A Novel Methodology for the Synthesis of Fumarates and Maleates

Abstract

The stereoselectivity of both the Wittig and the Horner-Wadsworth-Emmons reactions allows for the synthesis of ortho­gonally protected fumarates and maleates, respectively, from α-keto esters. This methodology has been shown to be useful in the synthesis of a derivative of the pH sensitive cis-aconitic linker, important for prodrug approaches in drug delivery. We have incorporated this linker into a cholesterol-based PEGylated lipid, for its potential use in liposomal drug and gene delivery. The synthesized lipids have also been shown to be pH degradable using HPLC analyses.

Key words

Wittig - Horner-Wadsworth-Emmons (HWE) - ozonolysis - cis-aconitic - PEGylated lipids

References and Notes

• 1a Kosterelos K. Miller AD. Chem. Soc. Rev.  2005,  970 
• 1b Fenske DB. Cullis PR. Methods Enzymol.  2005,  7 
• 2 Gabizon A. Papahadjopoulos D. Proc. Natl. Acad. Sci. U.S.A.  1998,  6949 
• 3 Woodle MC. Matthey KK. Newman MS. Hidayat JE. Collins LR. Redemann C. Martin FJ. Papahadjopolous D. Biochim. Biophys. Acta  1992,  1105:  193 
• 4a Holland JW. Hui C. Cullis PR. Madden TD. Biochemistry  1996,  35:  2618 
• 4b Keller M. Harbottle RP. Perouzel E. Colin M. Shah L. Rahim A. Vaysse L. Bergau A. Moritz S. Brahimi-Horn C. Coutelle C. Miller AD. ChemBioChem  2003,  4:  286 
• 5 Guo X. Szoka FC. Acc. Chem. Res.  2003,  36:  335 
• 6 Kirpotin D. Hong K. Mullah N. Papahadjopoulos D. Zalipsky S. FEBS Lett.  1996,  388:  115 
• 7 Pak CC. Ali S. Janoff AS. Meers P. Biochim. Biophys. Acta  1998,  1372:  13 
• 8 Kratz F. Beyer U. Schutte MT. Crit. Rev. Ther. Drug Carrier Sys.  1999,  245 
• 9 Zelphati O. Szoka FC. Proc. Natl. Acad. Sci. U.S.A.  1996,  93:  11493 
• 10 Askon A. Cho MJ. J. Pharm. Sci.  2002,  91:  903 
• 11a Guo X. Mackay JA. Szoka FC. Biophys. J.  2003,  84:  1784 
• 11b Guo X. Szoka FC. Bioconj. Chem.  2001,  12:  291 
• 12a Hamann PR. Hinman LM. Hollander I. Beyer CF. Lindh D. Holcomb R. Hallet W. Tsou HR. Upeslacis J. Shochat D. Mountain A. Flowers DA. Bernstein I. Bioconjugate Chem.  2002,  13:  47 
• 12b Ulbrich K. Etrych T. Chytil P. Pechar M. Jelinkova M. Rihova B. Int. J. Pharm.  2004,  277:  63 
• 12c Ulbrich K. Etrych T. Chytil P. Jelinkova M. Rihova B. J. Controlled Release  2003,  87:  33 
• 13a Shin J. Shum P. Thompson DH. J. Controlled Release  2003,  91:  187 
• 13b Boomer JA. Inerowicz HD. Zhang ZY. Berstrand N. Edwards K. Kim JM. Thompson DH. Langmuir  2003,  19:  6408 
• 14a Alshamkhani A. Duncan R. Int. J. Pharm.  1995,  122:  107 
• 14b Reddy JA. Low PS. J. Controlled Release  2000,  64:  2737 
• 14c Drummond DC. Daleke DL. Chem. Phys. Lipids  1995,  75:  27 
• 15 Bender ML. J. Am. Chem. Soc.  1957,  79:  1528 
• 16a Ulbrich K. Etrych T. Chytil P. Jelinkova M. Rihova B. J. Controlled Release  2003,  33 
• 16b Yoo HS. Lee EA. Park TG. J. Controlled Release  2002,  17 
• 16c Drummond DC. Daleke DL. Chem. Phys. Lipids  1995,  27 
• 17 Fletcher S. Jorgensen MR. Miller AD. Org Lett.  2004,  4245 
• 18 Shen WC. Ryser HJP. Biochem. Biophys. Res. Commun.  1981,  102:  1048 
• 19 Remenyi J. Balazs B. Toth S. Falus A. Toth G. Hudecz F. Biochem. Biophys. Res. Commun.  2003,  303:  556 
• 20 Wasserman HH. Ho W.-B. J. Org. Chem.  1994,  4364 
• 24 Diaz M. Branchadell V. Oliva A. Ortuno RM. Tetrahedron  1995,  51:  11841 
• 25 Gibson FS. Bergmeier SC. Rapoport H. J. Org. Chem.  1994,  59:  3216 

21

General Procedure for the Formation of the α-Keto Esters.
O₂ gas was bubbled through a solution of cyano keto phosphorane (2 mmol) in CH₂Cl₂ (25 mL) at r.t. for 10 min, cooled to -78 °C for 5 min. The solution then had O₃ bubbled through for 40 min at -78 °C until the color of the solution had changed to a murky green, via a bright yellow. Bubbling N₂ gas through the solution still at -78 °C for 10 min quenched the excess O₃, once the solution returned to bright yellow in color the alcohol (2.4 mmol) was added. The solution was then purged with N₂ and stirred at -78 °C for 2 h until the solution turned colorless. The crude product was purified using flash chromatography on silica gel to afford the α-keto ester as a clear oil.

22

Typical Procedure for the Formation of a Fumarate ( 6a). To a solution of the α-keto ester (0.25 mmol) in anhyd THF (6 mL) was added tert-butoxycarbonylmethylene triphenyl phosphorane (0.188 g, 0.5 mmol) dropwise (via syringe pump 2 mL/h) in anhyd THF (4 mL) at 0 °C under a nitrogen atmosphere. After 5 h the reaction was allowed to stir at r.t. for a further 10 h. The crude mixture was then concentrated in vacuo and purified using flash chromatography on silica gel to afford the desired fumarate.
IR (CH₂Cl₂ film): 3394, 2977, 2935, 1715, 1647, 1518, 1455, 1366, 1273, 1152, 974, 869 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 1.32-1.38 [2 H, m, CH₂ (CH₂)₂C=C], 1.40-1.52 (4 H, m, 2 ¥ CH₂), 1.43 (9 H, s, 3 ¥ CH₃), 1.48 (9 H, s, 3 ¥ CH₃), 2.72 (2 H, t, J = 7.6 Hz, CH₂C=C), 3.07-3.10 (2 H, m, CH₂NH), 3.77 (3 H, s, CH₃), 4.52 (1 H, br, NH), 6.65 (1 H, s, trans-alkene). ¹³C NMR (100 MHz, CDCl₃): δ = 26.73, 27.49 (2 ¥ CH₂), 28.08, 28.48 [2 ¥ C(CH₃)₃], 28.78 (CH₂), 29.72 (CH₂), 40.49 (CH₂NH), 52.35 (CH₃), 78.94, 81.34 [2 ¥ C(CH₃)₃], 128.80 (CH=C), 145.80 (CH=C), 155.93 (CO₂NH), 164.95, 167.65 (2 ¥ CO). MS (CI): m/z calcd for C₁₉H₃₄N₁O₆: 372.2386; found: 372.2382 [M⁺ + H]; m/z (%) 389 (15) [M⁺ + NH₄], 372 (100) [M⁺ + H], 333 (76) [M⁺ - (CH₃)₃ + NH₄], 272 (52) [M⁺ - (CH₃)₃CO₂].

23

Typical Procedure for the Formation of Maleates. To a stirred suspension of NaH [0.01 g, 0.25 mmol (60% in mineral oil, w/w)] in anhyd THF (1 mL) at 0 °C and under a nitrogen atmosphere, was added tert-butyl-P,P-dimethylphosphonoacetate (0.05 mL, 0.25 mmol) in anhyd THF (1 mL) dropwise. After 30 min, the α-keto phosphorane (0.25 mmol) in THF (2 mL) was added dropwise over 1 h. After 3 h the excess NaH was neutralized with H₂O (0.1 mL) and the crude mixture concentrated in vacuo, azetroped with MeOH. The crude mixture was purified using flash chromatography on silica gel (hexane-EtOAc, 7:1) to afford the desired maleate. IR (neat): 3400, 2977, 2934, 2863, 1715, 1519, 1367, 1248, 1159, 1066, 1003, 781 cm^-1. ¹H NMR (400 MHz, CDCl₃): δ = 1.32-1.38 [2 H, m, CH₂ (CH₂)C=C], 1.40-1.52 (4 H, m, 2 ¥ CH₂), 1.43 (9 H, s, 3 ¥ CH₃), 1.48 (9 H, s, 3 ¥ CH₃), 2.31 (2 H, t, J = 7.2 Hz, CH₂C=C), 3.07-3.10 (2 H, m, CH₂NH), 3.77 (3 H, s, CH₃), 4.52 (1 H, br, NH), 5.69 (1 H, t, J = 1.4 Hz, cis-alkene). ¹³C NMR (100 MHz, CDCl₃): δ = 25.98 (CH₂), 26.64 (CH₂), 27.91 [t-Bu C(CH₃)₃], 28.32 [Boc C(CH₃)₃], 29.68 (CH₂), 34.08 (CH₂C=C), 40.30 (CH₂NH), 52.00 (CH₃), 78.96 [Boc C(CH₃)₃], 81.33 [t-Bu C(CH₃)₃], 121.90 (CH=C), 147.83 (CH=C), 155.90 (CONH), 164.07 (CO), 169.25 (CO). MS (CI): m/z calcd for C₁₉H₃₇N₂O₆: 389.2651; found: 389.2645 [M⁺ + NH₄ ⁺]; m/z (%) = 389 (60) [M⁺ + NH₄ ⁺], 372 (80) [M⁺ + H], 333 (65), 316 (35), 277 (100) and 216 (33).