Absolute configuration Substitution

X-ray analysis of an optically active oxaziridine substituted at nitrogen with the 1-phenylethyl group of known configuration led to the absolute configuration (+)-(2R,3R)-2-(5-l-phenylethyl)-3-(p-bromophenyl)oxaziridine of the dextrorotatory compound as expected, C-aryl and A-alkyl groups were trans to each other (79MI50800). 

Metalated SAMP- or RAMP-hydrazones derived from alkyl- or arylethyl ketones 3 add to arylaldehydes both diastereo- and enantioselectively. Substituted / -hydroxy ketones with relative syn configuration of the major diastereomer are obtained with de 51-80% and 70-80% ee. However, recrystallization of the aldol adducts, followed by ozonolysis, furnishes diastereo- and enantiomerically pure (lS, S )-. yn-a-mcthyl-/3-hydroxy ketones 5 in 36-51% overall yield. The absolute configuration of the aldol adducts was established by X-ray crystallographic analysis. Starting from the SAMP- or RAMP-hydrazone either enantiomer, (S,S) or (R,R), is available using this methodology16. 

The stereochemical course of the enantioface differentiation on the aldehyde is dictated by the configuration of the sulfoxide group sulfinyl-subsdtuted dihydroisoxazoles epimeric at C-5 (e.g., 18) provide aldol adducts 19 with the same configuration at the hydroxy-substituted carbon (C-2 ) independent of the absolute configuration at C-5, however, with different degrees of stereoselectivity23. 

A similar trend was observed in the reaction of tri- and tetrasubstituted etiolates derived from 2-unsubstituted or 2-bromo substituted 3,4-dihydro-6-methoxy-1(2//)-naphthalenone16. The trisubstituted cnolate underwent addition to (—)-(2 )-2-(4-methylphenylsulfinyl)-2-cyclopen-tenone via attack on the nonchelated conformation to give an adduct of d.r. [(2S)/(2/ )] 77 23. The tetrasubstituted enolate underwent addition to the corresponding ( + )-(5)-enone via attack on the chelated conformation to give an adduct with the same absolute configuration at C-2 but with d.r. [(2R) (2S)] 95.5-97 4.5-3. 

From a variety of differently substituted compounds, best results were obtained with the catalysts 195a-c in combination with /-methyl diazoacetate and monoolefins, cyclopropanes were obtained with a relatively high trans/cis ratio and enantiomeric excesses of 44-89% (Table 12). The absolute configuration at the catalyst s chiral center determines the enantioselectivity for both diastereoisomers. 

The relative and absolute configurations of diepoxydicarbazoles involving the 2,6-dioxa-4,8-diazaadamantane system were determined in the course of a study on indole and indole alkaloids.242 Water-soluble azo, anthraquinone, and phthalocyanine dyes which are substituted by a 4-chloro-s-triazin-2-ylamino group can be quaternized with a l-aza-3-methyl-4,6,10-trioxa-adamantane unit in aqueous medium at 40 50"C.243 Dyes mixed with... 

Chiral sulfonium ylides have been known for some 30 years, and their stereochemistry and properties have been studied.15 Optically active selenonium ylides were obtained by reacting selenoxides with 1,3-cyclohexanedione under asymmetric conditions by Sakaki and Oae in 1976 for the first time,16 and also optically resolved by fractional recrystallization of the diastereomeric mixtures in the early 1990s.17 In 1995, optically active selenonium ylides 6 were obtained in over 99% de by nucleophilic substitution of optically active chloroselenurane or selenoxide with active methylene compounds with retention of configuration.18 The absolute configurations were determined by X-ray analysis of one... 

Yamamoto and co-workers found that BLA 76a is one of the best catalysts for the enantioselective and selective cycloaddition of a-substituted ajl-enals to highly reactive dienes such as cyclopentadiene. The results in the presence of (J )-76a are summarized in Table 5-2. The major enantiomer has, in several cases, been demonstrated to have the predicted absolute configuration. 

Introduction of a double bond between the triple bond and the leaving group leads to enyne electrophiles 45, which would give access to vinylallenes 46 if the attack of the nucleophile takes place at the triple bond in an SN2" (1,5) substitution reaction (Scheme 2.16). In addition to the regioselectivity, two types of stereoselectivity also have to be considered in this transformation, i.e. the configuration of the olefinic double bond of the vinylallene and the (relative or absolute) configuration of the allenic chirality axis. 

When the allene moiety of 2,3-allenylamines was substituted with Br, an intramolecular nucleophilic substitution reaction led to a chiral 2,3-ds-ethynylaziridine 323. The diastereoselectivity depends on the absolute configuration of the allene moiety, i.e. typically for a matched-mismatched pair the S,aR-isomer afforded the product with much higher stereoselectivity [155, 156],... 

Falk and Schlogl 42> as late as 1968 established the absolute configuration of compound 13a as (+) —(S)-4-carboxy[2.2]paracyclophane. They did this by kinetic resolution of the racemic carboxylic anhydride with ( —)-a-phenylethylamine this kinetically controlled amidation afforded the dextrorotatory compound (13) in 3.8% optical yield 42C The similar topology of the carboxyl-group environment in 13 and in a-substituted metallocene carboxylic acids (14), configurations con-... 

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