Total Synthesis of the Ortho-Hydroxylated Protoberberines (S)-Govaniadine, (S)-Caseamine, and (S)-Clarkeanidine via a Solvent-Directed Pictet–Spengler Reaction

Abstract

The common para regioselectivity in Pictet–Spengler reactions with dopamine derivatives is redirected to the ortho position by a simple change of solvents. In combination with a chiral auxiliary on nitrogen, this ortho-selective Pictet–Spengler produced the 1-benzyltetrahydroisoquinoline alkaloids (S)-crassifoline and (S)-norcrassifoline and the bioactive 1,2-dioxygenated tetrahydroprotoberberine alkaloids (S)-govaniadine, (S)-caseamine, and (S)-clarkeanidine with high enantiopurity. Ortho/para ratios up to 89:19 and diastereomeric ratios up to 85:15 were obtained during formation of the B-ring. The general applicability of this solvent-directed regioselectivity was demonstrated with a second Pictet–Spengler reaction as required for C-ring formation of caseamine (o/p = 14:86 in trifluoroethanol) and clarkeanidine (o/p = 86:14 in toluene).

Introduction

Most of the 1-benzyltetrahydroisoquinoline alkaloids found in nature are formed from dopamine and contain a 6,7-dioxygenated substitution pattern in the A-ring as a result of enzyme-catalyzed Pictet–Spengler condensations (Figure 1).^1,2 Isomeric 1-benzyltetrahydroisoquinolines with oxygen substituents at C-7 and C-8 are less abundant in nature but display interesting biological properties.³ Examples of more complex alkaloids derived from 7,8-dioxygenated 1-benzyltetrahydroisoquinolines are the parent compound crassifoline (3), several tetrahydroprotoberberines (e.g., govaniadine (4)), the cularine alkaloids (5),⁴ and pavine alkaloids such as neocaryachine (6). Labeling studies performed by Müller and Zenk⁵ to elucidate the biosynthesis of crassifoline and the cularine alkaloids showed that this unusual oxygenation pattern in the tetrahydroisoquinoline ring is not formed by oxygen transposition but most likely by an ortho-selective Pictet–Spenglerase, although this enzyme has not yet been described in literature.

Figure 1.

General biocatalytic Pictet–Spengler reactions and some examples of alkaloids based on the 7,8-dioxygenated tetrahydroisoquinoline structure.

The enantioselective chemical syntheses of 6,7-oxygenated 1-benzyltetrahydroisoquinolines preferably follow the lines of the biosynthesis. In particular, the Bischler–Napieralski method, in combination with asymmetric hydrogenation or by chiral auxiliary directed hydride reduction, is favored for enantioselective preparations (reviewed by Rozwadowska in 2004 and 2016, see ref (2)). A practical synthesis of the 7,8-dioxygenated tetrahydroisoquinoline ring system, however, is not accessible via the Bischler–Napieralski reaction, which exclusively yields para products. Likewise, the Pictet–Spengler approach with chiral (organo)catalysis, or with assistance of chiral auxiliaries, is only effective for the traditional 6,7-substitution pattern.^2,3,6,7 A few methods are described to prepare this 7,8-substitution pattern, and these are not based on Pictet–Spengler or Bischler–Napieralski approaches but require multistep quinoline ring construction. Rodrigues described an efficient build-up/chiral auxiliary approach to ortho-hydroxylated crassifoline and the cularine alkaloids.^4a Halogen atoms as temporary blocking substituents at positions in the aromatic ring that should stay unsubstituted are also applied.^4b

Ortho selectivity toward an activating substituent in Mannich-type cyclizations is more often observed, but in the Pictet–Spengler reaction, ring closure ortho to the phenolic substituent is always a minor process in comparison to the para position. The pH dependency of ortho/para ratios was investigated by Bates, who found pH 7 as an optimum for ortho product formation (o/p = 50:50) using formaldehyde or acetaldehyde.⁸

In a previous publication on the synthesis of javaberine alkaloids, we reported that the regioselectivity of the Pictet–Spengler reaction between secondary phenylethylamines and aldehydes depends strongly on the solvent and varies between 99% para selectivity in trifluoroethanol to 81% ortho selectivity in aprotic, apolar solvents without addition of external acids (Scheme 1).^7a

Scheme 1. Ortho-Selective Pictet–Spengler Reaction toward the Javaberine Synthesis (ref (7a)).

Furthermore, both ortho and para products were formed as single diastereomers. To translate this uncatalyzed Pictet–Spengler procedure⁹ to both the challenging ortho regioselectivity and enantioselectivity in the 1-benzyltetrahydroisoquinoline series, we herein disclose a chiral auxiliary approach starting from a (S)-(−)-α-methylbenzyl-functionalized dopamine analogue.¹⁰

Results and Discussion

The benzene ring in the dopamine part of the key precursor 10 (Scheme 2) requires activation by a free phenolic OH to allow non-acid-catalyzed Pictet–Spengler reactions with dopamine derivatives. If methoxy or methylenedioxy substituents are the activating substituents, strongly acidic catalysts are required that produce almost exclusively para-substituted Mannich-type products.² The required phenylethylamine 10 was prepared from phenylacetaldehyde 9 that was obtained after a convenient Wittig/hydrolysis homologation process^7,15 starting from isovanilline (7). Reductive amination of phenylacetaldehyde 9 with (S)-α-methylbenzylamine gave chiral dopamine analogue 10. To optimize the Pictet–Spengler conditions, we selected (S)-govaniadine 4, a 1,2-oxygenated tetrahydroprotoberberine alkaloid that has not been synthesized before (Scheme 2). Govaniadine is isolated from Corydalis govaniana Wall. and has been the subject of different studies on its biological activity since its discovery in 2013.¹¹ These studies revealed significant analgesic activity for govaniadine, similar to that of ibuprofen, due to its potential binding to the COX-2 enzyme.¹² Furthermore, high and selective leishmanicidal activity,¹³ antiurease activity,¹⁰ and glucoronidase inhibition were reported.¹⁴

Scheme 2. Ortho and Para Product Formation: Activation of the Enamine Intermediate in Aprotic and Protic Solvents.

The Pictet–Spengler reaction of aldehyde 11(16) with equimolar amounts of 10 in different solvents was monitored by NMR and shows a clear solvent-dependent ortho/para distribution of the product (Table 1). Protic solvents, with TFE as the strongest proton donor, gave fast reactions with high preference for the para isomer 17, which is typical for a process that is acid catalyzed. Reactions in toluene and dichloroethane, both performed at higher dilution to prevent intermolecular catalysis by the phenolic OH, were considerably slower but gave good selectivity for the ortho isomer 15.

Table 1. Ortho/Para Ratios in the Pictet–Spengler Cyclization of 10.

entry	solvent	T (°C)	time	ortho/paraa (15/17)	dr orthob (15/16)
1	TFE	75	1 h	10:90	53:47
2	methanol	65	2 d	38:62	60:40
3	MeCN	80	2 d	65:35	60:40
4	DCEc	80	4 d	72:28	85:15
5	toluenec	105	4 d	81:19	73:27

^aAt >80% conversion, determined by ¹H NMR.

^bThe para isomer was formed as a ca. 50:50 mixture of inseparable diastereomers.

^cPerformed at 40 mM. TFE = 2,2,2-trifluoroethanol, DCE = 1,2-dichloroethane.

Importantly, the diastereomeric ratio of the ortho isomers 15, with the required (S)-configuration at C-1¹⁷ and 16 (R-configuration at C-1, not shown) in toluene and dichloroethane was good, and the isomers were readily separable by chromatography. This is in sharp contrast with the para isomer 17, which was formed exclusively as an inseparable mixture of both diastereomers in nearly equal amounts. NMR spectra of the crude reaction mixtures at an early stage showed that the reactants were converted into the unstable, E-enamine 12. An explanation for the high (S)-preference at C-1 in the ortho isomer can be found in reaction intermediate 13, assuming that the intramolecular hydrogen-bonded structure shown in Scheme 2 is favored in aprotic solvents. The compact structure of intermediate 13 enhances approach of the aromatic ring from the top side of the iminium ion. In toluene, the highest ortho/para ratio was obtained, while DCE gave a better diastereomeric ratio. Since the yields in both solvents were comparable, toluene was selected for scaling up the synthesis, providing pure 15 in 48% isolated yield. Debenzylation of 15 to 18 and cyclization of the C-ring with formaldehyde under acidic conditions produced (S)-(−)-govaniadine 4, which was identical to the natural product (Scheme 3).¹¹ Final analysis of the recrystallized product (>99% ee) gave an optical rotation which is typical for such systems: [α]_D²⁰–339 (c = 0.55, methanol). This optical rotation is significantly larger than the one reported in the literature ([α]_D²⁰ −59.9 (c = 0.1, methanol), indicating that the product isolated from Corydalis govaniana Wall. was not optically pure.¹¹

Scheme 3. Synthesis of Govaniadine.

The next targets, norcrassifoline (23) and crassifoline (3), were readily prepared via the same sequence, starting from phenethylamine 10 and TBS-protected homoisovaniline 19(7b) (Scheme 4). Comparable yields and selectivities were obtained from the Pictet–Spengler reaction in toluene, producing 45% of the desired isomer 20 after chromatographic purification. Hydrogenolysis of the chiral auxiliary gave norcrassifoline 23, which was reductively methylated to crassifoline 3.

Scheme 4. Synthesis of Caseamine and Clarkeanide via Norcrassifoline.

Norcrassifoline (23) was also used as the synthetic precursor for the related protoberberines caseamine (24¹⁸), displaying activity against urease,¹¹ and clarkeanidine (25¹⁹). Biosynthetically, the ring closure of 1-benzyltetrahydroisoquinolines to tetrahydroprotoberberines does not proceed with formaldehyde but via the berberine bridging enzyme (BBE) catalyzed oxidation of N-methylated benzyltetrahydroisoquinolines, immediately followed by ring closure of the intermediate methylene iminium salt (Scheme 5).²⁰

Scheme 5. Ortho vs Para Selectivity in Tetrahydroberberine Synthesis.

A clear preference of this enzyme for ring-closure ortho to the phenolic OH (26) is observed, which hampers the synthesis of para products via biocatalytic routes.²¹ Similar to the synthetic Pictet–Spengler approaches under traditional protic conditions using formaldehyde and a free NH substrate, the para isomer is formed exclusively when alkoxy groups are used as the activating substituents,² as we also have shown in the govaniadine synthesis (Scheme 2). When a free phenolic OH is the activator, the para product (28) is always formed in excess, but is accompanied by some ortho product.²²⁻²⁴ Application of the solvent-directed Pictet–Spengler process (see Scheme 2) to the tetrahydroprotoberberine synthesis with norcrassifoline and formaldehyde selectively produced both isomers under mild conditions (Scheme 4). The para isomer (S)-caseamine 24 was obtained by reaction of 23 with formaline in trifluoroethanol [64%, o/p = 14:86, >99% ee after recrystallization, [α]_D²⁰ −314 (lit.¹⁸ [α]_D²⁰ −328)]. Starting from 23 under aprotic conditions using paraformaldehyde in toluene, the ortho isomer (S)-clarkeanide 25 was formed [55%, o/p = 86:14, 95% ee after crystallization, [α]_D²⁰ −442 (lit.¹⁹ [α]_D²⁰ −277)].

In conclusion, we have shown that Pictet–Spengler reactions under apolar conditions can produce the otherwise difficult to access ortho-oxygenated products. The chiral auxiliary-supported route is straightforward, scalable, and in particular, suitable for high diastereoselectivety in ortho-hydroxylated tetrahydroisoquinoline preparations. In addition, application of this solvent-directed Pictet–Spengler approach to regioselective tetrahydroprotoberberine synthesis provides a useful addition to existing methods.

Experimental Section

General Information

Anhydrous CH₂Cl₂ and CH₃CN were freshly distilled from CaH₂. Dried THF was obtained by distillation from sodium/benzophenone. DMF and DMSO on 4 Å molecular sieves were obtained from Sigma-Aldrich and stored under N₂ atmosphere. Toluene was distilled and stored on 4 Å molecular sieves. Reagents were purchased with the highest purity (usually >98%) from Sigma-Aldrich and Fluorochem and used as received. Reactions were monitored by thin-layer chromatography (TLC) carried out on 0.25 mm E. Merck silica gel plates (60F-254). SilaFlash P60 (particle size 40–63 μm) was used for silica column chromatography. NMR spectra were recorded on Bruker DRX-500, -400, and -300 MHz instruments and calibrated on residual undeuterated solvent signals as internal standard. The ¹H NMR multiplicities were abbreviated as follows: s = singlet, d = doublet, t = triplet, q = quartet, quint = quintet, m = multiplet. High-resolution mass spectra (HRMS) were recorded on a AccuTOF GC v 4g, JMS-T100GCV mass spectrometer (JEOL, Japan). An FD/FI probe equipped with a FD emitter of 10 μm. Current rate 51.2 mA/min over 1.2 min machine using field desorption (FD) as ionization method. IR spectra were recorded on a Bruker Alpha FTIR machine. Chiral HPLC was performed with a Shimadzu LC-20AD with Shimadzu SPD-M20A diode array detector using a Daicel Chiralcel AD column (eluent n-heptane/2-propanol 70/30, flow 1.000 mL/min, λ 230 nm).

Synthetic Procedures

2-Methoxy-5-(2-methoxyethenyl)phenol (8)¹⁵

KOt-Bu (22.4 g, 200 mmol) was added in three portions, with intervals of 3 min, to an efficiently stirred suspension of methoxymethyltriphenyl phosphonium chloride (34.3 g, 100 mmol) in dry THF (250 mL) with ice cooling. After additional stirring for 5 min, isovanillin 7 (13.7 gr, 90 mmol) was added in three portions, with intervals of 2 min, to the reaction mixture resulting in a rapid color change from red to yellow. The cooling bath was removed, and the mixture was stirred at rt for 5 h. Silica gel was added (150 g), the solvents were evaporated thoroughly, and the residue was put on top of a silica column. Flash chromatography (petroleum ether/ethyl acetate 4/1, 3/1 and 2.5/1) gave 8 (13.3 g, 73.9 mmol, 82%, 45:55 E/Z mixture) as an oil, which solidified upon standing. The spectra were identical with those of ref (15): ¹H NMR (400 MHz, CDCl₃) δ 7.09 (dd, J = 8.4, 2.1 Hz, 1H), 7.05- 6.93 (m, 2H), 6.87–6.69 (m, 3H), 6.12 (s, 1H), 6.08 (d, J = 7.0 Hz, 1H), 5.82 (d, J = 12.9 Hz, 1H), 5.20 (d, J = 7.0 Hz, 1H), 3.825 (s, 3H), 3.82 (s, 3H), 3.74 (s, 3H), 3.68 (s, 3H).

2-(3-Hydroxy-4-methoxyphenyl)acetaldehyde (9)

A mixture of TFA (5 mL) and water (5 mL) was added to a solution of enol ether 8 (7.39 g, 41.0 mmol) in DCM (200 mL). The resulting heterogeneous mixture was stirred vigorously overnight at rt. Water was added, and after separation the organic layer was washed with NaHCO₃ aq and dried over Na₂SO₄. Chromatographic separation (2/1 and 3/2 petroleum ether/ethyl acetate) gave pure 9 (3.75 g, 24.7 mmol, 60%) as an oil, which solidified in the freezer: ¹H NMR (400 MHz, CDCl₃) δ 9.66 (t, J = 2.4 Hz, 1H), 6.83 (dd, J = 8.2, 1.0 Hz, 1H), 6.78 (d, J = 2.1 Hz, 1H), 6.67 (dd, J = 8.2, 2.1 Hz, 1H), 6.12 (s, 1H), 3.83 (s, 3H), 3.55 (d, J = 2.4 Hz, 2H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 199.7, 145.9, 145.8, 124.6, 120.9, 115.7, 111.0, 55.7, 49.5.

(S)-2-Methoxy-5-(2-((1-phenylethyl)amino)ethyl)phenol (10)

Aldehyde 9 (3.74 g, 22.5 mmol) and (S)-(−)-α-methylbenzylamine (3.1 mL, 24 mmol) were dissolved in THF (75 mL) and stirred at 0 °C for 30 min. Sodium triacetoxyborohydride (10.6 g, 50 mmol) was added, and the mixture was stirred at 0 °C for 30 min and at rt for 14 h. The solvent was evaporated, and the residue was dissolved in ethyl acetate and washed with Na₂CO₃ solution and water. Next, the product was extracted from the organic layer with aqueous HCl (3 × 100 mL). The water layer was washed three times with ethyl acetate before the water layer was basified with Na₂CO₃ solution. Extraction with ethyl acetate, drying over Na₂SO₄, and evaporation of the solvent gave chiral amine 10 (5.02 g, 18.5 mmol, 82%) as a solid: mp 86–92 °C; ¹H NMR (400 MHz, CDCl₃) δ 7.45–7.14 (m, 5H), 6.81–6.73 (m, 2H), 6.66 (dd, J = 8.2, 2.3 Hz, 1H), 5.65 (bs, 1H), 3.88 (s, 3H), 3.78 (q, J = 6.7 Hz, 1H), 2.87–2.47 (m, 4H), 1.35 (d, J = 6.7 Hz, 3H); ¹³C{¹H} NMR (75 MHz, CDCl₃) δ 146.0, 145.6, 145.0, 132.8, 128.5, 127.0, 126.7, 119.7, 115.5, 111.1, 58.2, 55.9, 48.7, 35.3, 23.9; HRMS (ESI⁺) m/z calcd for C₁₇H₂₂NO₂ (M + H)⁺ 272.1651, found 272.1642.

(S)-5-(2-((Benzo[1,3]dioxol-5-yl)vinyl)(1-phenylethyl)amino)ethyl-2-methoxyphenol (12)

An equimolar solution of 10 and 11 in toluene was refluxed for 20 min. Evaporation of the solvent gave unstable enamine 12, mixed with small amounts of starting materials and Pictet–Spengler products: ¹H NMR (400 MHz, CDCl₃) δ; 7.42–7.13 (m, 10H), 6.87 (d, J = 14.0 Hz, 1H), 6.82–6.68 (m, 5H), 6.64 (dd, J = 8.1, 1.8 Hz, 2H), 5.92 (s, 2H), 5.31 (d, J = 14.0 Hz, 1H), 4.45 (q, J = 7.0 Hz, 1H), 3.89 (s, 3H), 3.20 (m, 2H), 2.72 (m, 2H), 1.56 (d, J = 7.0 Hz, 3H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 148.0, 145.7, 145.2, 143.8, 142.8, 137.8, 135.0, 134.8, 133.1, 129.1, 128.7, 128.5, 128.4, 128.4, 128.2, 127.5, 127.2, 127.0, 126.9, 126.9, 125.3, 120.1, 120.0, 116.9, 115.1, 115.0, 110.8, 108.5, 103.5, 100.7, 100.5, 97.1, 61.4, 55.9, 55.9, 55.9, 49.6, 33.2, 21.5, 19.1.

Pictet–Spengler of 10 with Aldehyde 11

A solution of 10 (0.542 g, 2.0 mmol) and homopiperonal 11(16) (0.345 g, 2.1 mmol) in anhydrous toluene (50 mL) was stirred at 105 °C for 4 days. Evaporation of the solvent and separation by flash chromatography (petroleum ether/ethyl acetate 19/1, 10/1, and 4/1) provided first the minor (R)-ortho isomer 16 (0.150 g, 0.360 mmol, 18%), then the desired isomer 15 (0.401 g, 0.962 mmol, 48%), and finally, an inseparable mixture of two para isomers 17 (0.135 g, 0.324 mmol, 16%). 16: [α]_D²⁰ −2.0 (MeOH, c = 2.1); ¹H NMR (500 MHz, CDCl₃) δ 7.31–7.24 (m, 2H), 7.25–7.19 (m, 3H), 6.97 (s, 1H), 6.87 (d, J = 7.9 Hz, 1H), 6.82 (d, J = 7.9 Hz, 1H), 6.77 (d, J = 8.2 Hz, 1H), 6.64 (d, J = 8.2 Hz, 1H), 6.11–5.92 (m, 2H), 5.80 (s, 1H), 4.50 (dd, J = 10.3, 3.0 Hz, 1H), 3.92 (s, 3H), 3.70 (q, J = 6.4 Hz, 1H), 3.31–3.15 (m, 1H), 3.06 (dd, J = 13.7, 3.0 Hz, 1H), 2.89–2.78 (m, 2H), 2.77–2.70 (m, 1H), 2.27–2.20 (m, 1H), 1.03 (d, J = 6.4 Hz, 3H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 146.9, 146.4, 145.3, 143.9, 142.3, 135.7, 128.4, 128.1, 127.2, 126.5, 125.0, 122.5, 119.4, 110.3, 108.8, 107.4, 100.5, 57.7, 56.0, 54.2, 39.6, 39.5, 22.5, 21.8; IR (neat) ν 3514, 1487 cm^–1; HRMS (FD⁺): m/z calculated for C₂₆H₂₈NO₄ (M + H)⁺ 418.2018, found 418.2006. 15: [α]_D²⁰ +41.9 (MeOH, c = 1.0); ¹H NMR (500 MHz, CDCl₃) δ 7.13–7.06 (m, 1H), 7.05 (t, J = 7.4 Hz, 2H), 6.84 (d, J = 7.4 Hz, 2H), 6.76 (d, J = 8.4 Hz, 1H), 6.71 (d, J = 7.7 Hz, 1H), 6.67 (d, J = 8.4 Hz, 1H), 6.64–6.56 (m, 2H), 5.97 (s, 2H), 5.63 (s, 1H), 3.96 (dd, J = 10.3, 2.9 Hz, 1H), 3.89 (s, 3H), 3.63 (q, J = 6.4 Hz, 1H), 3.47–3.38 (m, 1H), 3.33 (dd, J = 14.6, 5.8 Hz, 1H), 3.01–2.84 (m, 2H), 2.74 (dd, J = 13.7, 10.3 Hz, 1H), 2.53–2.34 (m, 1H), 1.29 (d, J = 6.4 Hz, 3H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 146.9, 146.1, 145.4, 144.0, 142.7, 135.1, 128.4, 127.8, 127.5, 126.2, 125.1, 122.7, 119.3, 110.4, 108.7, 107.5, 100.5, 58.9, 56.6, 56.0, 39.6, 38.7, 22.3, 22.1; IR (neat) ν 3533, 1489 cm^–1; HRMS (FD⁺) m/z calcd for C₂₆H₂₈NO₄ (M + H)⁺ 418.2018, found 418.2015. 17 (mixture of diastereomers: ¹H NMR (500 MHz, CDCl₃, selected signals) δ 7.34 (d, J = 4.6 Hz, 4H), 7.24–7.19 (m, 2H), 7.13 (t, J = 5.4 Hz, 2H), 6.74 (d, J = 7.8 Hz, 1H), 6.69 (dd, J = 10.7, 8.5 Hz, 2H), 6.60–6.58 (m, 1H), 6.52 (dd, J = 7.9, 1.7 Hz, 1H), 6.48–6.39 (m, 1H), 6.07 (s, 1H), 5.95 (d, J = 2.2 Hz, 2H), 4.02 (d, J = 8.1 Hz, 1H), 3.96–3.75 (m, 2H), 3.70 (s,1H), 3.69 (s, 3H), 3.63 (s, 1H), 3.34–3.21 (m, 1H), 3.19 (s, 1H), 3.17–3.01 (m, 1H), 2.85 (m, 3H), 2.80–2.61 (m, 2H), 2.52–2.27 (m, 2H), 1.39 (d, J = 6.6 Hz, 3H).

(S)-1-(Benzo[1,3]dioxol-5-ylmethyl)-7-methoxy-1,2,3,4-tetrahydroisoquinolin-8-ol 18

Pictet-Spengler product 15 (0.590 g, 1.41 mmol) was dissolved in ethanol (20 mL) and palladium hydroxide on carbon (10% w/w, 0.190 g) was added. The reaction flask was flushed with hydrogen and stirred for 18 h at 60 °C under hydrogen atmosphere. The mixture was filtered over Celite, the residue was washed with ethanol (100 mL) and the combined filtrates were evaporated. Chromatographic purification (silica gel, petroleum ether/ethyl acetate/Et₃N 50:50:3 gave 18 as a light yellow glass (0.438 g, 1.40 mmol, 99%): [α]_D²⁰+12 (MeOH, c = 0.28); ¹H NMR (400 MHz, CDCl₃) δ 6.85 (d, J = 1.7 Hz, 1H), 6.80 (d, J = 7.6 Hz, 1H), 6.78–6.70 (m, 2H), 6.63 (d, J = 8.3 Hz, 1H), 5.96 (s, 2H), 4.33 (dd, J = 10.6, 2.7 Hz, 1H), 3.85 (s, 3H), 3.23 (m, 2H), 2.99 (ddd, J = 12.1, 6.0, 2.6 Hz, 1H), 2.93–2.73 (m, 2H), 2.67 (dt, J = 16.1, 3.5 Hz, 1H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 147.6, 145.8, 144.2, 142.0, 134.2, 128.1, 125.5, 122.1, 119.5, 109.6, 108.9, 108.1, 100.7, 56.0, 53.2, 37.9, 37.5, 28.8; HRMS (FD⁺) m/z calcd for C₁₈H₂₀NO₄ (M + H)⁺ 314.1392, found 314.1400.

(S)-Govaniadine (4)^11,12

Amine 18 (44.8 mg, 0.142 mmol) was dissolved in ethanol, concd HCl (50 μL) was added, the volatiles were evaporated, and the residue was coevaporated with ethanol. The hydrochloride was redissolved in ethanol (1 mL), water (1.5 mL), and aqueous formaldehyde (37%, 1.5 mL), and this solution was refluxed for 6 h. The volatiles were evaporated, and the residue was stirred with aqueous Na₂CO₃ and ethyl acetate until dissolved. Extractive workup and chromatographic purification (silica gel, petroleum ether/ethyl acetate 50/50 and 0/100) gave (S)-govaniadine (28.1 mg, 0.0875 mmol, 61%) as a crystallizing glass. Recrystallization from methanol gave enantiopure (S)-govaniadine (4): ee > 99% (Chiralcel AD column, eluent n-heptane/2-propanol 70:30, flow 1.000 mL/min); [α]_D²⁰ −339 (MeOH, c = 0.55) [lit.¹¹ [α]_D²⁰–59.9 (c = 0.1, MeOH)]; mp 141–144 °C; ¹H NMR (400 MHz, CDCl₃ + 10% CD₃OD) δ 6.75 (d, J = 8.3 Hz, 1H), 6.65 (d, J = 8.3 Hz, 1H), 6.58 (s, 1H), 6.55 (s, 1H), 5.89 (s, 2H), 4.06–3.93 (m, 2H), 3.88 (s, 3H), 3.71–3.59 (m, 1H), 3.16–2.92 (m, 2H), 2.85–2.55 (m, 3H); ¹³C{¹H} NMR (101 MHz, CDCl₃ + 10% CD₃OD) δ 145.9, 145.6, 144.3, 142.4, 128.2, 128.1, 126.7, 124.4, 119.4, 108.9, 108.6, 106.0, 100.4, 57.7, 56.1, 55.9, 48.4, 32.2, 29.3; IR (neat) ν 3507, 1488 cm^–1; HRMS (FD+) m/z calcd for C₁₉H₁₉NO₄ (M)⁺ 325.1314, found 325.1325.

Pictet–Spengler with Aldehyde 19

A mixture of amine 10 (1.084 g, 4.0 mmol) and aldehyde 19(7a,7b) (1.12 g, 4.0 mmol) was heated at 105 °C in anhydrous toluene (100 mL, 40 mM) during 5 days. Separation by flash chromatography (petroleum ether/ethyl acetate, 12/1, 10/1) provided first the minor 1-(R)-ortho isomer 21 (0.462 g, 0.867 mmol, 21.7%), then the desired 1-(S)-ortho isomer 20 (0.962 g, 1.80 mmol, 45%), and finally an inseparable mixture of two para isomers in a ca. 1/1 ratio (0.221 g, 0.42 mmol, 16%). 21: [α]_D²⁰ −2.3; ¹H NMR (400 MHz, CDCl₃) δ 7.30–7.20 (m, 5H), 6.99–6.90 (m, 2H), 6.85 (d, J = 8.1 Hz, 1H), 6.77 (d, J = 8.3 Hz, 1H), 6.63 (d, J = 8.3 Hz, 1H), 5.78 (s, 1H), 4.52 (dd, J = 9.9, 2.9 Hz, 1H), 3.92 (s, 3H), 3.85 (s, 3H), 3.69 (q, J = 6.3 Hz, 1H), 3.29–3.12 (m, 1H), 3.04 (dd, J = 13.6, 3.0 Hz, 1H), 2.90–2.76 (m, 2H), 2.70 (dd, J = 14.0, 6.0 Hz, 1H), 2.21 (dd, J = 16.6, 4.7 Hz, 1H), 1.06 (s, 9H), 1.03 (d, J = 6.3 Hz, 3H), 0.21 (s, 3H), 0.20 (s, 3H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 149.0, 146.6, 144.3, 144.0, 142.4, 134.6, 128.6, 128.1, 127.3, 126.5, 125.3, 122.8, 122.6, 119.4, 111.5, 108.7, 57.8, 56.1, 55.7, 54.2, 39.5, 39.3, 25.8, 22.6, 21.9, 18.5; HRMS (FD⁺) m/z calcd for C₃₂H₄₄NO₄Si (M + H)⁺ 534.3034, found 534.3016. 20: [α]_D²⁰ +10.5; ¹H NMR (400 MHz, CDCl₃) δ 7.2–7.1 (m, 3H), 6.8 (m, 2H), 6.85–6.76 (m, 3H), 6.76–6.63 (m, 2H), 5.68 (s, 1H), 4.09 (dd, J = 9.9, 3.0 Hz, 1H), 3.91 (s, 3H), 3.915 (s, 3H, 3.73 (q, J = 6.5 Hz, 1H), 3.56–3.42 (m, 1H), 3.33 (dd, J = 14.4, 5.5 Hz, 1H), 3.05–2.89 (m, 2H), 2.89–2.72 (m, 1H), 2.49 (dd, J = 16.4, 4.3 Hz, 1H), 1.33 (dd, J = 6.5 Hz, 3H), 1.12 (s, 9H), 0.27 (s, 3H), 0.26 (s, 3H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 149.0, 146.1, 144.1, 143.9, 142.6, 133.9, 128.5, 127.8, 127.3, 126.1, 125.3, 123.0, 122.8, 119.2, 111.9, 108.5, 58.9, 56.4, 55.9, 55.9, 38.9, 38.7, 29.4, 25.8, 22.7, 22.0, 18.4; HRMS (FD⁺) m/z calcd for C₃₂H₄₄NO₄Si (M + H)⁺ 534.3040, found 534.3077. Para isomers: ¹H NMR (400 MHz, CDCl₃, selected signals) δ 6.01 (s, 1H), 5,85 (s), 4.02 (t, J = 6.8 Hz, 1H), 3.80 (s, 1H), 3.64 (s, 2H), 3.57 (s, 1H), 1.40 (d, J = 6.5 Hz, 2H), 1.28 (d, J = 6.5 Hz, 2H), 0.98 (s, 1H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 149.2, 149.1, 145.5, 144.5, 144.5, 143.9, 143.8, 143.7, 133.1, 132.8, 129.1, 128.4, 128.2, 128.2, 128.1, 127.6, 127.4, 127.3, 127.2, 127.1, 126.8, 126.5, 123.1, 123.0, 122.5, 122.3, 120.3, 119.9, 114.2, 114.2, 111.9, 111.8, 111.6, 110.8, 110.4, 60.7, 59.5, 59.1, 58.8, 55.8, 55.7, 55.6, 55.5, 55.5, 55.4, 55.3, 41.7, 40.6, 40.2, 39.7, 25.8, 25.7, 25.6, 25.6, 24.4, 23.6, 22.3, 22.0, 18.4, 18.3.

(S)-1-(3-Hydroxy-4-methoxybenzyl)-7-methoxy-2-((S)-1-phenylethyl)-1,2,3,4-tetrahydroisoquinolin-8-ol hydrochloride (22)

A solution of 20 (0.587 g, 1.1 mmol) in methanol (20 mL) was stirred with HCl concd (2 mL) during 18 h at rt. The solvents were evaporated, and the residue was evaporated three times with methanol to remove water and TBSOH to give 22 (hydrochloride, 0.453 g, 1.08 mmol, 98%) as a dark glass: ¹H NMR (400 MHz, CD₃OD) δ 7.53–7.45 (m, 1H), 7.43–7.36 (m, 2H), 7.11–7.05 (m, 2H), 7.03 (d, J = 8.4 Hz, 1H), 6.84 (d, J = 8.4 Hz, 1H), 6.80 (d, J = 8.0 Hz, 1H), 6.45–6.37 (m, 2H), 4.81–4.75 (m, 1H), 4.35 (q, J = 6.8 Hz, 1H), 4.01–3.91 (m, 1H), 3.90 (s, 3H), 3.88 (s, 3H), 3.87–3.77 (m, 2H), 3.30–3.04 (m, 2H), 2.94 (dd, J = 15.6, 10.2 Hz, 1H), 1.72 (d, J = 6.8 Hz, 3H); ¹³C{¹H} NMR (101 MHz, CD₃OD) δ 148.7, 147.9, 147.5, 144.6, 137.2, 131.0, 130.7, 129.0, 128.9, 124.1, 121.6, 120.7, 118.2, 117.3, 113.1, 112.9, 64.0, 60.0, 56.8, 56.6, 43.0, 38.8, 22.3, 18.6; HRMS (FD⁺) m/z calcd for C₂₆H₃₀NO₄ (M + H)⁺ 420.2169, found 420.2160.

Debenzylation to Norcrassifoline (23)

Compound 22 (0.400 g, 0.877 mmol) was stirred with 10% Pd/C (0.15 g) in 12 mL of ethanol under H₂ at atmospheric pressure during 18 h. Filtration over Celite and evaporation gave norcrassifoline 23 (hydrochloride, 0.276 g, 0.88 mmol, 100%) as a brownish glass: [α] = no transmission; ¹H NMR (400 MHz, CD₃OD) δ 7.07–6.92 (m, 2H), 6.92–6.79 (m, 2H), 6.74 (d, J = 8.3 Hz, 1H), 3.89 (s, 3H), 3.88 (s, 3H), 3.54 (m, 2H), 3.3–3.2 (m, 2H), 3.03 (m, 2H), 2.91 (dd, J = 15.1, 10.5 Hz, 1H); ¹³C{¹H} NMR (101 MHz, CD₃OD) δ 148.7, 148.1, 147.4, 144.3, 129.9, 125.1, 121.8, 120.7, 120.2, 117.2, 113.3, 112.7, 58.4, 56.8, 56.6, 55.0, 50.0, 49.8, 49.6, 49.4, 49.1, 48.9, 48.7, 48.5, 39.0, 37.6, 25.7, 18.5; HRMS (FD⁺) m/z calcd for C₁₈H₂₂NO₄ (M + H)⁺ 316.1549, found 316.1555.

(S)-(+)-Crassifoline (3)⁴

A mixture of 22 (hydrochloride, 35.1 mg, 0.10 mmol), paraformaldehyde (35 mg, 0.80 mmol), sodium acetate (33 mg, 0.40 mmol), sodium cyanoborohydride (33.0 mg, 0.54 mmol), and zinc chloride (35.0 mg, 0.26 mmol) was stirred in methanol (4 mL) for 24 h at rt.^7b Silica gel was added, and the residue obtained after evaporation was applied to a silica column. Elution with ethyl acetate, ethyl acetate/MeOH/Et₂NH 95/3/2, and ethyl acetate/MeOH/Et₂NH 90/7/3 gave crassifoline (3) (23.7 mg, 0.072 mmol, 72%) as a glass: [α]_D²⁰ +17.6 (c = 0.5 in MeOH) [lit.^4,5 [α]_D²⁰ +20 (c = 0.5 in MeOH)]; ¹H NMR (400 MHz, CDCl₃) δ 6.96 (d, J = 1.8 Hz, 1H), 6.78 (d, J = 1.7 Hz, 1H), 6.75 (d, J = 8.5 Hz, 1H), 6.63 (d, J = 8.3 Hz, 1H), 5.79 (bs, 2H), 4.10 (dd, J = 9.4, 3.0 Hz, 1H), 3.90 (s, 3H), 3.88 (s, 3H), 3.30 (ddd, J = 12.9, 10.5, 5.0 Hz, 1H), 3.01 (dd, J = 14.3, 3.0 Hz, 1H), 2.95–2.70 (m, 3H), 2.51–2.41 (m, 1H), 2.37 (s, 3H); ¹³C{¹H} NMR (101 MHz, CDCl₃) δ 145.2, 144.9, 144.2, 142.5, 134.3, 127.2, 124.3, 120.5, 119.2, 115.6, 110.4, 109.0, 60.2, 56.1, 55.9, 44.9, 42.4, 38.8, 22.9; HRMS (FD⁺) m/z calcd for C₁₉H₂₄NO₄ (M + H)⁺ 330.1700, found 330.1689.

(S)-(−)-Caseamine (24)¹⁸

A solution of norcrassifoline (23) (free base, 45 mg, 0.125 mmol) and 37% aqueous formaldehyde (30 μL, 0.4 mmol) in trifluoroethanol (1.0 mL) was stirred during 5 h at rt. Caseamine 24 (21.8 mg, 0.066 mmol, 53%) directly crystallized from the reaction mixture. Chromatography (ethyl acetate and ethyl acetate/MeOH 97/3 gave additional caseamine (4.5 mg, total yield 0.080 mmol, 64%) and clarkeanidine 25 (4.4 mg, 0.013 mmol, 10%, spectra see next experiment). Caseamine 24(18) ee 99% (Chiralcel AD column, eluent n-heptane/2-propanol 70:30, flow 1.000 mL/min): [α]_D²⁰ −314 (CHCl₃ + MeOH, c = 0.15) [lit.¹⁸ [α]_D–328 (c = 0.04, CHCl₃)]; mp 246–250 °C (lit.¹⁸ mp 246–247 °C); ¹H NMR (300 MHz, d₆-DMSO, partial overlap by solvent peaks) δ 8.64 (s, 1H), 8.54 (s, 1H), 6.79 (d, J = 8.2 Hz, 1H), 6.62 (s, 1H), 6.55 (d, J = 8.2 Hz, 1H), 6.46 (s, 1H), 3.80 (s, 2H), 3.77 (s, 3H), 3.73 (s, 3H), 3.46–3.36 (m, 1H), 2.96 (dt, J = 10.4, 4.8 Hz, 1H), 2.83 (dt, J = 13.2, 5.6 Hz, 1H), 2.67 (dt, J = 15.8, 4.7 Hz, 1H), 2.40 (dd, J = 16.1, 11.3 Hz, 1H); ¹³C{¹H} NMR (75 MHz, d₆-DMSO) δ 145.8, 145.2, 144.6, 142.8, 127.8, 127.0, 125.6, 124.7, 118.7, 115.2, 110.0, 109.8, 57.0, 56.0, 55.7, 55.6, 47.9, 31.4, 29.3; HRMS (FD⁺) m/z calcd for C₁₉H₂₁NO₄ (M⁺) 327.1471, found 327.1499.

(S)-(−)-Clarkeanidine (25)¹⁸

A solution of norcrassifoline (23) (free base, 63 mg, 0.20 mmol) in anhydrous toluene (4 mL) was stirred with paraformaldehyde (9.0 mg, 0.30 mmol) at 105 °C for 3 h. The solvent was evaporated, and the isomers were separated by chromatography: petroleum ether/ethyl acetate 1/1 and ethyl acetate for the ortho isomer clarkeanidine 25 (36.1 mg, 0.11 mmol, 55%) and then ethyl acetate/MeOH 97/3 for the para isomer caseamine 24 (6.0 mg, 0.018 mmol, 9%). Clarkeanidine (25): mp 177–180 °C (recrystallized from DCM/petroleum ether), (lit.¹⁹ mp 178–179 °C); ee 95% (Chiralcel AD column, eluent n-heptane/2-propanol 70:30, flow 1.000 mL/min); [α]_D²⁰ −442 (c = 0.1, CHCl₃) [lit.^17,18 [α]_D²⁰ −277 (CHCl₃)]; ¹H NMR (300 MHz, CDCl₃) δ 6.75 (m, 2H), 6.66 (m, 2H), 5.78 (bs, 2H), 4.24 (d, J = 16.0 Hz, 1H), 3.99 (dd, J = 11.2, 3.5 Hz, 1H), 3.90 (s, 3H), 3.89 (s, 3H), 3.84 (d, J = 16.0 Hz, 1H), 3.72 (dd, J = 16.2, 3.6 Hz, 1H), 3.22–3.12 (m, 1H), 3.12–3.00 (m, 1H), 2.89–2.64 (m, 3H); ¹³C{¹H} NMR(75 MHz, CDCl₃) δ 144.3, 143.8, 142.5, 141.7, 129.0, 128.8, 124.6, 121.0, 119.4, 108.9, 56.2, 56.2, 56.1, 53.0, 49.1, 32.2, 29.8; HRMS (FD⁺) m/z calcd for C₁₉H₂₂NO₄ (M + H)⁺ 328.1549, found 328.1558.

Supporting Information Available

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.joc.8b02378.

• ¹H and ¹³C NMR spectra of all new products and intermediates; chiral HPLC traces of the tetrahydroprotoberberines (PDF)

The authors declare no competing financial interest.