Stereo-controlled synthesis of novel photoreactive gamma-secretase inhibitors

Abstract

The stereoselective synthesis of novel photoreactive gamma-secretase inhibitors 2 and 3 has been achieved. Key steps of the strategy involve preparation of alpha-N-Boc-epoxide 8 and formation of lactone 14 in a practical and stereo-controlled fashion. Compounds 2 and 3 are potent gamma-secretase inhibitors and directly interact with presenilin-1, a catalytic subunit of gamma-secretase.

Figure 1

Structure of L-685,458 (1). The side chains corresponding to the P and P′ sites are marked.

Figure 2

X-Ray Crystallographic Structure of 12: C₃₇H₄₉NSiO₅, M 615.86, orthorhombic P2₁2₁2₁ (No. 19), a = 5.9239(12) Å, b = 12.923(3) Å, c = 46.419(9) Å, V = 3553.0(12) ų, D_c (Z = 4) = 1.151 g/cm³, T = 100 K, μ = 0.106 cm⁻¹. The final R value is 0.2116 for 3442 independent reflections with I > 2σI and 398 parameters. (The crystal structure of 12 has been deposited at the Cambridge Crystallographic Data Centre with the deposition number: CCDC 710680)

Figure 3

Both 2 and 3 are potent γ-secretase inhibitors that directly bind to Presenilin-1. A) Inhibitory potencies of compounds 2 and 3 against γ-secretase. B) Scheme of photoaffinity labeling procedure. After photo-crosslinking, the biotinylated proteins were captured, eluted and analyzed by Western analysis. C) Analysis of photolabeled proteins. The photo-crosslinked proteins were resolved by SDS-PAGE and probed with PS1-NTF (N-terminal fragment) antibody.

SCHEME 1. Synthesis of Epoxide 8

Reagents and conditions: (a) TMSCHN₂, MeOH, 0 °C to rt, 18 h, 90%; (b) NaBH₄, MeOH, −60 °C, 86%; (c) TBSCl, CH₂Cl₂, imidazole, rt, 95%; (d) 4 equiv. CH₂ICl, 5 equiv. LDA, THF, −78 °C; (e) 4 equiv. NaBH₄, MeOH, −78 °C to 0 °C, 65%; (f) KOH, EtOH, 0 °C to rt, 95%.

SCHEME 2. Synthesis of Acid 15

Reagents and conditions: (a) CH₂(CO₂Et)₂, NaOEt, EtOH, rt, 70%; (b) LiOH/DME-H₂O, 50 °C, 6 h; (c) toluene, reflux, 8 h, 60%; (d) LDA, PhCHO, THF, −78 °C; (e) Ac₂O, Et₃N, 120 °C, 80% for 2 steps; (f) H₂, 10% Pd/C, EtOAc, rt, 6 h, 90%; (g) HF·Py, THF, 18 h, 83%; (h) MnO₂, CH₂Cl₂, 18 h, 76%; (i) (ia) LiOH/DME-H₂O, rt; (ib) TBSCl, imidazole, DMF, rt; (ic) MeOH, 79% for 3 steps.

SCHEME 3. Synthesis of Compounds 2 and 3

Reagents and conditions: (a) TMSCl, MeOH, 0 °C to rt, 18 h, 80%; (b) 15, EDC, HOBt, i-Pr₂NEt, DMF, 57% (c) n-Bu₄NF, THF, rt, 85%; (d) LiOH, THF/H₂O, rt, 90%; (e) 5-biotinamido pentylamine, EDC, HOBt, DMF, rt, 50%.

Chart 1