N-Acyl-α -triphenylphosphonio-α -amino Acids: Synthesis and Decarboxylation to α -(N-Acylamino)alkyltriphenylphosphonium Salts

Abstract

4-Phosphoranylidene-5(4H)-oxazolones 1 undergo hydrolysis in THF in the presence of HBF ₄ at room temperature to give N-acyl-α -triphenyphosphonioglycines 3 (R ² = H) in very good yields. 4-Alkyl-4-triphenylphosphonio-5(4H)-oxazolones 2 react with water in CH ₂ Cl ₂ /THF solution without any acidic catalyst at 0–5°C in a few days yielding N-acyl-α -triphenylphosphonio-α -amino acids 3 (R ² = Me) or α -(N-acylamino)alkyltriphenylphosphonium salts 4 (R ² = alkyl, other then Me). α -Triphenylphosphonio-α -amino acids 3 upon heating to 105–115°C under reduced pressure (5 mm Hg) or upon treatment with a Hünig base in CH ₂ Cl ₂ at 20°C undergo decarboxylation to give the corresponding α -(N-acylamino)-alkyltriphenylphosphonium salts 4 , usually in very good yields.

Keywords:

• 4-Alkyl-4-triphenylphosphonio-5(4H)-oxazolone hydrolysis
• 4-triphenylphosphoranylidene-5(4H)-oxazolone hydrolysis
• α-(N-acylamino)alkyltriphenylphosphonium salts
• decarboxylation
• N-acyl-α-triphenyphosphonio-α-amino acids

Dedicated to Professor Marian Mikołajczyk from the CBMiM PAN in Łódź, Poland, on the occasion of his 70th birthday.

The financial help of the Ministry of Science and Higher Education of Poland (Grant No. N N204 238334) is gratefully acknowledged.

Notes

^a For the formula C₂₆H₂₉BF₄NO₄P (crystals contains 1 mol of THF per 1 mol of compound 3c).

^b In CH₃CN.

^c In Nujol.

^a N-pivaloyl-β -triphenylphosphonioalanine iodide 6 was also identified in the reaction mixture (yield 59 %). ¹H NMR [CD₃CN, δ (ppm)]: 7.90–7.67 (m, 15H, Ph), 7.23 (d, br, J = 8.4 Hz, 1H, NH), 4.74–4.63 (m, 1H, Ph₃P⁺CH₂CH), 4.31 (ddd, J _PH = 10.8 Hz, J _HH = 16.0 Hz, J _HH = 10.8 Hz, 1H, Ph₃P⁺CH ₂—one of two diastereotopic protons), 3.95 (ddd, J _PH = 14.1 Hz, J _HH = 16.2 Hz, J _HH = 3.3 Hz, 1H, Ph₃P⁺CH ₂—one of two diastereotopic protons), 0.90 (s, 9H, t-Bu). ¹³C NMR [CD₃CN, δ (ppm)/J _PC (Hz)]: 180.2 (HNC = O), 171.1/15.6 (HOC = O), 136.1/3.0 (Ph₃P⁺, C⁴), 134.8/10.0 (Ph₃P⁺, C²), 131.2/13.1 (Ph₃P⁺, C³), 118.8/91.1 (Ph₃P⁺, C¹), 48.2/3.0 (Ph₃P⁺CH₂ CH), 39.1 (CMe₃), 27.3 (CMe ₃), 25.5/54.8 (Ph₃P⁺ CH₂).

^b α -(N-Pivaloylamino)vinyltriphenylphosphonium iodide 5 was also isolated from the reaction mixture in 18% yield (see the Experimental section).

^c One of two diastereotopic protons of the methylene group.

^a One of two diastereotopic protons of the methylene group.