Mechanism of formation of serine β-lactones by Mitsunobu cyclization: synthesis and use of L-serine stereospecifically labelled with deuterium at C-3

The ring closure of N-benzyloxycarbonyl-L-serine (1) under Mitsunobu conditions (Ph3P, dimethyl azodicarboxylate, −78 °C) to give the corresponding β-lactone (2) is shown by deuterium and oxygen-18 labelling studies to proceed by hydroxy group activation, in contrast to analogous cyclizations of more hindered β-hydroxy acids, which usually occur by carboxy group activation. Samples of 1 stereospecifically labelled with deuterium at C-3 were prepared by hydrogenation of (Z)-2-acetamido-3-methoxyacrylic acid (9) with deuterium, followed by selective Acylase I deacetylation of the 2S isomer, removal of the protecting groups, and N-acylation of the resulting L-serine with benzyl chloroformate. Mitsunobu cyclizations of this 3R deuterated N-acyl serine, of the [hydroxy-18O] analog 1g, and of the [carboxy-18O] derivative 1f show that lactonization occurs with inversion of configuration at C-3, loss of the hydroxy oxygen, and retention of the carboxy oxygens. Similar labelling experiments demonstrate that aqueous sodium hydroxide opens the β-lactone ring by exclusive attack at the carbonyl to regenerate 1, whereas acidic hydrolysis proceeds primarily by attack of water at the C-3 methylene group of 2. This information allows interconversion of L-serines that are stereospecifically labelled at C-3 with hydrogen isotopes and affords access to other labelled (β-substituted alanines.