Configurations racemization

Having retraced the efficient and elegant sequences of reactions that have led to the synthesi of key intermediates 11 and 12, we are now in a position to address their union and the completion of the total synthesis of erythronolide B. Taken together, intermediates 11 and 12 contain all of the carbon atoms of erythronolide B, and although both are available in optically active form of the required absolute configuration, racemic 11 and enantiomerically pure 12... 

It would usually be assumed that abstraction/recombination reactions such as those illustrated in this section would proceed with racemization at the reacting centers. It has been reported22, however, that photocyclization of amide 1 proceeds with complete retention of absolute configuration. Racemization at the site of abstraction requires orbital rehybridization, passing through a planar intermediate. In this case rehybridization appears to be markedly slowed. This may be an electronic effect due to the heteroatom substituent on the intermediate radical, or simply a steric effect. The structure of product 2 was established by X-ray crystallographic analysis. 

If a nucleophile attacks the carbocation in Figure 6-10 from the front side (the side the leaving group left), the product molecule shows retention of configuration. Attack from the back side gives a product molecule showing inversion of configuration. Racemization is simply a combination of retention and inversion. When racemization... 

The pathways differ in outcome with substrates of defined configuration racemization for the radical path, inversion for the Sn2, and retention for the concerted route. 

How would such a species explain the stereochemical results racemization and retention of the exo configuration Racemization was easy, one had only to permit the initially formed ion to equilibrate rapidly with the equivalent, mirror-image form as noted in Figure 21.56. 

Cyclohexenylethylamine (14) (Schnider and Hellerbach 1950) provided a facile route to octahydroisoquinolines, such as (15), and its optically active analogs (17), (18) and (19) with morphine configuration. Racemization of (18) was achieved by refluxing the unwanted optical isomer in presence of a metal catalyst (Hindely 1961). 

Stereochemical s. Steric Stereochemistry of electrophilic addition 21, 593 -, preparative, review 22, 693 Stereoisomers (s. a. Asym..., Configuration, Racem...)... 

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