Absolute configuration from solution reactions

Absolute Configurations from Solution Reactions The Tris(diimine)iron(II)/Cyanide Inversion Reaction... 

Chiral nitrosoureas can be easily prepared starting from disubstituted pyrrolidines possessing C2 symmetry. Thus (27 , 57 l-dimcthyI pyrrolidine can be smoothly converted, by reaction with phenoxyhydroxamic acid in pyridine solution to the corresponding (V-hydroxyurea 1. Oxidation of 1 with tetrapropylammonium periodate in dichloromethane gives the short-lived nitroso dienophile 2 which reacts with 1,3-cyclohexadiene to give the bicyclic 3,6-dihydro-2//-1,2-ox-azine 3 in 81% yield and d.r. 99 1, The absolute configuration of the reaction product was ascertained unequivocally by synthetic methods as (17 ,45)107. 

Reactions of Boc BMI with Electrophiles. The enolate of Boc-BMI is generated with Lithium Diisopropylamide in THF at -75 °C the resulting solutions of this highly nucleophilic reagent are stable up to 0 °C. All reactions occur from the face of the enolate trans to the t-Bu group at C(2). Alkylations even with secondary alkyl halides are so efficient, to give (3), that one-pot double alkylations, which yield (4), are possible the sequence in which two different alkyl halides are employed determines the absolute configuration of the ot-branched ot-amino acids eventually obtained. 

The stereochemical course of further transformations with the lithiated sUane (RJS)-2 is not affected by crystallization and isolation of the product reactions starting with the product as prepared and maintained in solution give the same d.r. values. Beginning with a known absolute configuration at the metalated carbon center, we were able to examine and explain the stereochemical course of an integral sequence of transformations by experimental and computational methods, starting from the... 

For the determination of the absolute configuration of the major diastereomer of silane (5,S)-3, the hydrogen iodide adduct (5,5)-3 HI was formed by reaction of the compound with anhydrous hydrogen iodide (solution in diethyl ether) at 0 °C. Colorless needles of (5,5)-3-HI were obtained as single crystals from CH2Cl2/n-pentane. 

To a solution of [Rh(cod)2]Bp4 (5.0 mg, 0.012 mmol) in MeOH (10 mL) in a glove box was added (, 5, 7 ,5)-Me-PennPhos (125) (0.15 mL of a 0.1-M solution in MeOH, 0.015 mmol). After the mixture was stirred for 30 min, the enol acetate 172 (1.2 mmol) was added. The hydrogenation was performed at room temperature under 1.7 atm of hydrogen for 24 h. After the hydrogen was released, the reaction mixture was passed through a short silica gel column to remove the catalyst. The enantiomeric excess of 173 was measured by capillary GC without any further purification. The absolute configurations of the product was determined by comparing the observed optical rotation with that of chiral acetate made from a readily available secondary alcohol. ... 

Next, the asymmetric bromination of the cyclodextrin complex in the solid state was compared with the homogeneous reaction in (CH3)2S0 solution (see Table III). The optical yield from the homogeneous reaction was much smaller than that from the gas-solid reaction, although the absolute configurations were the same. This result shows that the gas-solid reaction is topochemically controlled by the crystalline lattice of the cyclodextrin complex. 

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