Studies Towards the Construction of Alkylidene Quinolizidines . The Total Synthesis of Homopumiliotoxin 223 G

A adição de 5-metil-2-tri-isopropilsililoxifurano (5) a N-carbobenziloxi-2-metoxipiperidina (6a) forneceu uma mixtura dos isômeros eritro e treo 7a and 8a, respectivamente, em rendimentos de moderado a bom (42-85%) e razão diastereoisomérica (7a : 8a) variando entre 1,1:1 – 6:1, dependendo do sistema de solvente e do ácido de Lewis empregados. O isômero treo 8a foi transformado na (+/-)-homopumiliotoxina 223G (1) que foi obtida a partir de 6a em 5 etapas e 13% de rendimento total.


Introduction
A wide range of biologically active compounds is found in the skin secretions of amphibians.Many of these alkaloids have unique profiles of pharmacological activities and therapeutic potential.A remarkable variety of alkaloids have been isolated from skin extracts of the frogs of the Dendrobatidae family, which are used as antimicrobial agent and chemical defense against predators.It appears likely that all of the frog skin alkaloids are taken up from diet, which for such amphibians consists mainly of small arthropods. 1 Homopumiliotoxins 223G (1), 235C, 319A, 319B and 321B (Figure 1) featuring a quinolizidine core have been isolated in such minute amounts from Dendrobatidae frogs which precluded the structural elucidation of several representatives to be carried out. 2 Prior to our efforts in this area, a single synthetic route to homopumiliotoxin 223G (1) had been reported by Kibayashi and coworkers 3 along the synthetic scheme depicted in Figure 2. The construction of the quaternary stereogenic center was centered around the TiCl 4 -mediated addition of allenylsilanes to a methyl ketone derived from (S)-pipecolic acid.After hydrostannylation of the triple bond and iodine-tin exchange, the construction of the quinolizidine ring incorporating the (Z)-alkylidene side chain was achieved through palladium-catalyzed carbonylation of the vinyl iodide intermediate. 3ur approach to homopumiliotoxin 223 (1) is based on the preparation of bicyclic lactam 2a through the addition of 5-methylsilyloxyfuran 5 to an N-acyliminium ion 4, followed by the installation of the (Z)-alkylidene side chain based on a stereoselective aldol reaction followed by stereospecific elimination (Figure 3).

Results and Discussion
][6] Additionally, the hitherto not observed regioisomer 9b (relative configuration not determined) was formed when N-Boc precursor of the N-acyliminium ion was employed due to increased steric hindrance involving the methyl group at C-5 in silyloxyfuran 5 and the N-Boc group (Table 1).
The relative configuration at the two newly generated stereogenic centers was established after catalytic hydrogenation of 7a,b and 8a,b, followed by methanolysis, to give quinolizidinones 12 and 13, as illustrated below for 7a and 8a (Scheme 1).Comparison of the nOe experiments performed with quinolizidinones 12 (no increment on H-9a upon irradiation of the methyl group at C-1) and 13 (3.4% increment of the signal of H-9a upon   irradiation of the methyl group at C-1) allowed us to establish the erythro relative configuration for the major diastereoisomer 7a formed in the coupling reaction of 6a and silyloxyfuran 5.
The stereochemical outcome of the above reaction came to us as a surprise as previous results from our laboratory 8 and elsewhere 5 with 1-silyloxyfurans led us to predict the preferential formation of the threo isomer.Additionally, theoretical calculations of the transition state geometries associated with the addition of 5-methylsilyloxyfuran 5 to the N-acyliminium ion precursors at DFT level (B3LYP/3-21G*) showed that array A (relative energy: 1.52 kcal mol -1 ) displaying an antiperiplanar approach of the π systems of the nucleophile and N-acyliminium ion leads to the lowest energy transition state for the erythro isomer while array E (relative energy: 0 kcal mol -1 ) with a synclinal arrangement is prefered for the transition state leading to the threo isomer.Martin and coworkers have found a similar result for the transition state calculations (RHF/3-21G*) in the addition of 2-methoxyfuran to 5-membered N-carbomethoxy-Nacyliminium ion. 7Although at this point, we are not able to rationalize the reversal of the stereochemical outcome observed when 5-methyl-2-silyloxyfuran 5 was employed, the unexpected preference for the erythro isomer may be due to the steric hindrance posed by the methyl group at C-5 which has not being properly taken into account in the DFT calculations.
The addition of 5-methylsilyloxyfuran 5 to the Nacyliminium ion derived from chiral 2-methoxypiperidine carbamate 6c (Scheme 2) afforded butenolide 7c as the major diastereoisomer (diastereoisomeric ratio 12:1 determined by capillary GC analysis).Surprisingly, the regioisomer 9c (relative configuration not determined) was formed upon changing the order of addition of the reagents: whereas none of regioisomer 9c was observed when TiCl 4 was added to a solution of methoxycarbamate 6c in dichloromethane, followed by the addition of silyloxyfuran 5, significant amounts were formed when the Lewis acid was added to a mixture of 6c and 5.
The relative configuration at the two newly generated stereogenic centers was established after catalytic hydrogenation to 10c, followed by methanolysis to give quinolizidinone 12 and the recovery of the chiral auxiliary.However, the absolute configuration has not being unambiguously established yet.The Si-face selectivity of the chiral N-acyliminium ion derived from 6c was proposed based on our previous results with 8-phenylmenthyl chiral auxiliaries 8 and was rationalized through the kinetically preferred attack of the nucleophile to the s-cis conformation of N-acyliminium ions (Scheme 2), 9 that might be enforced by π-stacking interactions 10 involving the low-lying LUMO of the carbamoyl group and HOMO of the phenyl substituent.

The construction of the (Z)-alkylidene side chain and the synthesis of homopumiliotoxin 223G (1)
With an access to the heterocyclic core of homopumiliotoxin 223G secured, we focused on the aldol reaction as the central strategy to install the (Z)-alkylidene side chain characteristic of this family of alkaloids.In order to evaluate the stereochemical outcome of the aldol reaction of lithium enolates derived from six-membered lactams, we first examined the addition of the lithium enolate of readily available N-ethyl-δ-valerolactam, (prepared in 96% yield from δ-valerolactam and ethyl iodide) to isobutyraldehyde.The reaction of its lithium enolate (generated in THF at -78 °C with LDA or LiHMDS) with isobutyraldehyde afforded two aldol products 14a:14b in 3.9:1 ratio and 75% yield when LDA was employed and 4.4:1 ratio and 60% yield with LiHMDS.The determination of diastereoisomeric ratio was achieved by GC and confirmed by 1 H-NMR.The relative configuration of the major diastereoisomer was tentatively assigned at this point as anti-14a based on the magnitude of the coupling constant (9.2 Hz) between H-3 and H-1' and the relative shielding of C-3 and C-1' (δ 44.0 and 76.3, respectively) in the major adduct as compared to the minor one (δ 44.8 and 76.6, respectively).The deshielding of the hydroxylic hydrogen in the 1 H-NMR spectrum of 14a (δ 5.90) and the lower stretching frequencies of the hydroxyl and carbonyl groups (3338 and 1610 cm -1 , respectively) in 14a as compared to 14b (3423 and 1622 cm -1 , respectively) are consistent with a hydrogen-bonded hydroxyl group in 14a.
The relative stereochemistry was eventually established after syn elimination carried out under the conditions described by Corey and coworkers. 11Treatment of the major diastereoisomer 14a with dicyclohexylcarbodiimide (DCC) and cuprous chloride in refluxing toluene stereospecifically provided (E)-isobutylidene piperidinone 15a in 88% yield, while under the same conditions (Z)-isobutylidene piperidinone 15b was formed in 87% yield from the minor aldol adduct 14b.
The relative configuration of the isobutylidene piperidinones 15a and 15b could be straigthforwardly assigned by inspection of the corresponding 1 H-NMR spectra, particularly from the H-1' signal which appeared deshielded in 15a (δ 6.65) in comparison with 15b (δ 5.49) as the result of the anisotropic effect of the carbonyl group.
Next we evaluated the reaction of the preformed lithium enolate of quinolizidinone 13 with isobutyraldehyde which produced a 20:1 mixture of two aldol adducts 16a:16b in 85% yield, as depicted in Scheme 5.Only two out of the four possible stereoisomers were formed.The syn and anti stereochemistries were assigned by analogy to the above results.Moreover, an outstanding selectivity was observed: the diastereoisomeric ratio was determined by GC and 1 H-NMR to be 20:1 and 32:1 for the lithiumand titanium (IV)-mediated reactions, respectively.
The relative configuration of the two newly created stereogenic centers was unequivocally established after syn elimination to the corresponding isobutylidene derivatives.Treatment of the major aldol product anti-16a with DCC and cuprous chloride in refluxing toluene afforded (E)-17 in 95% yield while (Z)-17 was formed in 95% yield from the minor aldol adduct syn-16b.The assignment of the configuration of the double bond was possible upon inspection of the 1 H-NMR spectra which displayed H-1' deshielded in (E)-17 (δ 6.81) when compared to (Z)-18 (δ 5.60).Alternatively, Mukaiyama aldol reaction of the N,O-silylketene acetal derived from 13 12 let to a reversal in the stereochemical outcome and aldol syn-16b was formed as the major isomer (3:1 mixture of syn-16b:anti-16a) in 70% yield, Scheme 5.
The high diastereoselection observed in the aldol reactions with lithium and titanium (IV) enolates led us to consider that these metal enolates provided highly selective aldol reaction under chelation control according to a Zimmerman-Traxler model (Scheme 7).The formation of diastereoisomers anti-16a and syn-16b was accounted for based on the approach of the aldehyde cis to the lithiated hydroxyl group of quinolizidinone 13.
Thus, the relative stereochemistry of the aldol adduct lactam at C-3 and C1' was generated by the attack of the enolates into the aldehyde through its concave face.These results could be rationalized by the preformed quaternary lithium alkoxide in the enolate formation step, stabilizing the quinolizidinone enolate through a possible lithium dimer interaction, affording anti-16a preferentially due to the relief of the steric hindrance involving the isopropyl group which is axially positioned in the transition state model leading to syn-16b (Scheme 7).Theoretical analysis through geometry optimization of the possible conformers of anti-16a using DFT method (B3LYP/STO-3G, Gaussian98 program), 13 as well as the semi empiric methods PM3 and AM1, showed an increase in the stability ranging from 1.8 to 4 kcal/mol when a hydrogen bond involving the carbonyl and hydroxyl groups is present.Additionally, geometry optimization using semi-empirical and ab initio Scheme 5. Aldol reaction of lithium, titanium(IV) and silicon enolates of 13 with isobutyraldehyde.Scheme 6. syn-Elimination from anti-16a and syn-16b.
(DFT) methods showed that anti-16a is more stable by 0.39 (using PM3), 2.18 (using AM1) and 8.39 kcal mol -1 (using DFT) than its corresponding epimer at C-3 and C-1' which would require cis approach of the aldehyde to the methyl group at the quaternary center through a Zimmerman-Traxler transition state.
The preferential formation of syn-16b when the N,Osilylketeneacetal from quinolizidinone 13 was employed (Mukaiyama conditions) may be rationalized through a preferential open transition state model with antiperiplanar approach of the N,O-silylketeneacetal to the aldehyde so as to relieve the steric strain between the isopropyl group of the aldehyde and the quinozilidine ring (Scheme 7).
At this point, we needed to secure an efficient and stereospecific anti elimination methodology in order to benefit from the highly stereoselective formation of anti-16a when the lithium or titanium(IV) enolates were employed (Scheme 5) and we that goal in mind several reaction conditions were investigated.
Initially, we employed anti-14a as our model compound and the results are summarized in Scheme 8.The conversion of anti-14a to the corresponding mesylate 19, followed by elimination in refluxing pyridine provided only (E)-15a.The same stereochemical outcome was observed by Gallagher 16 (1.Me 2 SO 2 Cl, py, 0 °C to rt; 2. HCl; 3. MeOH/ KOH, reflux) (Scheme 8, equation 2).Reasoning that the preferential formation of (E)-15a resulted from a competitive ElcB mechanism which would be enforced over the expected E2 by polar solvents, we decided to investigate elimination of 19 in hexane with DBU as base.Compared to bases such as pyridine, this amidine base (DBU) is particularly effective in promoting elimination reactions. 14In fact, under these conditions a 3:1 mixture of stereoisomers 15a:15b was formed in 88% yield (Scheme 8, equation 3).The exclusive formation of stereoisomer 15b was eventually achieved using Stork protocol, 15 suggesting that only E2 mechanism was operative under these experimental conditions (Scheme 8, equation 4).
The preference for the formation of (E)-isobutylidene side chain was also observed when anti-16a was submitted to the conditions described by Gallagher et al. 16 which provided (E)-17 in 83% yield.However, as observed above for anti-14a, under Stork conditions only the desired (Z)isomer 18 was formed in 80% yield (Scheme 9).
The vinylogous Michael addition of silyloxyfuran 5 to α-methoxycarbamate 6a was employed in the total synthesis of (±)-homopumiliotoxin 223G (1) alkaloid which was accomplished in 5 steps and 13% overall yield.The approach may find good use also in the preparation of indolizidine alkaloids and provide a good solution to the installation of the (Z)-alkylidene side chain in heterocyclic systems.Additionally, our initial results on the asymmetric version of the vinylogous Mannich reaction employing chiral carbamate 6c may be considered for the preparation of the asymmetric version by the route described above.

General
All experiments were carried out under an argon atmosphere except for hydrolysis under acid conditions.Dichloromethane was distilled from CaH 2 , tetrahydrofuran previously treated with CaH 2 and distilled from sodium, methanol was distilled from Mg tunnings.The normal extracts consisted of drying over MgSO 4 , filtration and concentration under reduced pressure with a rotatory evaporator.The compounds were purified by column chromatography on silica gel (70-230 mesh).The 1 H-NMR and 13 C-NMR spectra were recorded on a Varian Gemini (7.05T), Varian Inova (11.7T) spectrometers.Chemical shifts (δ) are recorded in ppm with the solvent resonance as the internal standard and coupling constants (J) recorded in Hz.Signals for rotational and/or configuration isomers are denoted inside brackets.The infrared spectra were recorded as films in KBr cells on a Nicolet Impact 410 (FTIR).High resolution mass spectroscopy (HRMS) were performed on a Autoespec-Micromass-EBE. Optical rotations were measured on a polarimeter Polamat A Carl Zeiss Jena using a quartz cell and a mercury or sodium lamp.The melting points were measured on an Eletrothermal 9100 apparatus.The gas chromatography analyses (FID detector) were performed using a HP-5890-II equipament.Gas chromatography-mass spectrometry (GC-MS) analyses were performed on a Hewlett Packard 5890/ Hewlett Packard 5970 MSD.

Preparation of lithium aluminum hydride solution in THF
Solution of LiAlH 4 (5.0 g, 0.125 mol) in THF was prepared by adding an excess of the hydride to dry THF (80.0 mL) and stirred the mixture at least 2 h under a dry argon atmosphere.The resulting solution was then filtered under a slight positive argon pressure through a 2 cm bed of tightly packed Celite prepared on a Schlenk system.After following the above procedure, a crystal-clear 1.55 mol L -1 solution was obtained.

(E)-1 Hydrochloride
The same procedure was employed to 17 affording (E)-1 in 80% yield as a colorless crystal.

Table 1 .
Addition of 5-methylsilyloxyfuran(5)to N-acyliminium ions a Diastereoisomeric ratio determined by GC and confirmed by 1 H-NMR analyses; b Yields determined after column chromatography on silica gel of the crude product.Scheme 1. Conversion of butenolides 7a and 8a to the corresponding quinolizidinones 12 and 13.J. Braz.Chem.Soc.