Building polyfunctional piperidines: a stereoselective strategy of a three-component Mannich reaction inspired by biosynthesis and applications in the synthesis of natural alkaloids (+)-241D; (−)-241D; isosolenopsin A and (−)-epimyrtine

A general method to assemble multi-substituted chiral piperidines was developed, inspired by the biosynthesis of piperidine natural products. In biosynthesis, D-piperideine 4 plays a key role as a common intermediate giving rise to a variety of piperidine-based natural alkaloids. Nature uses L-lysine as a building block, enzymatically transforming it into a d-amino carbonyl intermediate 3 as the precursor to cyclize into D-piperideine 4. We envisioned that such a process could be accomplished by a vinylogous type Mannich reaction if a functionalized dienolate was employed. A stereoselective threecomponent vinylogous Mannich-type reaction (VMR) of 1,3-bis-trimethylsily enol ether 7 was therefore investigated and was found to give cyclized chiral dihydropyridinone compound 9 as an adduct. Like Dpiperideine in biosynthesis, the chiral 2,3-dihydropyridinone compound 9 from VMR is a versatile intermediate for building a variety of new chiral piperidine compounds. The method was showcased by concise two-step approaches in the synthesis of the bioactive natural alkaloids (+)-241D; ( )-241D and isosolenopsin A. Furthermore, when properly functionalized substrate aldehyde 24 was employed, the corresponding dihydropyridinone adduct 25 cyclized to form a second piperidine ring, leading to a chiral polyfunctional quinolizidine enaminone 27. This versatile intermediate was used to prepare a variety of new chiral quinolizidine compounds, including natural alkaloid ( )-epimyrtine.


Introduction
Functionalized piperidine rings are common moieties incorporated in a variety of natural alkaloids and pharmaceutical molecules. 1 In fact, piperidine is the most frequently used nonaromatic ring in small molecule drugs listed in the FDA orange book. 2 Developing synthetic approaches for the stereoselective construction of these ring systems has been an area of intense research in synthetic organic chemistry for decades. 3 Among the various piperidine derivatives, 2 and/or 6 substituted piperidines are particularly common and interesting 4 since such substitution patterns block the metabolism of the piperidine ring and potentially have a signicant impact on the ring's 3D conformation. For such reasons, installation of a substitutions adjacent to the piperidine nitrogen are commonly employed as a strategy in medicinal chemistry research to tune either biological activities or pharmacological properties. In practice, the methyl group is one of the most common and simplest substituents serving this purpose. Interestingly, a-methyl multisubstituted piperidines are also commonly found in naturally occurring piperidine alkaloids such as (À)-pinidinol, (+)-241D and isosolenopsin A etc. (Fig. 1). Some of these natural alkaloids have demonstrated interesting pharmacological properties and served as valuable starting points for new drug discovery. 5 The biosynthetic pathway of many piperidine-based natural alkaloids has been studied. D 1 -Piperideine 4, which forms from an intramolecular imine cyclization of a d-amino pentanal precursor 3, was believed to be a key common intermediate in the pathway. Studies have shown that further transformations on this prototype piperidine ring lead to a variety of structurally diversied piperidine, quinolizidine and indolizidine alkaloids in nature. 6 The basic starting building block in this pathway is Llysine, which undergoes several enzymatically catalyzed transformations, including decarboxylation by LDC (lysine decarboxylase) and oxidative deamination by CuAO (copper amine oxidase). The resulting d-amino pentanal 3 then gives rise to the key D 1 -piperideine ring (Fig. 2). However, without nature's powerful enzyme tools, chemical synthesis of D 1 -piperideine is tedious 7 due to its instability and such intermediate is therefore not practical to be widely applied in synthesis lab like its role in biosynthesis. 8 We envisioned however that similar d-amino carbonyl precursor for D 1 -piperideine can be assembled conveniently via a vinylogous Mannich-type reaction (VMR) with an aldimine if a properly functionalized dienolate was employed. As shown in Fig. 2, cyclization of the initial d-amino carbonyl adduct would lead to a 2,3-dihydropyridinone, which could also be viewed as a tautomeric form of cyclic imine, but more stable and easier to handle (Fig. 2). In fact, the synthetic utility of dihydropyridinones has been extensively investigated by the Comins group, but to date the methodology for preparation of these intermediates has been limited. 9 Here we report the successful implementation of the VMR strategy to generate useful chiral dihydropyridone intermediates, and their subsequent transformation to a variety of interesting piperidinecontaining natural products and compounds of medicinal interest.

Results and discussion
The simple 1,3-bis-trimethylsily enol ether 7 has been employed as a vinylogous nucleophilic reagent in several organic transformations such as cyclization with 1,2-dielectrophiles, bromination, and vinylogous aldol reaction. 10 Surprisingly, the use of 7 as dienolate in a Mannich-type reaction has never been reported. 11 To ensure stereoselective control in VMR, inexpensive commercially available chiral a-methyl benzylamine 6 was employed to form chiral aldimines in situ. The three-component VMR reaction of 6 and 7 with various aldehydes 5 was carried out in the presence of Sn(OTf) 2 in DCM at À78 C to 0 C. Corresponding adducts 8 were observed from reaction LC-MS analysis, however in a mixture with cyclized 2,3-dihydropyridinone products 9. Treatment of the crude mixture with a catalytic amount of acetic acid in DCM led to complete conversion of acylic adducts 8 into 9 (Scheme 1).
The results of the VMR reaction of 7 with various aldehydes are summarized in Table 1. Most of the reactions showed moderate to good yields. A variety of functional groups were well tolerated. The reactions showed excellent diastereoselectivities since in all cases only single isomers were observed and isolated from the reaction mixtures. In order to conrm that the stereoselectivities of the reaction were auxiliary directed, compounds 9d-I and 9d-II were prepared from the same chiral substrate aldehyde 5d, in the presence of chiral amine auxiliary 6a and its enantiomer 6b. The proton NMR spectra of these compounds showed that the J Ha/Hb value for 9d-I was 8.80 Hz while the corresponding value for 9d-II was 9.2 Hz, suggesting that 9d-I and 9d-II were the erythro and threo isomers respectively, based on literature precedent. 12 These results conrmed auxiliary directed stereoselectivities and further supported the established sense of stereochemical induction in such VMR 13 (Fig. 3).
To examine the synthetic utility of 2,3-dihydropyridinones obtained from the VMR, adduct compound 9h was selected to probe further transformations. When the compound 9h was treated with TFA at room temperature, the chiral benzyl directing group was cleaved to give cyclic enaminone 10 in quantitative yield (Scheme 2). We also found that the corresponding chiral substituted piperidine could be obtained from 9h via palladium catalyzed hydrogenation. Interestingly, under different hydrogenation conditions, the reduction of 9h yielded different major piperidine products. When hydrogenation was performed in MeOH in presence of palladium on carbon at room temperature, the reaction cleaved the chiral benzyl group and saturated the 2,3-dihydro-4-pyridinone simultaneously to give cis-3-hydroxy 2,6-disubstituted piperidine compound 11 stereospecically as the major product, accompanied by deoxygenated piperidine compound 12 as the minor product (11/12, ratio 10 : 1) 14 (Scheme 2). However, when the hydrogenation was performed in a Parr hydrogenator under 40 psi hydrogen pressure in a mixture of methanol and acetic acid (1/1), the major product was deoxygenated cis-2,6-dialkylated piperidine 12 accompanied by 11 as the minor product (12/11, 5 : 1) (Scheme 2). The results could be explained by a shi in the equilibrium between 2,3-dihydropridinone and 2,3-dihydropridinium under different conditions. 15 Presumably, 2,3dihydropridinone is the major species present under neutral conditions and hydrogenation led to 4-hydroxy piperidine product 11. However, under acidic conditions, the protonated 2,3-dihydropridinium species is the major (or more reactive) species present, and hydrogenation gives the corresponding deoxygenated piperidine 12 as the major product 16 (Fig. 4). Similar hydrogenations of 2,3-dihydropyridinones have been reported to yield 4-hydroxy piperidine compounds stereospe-cically. 14 However, to our knowledge the direct deoxygenative reduction of 2,3-dihydropridinones is rarely reported. The results allowed accessing different substitution type piperidine compounds from 2,3-dihydropyridinones 9 by simply switching different reduction conditions. We further probed the utility of our chiral piperidine intermediates by applying the VMR methodology to the asymmetric synthesis of natural piperidine-containing alkaloids. Dendrobate alkaloid (+)-241D and its enantiomer (À)-241D were among our rst targets. Dendrobate alkaloid (+)-241D was isolated from the methanolic skin extracts of the Panamanian poison frog Dendrobates speciosus. 17 The alkaloid shows interesting bioactivity as a potent non-competitive blocker of acetylcholine and ganglionic nicotinic receptor channels. 18 The structure of (+)-241D features an all-cis 2,4,6-trisubstituted piperidine core bearing three chiral centers. The asymmetric synthesis of (+)-241D has been reported by multiple research groups via a variety of synthesis routes employing between eight and eighteen steps. 19 We were delighted to nd that using the newly developed VMR strategy, the asymmetric synthesis of (+)-241D and its enantiomer could be accomplished simply in two steps from inexpensive commercial materials. Using chiral a-methyl benzylamines 6a & 6b to control stereochemistry, the reaction of bis-trimethylsily enol ether 7 with decanal 13 yielded chiral adducts 14 & 15 respectively. Subsequent reduction of 2,3dihydro-4-pyridones 14 & 15 by palladium-catalyzed hydrogenation in methanol gave (+)-241D and (À)-241D in good yield (Scheme 3).
The versatile utility of such VMR approach in assembling piperidine was further exampled in asymmetric synthesis of another natural alkaloid isosolenopsin A which incorporate cis-2,6-dialkylpiperidine as a core. isosolenopsin A was isolated from the venom of the re ant solenopsis and was found to have a variety of interesting bioactivities including antibiotic, antifungal, anti-HIV, blockade of neuromuscular transmission and potent and selective inhibition of the neuronal nitric oxide synthase. 20 By the similar strategy, corresponding VMR adduct 2,3-dihydro-4-pyridones 17 was obtained when dodecanal 16 and chiral amine 6b were employed. The palladium-catalyzed reduction on 2,3-dihydro-4-pyridone 17 was carried out in methanol in presence of acetic acid (50%) under 40 psi hydrogen pressure in a Parr hydrogenator. Corresponding deoxygenated product isosolenopsin A was obtained as the major product in moderate yield (45%) (Scheme 4). The current approach presented the shortest route for asymmetric synthesis of isosolenopsin A than any other reported methods. 21 Beyond applying to building simple multi-substituted piperidine compounds, current VMR strategy also provides potentials in synthesizing chiral quinolizidine compounds. Quinolizidine compounds structurally incorporate two fused piperidine rings sharing common nitrogen. Like piperidine, quinolizidine represent both a class compound of pharmaceutical interest and an important family of natural alkaloids. In nature, several hundred structurally related quinolizidine compounds have been identied from a variety of natural sources, predominately from plants and amphibian skin. 22 Some natural quinolizidine alkaloids exhibit interesting pharmacological properties 23 and serve as important starting points for the drugs discovery. 24 Interestingly, the biosynthesis of some quinolizidine alkaloids shares the same pathway of the natural piperidine alkaloids that undergo the same D 1 -piperideine intermediate 4, enzymatically starting from L-lysine. 25 As an example, in the biosynthesis of quinolizidine alkaloids lupinine, D 1 -piperideine is also the key intermediate to assemble the rst piperidine ring for the quinolizidine core. To build the second piperidine ring, two D 1 -piperideine intermediates undergo a cross aldol-type coupling and one of the imine systems gets hydrolyzed aer coupling and undergoes oxidation resulting in primary amine function 20. Ultimately the formation of the quinolizidine nucleus in biosynthesis is accomplished by another intramolecular imine formation (Fig. 5). We however envisioned that in the VMR we developed, if the aldimine substrate has been properly functionalized, piperidinelike adducts arising from the asymmetric VMR may further conveniently cyclize to form second piperidine ring to give the desired quinolizidine product. As shown in Fig. 5, if a d-leaving group is incorporated in the aldimine substrate and can be tolerated in the asymmetric VMR for the rst piperidine ring construction, the quinolizidine structure 22 should be readily formed by a subsequent intramolecular SN2 cyclization (Fig. 5).
To test this idea 5-chloropentanal 24 was prepared from the oxidation of 5-chloropentan-1-ol 23 and the corresponding three-component VMR reaction was carried out. The reaction gave adduct 25 in expected excellent diastereoselectivity as a single stereoisomer. The d chloride group which serve as a future leaving group on the substrate, was well tolerated (Scheme 5). With dihydropyridinone 25 in hand we set out to construct the second ring for a quinolizidine core. The a-methyl benzyl group was cleaved cleanly upon the treatment with TFA at room temperature overnight to give compound 26 in quantitative yield. In presence of sodium hydride in DMF, intramolecular SN2 cyclization by 26 led to a quinolizidine intermediate 27 as a cyclic enaminone (Scheme 5). We envisaged that such cyclic enaminone 27 could be a valuable polyfunctional quinolizidine intermediate since different organic transformations can be carried out at different positions on this molecule. It provides convenient entries to access different types chiral quinolizidine compounds (Fig. 6).
The reduction of cyclic enanminone 27 was rst explored. It was found that under the conditions of either palladiumcatalyzed hydrogenation in methanol or treating L-selectride (LiBu i 3 BH) in THF, both alkene and carbonyl were reduced affording cis-2-hydroxyl-4-methly quinolizidine 28 as product (Scheme 6, eqn (1)). Similarly as in the reduction of 2,3-dihydropyridinone, the reduction on quinolizidine enanminone also proceeded in stereoselective manner which is in agree with literature precedents. 26 When the reduction was carried out with "super hydride" (LiEt 3 BH) in presence of BF 3 $Et 2 O in THF, the alkene functionality was selectively reduced, giving 29 as natural quinolizidine alkaloid (À)-epimyrtine as the product in good yield 27 (Scheme 6, eqn (2)). The results provided a concise approach for the enantioselective synthesis of such natural alkaloid. 28 Conjugate additions to quinolizidine enaminone 27 were also explored. Although direct conjugate addition of Grignard reagents to cyclic enaminones has been previously reported, 29 in our hands, treatment of 27 with methyl magnesium bromide   31 Finally, to further probe structural diversication, alkylation reaction on the methyl side chain of 27 was investigated. It was found that a corresponding enolate can be generated by treating 27 with LiN(SiMe 3 ) 2 (LiHMDS) in THF at low temperature. By subsequently treating such enolate with alkylating agents 31, corresponding alkylation products 32 could be obtained smoothly. The results of such reaction were summarized in Table 3. The reactions gave moderate to good yields by showing the tolerance toward different functional groups. No epimerization was observed in such enolate alkylation. Such alkylation reaction led to the side chain extension and provided opportunities to synthesize more structural diversied quinolizidine-based compound beyond a methyl substituted type (Scheme 8).

Conclusion
In summary, inspired by the biosynthesis pathway of natural piperidine-based alkaloids, a general and practical approach to synthesize multi-substituted chiral piperidine was developed via a stereoselective three-component vinylogous Mannich-type reaction (VMR) by using 1,3-bis-trimethylsily enol ether 7 as a dienolate. The corresponding VMR adduct was chiral 2,3dihydropyridinones 9 which played the role of cornerstone in building new targeted chiral piperidine compounds. The efficiency of such stereoselective synthesis approach was exampled in developing novel synthesis of bioactive natural alkaloids: dendrobate alkaloids (+)-241D; (À)-241D, and isosolenopsin A in highly concise manners. Beyond simple piperidine compound synthesis, the method also provided rapid route for chiral quinolizidine construction. When prefunctionalized substrate aldehyde 24 was employed, the corresponding VMR adduct could cyclize to give versatile quinolizidine cyclic enaminone 27. The different types transformations carried out on such polyfunctional intermediate gave rise a variety of new chiral quinolizidine compounds, including natural alkaloid (À)-epimyrtine. We believe the presented VMR approach offers a general; stereoselective and efficient way to assemble multi-substituted chiral piperidine-based compound in organic synthesis.

General methods
All commercial reagents and solvents were used without puri-cation. 1 H and 13 C NMR spectra were recorded on a Bruker 400 MHz spectrometer using TMS as the internal standard (0 ppm). TLC analyses were carried out on aluminum sheets precoated with silica gel 60 F254, and UV radiation was used for detection. Flash column chromatography was performed on silica gel (SiliaFlash F60, 230-400 mesh). LC/MS analysis was performed on an Agilent 1100 series system equipped with an Agilent 1100 series binary pump, Agilent 1100 series autosampler, Agilent 1100 series DAD UV detector, Agilent 1100 series single quadrupole mass spectrometer with ESI source, and a SEDEX 75 ELSD. The mass spectrometer was set to scan from 100 to 1000 AMU. Mass spectrometric data were acquired in the positive ionization mode. The mobile-phase solvents used were (A) 0.05% aq. TFA; and (B) 0.035% TFA in MeCN. The total mobile phase ow rate was 1.0 mL min À1 . The gradient was 10-90% in 3 min with an isocratic hold of 100% mobile-phase B for 0.49 min at the end of the gradient. A Waters Atlantis T3 (5 mm, 2.1 Â 50 mm) column was used. Chemical shis of NMR spectra are reported in ppm, multiplicities are indicated by s (singlet), d (doublet), t (triplet), q (quartet), and m (multiplet). All vinylogous Mannich reactions were carried out in oven-dried glassware under air atmosphere. 1,3-Bis-trimethylsily enol ether 7 was prepared freshly following the procedure from literature. 10d Diastereo-selectivities of VMR described in the manuscript were determined both by HPLC and NMR.