Asymmetric carbon atoms, relative stereochemistry

Pyrrolizidine derivatives with at least one substituent, and particularly the pyrrolizidine alkaloid components, have one or more asymmetric carbon atoms. The stereochemistry of pyrrolizidine was clarified for the most part in the course of investigation of the naturally occurring pyrrolizidine alcohols. Here, the problems of relative and absolute configuration and of stereoisomeric transformations will be considered. 

For achiral metallocene-based catalysts Czv and achiral Q metallocenes in Chart 2) the chain-end control is present as the only stereocontrol mechanism. It derives from the presence of an asymmetric carbon atom on the last inserted monomer. The chirality R or 5) of this atom is related to the enantiotopic face of the olefin where the insertion took place (Scheme 34). In the NMR spectrum of the polymer we lose this kind of information, as two successive insertions of the re olefin face and two successive insertions of the si face produce the same m diad (see section II.G). As a consequence, we can observe only the relative chirality between consecutive inserted monomer units (5,5 or R,R as m diads and S,R or R,S as r diads) disregarding the absolute configuration of tertiary atoms. We prefer to use the re and si nomenclature indicating the stereochemistry of the methines in the polymer chain (Scheme 35), bearing in mind that the insertion of the re propene enantioface will produce an 5 configuration on the methine. 

Until relatively recently, interest in chiral chemistry has been largely academic and, as a consequence, has occupied a relatively minor position in the analytical chemistry syllabuses of most universities. Despite the emphasis that has been placed on the recent advances in chiral chemistry, optical isomers have been know for many years and were first identified by Biot [1] in the early 1800s, and their existence was established by the work of Pasteur [2] in 1848. Both van t Hoff [3] and Le Bel [4] proposed the existence of the asymmetric carbon atom and used it to explain the cause of optical rotation. However, it was Emil Fisher [5], who made the first serious attempts to relate the absolute stereochemistry of optical isomers and determined the configuration of (+)-glucose for which he received the Nobel prize. Fisher predicted that the (+)-isomer of glyceraldehyde was the D-isomer and arbitrarily assigned the stereochemistry as ... 

A useful catalyst for asymmetric aldol additions is prepared in situ from mono-0> 2,6-diisopropoxybenzoyl)tartaric acid and BH3 -THF complex in propionitrile solution at 0 C. Aldol reactions of ketone enol silyl ethers with aldehydes were promoted by 20 mol % of this catalyst solution. The relative stereochemistry of the major adducts was assigned as Fischer- /ir o, and predominant /i -face attack of enol ethers at the aldehyde carbonyl carbon atom was found with the (/ ,/ ) nantiomer of the tartaric acid catalyst (K. Furuta, 1991). 

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