Novel 5, 6-Dihydropyrrolo[2,1-a]isoquinolines as Scaffolds for Synthesis of Lamellarin Analogues

As core skeletons of lamellarins: 5,6-Dihydropyrrolo[2,1-a]isoquinolines are one of the important alkaloids that exhibit significant biological activities, in this study, an efficient synthetic route was described for two novel compounds, 5,6-dihydropyrrolo[2,1-a]isoquinolines I and II. Compound I was synthesized from isovanillin with 28.3% overall yield by a six-step reaction while II from 2-(3, 4-dimethoxyphenyl) ethanamine was with 61.6% overall yield by a three-step reaction. And the structures of these two compounds were confirmed by means of IR spectrum, 1H NMR, 13C NMR, MS, HRMS, and melting point measurements.


Introduction
Lamellarins are a group of hexacyclic marine alkaloids that were initially isolated from a prosobranch mollusk by Faulkner and coworkers in 1985 [1]. Since then, over 70 compounds belonging to this group have been isolated and identified [2].
Some of these lamellarins and related compounds exhibit interesting biological activities in multidrug resistance (MDR) and their corresponding parental cell lines [3]. As well known [4], Lamellarin D (LMD) exhibits a significant cytotoxicity against a large panel of cancer cell lines and is a potential non-CPT (camptothecin) topoisomerase 1 poison [5,6]. LMD affects cell cycle and acts on cancer cell mitochondria to induce apoptosia [7].
Due to the fascinating novel structures and biological activities, more and more researchers have devoted into the synthetic studies of lamellarins [8] and related 3,4diarylpyrrolo derivatives. As one of the important alkaloids, 5,6-Dihydropyrrolo[2,1-a]isoquinolines exhibits pronounced biological activities. The biological activity of 5,6dihydropyrrolo[2,1-a]isoquinolines I and II was evaluated by their effects on the proliferation of MDA-MB-231 (breast cancer cell line) by MTT assay. Our results showed that compound I could significantly inhibit the proliferation of MDA-MB-231 at the concentration of 40 μg/mL, in contrast, compound II could enhance the proliferation of the MDA-MB-231 at the same concentration. In addition, they are also scaffolds for synthesis of lamellarin analogues [9].

Analysis Means of Compounds.
Melting points (uncorrected) were determined by a Gongyi X-4 apparatus. Infrared spectra(IR) were determined by Nicolet 550 spectrometer. NMR spectra were recorded by Bruker DRX500 or Bruker DRX400 spectrometer. All data were calibrated at δ 0.00 ppm for 1 H spectra and 13 C spectra from the original spectra (TMS). Low resolution mass spectra (LRMS) were recorded with an HP 6890/5973 GC-MS mass spectrometer. High resolution mass (HRMS) for unreported compounds were recorded with a Micromass GTC Gas Chromatography/TOF Mass spectrometer. All solvent were redistilled prior to use, unless otherwise stated, all other commercially available chemicals were used without further purification.

Results
The target compounds I and II had been synthesized by our route and their structures were determined by interpretation    of spectral data. The 1 H NMR and 13 C NMR spectra of them were assigned as indicated in Figures 1, 2, 3, and 4. An initial 1 H-NMR spectrum of I (in CDCl 3 ) revealed four -OMe-H signals at 3.94 (s, 3H), 3.91 (s, 3H), 3.91 (s, 3H), 3.88 (s, 3H). These peaks are the featured signals of the -OMe-. 2.95 and 4.05 doublets (J = 6.6 Hz) indicate -CH 2 Nand -CH 2 -moieties connected with it in the isoquinoline ring. It can be seen that the distinguishing feature of Ar-CH 2 O 5.14 (s, 2H) is shown in Figure 1. There are several groups of signals in the aromatic region; they are 7.12 (d, J = 1.6 Hz, 1H), 7.11 (s, 1H), 7.10 (s, 1H), 6.73 (s, 1H), 6.69 (d, J = 1.6 Hz, 1H), and 6.60 (s, 1H), respectively. Among them, 7.12 (d, J = 1.6 Hz, 1H) and 6.69 (d, J = 1.6 Hz, 1H) are the signals in the pyrrole ring; this can be estimated from the peak type. Since Ar-H in the Ar-CH 2 O are influenced by other protons more slightly, they will overlap together and show the multiplet in the spectra. So 7.29-7.46 (m, 5H) is the signal of Ar-H in the Ar-CH 2 O. A molecular formula 6 Evidence-Based Complementary and Alternative Medicine of C 29 H 29 NO 5 , resulted from HR-MS data of I. The 13 C NMR spectrum of I displayed twenty-seven signals, which represented all twenty-nine C-atoms, eighteen of which were assignable to three aromatic-C moieties and accounted for sixteen spectral signals. Of the remaining eleven signals, four were from OMe (56.96, 56.97, 57.14, 57.43 ppm), and seven were from isoquinoline and pyrrole ring C-atoms.
NMR data of II (see Figures 3 and 4) indicated a C 23 H 25 framework, which HR-MS analysis expanded to a molecular formula of C 23 H 25 NO 5 . The simplest assumed relationship between the two isoquinoline, I as an BnOsubstituted II, was reinforced by characterization of the NMR data, which exhibited many similar signals. Specifically, too many shifts of H and C resonances are very similar to each other which proved the basic framework between I and II. The NMR signals which distinguished I from II were those of three aromatic protons appropriate for Ar-H (7.29-7.46 ppm, m, 5H) and -CH 2 -in the Ar-CH 2 O. The remaining distinguishing feature was the number of -OMesignal in 13 C NMR at 56-57 ppm.

Discussions
I and II from 1-methyl-3,4-dihydroisoquinoline and 2,4,5trimethoxy-α-halogen-acetophenone were obtained with high yields under mild conditions for the first time. This novel method, as the key reaction step, provides a general and highly efficient method for the preparation of 5,6-dihydropyrrolo[2,1-a]isoquinolines. We envisaged that the 5,6-dihydropyrrolo[2,1-a]isoquinolines could be constructed by the formation of quaternary ammonium salt, and subsequent lactonization in the presence of anhydrous K 2 CO 3 . The negative carbon ion of 1-methyl-3,4dihydroisoquinoline is also active in the Knorr reaction. Both 2,4,5-trimethoxy-α-bromoacetophenone and 2,4,5trimethoxy-α-chloracetophenone were employed. We found that the yield of the former is about 5% higher than the later. Therefore, 2,4,5-trimethoxy-α-bromoacetophenone is used in the synthesis of I and II.