Function through bio-inspired, synthesis-informed design: step-economical syntheses of designed kinase inhibitors†Dedicated to Max Malacria, a friend and scholar whose science and creative contributions to step-economical synthesis have inspired us all and moved the field closer to the ideal.‡Electronic supplementary information (ESI) available: Synthetic procedures and spectral data. See DOI: 10.1039/c4qo00228hClick here for additional data file

Abstract

The human kinome comprises over 500 protein kinases. When mutated or over-expressed, many play critical roles in abnormal cellular functions associated with cancer, cardiovascular disease and neurological disorders. Here we report a step-economical approach to designed kinase inhibitors inspired by the potent, but non-selective, natural product staurosporine, and synthetically enabled by a novel, complexity-increasing, serialized [5 + 2]/[4 + 2] cycloaddition strategy. This function-oriented synthesis approach rapidly affords tunable scaffolds, and produced a low nanomolar inhibitor of protein kinase C.

Fig. 1. Design of staurosporine analogs based on [5 + 2]/[4 + 2] serialized cycloadditions.

Scheme 1. Synthesis of 5-chloro-2-ethynylindole 4. Reagents and conditions: (a) i. n-BuLi, THF, –78 °C, then CO₂ (g); ii. t-BuLi, –78 °C, 1 h, then 1,2-diiodoethane, –78 °C, 20 min, 73%. (b) 5 mol% Pd(PPh₃)₂Cl₂, 10 mol% CuI, (trimethylsilyl)acetylene, Et₃N, 40 °C, 30 min, 98%. (c) NaOH (aq.), i-PrOH, RT, 15 min, 92%.

Scheme 2. Synthesis of the key core scaffold 6. Reagents and conditions: (a) 5 mol% [Rh(CO)₂Cl]₂, DCE, RT, 18 h, then H⁺, 61%. (b) Dimethyl acetylenedicarboxylate, PhMe, reflux, 72 h; then DDQ, RT, 30 min, 57%.

Scheme 3. Completed synthesis of pentacyclic common intermediate 9. Reagents and conditions: (a) Cs₂CO₃, 7, DMF, 75 °C, 14 h, 77%. (b) KOH (aq.), EtOH, 70 °C, 22 h; then citric acid, RT, 10 min. (c) HMDS, MeOH, CH₃CN, reflux, 12 h, 57% (two steps).

Scheme 4. Ketone diversification of 9 for analog generation. Reagents and conditions: (a) NaBH₄, MeOH–CH₂Cl₂ (1 : 1), 0 °C, 10 min. (b) TFA, i-Pr₃SiH, CH₂Cl₂, RT. (c) NH₄OAc, Et₃N, NaCNBH₃, MeOH–CH₂Cl₂ (1 : 1), RT, 4 h. (d) MeNH₃Cl, Et₃N, NaCNBH₃, MeOH–CH₂Cl₂ (1 : 2), RT, 3 h. (e) Me₂NH₂Cl, Et₃N, NaCNBH₃, MgSO₄, MeOH–CH₂Cl₂ (1 : 2), RT, 48 h.

Scheme 5. De-chlorination to provide analog 14. Reagents and conditions: (a) Pd(OAc)₂, KF, PMHS, H₂O–THF (2 : 5), RT, 1 h. (b) Me₂NH₂Cl, Et₃N, NaCNBH₃, RT, 48 h. (c) TFA, i-Pr₃SiH, DCM, RT, 30 min, 43% (3 steps).

Fig. 2. Design of a simplified staurosporine analog.

Scheme 6. Synthesis of a new des-indole analog 19. Reagents and conditions: (a) n-BuLi, KOtBu, LiBr, THF, –78 °C to –20 °C to 0 °C, 1.5 h, 66%. (b) [(C₁₀H₈)Rh(COD)]SbF₆, DCE, RT, 45 min; then DMAD, RT, 15 min; then H⁺, 75%. (c) DDQ, PhMe, 80 °C, 6.5 h, 84%. (d) 1 N KOH (aq.), EtOH, 80 °C, 2–4 h; then Ac₂O, 50 °C, 18 h; then HMDS–MeOH (2 : 1), DMF, 80 °C, 3 h, 41% (3 steps). (e) NaBH₃CN, Me₂NH₂Cl, Et₃N, mol. sieves, MeOH–DCM (1 : 2), RT, 48 h; then TFA, i-Pr₃SiH, DCM, RT, 30 min, 67%.

Fig. 3. Computer docking of energy-minimized analog 19 into the active site of PKCα.