Kristin Hammer

RUTHENIUM(II) IN RING CLOSING METATHESIS FOR THE STEREOSELECTIVE PREPARATION OF CYCLIC 1-AMINO-1-CARBOXYLIC ACIDS

Abstract Stereoselective synthesis of α-amino acids where the α-carbon of the amino acid is incorporated into a five-, six- or seven-membered ring is described. The stereoselective control results from stepwise bisalkenylation of (R)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine. Ring closing metathesis was effected by ruthenium(II)-catalysis. The spiro-cycloalkene intermediates were further transformed into 1-aminocycloalkene-1-carboxylic acid derivatives by mild acid hydrolysis.

Synthesis of conformationally restricted serine derivatives through ruthenium(II)-catalyzed ring closing metathesis

Abstract Stereoselective synthesis of α-amino-β-hydroxy acids where the α-carbon of the amino acid is incorporated into a five-, six- or seven-membered ring is described. The stereoselective control at the chiral auxiliary carbon results from stepwise alkenylation and aldol formation using metalated species of the chiral auxiliary ( R )-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine. Ring closing metathesis was effected by ruthenium(II)-catalysis. The spiro-cycloalkene intermediates were transformed into 1-amino-2-hydroxycycloalkene-1-carboxylic acid derivatives by mild acid hydrolysis.

RUTHENIUM-CATALYZED ENYNE METATHESIS IN STEREOSELECTIVE PREPARATION OF CYCLIC 1-AMINO-1-CARBOXYLIC ACIDS

Abstract Stereoselective syntheses of α-amino acids are described. The α-carbon of the amino acid is incorporated into a five- or six-membered ring which is a conjugated diene with one exocyclic double bond. Enynes were intermediates for the metathesis cyclization reaction effected by ruthenium(II)-catalysis. The enyne substrates were available in stereochemically pure form by stepwise alkylations of the chiral auxiliary ( R )-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine with bromo-alkenes and -alkynes.

Ru(II)-catalyzed RCM reactions with electrophilic diene substrates

Abstract Electrophilic dienes from α,β-unsaturated ketones readily undergo the Ru(II)-benzylidene initiated RCM reaction. Cyclohexenone spirane formation was faster than cycloheptenone spirane formation. The products were pure stereoisomers. The products after hydrolysis were α,β-unsaturated oxo derivatives of cyclic α-amino acids.

Hydroxymethylated cyclic α-amino acid dipeptides by ruthenium ring closing metathesis

Abstract Stereoselective syntheses of cyclic α-amino-β-hydroxymethylcyclohexene-α-carboxylic acids are described. The acids were isolated as dipeptides. RCM reactions were effected by Ru(II)-catalysis on hydroxymethylated dienes. The diene substrates were available in stereochemically pure form by stepwise alkylations of ( R )-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine with 4-bromo-1-butene and vinyloxirane.

RU(II)-CATALYZED RING CLOSING METATHESIS IN STEREOSELECTIVE SYNTHESES OF CONSTRAINED HOMOSERINE ANALOGUES

Abstract Stereoselective syntheses of cyclic α-amino-°-hydroxycyclohexene- and cycloheptene-α-carboxylic acids are described. RCM cyclization reactions were effected by Ru(II)-catalysis on sterically homogenous hydroxylated dienes. The diene substrates were available by stepwise, stereoselective alkylations of (R)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine with bromoalkenes, ethylene oxide and subsequent transformations.

SYNTHESIS OF CYCLIC CONSTRAINED β-OXO-SUBSTITUTED α-AMINO ACIDS

ABSTRACT Ru(II)-catalyzed RCM reaction with a 5,5-diene of (2R)-2,5-dihydro-3,6-dimethoxy-2-isopropylpyrazine provided a spiroannulated cyclohexenone as substrate for conjugate additions of organocuprates. Acid hydrolysis gave β-oxo-substituted α-amino acids. Assignments of configurations are based on single crystal X-ray analyses.

Stereoselectivity in the synthesis of conformationally constrained vicinally dihydroxylated cyclic α-amino acids

Stereoselective syntheses of precursors to vicinal cis-dihydroxy-1-aminocyclopentane- and -cyclohexane-carboxylic acid methyl esters and their methoxy analogues are described. The chiral products are isolated as pure enantiomers. The absolute configurations at the new stereogenic centres are established by X-ray analysis.