A Simple Stereoselective Synthesis of a Novel Cytotoxic Alkaloid Barrenazine A [¹]

Abstract

The cytotoxic marine alkaloid barrenazine A was synthesized stereoselectively in eight simple steps from octanal. A key step involved the preparation of a functionalized 4- aminopiperidin-5-ol that underwent oxidative dimerization on treatment with 2-iodoxybenzoic acid.

The novel alkaloids barrenazine A (1) and barrenazine B (2) (Figure  [¹] ) were isolated in small quantities from an unidentified tunicate collected in the Barren Islands (Madagascar). [²] These compounds possess a C ₂-symmetric structure in which a central pyrazine moiety is fused to two piperidine rings. Each barrenazine contains two stereogenic centers, and the two compounds have different side chains. Barrenazine A (1) exhibits cytotoxic activity against LOVO-DOX colon carcinoma. Because of the interesting structures, lack of availability, and bioactivity of barrenazines, the synthesis of these compounds has attracted the interest of organic chemists. [³] In a continuation of our work on the construction of bioactive natural products, [4] we report on the total synthesis of barrenazine A (1) by a series of simple steps starting from octanal.

A retrosynthetic analysis of barrenazine A (Scheme  [¹] ) indicated that the compound might be prepared from functionalized 4-aminopiperidin-5-ol (3) by oxidative dimerization. Compound 3 could be obtained from the N-allyl amine 4, generated from octanal (5).

Our synthesis of barrenazine A (1; Scheme  [²] ) began from octanal (5), which was subjected to enantioselective Maruoka allylation [5] with the titanium complex (R,R)-I (Figure  [²] ) and allyl(tributyl)stannane to give the homoallylic alcohol 6 (97% ee). This was treated with mesyl chloride and triethylamine, and then with allylamine, to give the amino compound 7 (94% ee). The amine group in 7 was protected by treatment with di-tert-butyl dicarbonate in the presence of a catalytic amount of 4-(N,N-di­methylamino)pyridine to give the protected dialkenyl­-amine 4. Ring-closing matathesis [6] of amine 4 in the presence of the second-generation Grubbs catalyst gave the tetrahydropyridine 8. This was subjected to epoxidation with 3-chloroperbenzoic acid at 0 ˚C to room temperature to give, after workup, the major compound 9. [7] The configuration of product 9 was confirmed by nuclear Overhauser effect spectroscopic studies. Diaxial opening [8] of the epoxide ring of 9 with ammonium hydroxide in the presence of lithium perchlorate gave the single product 3. Oxidative dimerization of 3 by treatment with 2-iodoxybenzoic acid (IBX; 1-hydroxy-1λ [5] ,2-benziodoxol-1,3-dione) [9] at room temperature gave the pyrazine compound 10. Finally, deprotection of 10 with trifluoroacetic acid at 0 ˚C to room temperature gave barrenazine A (1).

The structures of all the products were established by spectroscopic and spectrometric methods (IR, ^¹H and ^¹³C NMR, and MS) and elemental analysis. The properties of the synthetic compound 1 were in good agreement with those reported earlier. [³a]

In conclusion, we have developed an efficient synthesis of the novel cytotoxic marine alkaloid, barrenazine A from octanal by a series of simple steps using readily available reagents. High yields were obtained in each step of the conversion. The synthesis was completed in eight steps with an overall yield of 22%. The method can be adapted to the preparation of various analogues of barrenazine A.

Silica gel F₂₅₄ plates were used for TLC; the spots were examined under UV radiation and then developed with I₂ vapor. Column chromatography was performed on silica gel (BDH 100-200 mesh). Solvents were purified according to standard procedures. The spectra were recorded with the following instruments: IR, Perkin-Elmer RX FT-IR spectrophotometer; NMR, Varian Gemini 200 MHz (^¹H) and 50 MHz (^¹³C) spectrometers; ESI-MS, VG-Autospec Micromass. Organic extracts were dried over anhyd Na₂SO₄.

(4 S )-Undec-1-en-4-ol (6)

Ti(Oi-Pr)₄ (0.429 g, 1.5 mmol) was added to a stirred soln of TiCl₄ (0.095 g, 0.5 mmol) in CH₂Cl₂ (8 mL) at 0 ˚C under N₂, and the mixture was allowed to warm to r.t. After 1 h, AgO (0.124 g, 1 mmol) was added and the reaction was continued for 5 h with exclusion of direct light. The mixture was diluted with CH₂Cl₂ (30 mL) then treated with (R)-(+)-1,1′-bi-2-naphthol [(R)-BINOL; 0.546 g, 2 mmol] at r.t. for 2 h to give the chiral oxide (R,R)-I. This complex was cooled to -15 ˚C and treated sequentially with octanal (5; (1.28 g, 10 mmol) and allyl(tributyl)stannane (4.303 g, 13 mmol) at -15 ˚C. The mixture was then allowed to warm to 0 ˚C and stirred for 20 h. The reaction was quenched with sat. aq NaHCO₃ (15 mL), and the mixture was extracted with Et₂O (3 × 40 mL). The organic extracts were dried (Na₂SO₄) and concentrated to give a residue that was purified by column chromatography [silica gel, EtOAc-hexane (1:9)] to give a colorless liquid; yield: 1.46 g (86%).

FTIR: 3413, 1636, 1461, 1377 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 5.78 (m, 1 H), 5.14-5.02 (m, 2 H), 3.59 (m, 1 H), 2.22 (m, 1 H), 2.10 (m, 1 H), 1.81 (br s, 1 H), 1.48-1.20 (m, 12 H), 0.88 (t, J = 7.0 Hz, 3 H).

^¹³C NMR (50 MHz, CDCl₃): δ = 135.0, 118.2, 70.9, 42.2, 37.0, 31.9, 30.1, 29.5, 25.6, 23.0, 14.2.

ESI-MS: m/z 171 [M + H]⁺.

Anal. Calcd for C₁₁H₂₂O: C, 77.65; H, 12.94. Found: C, 77.76; H, 12.88.

(4 R )- N -Allylundec-1-en-4-amine (7)

Anhyd Et₃N (1.071 g, 10.6 mmol) was added to a stirred soln of enol 6 (1.4 g, 8.2 mmol) in CH₂Cl₂ (8 mL) at r.t. under N₂. A catalytic amount of DMAP was added to the soln and the mixture was stirred for 15 min then allowed to warm to 0 ˚C. MsCl (1.41 g, 9.84 mmol) was added and the mixture was stirred for 1 h. The reaction was then quenched with sat. aq NaHCO₃ (10 mL) and extracted with CH₂Cl₂ (3 × 20 mL). The extracts were dried (Na₂SO₄) and concentrated in vacuo to give a crude mesylate that was used without further purification; yield: 1.878 g (92%). The crude mesylate (1.878 g, 7.6 mmol) was dissolved in anhyd DMF (40 mL), and allylamine (8.664 g, 152 mmol) was added at r.t. The mixture was then heated to 50 ˚C and refluxed for 36 h. The mixture was allowed to cool to r.t. then extracted with Et₂O (3 × 40 mL). The combined organic extracts were washed with brine (2 × 20 mL), dried (Na₂SO₄), and concentrated in vacuo. The residue was purified by column chromatography [silica gel, EtOAc-hexane (4:6)] to give amine 7 as yellow liquid; yield: 1.345 g (85%).

FTIR: 3445, 1643, 1458, 1242 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 5.89 (m, 1 H), 5.74 (m, 1 H), 5.28-5.07 (m, 4 H), 3.33 (d, J = 8.0 Hz, 2 H), 2.73 (m, 1 H), 2.32-2.18 (m, 3 H), 1.48 (m, 1 H), 1.38-1.20 (m, 11 H), 0.87 (t, J = 7.0 Hz, 3 H).

^¹³C NMR (50 MHz, CDCl₃): δ =133.5, 132.0, 120.6, 118.9, 56.0, 48.1, 36.2, 31.9, 30.0, 29.9, 29.1, 25.2, 22.9, 14.1.

ESI-MS: m/z 210 [M + H]⁺.

Anal. Calcd for C₁₄H₂₄N: C, 80.38; H, 12.92; N, 6.70. Found: C, 80.45; H, 12.89; N, 6.74.

tert -Butyl Allyl[(1 R )-1-heptylbut-3-en-1-yl]carbamate (4)

Et₃N (0.814 g, 8.1 mmol) was added to a stirred soln of amine 7 (1.3 g, 6.2 mmol) in anhyd CH₂Cl₂ under N₂ at r.t, and the mixture was stirred for 15 min. (Boc)₂O (1.486 g, 6.8 mmol) was then added, and the mixture was stirred for a further 3 h. The reaction was then quenched with sat. aq NH₄Cl (6 mL) and the mixture was extracted with EtOAc (2 × 30 mL). The combined organic extracts were dried (Na₂SO₄) and concentrated in vacuo to give a residue that was purified by column chromatography [silica gel, EtOAc-hexane (1:19)] to give a colorless liquid; yield: 1.787 g (93%).

FTIR: 1691, 1459, 1370, 1252, 1212 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 5.88-5.61 (m, 2 H), 5.17-4.93 (m, 4 H), 4.09 (m, 1 H), 3.70 (m, 1 H), 3.59 (m, 1 H), 2.32-2.10 (m, 2 H), 1.43 (s, 9 H), 1.35-1.19 (m, 12 H), 0.88 (t, 3 H, J = 7.0 Hz).

^¹³C NMR (50 MHz, CDCl₃): δ = 155.6, 136.9, 135.8, 116.9, 115.5, 79.0, 55.2, 45.9, 38.2, 33.1, 32.0, 29.1, 29.0, 28.7, 27.8, 26.5, 22.9, 14.4.

ESI-MS: m/z 310 [M + H]⁺.

Anal. Calcd for C₁₉H₃₅NO₂: C, 73.79; H, 11.33; N, 4.53. Found: C, 73.85; H, 11.42; N, 4.57.

tert -Butyl (2 R )-2-Heptyl-3,6-dihydropyridine-1(2 H )-carboxylate (8)

N₂ was bubbled through a soln of carbamate 4 (1.74 g, 5.6 mmol) in anhyd CH₂Cl₂ (150 mL) and then the Grubbs second-generation catalyst (10 mol%) was added. The mixture was heated under N₂ at 50 ˚C for 6 h then cooled and concentrated in vacuo. The residue was purified by column chromatography [silica gel, EtOAc-hexane (1:19)] to give a colorless liquid; yield: 1.439 g (91%).

FTIR: 1697, 1602, 1459, 1369, 1253 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 5.72-5.51 (m, 2 H), 4.40-4.04 (m, 2 H), 3.40 (m, 1 H), 2.41 (m, 1 H), 1.88 (m, 1 H), 1.43 (s, 9 H), 1.35-1.19 (m, 12 H), 0.88 (t, J = 7.0 Hz, 3 H).

^¹³C NMR (50 MHz, CDCl₃): δ = 155.0, 123.8, 123.1, 79.2, 49.0, 40.2, 32.2, 32.0, 29.8, 29.6, 28.9, 27.7, 26.2, 22.9, 14.2.

ESI-MS: m/z 281 [M]⁺.

Anal. Calcd for C₁₇H₃₁NO₂: C, 72.60; H, 11.03; N, 4.98. Found: C, 72.73; H, 11.12; N, 4.92.

tert -Butyl (1 S ,4 R ,6 R )-4-Heptyl-7-oxa-3-azabicyclo[4.1.0]heptane-3-carboxylate (9)

A soln of MCPBA (1.035 g, 6 mmol) in anhyd CH₂Cl₂ (5 mL) was added to a stirred soln of ester 8 (1.4 g, 5.0 mmol) in anhyd CH₂Cl₂ (5 mL) under N₂ at 0 ˚C, and the mixture was stirred for 2 h. When the reaction was complete, it was quenched with sat. aq NaHCO₃ (6 mL) and the mixture was stirred for 1 h then extracted with CH₂Cl₂ (2 × 30 mL). The combined organic layers were dried (Na₂SO₄) and concentrated in vacuo. The residue was purified by column chromatography [silica gel, EtOAc-hexane (3:7)] to give a colorless liquid; yield: 1.005 g (85%; 80% de).

FTIR: 1694, 1413, 1367, 1249, 1173 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 4.32 (m, 1 H), 4.10 (m, 1 H), 3.22 (m, 1 H), 3.18-3.04 (m, 2 H), 2.14-2.10 (m, 2 H), 1.72-1.68 (m, 2 H), 1.42 (s, 9 H), 1.37-1.18 (m, 10 H), 0.86 (t, J = 7.0 Hz, 3 H).

^¹³C NMR (50 MHz, CDCl₃): δ = 155.7, 79.8, 49.9, 49.1, 46.5, 37.6, 32.4, 32.0, 29.8, 29.3, 28.4, 28.1, 26.3, 22.8, 14.2.

ESI-MS: m/z 320 [M + Na]⁺.

Anal. Calcd for C₁₇H₃₁NO₃: C, 68.69; H, 10.44; N, 4.71. Found: C, 68.77; H, 10.37; N, 4.65.

tert -Butyl (2 R ,4 S ,5 S )-4-Amino-2-heptyl-5-hydroxypiperidine-1-carboxylate (3)

LiClO₄ (0.678 g, 6.4 mmol) and a 30% soln of NH₃ in H₂O (15 mL) were added to a stirred soln of ester 9 (0.96 g, 3.2 mmol) in anhyd THF (10 mL) under N₂ at r.t. The mixture was heated at 120 ˚C for 20 h then cooled. The solvent was evaporated under reduced pressure and the mixture was extracted with CH₂Cl₂ (3 × 20 mL). The organic layers were washed with brine (2 × 20 mL), dried (Na₂SO₄), and concentrated under in vacuo. The residue was purified by column chromatography [silica gel, EtOAc-hexane (6:4)] to give a pale-yellow liquid; yield: 0.79 g (78%).

FTIR: 3427, 1668, 1422, 1369, 1250 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 4.12 (m, 1 H), 3.95 (m, 1 H), 3.79-3.65 (m, 2 H), 3.20 (m, 1 H), 2.18 (m, 1 H), 1.81 (br s, 1 H), 1.46 (s, 9 H), 1.33-1.22 (m, 14 H), 0.83 (t, J = 7.0 Hz, 3 H).

^¹³C NMR (50 MHz, CDCl₃): δ = 156.2, 80.1, 68.0, 59.5, 50.1, 41.0, 32.1, 31.9, 30.1, 29.3, 29.0, 28.2, 26.5, 22.2, 14.1.

ESI-MS: m/z 314 [M + H]⁺.

Anal. Calcd for C₁₇H₃₄N₂O₃: C, 64.96; H, 10.82; N, 8.91. Found: C, 64.86; H, 10.94; N, 8.79.

Di- tert -butyl (3 R ,8 R )-3,8-Diheptyl-1,3,4,6,8,9-hexahydrodipyrido[3,4- b :3′,4′- e ]pyrazine-2,7-dicarboxylate (10)

IBX (1.286 g, 4.8 mmol) was dissolved in anhyd DMSO (6 mL) under N₂ at r.t., and the soln was stirred for 0.5 h. To this stirred soln was added a soln of ester 3 (0.75 g, 2.4 mmol) in anhyd DMSO (3 mL), and the mixture was heated to 60 ˚C for 12 h. The reaction was quenched with sat. aq Na₂S₂O₃ (5 mL). The suspension was then stirred for 0.5 h and the precipitate was filtered off on a Celite pad. The filtrate was extracted with EtOAc (3 × 20 mL) and combined organic layers were washed with sat. aq NaHCO₃ (3 × 20 mL). The organic layers were dried (Na₂SO₄) and concentrated in vacuo, and the residue was purified by column chromatography [silica gel, EtOAc-hexane (4:6)] to give a yellow liquid; yield: 0.5 g (72%).

FTIR: 1692, 1460, 1371, 1243, 1163 cm^-¹.

^¹H NMR (200 MHz, CDCl₃): δ = 4.70-4.59 (m, 2 H), 4.33-4.26 (m, 2 H), 4.19-4.10 (m, 2 H), 3.41-3.29 (m, 4 H), 1.68-1.41 (m, 8 H), 1.37-1.18 (m, 34 H), 0.83 (t, J = 7.0 Hz, 6 H).

^¹³C NMR (50 MHz, CDCl₃): δ = 155.7, 147.2, 146.9, 79.6, 51.2, 42.8, 32.3, 31.8, 30.0, 29.8, 29.3, 27.8, 26.0, 22.9, 15.1.

ESI-MS: m/z 609 [M + Na]⁺.

Anal. Calcd for C₃₄H₅₈N₄O₄: C 69.63; H, 9.90; N, 9.56. Found: C, 69.78; H, 9.82; N, 9.52.

Barrenazine A (1)

TFA (0.059 g, 0.52 mmol) was added to a stirred soln of the protected compound 10 (0.156 g, 0.26 mmol) in anhyd CH₂Cl₂ (3 mL) under N₂ at r.t., and the mixture was stirred for 2 h. The reaction was quenched with sat. aq NaHCO₃, and the mixture was stirred for 0.5 h and then extracted with CH₂Cl₂ (3 × 10 mL). The organic layers were dried (Na₂SO₄) and concentrated under reduced pressure, and the residue was purified by column chromatography [silica gel, MeOH-CHCl₃ (2:8)] to give a white precipitate; yield: 0.083 g (82%); mp 90-92 ˚C; [α]_D ^²5 -56.2 (c 0.6, MeOH). The spectral data (IR, ^¹H and ^¹³C NMR, and MS) for the compound were identical to those reported earlier. [³a]

Anal. Calcd for C₂₄H₄₂N₄: C 74.56; H, 10.95; N, 14.49. Found: C 74.43; H, 10.85; N, 14.35.

Acknowledgment

The author thanks CSIR and UGC New Delhi for financial assis­tance.

References

• 2 Chill L. Aknin M. Kashman Y. Org. Lett.  2003,  5:  2433 
  CrossrefSearch in Google Scholar
• 3a Focken T. Charette AB. Org. Lett.  2006,  8:  2985 
  Search in Google Scholar
• 3b Montserrat Martinez M. Sarandeses LA. Pérez Sestelo J. Tetrahedron Lett.  2007,  48:  8536 
  Search in Google Scholar
• 3c Buron F. Turck A. Plé N. Bischoff L. Marsais F. Tetrahedron Lett.  2007,  48:  4327 
  Search in Google Scholar
• 3d Peña-López M. Montserrat Martínez M. Sarandeses LA. Pérez Sestelo J. Org. Lett.  2010,  12:  852 
  Search in Google Scholar
• 4a Das B. Krishnaiah M. Sudhakar Ch. Bioorg. Med. Lett.  2010,  20:  2303 
  Search in Google Scholar
• 4b Das B. Veeranjaneyulu B. Balasubramanyam P. Srilatha M. Tetrahedron: Asymmetry  2010,  21:  2762 
  Search in Google Scholar
• 4c Das B. Kumar DN. Tetrahedron Lett.  2010,  51:  6011 
  Search in Google Scholar
• 5 Hanava H. Hashimoto T. Maruoka K. J. Am. Chem. Soc.  2003,  125:  1708 
  CrossrefSearch in Google Scholar
• 6a Chattarjee AK. Morgan JP. Scholl N. Grubbs RH. J. Am. Chem. Soc.  2000,  122:  3783 
  Search in Google Scholar
• 6b Trnka TM. Grubbs RH. Acc. Chem. Res.  2001,  34:  18 
  Search in Google Scholar
• 7 Coombs TC. Lushington GH. Douglas J. Aube J. Angew. Chem. Int. Ed.  2011,  50:  2734 
  CrossrefSearch in Google Scholar
• 8 Fürst A. Plattner PA. Helv. Chim. Acta  1949,  32:  275 
  CrossrefPubMedSearch in Google Scholar
• 9a Nicolaou KC. Mathison CJN. Montagnon T. Angew. Chem. Int. Ed.  2003,  42:  4077 
  Search in Google Scholar
• 9b Fontaine P. Chiaroni A. Massou G. Zhu J. Org. Lett.  2008,  10:  1509 
  Search in Google Scholar

Part 47 in the series Synthetic Studies on Natural Products.

References

• 2 Chill L. Aknin M. Kashman Y. Org. Lett.  2003,  5:  2433 
  CrossrefSearch in Google Scholar
• 3a Focken T. Charette AB. Org. Lett.  2006,  8:  2985 
  Search in Google Scholar
• 3b Montserrat Martinez M. Sarandeses LA. Pérez Sestelo J. Tetrahedron Lett.  2007,  48:  8536 
  Search in Google Scholar
• 3c Buron F. Turck A. Plé N. Bischoff L. Marsais F. Tetrahedron Lett.  2007,  48:  4327 
  Search in Google Scholar
• 3d Peña-López M. Montserrat Martínez M. Sarandeses LA. Pérez Sestelo J. Org. Lett.  2010,  12:  852 
  Search in Google Scholar
• 4a Das B. Krishnaiah M. Sudhakar Ch. Bioorg. Med. Lett.  2010,  20:  2303 
  Search in Google Scholar
• 4b Das B. Veeranjaneyulu B. Balasubramanyam P. Srilatha M. Tetrahedron: Asymmetry  2010,  21:  2762 
  Search in Google Scholar
• 4c Das B. Kumar DN. Tetrahedron Lett.  2010,  51:  6011 
  Search in Google Scholar
• 5 Hanava H. Hashimoto T. Maruoka K. J. Am. Chem. Soc.  2003,  125:  1708 
  CrossrefSearch in Google Scholar
• 6a Chattarjee AK. Morgan JP. Scholl N. Grubbs RH. J. Am. Chem. Soc.  2000,  122:  3783 
  Search in Google Scholar
• 6b Trnka TM. Grubbs RH. Acc. Chem. Res.  2001,  34:  18 
  Search in Google Scholar
• 7 Coombs TC. Lushington GH. Douglas J. Aube J. Angew. Chem. Int. Ed.  2011,  50:  2734 
  CrossrefSearch in Google Scholar
• 8 Fürst A. Plattner PA. Helv. Chim. Acta  1949,  32:  275 
  CrossrefPubMedSearch in Google Scholar
• 9a Nicolaou KC. Mathison CJN. Montagnon T. Angew. Chem. Int. Ed.  2003,  42:  4077 
  Search in Google Scholar
• 9b Fontaine P. Chiaroni A. Massou G. Zhu J. Org. Lett.  2008,  10:  1509 
  Search in Google Scholar

Part 47 in the series Synthetic Studies on Natural Products.