Mechanisms of stereomutation and thermolysis of spiro-1,2-oxaphosphetanes: new insights into the second step of the Wittig reaction.

The experimentally observed stereomutation of spiro-1,2-oxaphosphetanes is shown by DFT calculations to proceed through successive M(B2) or M(B4) and M(B3) mechanisms involving two, four, and three Berry pseudorotations at phosphorus, respectively. Oxaphosphetane decomposition takes place in a single step via a polar transition state. The calculated activation parameters for this reaction are in good agreement with those determined experimentally.

The role of fluorine in the stereoselective tandem aza-Michael addition to acrylamide acceptors: an experimental and theoretical mechanistic study.

Aza-Michael additions of alpha-amino esters to fluorinated acceptors take place in a highly stereoselective manner, to give partially modified Psi[NHCH2]retropeptides incorporating a hydrolytically stable trifluoroalanine mimic. The reaction mechanism has been investigated experimentally and theoretically, in order to explain the effect of the trifluoromethyl group on the reactivity and the origins of the experimentally observed stereocontrol. The reaction is a two-step process, involving a tandem aza-Michael addition followed by a stereoselective hydrogen transfer.

Synthesis of enantiopure pyrrolidine-derived peptidomimetics and oligo-beta-peptides via nucleophilic ring-opening of beta-lactams.

The synthesis of the two enantiomers of pyrrolidine-derived spiro beta-lactams by resolution with D- and L-Boc phenylalanine is described. The potential of these optically active spiro beta-lactams on the synthesis of peptidomimetics as analogues of melanostatin is evaluated. Theoretical studies of several models, at the Becke3LYP/6-31+G* level of theory, together with previous experimental evidences from our group, gathered by NMR, allow us to design structures that can efficiently mimic some biologically active peptide-type molecules.

Mechanism of anionic dearomatizing reactions of diphenylphosphinamide derivatives: a theoretical and experimental study.

The mechanism of the anionic dearomatisation of phosphinamide derivatives has been investigated both theoretically and experimentally. The potential-energy surface of model reactions was studied at the Becke3LYP/6-31+G* level of theory, and according to this study, a pre-reactive complex is formed between the alkyllithium and the phosphinamide. This complex evolves preferentially through NC(alpha)-metalation of the phosphinamide.

Ionic vs pericyclic transition states for the cyclization reactions of C alpha-N anions derived from amides and phosphinamides.

[structure: see text] Ab initio and DFT calculations indicate that the anionic cyclization of vinyl- and phenyl-carboxamides and phosphinamides metalated at the C(alpha)-N carbon fits better to a Michael-type ionic addition than to an electrocyclic ring closure. The lithium atom has no influence on the type of reaction mechanism.