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.
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.