Search results for "Stereoselectivity"

Amino Sugars and Glycosylamines as Tools in Stereoselective Synthesis

2005

ChemInform Abstract: Stereoselective Syntheses of Chiral Heterocycles and Alkaloids Using Carbohydrate Auxiliaries

2010

Star-shaped conjugated compounds forming nematic discotic systems

2004

Abstract Star-shaped compounds, having a benzene ( 9a,b ) or a 1,3,5-triazine ( 11a,b ) core and stilbenoid arms were prepared. Hexyloxy chains, attached in the middle of the arms, provide nematic discotic phases ND, which are unusual for such systems. The position of the sidechains prevents the micro-segregation, which is valid for star-shaped discs of columnar phases. The stilbenoid character of 9a,b and 11a,b guarantees a high light sensitivity. Apart from the statistical CC bond formation by irradiation in solution or in the LC phases, a topochemically controlled chemo-, regio- and stereoselective photocyclodimerization 11a → 12 was found in the crystalline state. The structure determin…

Regioselective Epoxide Ring Opening. Steroselective Synthesis of a Tetrahydropyran Ring

1998

The stereoselective synthesis of a 2-substituted tetrahydropyran with adjacent alkoxy-bearing stereogenic centre is described. The key steps of this synthesis were the stereoselective epoxidation of an allylic alcohol and the regioselective epoxide ring opening by lithium aluminum hydride. The regio and stereoselective synthesis of a trihydroxyselenide and a trihydroxysulfide is also described. The latter compounds are not suitable for cyclization to tetrahydrofuran ring.

Stereoselective Synthesis of (+)-11βH,13-Dihydroestafiatin, (+)-11βH,13-Dihydroludartin, (−)-Compressanolide, and (−)-11βH,13-Dihydromicheliolide fro…

2002

Starting from 2 and 3, obtained from santonin (1), we have synthesized natural guaianolides 4-7. Chemoselective epoxidation of 2 gave (+)-11betaH,13-dihydroestafiatin (4), and epoxidation of 3 followed by regioselective elimination of the hydroxyl group afforded (+)-11betaH,13-dihydroludartin (5). Sharpless' mild regioselective ring-opening of 4 and 5 followed by hydrogenolysis yielded (-)-compressanolide (6) and (-)-11betaH,13-dihydromicheliolide (7), respectively.

Book Review: Stereoselective Synthesis. By Mihály Nógrádi

1987

A density functional theory study of the regio- and stereoselectivity of the 1,3-dipolar cycloaddition of C-methyl substituted pyrazinium-3-olates wi…

2013

Abstract A DFT [B3LYP/6-31G(d)] study was carried out on the 1,3-dipolar cycloaddition (13DC) reactions of multi C-methyl substituted pyrazinium-3-olates with methyl acrylate (MA) and methyl methacrylate (MMA). Thermodynamic and kinetic parameters of the possible endo/exo stereoisomeric and 6-ester/7-ester regioisomeric pathways have been determined. The skeleton rearrangement of the 6-exo [3 + 2] cycloadducts into the [4 + 2] adducts is also considered. The electrophilic, P k + , and nucleophilic, P k - , Parr functions are used to have better understanding of the regioselectivity of these 13DC reactions. In all cases the exo pathways are more favourable compared to the endo alternatives. …

Stereoselective synthesis and determination of the cytotoxic properties of spicigerolide and three of its stereoisomers.

2003

Stereoselective syntheses of the naturally occurring, cytotoxic lactone spicigerolide and three nonnatural stereoisomers thereof are described. The commercially available sugar l-rhamnose was in all cases the chiral starting material. Key steps in each of these syntheses were asymmetric Brown allylations and ring-closing metatheses. The cytotoxic activities of the four lactones against a range of tumoral lines were then determined.

Structure elucidation of the adducts formed by fjord region Dibenzo[a,l]pyrene-11,12-dihydrodiol 13,14-epoxides with deoxyguanosine.

1999

Model adducts to be used in the identification of biologically formed adducts were synthesized by reaction of fjord-region dibenzo[a,l]pyrene 11,12-dihydrodiol 13,14-epoxides (DB[a,l]PDE) and deoxyadenosine (dA). The (+/-)-anti-DB[a,l]PDE was reacted with dA in dimethylformamide at 100 degrees C for 30 min to give four DB[a, l]PDE-14-N(6)dA adducts: (-)-anti-trans (26%), (+)-anti-trans (26%), (-)-anti-cis (17%), and (+)-anti-cis (17%). The (+/-)-syn-DB[a,l]PDE was reacted with dA under the same conditions to yield four DB[a, l]PDE-14-N(6)dA adducts and one N7Ade adduct: (+)-syn-cis (19%), (+)-syn-trans (13%), (-)-syn-cis (19%), (-)-syn-trans (13%), and (+/-)-syn-DB[a,l]PDE-14-N7Ade (22%). T…

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

2007

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. Both steps are base-catalyzed. The high level of stereocontrol is the result of a combination of electrostatic interactions a…