Strategy V Stereoselectivity

Background Needed for this Chapter Reference to Clayden, Organic Chemistry  

We shall discuss two strategies in the making of single enantiomers. Either we can resolve a racemic compound somewhere in the course of the synthesis or we can use a single enantiomer as starting material. Other strategies are discussed in detail in Strategy and Control. 

Enantiomers cannot be separated by the normal processes of purification crystallisation, distillation or chromatography. But diastereoisomers can. Resolution involves using an enantiomerically 

Organic Synthesis The Disconnection Approach. Second Edition Stuart Warren and Paul Wyatt 2008 John Wiley Sons, Ltd 

He prepared amine 8 by a kind of reductive amination of the ketone 9 via the N-formyl amine 10 and made it enantiomerically pure by resolution with malic acid 11-a cheap enantiomerically pure compound.4 

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The disadvantage of the intermolecular dipolar cycloaddition strategy is nonstereoselectivity. A recent stereoselective synthesis of lasubine 1 (2) utilizes the intramolecular tt cyclization of an /V-acyliminium ion as a key step (Scheme 4) (16). The reaction of carbinol 38, prepared from 3,4-dimethoxybenzaldehyde (33) and allylmagnesium bromide, with glutarimide under Mitsunobu conditions... 

In conclusion, eight research groups have been involved in the total synthesis research of Maoecrystal V. In most occasions, Diels-Alder reaction was used to construct the core structure of Maoecrystal V, in which oxidative dearomatization and IMDA played a crucial role in the construction of the core structure. Based on the analysis of previous synthesis strategies, it can be confirmed that, Diels-Alder reaction as the ancient and classical reaction plays a very important role in the total synthesis. It also has incomparable superiority in constructing quaternary carbon center stereoselectively. 

An excess amount of pyridine (3 equiv) reacts with 4,4-dimethyl-2-pent)uie to provide the olefinated product in 87% yield with high regio- and stereoselectivities (E/Z >99 1). Here, diphenylzinc is a Lewis acid catalyst (20 mol %), and its role is thus different compared with the aforementioned arylation procedure where it acts as an aryl donor. The in situ activation of pyridine by a Lewis acid, such as diphenylzinc, is an operationally practical strategy toward C2 olefinated pyridines. This avoids the less practical classical alternative that involves a three-step procedure (i) activation of pyridine using stoichiometric oxidant to form pyridine IV-oxide, (ii) nickel-promoted alkenylation, and (iii) stoichiometric reduction of the V-oxide. ... 

Ling, T., Potts, B.C., and Macherla, V.R. (2010) Concise formal synthesis of (-)-salinosporamide A (Marizomib) using a regio- and stereoselective epoxidation and reductive oxirane ring-opening strategy./. Org. Chem., 75, 3282-3285. 

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