Synthesis of antiproliferative Cephalotaxus esters and their evaluation against several human hematopoietic and solid tumor cell lines: uncovering differential susceptibilities to multidrug resistance

Abstract

Deoxyharringtonine (2), homoharringtonine (3), homodeoxyharringtonine (4), and anhydroharringtonine (5) are reported to be among the most potent members of the antileukemia alkaloids isolated from the Cephalotaxus genus. Convergent syntheses of these four natural products are described, each involving novel synthetic methods and strategies. These syntheses enabled evaluation of several advanced natural and non-natural compounds against an array of human hematopoietic and solid tumor cells. Potent cytotoxicity was observed in several cell lines previously not challenged with these alkaloids. Variations in the structure of the ester chain within this family of alkaloids confer differing activity profiles against vincristine-resistant HL-60/RV+, signalling new avenues for molecular design of these natural products to combat multi-drug resistance.

Figure 1

Figure 2

Figure 3

Comparative antitumor effects of cephalotaxus esters against sensitive (filled symbols) and vincristine-resistant (open symbols) HL-60: ●/○: 4, ▼/▽: 3, ■/□: 2, ◆/◇: 89, ▲/△: 87.

Figure 4

Comparative antitumor effects of cephalotaxus esters against sensitive and vincristine-resistant HL-60.

Figure 5

Correlation of calculated logP values and MDR ratio for deoxyharringtonine (2), homoharringtonine (3), homodeoxyharringtonine (4), benzyldehydrohomoharringtonine 87, and bis(demethyl)deoxyharrintonine 89.

Scheme 1

a) HONH₂·HCl, NaOH, EtOH, H₂O, 60–80 °C; b) iPr₂NH, LiAlH₄, THF, 60 °C; c) Et₃N, THF, 23–60 °C; d) Cs₂CO₃, 1,4-dioxane, 100–150°C.

Scheme 2

(a) DMAD, PhH, 23°C, 57%; (b) Cs₂CO₃, 1,4-dioxane, 100 °C, 92%.

Scheme 3

a) Et₃N, THF, 60 °C, 64%; b) Cs₂CO₃, 1,4-dioxane, 100 °C, 68%.

Scheme 4

a) TMSCH₂I, NaH, THF, 50°C, 62%; b) Tf₂O; DMAD; TBAT, CH₂Cl₂, 23 °C, 53%.

Scheme 5

Scheme 6

a) Tf₂O; PhCO₂H·NEt₃; DMAD; TBAT, CH₂Cl₂, 23 °C, 35%; b) Tf₂O; PhCO₂Cs; DMAD; TBAT, CH₂Cl₂, 23 °C, 64%; c) 31, Tf₂O; 44, Cs₂CO₃; DMAD; TBAT, CH₂Cl₂, 23 °C.

Scheme 7

Scheme 8

a) KHMDS, Ph₃PMeBr, THF, 60 °C, 75%; b) DMSO, Et₃N, SO₃·Pyr, CH₂Cl₂, 23 °C, 88%; c) CH₂=CHMgBr, THF, −78→23 °C, 93%, dr 8:1; d) Grubbs II, CH₂Cl₂, 23°C, 95%; e) PhSeCl, MeCN, 0°C; mCPBA, Et₃N, CH₂Cl₂, 0→23 °C; 98%; f) TBAF, THF, 23°C, 99%; g) NaIO₄, CH₂Cl₂, H₂O, 23 °C, 90%.

Scheme 9

a) Et₃N, THF, 23 °C, 85%; b) Cs₂CO₃, 1,4-dioxane, 100 °C, 76%.

Scheme 10

Scheme 11

a) Cs₂CO₃, TMSCH₂I, MeCN, 23 °C, 75%; b) Me₃CCOCl, AgOTf; PhSO₂CH=CH₂; TBAT, CH₂Cl₂, −45→23 °C, 77%.

Scheme 12

a) SmI₂, HMPA, tBuOH, THF, −45 °C, 74%; b) Cp₂ZrHCl, THF, 40°C, 99%; c) Et₃N, CbzCl, CH₂Cl₂, 23 °C, 50%; d) KHMDS, CbzCl, THF, 0°C, 86%; e) 2N HCl, MeOH, 23°C; Boc₂O, Yb-(OTf)₃·xH₂O, CH₂Cl₂, 0 °C; f) IBX, DMSO, 23°C, 50% (2 steps); g) CrCl₂, acetone, H₂O, 23 °C; h) H₂, Pd/C, EtOAc, 23 °C, 42% (2 steps); i) HC(OMe)₃, pTsOH, CH₂Cl₂, 55 °C, 90%; j) NaBH₄, MeOH, −78→23 °C, 95%.

Scheme 13

a) TMSCl, TMS₂NH, CH₂Cl₂, 23 °C; Me₃CCHO, TMSOTf, CH₂Cl₂, −25 °C, 82%; b) LHMDS, Me₂C=CHCH₂Br, THF, −78 °C, 66%; c) NaH, BnOH, THF, 0°C; 88%; d) 2,4,6-Cl₃C₆H₂COCl, DMAP, CH₂Cl₂, 23°C, 50%; e) H₂, Pd/C, EtOAc, 23 °C, 99%; f) 2,4,6-Cl₃C₆H₂COCl, DMAP, 1, CH₂Cl₂, 23 °C, 81%; g) NaOMe, MeOH, 23°C, 76%; h) TMSCHN₂, 7:2 PhH/MeOH, 23 °C, 100%; i) Ac₂O, DMAP, pyr, 23 °C, 74%; j) Pd/C, H₂, EtOAc, 23 °C, >99%.

Scheme 14

a) Hg(OAc)₂, NaBH₄, 1:1 THF:H₂O, 23 °C, 77%; (b) Pd/C, H₂, EtOAc, 23°C, 99%; (c) 2,4,6-trichlorobenzoyl chloride, DMAP, TEA, 1, CH₂Cl₂, 23 °C, 99%.

Scheme 15

a) LHMDS, allyl bromide, THF, −78°C, 59%; b) BnOH, NaH, THF, 0→23°C, 85%; c) 2,4,6-trichlorbenzoyl chloride, DMAP, Et₃N, CH₂Cl₂, 23 °C, 67%; d) Grubbs II, 23°C, 61%; e) Pd(OAc)₂, Et₃SiH, Et₃N, CH₂Cl₂, 23 °C, 85%; f) 2,4,6-trichlorobenzoyl chloride, DMAP, Et₃N, 1, CH₂Cl₂, 23 °C, 97%; g) NaOMe, MeOH, 0°C, 79%; (h) Pd/C, H₂, MeOH then 9:1 MeOH/HOAc, 23 °C, 79%; i) Pd/C, H₂, HOAc, 23°C, 69%.

Scheme 16

a) Pd/C, H₂, EtOAc, 23 °C, 97%; b) 2,4,6-trichlorobenzoyl chloride, DMAP, TEA, 1, CH₂Cl₂, 23 °C, 81%; c) NaOMe, MeOH, 23°C, 93%.