An Intramolecular Hydroaminomethylation-Based Approach to Pyrrolizidine Alkaloids under Microwave-Assisted Heating

A general method for the synthesis of pyrrolizidine derivatives using an intramolecular hydroaminomethylation protocol (HAM) under microwave (MW) dielectric heating is reported. Starting from a 3,4-bis(benzyloxy)-2-[(benzyloxy)methyl]-5-vinylpyrrolidine, MW-assisted intramolecular HAM in the presence of gaseous H2 and CO gave the natural alkaloid hyacinthacine A2 protected as benzyl ether. The same approach gave a lentiginosine analogue starting from the corresponding vinyl N-hydroxypyrrolidine. The nature of the reaction products and the yields were strongly influenced by the relative stereochemistry of the starting pyrrolidines, as well as by the catalyst/ligand employed. The use of ethanol as a solvent provides environmentally friendly conditions, while the ligand/catalyst system can be recovered by separating the alkaloid product with an SCX column and recycling the ethanolic solution. HAM worked up to three times with the recycled catalyst solution without any significant impact on yield.


Introduction
The pyrrolizidine nucleus can be considered as a privileged scaffold, since in nature more than 6000 animal or plant species hold pyrrolizidine alkaloids ( Figure 1) [1,2], which are responsible for a wide range of biological activities depending on substituents and stereochemistry [3] Highly polyhydroxylated pyrrolizidine alkaloids can act as sugar mimics that inhibit glycosidases [4,5] The main pharmaceutical application of pyrrolizidinebased compounds is in traditional medicine (with the plant extracts used as ingredients), although several concerns have been posed regarding their safety on humans [3] In addition, some pyrrolizidine alkaloids (necine bases) and analogues can be used as pesticides thanks to their toxicity and natural deterrent effect on insects [6,7]. We envisaged that a much more direct access to pyrrolizidine alkaloids such as hyacinthacine A 2 (1) and nor-lentiginosine (3) might be provided by a hydroaminomethylation reaction (HAM) carried out on the same intermediates C [15] and D [17], respectively (Scheme 1). [11,15,[17][18][19][20][21][22][23][24] This could be an elegant approach for the preparation of polycyclic derivatives with promising large-scale applications, as the HAM process is based on the hydroformylation reaction (HF), the most commonly used catalytic transformation in the industry [25].

Results and Discussion
Our retrosynthetic strategy for the synthesis of generic pyrrolizidine derivatives is shown in Scheme 2. The pyrrolizidine skeleton can be obtained by the HAM of olefin E or F derived from the reduction of hydroxylamine C or D, which in turn was obtained by the stereoselective addition of vinylmagnesium bromide to enantiomerically pure nitrones A or B. In the intramolecular HAM process, we expected the aldehyde to form an iminium/ enamine intermediate after HF on the olefin [28,29,46]. Nucleophilic binding of the amine, followed by reduction catalyzed by the Rh catalysts, would give the expected bicyclic pyrrolizidine product. To set up the process, we investigated the use of both hydroxylamine C and D and the corresponding amine E and F, possibly formed by the in-situ reduction in the reaction conditions. We started from nitrone 5 [47], an intermediate suitable for the synthesis of the norlentiginosine (3). A highly diastereoselective addition of vinylmagnesium bromide 6 gave hydroxylamine 7 in 96% yield and allowed the incorporation of a new (S)-configured stereogenic center in C-2 position (Scheme 3) [17]. Compound 7 and the corresponding amine 9, obtained by reduction with catalytic In and Zn in NH 4 Cl solution [48,49] [45], 7 gave the aldehyde 10 as the main reaction product (44%) together with a 10% of the desired bicyclic product 12 and traces of enamine 11 (Table 1, entry 1).  [a] all reactions are performed by setting the specific temperature required for the experiment and by setting at 150 Watt the maximum power to be used to reach the expected temperature.
Under the same reaction conditions, the amine 9 furnished solely the enamine 11, but with a poor 27% conversion. Better results were obtained with EtOH as solvent, especially starting from hydroxylamine 7 that gave the aldehyde 8a, in equilibrium with its cyclic hemiacetal form 8b, as the major reaction product in 65% isolated yield, while 9 showed almost no reactivity (Table 1, entry 2). In toluene alone, at lower temperatures (80 • C), no conversion was observed starting from 7, while starting from 9 only the hydrogenation by-product 13b was formed (Table 1, entry 3). At higher temperatures (130 • C), a relatively small amount of the enamine 11 was obtained from the two vinylpyrrolidines 7 and 9 (Table 1, entry 4). HAM with Biphephos or PPh 3 as ligands resulted in the hydrogenation of the C=C double bond and product 13 was obtained as the major component of the reaction mixture (40-90%) regardless of the starting material (Table 1, entries 5-6). Other variations in conditions, such as [RhCl(COD)] 2 as catalysts, pressure of syngas (3-14 bar) and reaction times (40-60 min) were tested without improvement in conversion or selectivity. The better result was then the conversion of hydroxylamine 7 into the mixture of 8a and 8b using EtOH as solvent. Treatment of this mixture with PTSA in refluxing toluene (30 min, MW) reversed the equilibrium towards aldehyde 8a, which could be hydrogenated (H 2 , 1 bar, MeOH MW, 40 • C, 15 min) to give compound 12 that, upon deprotection with trifluoroacetic acid, ref. [17] afforded nor-lentiginosine (3) (Scheme 4). However, when we explored the possible application of this methodology to a differently protected 2-vinyl-N-hydroxypyrrolidine hydroxylamine (14, Scheme 5), we were able to obtain only the intermediate aldehyde mixture 15a/15b in just 37% isolated yields. The following transformation into the corresponding bicyclic derivative 16 by reducing the hydroxylamine and subjecting the product to intramolecular reductive amination conditions did not occur using H 2 and Pd/C under both traditional heating or MW dielectric heating. For the synthesis of hyacinthacine A 2 (1), we started from the known hydroxylamine 17 obtained from the corresponding nitrone and vinylmagnesium bromide [15].  Table 2, entry 1), while most of the starting material remained unreacted. Increasing the temperature to 130 • C resulted in only 27% conversion to the desired compound 22 together with 2% of the enamine 21, but 56% of the starting material 17 was still present in the final reaction mixture (Scheme 6 and Table 2, entry 2). With EtOH as solvent instead of toluene, compound 17 did not react at all ( Table 2, entry 3). Increasing the syngas pressure had no effect, while using Biphephos as a ligand gave a complete conversion to the hydrogenation compound 18a ( Table 2, entry 4). Pyrrolidine 19 was then prepared following our well-established protocol that employs catalytic indium and stoichiometric Zn as reducing agent [48,50], and HAM was investigated under different conditions (Scheme 5). The unsaturated pyrrolizidine 21 was isolated from 19 in toluene at 110 • C in 43 % yield (  [15,[51][52][53] and is here used together with MW irradiation. Moreover, as we worked in a very small scale, the final product was obtained in such a low amount that good quality NMR spectra could not be recorded. Furthermore, both elemental analysis and [α] D are in agreement with data reported in the literature.
To investigate the possibility of reusing both the catalyst and the solvent used, the reaction was carried out on a 100-mg scale and the crude product filtered directly through an SCX column. Pyrrolizidine 22 was retained by the column and further recovered by washing the column with NH 3 in EtOH. The first ethanolic fraction containing the ligand and catalyst was then recycled by addition of amine 19 and the solution was subjected to a new HAM cycle under the conditions given in entry 7 of Table 2. The expected product 22 was obtained with 80% yield. The recycle was repeated 3 times without any effect on the reaction yield, proving that the catalyst can be recycled efficiently. It is worth noting that, for all substrates studied, the CO/H 2 addition to the C=C bond occurs regioselectively at the terminal position, as no branched aldehydes or the corresponding hemiacetals were observed.
In summary, we have shown that intramolecular HAM is a transformation that can be applied to the synthesis of pyrrolizidine alkaloids and their derivatives with high atom economy. We investigated both the use of hydroxylamines obtained from the reaction of nitrones with vinylmagnesium bromide and the corresponding amines. The use of the hydroxylamine as a substrate for HAM gave an intermediate aldehyde that was directly reduced to pyrrolizidine derivatives. In our experiments with different substrates, we found that the decoration of the A ring and/or the configuration at the different stereogenic centres have an influence on the reaction outcome. The hydroxylamine proved to be effective for our purposes only in the synthesis of the nor-lentiginosine 3 by a two-step process. Indeed, the intermediate aldehyde (8a) formed by HF remains in the corresponding unreactive hemiacetal form (8b), which requires further treatment and subsequent reducing conditions to afford 3. However, once a substituent was introduced at C-3 of the pyrrolidine ring, the amine performed better than the corresponding hydroxylamine and afforded the pyrrolizidine derivative in a one-pot reaction with 82% isolated yield. In all cases, intermediate enamine derivatives, potentially useful for other synthetic aims, were formed in different ratios, depending on the reaction conditions and catalyst/ligand used.

Materials and Methods
All reagents were used as purchased from commercial suppliers without further purification. Flash column chromatography was performed in glass columns using Merk silica gel 60 Å, 230-400 mesh particle size. For analytical thin-layer chromatography, Merck aluminium-backed plates pre-coated with silica gel 60 (UV254) were used and visualised by staining with a solution of p-anisaldehyde in EtOH or a KMnO 4 solution. 1 H NMR and 13 C NMR spectra were recorded using a 400 MHz Brucker Advance NMR spectrometer (Bruker BioSpin AG, Fällanden, Switzerland). Deuterated chloroform and methanol were used as solvents. Chemical shift (δ) values are given in parts per million (ppm) and refer to the residual signals of the deuterated solvent (δ 7.26 for 1H and δ 77.6 for 13 C in CDCl 3 , δ 3.34 for 1 H and δ 49.00 for 13 C in CD 3 OD). The data are presented as follows: chemical shift, multiplicity (s = singlet, d = doublet, dd = doublet of doublets, dt = doublet of triplets, t = triplet, q = quartet, m = multiplet or multiple resonances, bs = broad singlet), coupling constant (J) in hertz and the integration in ppm. Mass spectrometry data were collected using an Agilent 1100 LC /MSD VL system. MW assisted reactions are performed in a CEM Discover MW (CEM s.r.l., Cologno Al Serio (BG), Italy) oven equipped with a 10 mL tube for reactions under pressure and an external IR sensor to record the reaction temperature during irradiation (CEM s.r.l., Cologno Al Serio (BG), Italy). This glass vial, tested to withstand pressures up to 250 psi (17 bar, 1723 KPa), is equipped with a tubing connection to an external pressure control system that includes a valve and output tubing to vent the vial at the end of the reaction. The output tubing was connected via a three-way connector to a cylinder containing CO/H 2 (1:1) equipped with two taps to pre-purge the system prior to MW irradiation.
The nitrone derivatives were synthesised as previously reported [47,51,54]. General method for the preparation of 1-hydroxy-2-vynylpyrrolidine derivatives 7, 14, 17 [17]: To a stirred solution of the proper nitrone (4.37 mmol) in dry Et 2 O (40 mL), a 1 M solution of vinyl magnesium bromide in THF (5.5 mL, 5.5 mmol), was slowly added under nitrogen atmosphere at 20 • C. After stirring at 20 • C for 1 h and 45 min, 15 mL of saturated aqueous NaHCO 3 were added. The precipitate was filtered and the mixture extracted with diethyl ether (3 × 15 mL). The combined organic extracts were dried over dry Na 2 SO 4 , and the solvent evaporated under vacuum and directly used in the next step. Single 1 H-NMR spectra in Supplementary Materials.
General procedure for the synthesis of 2-vinylpirrolidine 9 and 19 [17]: To a stirred solution of the proper 1-hydroxy-2vinylpirrolidine (0.83 mmol) in dry methanol (10 mL), a saturated solution of NH 4 Cl (15 mL), powdered Zn (218 mg, 3.33 mmol) and a catalytic amount of indium dust (1.7 mg, 0.015 mmol) were added at 20 • C. The mixture was reflux overnight under N 2 . The solvent was evaporated under vacuum and a saturated aqueous solution of Na 2 CO 3 (15 mL) was added. The mixture was extracted with Et 2 O (3 × 5 mL) and the combined organic phases were dried over dry Na 2 SO 4 , and the solvent evaporated under vacuum. The brownish oil (was used in the next step. Single 1 H-NMR spectra in Supplementary Materials.