1 H, dd, Hꢀ2, 3J 7.9, 4J 1.0), 8.39 (1 H, d, H5, 4J 2.2), 9.15 (1 H, s, NH).
m/z (ES) 437 (M + H)+, calc. for C24H18F2N2O4 436.41.
1 h at room temperature and then cooled once again to 0◦C. A CH2Cl2
solution of the benzoxazepine piperidine derivative 8 (129 mg, 1.0
equiv., 0.46 mmol) and DIEA(240 µL, 3.0 equiv., 1.38 mmol)wasadded
dropwise. The solution was allowed to warm and stirred overnight at
room temperature. The solvent was removed under reduced pressure
and the residue was dissolved in EtOAc (20 mL). The organic layer
was washed successively by using H2O (10 mL), brine (10 mL), 5%
aqueous NaHCO3 (2 × 10 mL), and brine (10 mL), was dried over anhy-
drous MgSO4, filtered, and concentrated under reduced pressure. The
crude residue was purified by flash chromatography (CH2Cl2/MeOH,
95 : 5) to give the title compound 9 (76 mg, yield: 37%) as a white
solid. (Found: C 64.9, H 6.0, N 9.6%. C24H27F2N3O3 requires C
65.0, H 6.1, N 9.5%.) Rf 0.48 (CH2Cl2/MeOH, 95 : 5). δH (250 MHz,
CDCl3) 1.32 (3 H, d, CH3 β Ala, 3J 6.6), 1.38–1.46 (2 H, m, –(CHH–
CH2)2–N–), 1.88–1.92 (2 H, m, –CH2–CH2–NH–), 2.05–2.16 (2 H,
m, –(CHH–CH2)2–N–), 3.05–3.19 (1 H, m, –(CH2–CHH)2–N–), 3.28–
3.33 (2 H, m, –CH2–CH2–NH–), 3.51 (2 H, s, –CH2–C(O)–), 3.59–
3.64 (2 H, m, –(CH2–CHH)2–N–), 4.25–4.42 (1 H, br s, NH aniline),
4.85–4.90 (1 H, m, CH α Ala), 6.63–6.92 (7 H, m, ArH), 8.05 (1 H, br
s, NH Ala). m/z (ES) 444 (M + H)+, calc. for C24H27F2N3O3 443.49.
3
4
3
6-N-[(3,5-Difluoromandelyl)-L-alanyl]-
3,4-benzocoumarin, 7a/7b
6-Amino-3,4-benzocoumarin
was acylated by Boc-Ala (Boc = tert-butyloxycarbonyl) accord-
ing to similar procedure to that described previously for
the synthesis of compound 6. This procedure involved the
use of DCC/HOBt as coupling system and led to the
5 (100 mg, 1.0 equiv., 0.47 mmol)
a
a
corresponding N-Boc-alanyl coumarine derivative, 6-N-[(N-tert-
butoxycarbonyl)-l-alanyl]-3,4-benzocoumarin, which was quantita-
tively isolated (180 mg) as a pale yellow solid. (Found: C 65.8,
H 6.1, N 7.6%. C21H22N2O5 requires C 66.0, H 5.8, N 7.3%.)
Rf 0.31 (EtOAc/hexane, 1 : 1). δH (250 MHz, [D6]DMSO) 1.30 (3 H,
d, CH3 β Ala, 3J 7.0), 1.39 (9 H, s, CH3 Boc), 4.11–4.17 (1 H, m,
CH α Ala), 7.16 (1 H, d, NH Ala, 3J 7.3), 7.42 (1 H, d, ArH, 3J 9.0),
7.69–7.74 (2 H, m, ArH), 7.98 (1 H, td, ArH, 3J 8.0, 4J 1.4), 8.17
(1 H, d, ArH, 3J 8.3), 8.28 (1 H, dd, ArH, 3J 8.0, 4J 1.3), 8.58 (1 H,
d, ArH, 4J 2.5), 10.18 (1 H, br s, NH). m/z (ES) 383 (M + H)+, calc. for
C21H22N2O5 382.41. The previous N-Boc-alanyl coumarine derivative
(137 mg, 1.0 equiv., 0.36 mmol) was deprotected by usingTFA (276 µL,
10.0 equiv., 3.60 mmol) in CH2Cl2 (5 mL). The reaction mixture was
stirred overnight at room temperature and the solvent and excess TFA
were removed under reduced pressure. The resultingTFA salt was quan-
titatively isolated as a white solid (170 mg) after trituration in ether. δH
(250 MHz, [D6]DMSO) 1.50 (3 H, d, CH3 βAla, 3J 7.0), 4.00–4.11 (1 H,
m, CH α Ala), 7.47 (1 H, d, ArH, 3J 8.8), 7.70–7.78 (2 H, m, ArH), 8.01
(1 H, td, ArH, 3J 7.6, 4J 1.3), 8.10–8.20 (4 H, m, 2 ×ArH + NH2), 8.51
(1 H, d,ArH, 4J 2.3), 10.74 (1 H, br s, NH). m/z (ES) 283 (M + H)+, calc.
for C16H14N2O3 282.29 and for C16H14N2O3·CF3COOH 396.32. The
previous TFA salt (150 mg, 1.2 equiv., 0.36 mmol) was acylated by 3,5-
difluoromandelic acid (56 mg, 1.0 equiv., 0.30 mmol) in the presence of
BOP reagent as a coupling agent, according to a similar procedure to
that described previously for the synthesis of 1. The desired compound
was quantitatively isolated as a mixture of diastereomers (140 mg). Both
diastereomers were separated by flash chromatography (EtOAc/hexane,
2 : 1) but their respective stereochemistry was not fully attributed. Com-
pound 7a (70 mg) (Found: C 63.6, H 4.0, N 6.3%. C24H18F2N2O5
requires C 63.7, H 4.0, N 6.2%.) Rf 0.27 (EtOAc/hexane, 2 : 1). δH
(250 MHz, [D6]DMSO) 1.37 (3 H, d, CH3 β Ala, 3J 7.3), 3.30 (1 H,
br s, CHOH), 4.43–4.55 (1 H, m, CH α Ala), 5.09 (1 H, d, CHOH, 3J
5.0), 6.59 (1 H, d, NHAla, 3J 5.5), 7.11–7.21 (3 H, m,ArH), 7.43 (1 H, d,
ArH, 3J 9.3), 7.66–7.76 (2 H, m, ArH), 7.96–8.04 (1 H, m, ArH), 8.17–
8.30 (2 H, m, ArH), 8.56 (1 H, d, ArH, 4J 2.5), 10.31 (1 H, br s, NH).
m/z (ES) 453 (M + H)+, calc. for C24H18F2N2O5 452.12. Compound
7b (70 mg) (Found: C 63.8, H 4.1, N 6.3%. C24H18F2N2O5 requires
C 63.7, H 4.0, N 6.2%.) Rf 0.22 (EtOAc/hexane, 2 : 1). δH (250 MHz,
[D6]DMSO) 1.40 (3 H, d, CH3 β Ala, 3J 7.0), 3.30 (1 H, br s, CHOH),
4.40–4.51 (1 H, m, CH α Ala), 5.09 (1 H, d, CHOH, 3J 5.3), 6.56 (1 H,
d, NH Ala, 3J 5.0), 7.09–7.19 (3 H, m, ArH), 7.42 (1 H, d, ArH, 3J 9.0),
7.64–7.75 (2 H, m, ArH), 7.95–8.03 (1 H, m, ArH), 8.16–8.30 (2 H, m,
ArH), 8.54 (1 H, d, ArH, 4J 2.3), 10.31 (1 H, br s, NH). m/z (ES) 453
(M + H)+, calc. for C24H18F2N2O5 452.12.
1ꢀ-N-[N-(3,5-Difluoromandelyl)-L-alanyl]-4,5-dihydro-
spiro[1,5-benzoxazepine-2(3H),4ꢀ-piperidine], 10
The title compound was prepared according to a procedure similar to
that described for the synthesis of the previous derivative 9 by using
the mandelyl derivative 4 (119 mg, 1.0 equiv., 0.46 mmol) as an acidic
moiety and BOP reagent (244 mg, 1.2 equiv., 0.55 mmol). The desired
compound 10 was isolated as a white solid (80 mg, yield: 38%) after
flash chromatography (CH2Cl2/MeOH, 95 : 5). (Found: C 62.5, H 6.1,
N 9.3%. C24H27F2N3O4 requires C 62.7, H 5.9, N 9.2%.) Rf 0.47
(CH2Cl2/MeOH, 95 : 5). δH (250 MHz, CDCl3) 1.25–1.60 (5 H, m, CH3
β Ala + –(CHH–CH2)2–N–), 1.88–1.93 (2 H, m, –CH2–CH2–NH–),
2.04–2.12 (2 H, m, –(CHH–CH2)2–N–), 3.07–3.19 (2 H, m, –(CH2–
CHH)2–N–), 3.25–3.35 (2 H, m, –CH2–CH2–NH–), 3.56–3.81 (3 H,
m, –(CH2–CHH)2–N– + OH), 4.31–4.42 (1 H, m, NH aniline), 4.83–
4.89 (1 H, m, CH α Ala), 5.04 (1 H, br s, –CH(OH)–), 6.64–7.41 (7 H,
m, ArH), 7.95 (1 H, br s, NH Ala). m/z (ES) 460 (M + H)+, calc. for
C24H27F2N3O4 459.49.
5-[ N-(3,5-Difluorophenylacetyl)-L-alanyl]-amino-2-(4-aminobenzyl)-
tetrazole, 12
The substituted N-Boc protected 5-aminotetrazole derivative 11[10]
(140 mg, 1.0 equiv., 0.36 mmol) was dissolved in CH2Cl2 (5 mL). Tri-
fluoroacetic acid (275 µL, 10.0 equiv., 3.60 mmol) was then added at
0◦C and the reaction mixture was stirred overnight at room temperature.
The solvent and excess TFA were removed under reduced pressure. The
resulting trifluoroacetic acid salt was isolated as a white solid after
trituration in ether (140 mg, yield: 97%). δH (250 MHz, CD3OD) 1.62
(3 H, d, CH3 β Ala, 3J 7.0), 4.15 (1 H, br s, CH α Ala), 6.02 (2 H,
s, –CH2–), 7.64 (2 H, d, CHm-NO
,
3J 8.8), 8.28 (2 H, d, CHo-NO
,
2
2
3J 8.8). m/z (ES) 292 (M + H)+, calc. for C11H13N7O3 291.27 and
for C11H13N7O3·CF3COOH 405.29. The following coupling reaction
involving this trifluoroacetic acid salt was performed similarly to the
procedure described for the synthesis of 1 by using BOP as a coupling
agent. The tetrazole derivative was the result of a coupling reaction
between 3,5-difluorophenylacetic acid (34 mg, 1.0 equiv., 0.20 mmol)
and the previous TFA salt (80 mg, 1.0 equiv., 0.20 mmol). The resulting
nitro derivative was isolated by flash chromatography as a yellow solid
(60 mg, yield: 68%). (Found: C 51.1, H 4.1, N 21.9%. C19H17F2N7O4
requires C 51.2, H 3.9, N 22.0%.) Rf 0.50 (EtOAc). δH (250 MHz,
CDCl3) 1.36 (3 H, d, CH3 β Ala, 3J 7.0), 3.50 (2 H, s, –CH2–C(O)–),
4.90–5.01 (1 H, m, CH α Ala), 5.76 (2 H, s, N–CH2–), 6.52–6.74
1ꢀ-N-[N-(3,5-Difluorophenylacetyl)-L-alanyl]-4,5-dihydro-
spiro[1,5-benzoxazepine-2(3H),4ꢀ-piperidine], 9
(3 H, m, ArH), 7.47–7.50 (3 H, m, NH Ala + Hm-NO
,
3J 8.8), 8.10
2
The 3,5-difluorophenylacetyl alanine acid 3 (114 mg, 1.0 equiv.,
0.46 mmol) was dissolved in freshly distilled CH2Cl2 (5 mL) in the
presence of BOP reagent (244 mg, 1.2 equiv., 0.55 mmol). The reac-
tion mixture was cooled to 0◦C and then DIEA (80 µL, 1.0 equiv.,
0.46 mmol) was added dropwise. The reaction mixture was stirred for
(2 H, d, Ho-NO ,
3J 8.8), 10.55 (1 H, br s, NH tetrazole). m/z (ES)
2
446 (M + H)+, calc. for C19H17F2N7O4 445.38. This nitro derivative
(45 mg, 1.0 equiv., 0.10 mmol) was heated to reflux in a H2O/EtOH
solution (5 mL, 3 : 5 v/v). Powdered iron (28 mg, 5.0 equiv., 0.51 mmol)
and NH4Cl (11 mg, 2.0 equiv., 0.20 mmol) were then added. After 2 h,