Iodomethylation of Chiral α-Amino Aldehydes by Means of Samarium/Diiodomethane. Application to the Synthesis of Various Enantiomerically Pure Compounds

A case study of the MAC (masked acyl cyanide) oxyhomologation of N,N-dibenzyl-L-phenylalaninal with anti diastereoselectivity: preparation of (2S,3S)-allophenylnorstatin esters

The three-component reaction between a protected α-amino aldehyde, an alcohol and an α-silyloxymalononitrile provides an expedient access to protected α-hydroxy-β-amino acid derivatives. The prototypical process, performed on N-Cbz-phenylalaninal, is known to proceed with syn diastereoselectivity. The present study demonstrates that the diastereoselectivity of the reaction can be inverted, using the rationale of a Felkin-Anh interaction model. Reactions performed on N,N-dibenzyl-L-phenylalaninal proceed with a high anti diastereoselectivity, providing a panel of synthetically useful ester derivatives of (2S,3S)-allophenylnorstatin. The procedure is exploited to accomplish one of the most efficient syntheses of the title compound to date, in 3 steps (66% yield) from N,N-dibenzyl-L-phenylalaninal.

Direct stereoselective synthesis of enantiomerically pure anti-β-amino alcohols

Enantiomerically pure anti-β-amino alcohols were synthesized from optically pure α-(N,N-dibenzylamino)benzyl esters, derived from α-amino acids, by the sequential reduction to aldehyde with DIBAL-H at -78 °C and subsequent in situ addition of Grignard reagents. Besides anti-β-amino alcohols, anti-2-amino-1,3-diols and anti-3-amino-1,4-diols were obtained in good yields (60-95%) and excellent stereoselectivity (de > 95%). Our technique is compatible with free hydroxyl groups present in the substrate. To demonstrate the versatility of the method, spisulosine and sphinganine were synthesized in two steps from the appropriate N,N-dibenzyl-l-aminobenzyl ester in 42% and 45% yield, respectively.

Asymmetric synthesis of 2-(2-pyridyl)aziridines from 2-pyridineimines bearing stereogenic N-alkyl substituents and regioselective opening of the aziridine ring

The addition of chloromethyllithium to the imine derived from 2-pyridinecarboxaldehyde and (S)-valinol, protected as its O-trimethylsilyl ether, gave the 1,2-disubstituted aziridine with good yield and diastereoselectivity. The analogous reaction performed on the imine derived from (S)-valine methyl ester gave the product containing the aziridine ring and the alpha-chloro ketone group coming from the attack of chloromethyllithium to the ester function. Other stereogenic alkyl substituents at nitrogen gave less satisfactory results. Moreover, the aziridination protocol did not work on other aromatic imines which were not capable of bidentate chelation, e.g., 3- and 4-pyridineimine and benzaldimine. Preliminary studies showed the possibility to carry out regio- and stereospecific opening reactions of 2-(2-pyridyl)aziridines by attack of internally generated or external nucleophiles.

One-step highly diastereoselective synthesis of gamma-aminoalkyl-substituted gamma-butyrolactones by an asymmetric samarium-mediated ketyl-alkene coupling reaction

The samarium(II) iodide mediated reaction of N,N-dibenzyl-protected (S)-alpha-amino aldehydes with (1S,2R)-N-methylephedrinyl acrylate gave the (4R,1'S)-gamma-(aminoalkyl)-gamma-butyrolactones in good yields with high diastereoselectivities (up to 80% de); (4R,1'S)-gamma-amino-(2-phenylethyl)-gamma-butyrolactone (6a), which should be a potent precursor for gamma-secretase inhibitors, was obtained with high de value.