Ring activity, stereochemical

A number of recent reviews exist about intermolecular and intramolecular reactions of the iV-acyl-iminium intermediate. Moreover, detailed accounts of the application in alkaloid synthesis have recently appeared. This chapter deals with reactions of species (1) with nucleophilic alkenes (and alkynes). Other synthetically useful nucleophiles like aromatic rings, active methylene compounds and organome-tallics will not be discussed here. In (1) R, and R are hydrogen or carbon substituents, and R may also be a hetero substituent, such as alkylamino or alkoxy. This chapter differs from previous reviews, as the material is ordered here on the basis of the structural features of the A -acyliminium intermediate. Major emphasis is placed on recent developments and stereochemical details. 

In the case of chloro componnd 25, reaction with trimethylphosphine and base (NaHC03) yields the phosphorus ylide 27. Yield from 24 is 80%. This is readily cycUzed by gentle warming at 30°C to form componnd 29. Optimum cyclization yield -76% is achieved between pH 7.5 and 8.0. The bicyclic entity 29 is a doubly protected form of the cephalosporin ring system with all optically active stereochemical centers in place. 

The bicyclic entity 29 is a doubly protected form of the cephalosporin ring system with all optically active stereochemical centers in place. 

Stannylselenides 17, 693 Stereochemical effect of ring activity on cyclodehydration 17, 941... 

Scheme 8 presents the sequence of reactions that led to the synthesis of the B-ring of vitamin B12 by the Eschenmoser group. An important virtue of the Diels-Alder reaction is that it is a stereospecific process wherein relative stereochemical relationships present in the diene and/or the dienophile are preserved throughout the course of the reaction.8 Thus, when the doubly activated dienophile 12 (Scheme 8) is exposed to butadiene 11 in the presence of stannic chloride, a stereospecific reaction takes place to give compound 27 in racemic form. As expected, the trans relationship between... 

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