Synthesis and Reactivity of Oligo(ethylene glycol)-Tethered Morita-Baylis-Hillman Dimers in the Formation of Macrocyclic Structures Showing Remarkable Cytotoxicity

Abstract

Background/Objectives: Crown ethers have received increasing interest owing to their ability to form stable complexes with cations. This molecular feature has been successfully exploited in the development of biologically relevant ionophores. Methods: In order to obtain innovative crown ethers derivatives, a Morita-Baylis-Hillman adduct (MBHA) acetate (4) bearing a phenylacetylene moiety was dimerized via the click-chemistry CuAAC reaction with oligo(ethylene glycol) diazide derivatives to build-up a small series of dimeric MBHA derivatives (5a-d). These dimeric MBHA derivatives were reacted with n-butylamine to afford tunable macrocyclic crown ether-paracyclophane hybrid architectures (6a-d). Results: Compounds (E,Z)-6a, (E,E)-6a, 6b-d showed, in human breast cancer MDA-MB-231 and human melanoma A375 cells, IC₅₀ values comparable with those of reference anticancer agent Doxorubicin. Conclusions: This exploration approach provides original new macrocyclic architectures potentially useful as anticancer agents.

Keywords: anticancer agents; crown ethers; cytotoxicity; macrocyclic compounds; paracyclophane derivatives.

Figure 1

Structure of MBHA derivatives 1a,b, 2a,b, 3, and 4.

Scheme 1

Synthesis of MBHA dimer 5a and reaction with n-butylamine leading to macrocyclic crown ether-paracyclophane hybrid structures 6a. Reagents: (i) N₃-CH₂CH₂O-CH₂CH₂O-CH₂CH₂-N₃, CuBr(I), DIPEA, CH₃CN; (ii) CH₃CH₂CH₂CH₂NH₂, CHCl₃.

Scheme 2

Procedure for the preparation of dimeric MBHA derivatives 5a-d. Reagents: (i) CuBr(I), DIPEA, CH₃CN. 5a and 7a: n = 1; 5b and 7b: n = 2; 5c and 7c: n = 3, 5d, and 7d: n = 4.

Scheme 3

Reaction of oligo(ethylene glycol)-tethered MBHA dimers 5b-d with n-butylamine. Reagents: (i) CH₃CH₂CH₂CH₂NH₂; CHCl₃.

Figure 2

Comparison of the ¹H NMR spectra recorded (500 MHz, DMSO-d₆) with crystalline samples of (E,E)-6a and (E,Z)-6a. The signal assignment is reported in red for (E,Z)-6a and in black for (E,E)-6a.

Figure 3

Comparison of the ¹H NMR spectra recorded (400 MHz, DMSO-d₆) with the samples obtained from the reaction of dimeric MBHA derivatives 5b-d with n-butylamine. The ¹H NMR spectrum in the bottom trace was obtained with the (2:1) mixture of (E,E)-6a (the assignment is reported in black) and (E,Z)-6a (the assignment is reported in red) and is reported as the reference spectrum.

Figure 4

Comparison of the aromatic regions of the ¹H NMR spectra recorded (400 MHz, DMSO-d₆) with the samples obtained from the reaction of dimeric MBHA derivatives 5b-d with n-butylamine. The ¹H NMR spectrum in the bottom trace was obtained with the (2:1) mixture of (E,E)-6a (the assignment is reported in black) and (E,Z)-6a (the assignment is reported in red) and is reported as the reference spectrum.

Figure 5

Comparison of (A) the X-ray structure of (E,E)-6a [32] with the computed 3D structures (B) of (E,E)-6a (left) and (E,E)-6d (right).

Figure 6

Percentage of viable (A) human breast cancer MDA-MB-231 and (B) human melanoma cells A375 after 24 h of contact with different concentrations of test compounds, as determined by the Neutral Red Uptake. Negative control: complete medium; positive control: 4 µL of Lysis Solution (a 1:250 dilution of 9% wt:vol Triton X-100) per 100 μL of cells in culture medium. Data are mean ± SD of three experiments run in six replicates. * Values are statistically different versus Doxorubicin, p < 0.05.