Enantiomeric group elaboration

It is of interest to compare the occurrence of the eburnane alkaloids in Malaysian Kopsia with that of the Chinese species (118]. From such a comparison, it would appear that the Malaysian Kopsia (and Leuconotis) species elaborate exclusively eburnane alkaloids of one enantiomeric group (20R, 21R or 20P, 2ip configuration) [18-20,58,110-112,115-119], while the Chinese species appear to elaborate eburnane alkaloids of the opposite enantiomeric group (205, 215 or 20a, 21a configuration) [118,120-122]. Notable exceptions are the dimeric alkaloids kopsoffinol and kopsoffine isolated from the Malaysian Borneo species Kopsia pauciflora [124] which were reported to be constituted from union of kopsinine and dihydroebumamenine units with the 20a, 21a configuration [vide infl-a). 

C of the acetal and the Lewis acid. Enantiomerically pure cyclic acetals 49 deriving from chiral diols open a route to adducts 50, which can be successively elaborated to chiral /9-hydroxyesters upon oxidation of the alcoholic group followed by alkaline elimination (equation 33). 

Conjugate addition reactions to enantiomerically pure tolylsulfinyl trifluoro-propene were used recently to prepare a-trifluoromethyl aldehydes [249]. Conjugate adducts were elaborated by Pummerer rearrangement of the sulfoxyl group to install the aldehydic carbonyl (Eq. 83). 

Starting with tropone 330, the azido compound 331 was first synthesized, as shown in Scheme 55. Then, compound 331 was chemoselectively reduced to unsaturated amine 332 by Lindlar catalyst, and this material was elaborated to the meso carbamate 333, ready for enzymatic asymmetrization. Treatment of 333 with Amano P-30 lipase in the presence of isopropenyl acetate resulted in formation of the enantiomerically pure (>98% ee) monoacetate 334, the common intermediate to both calystegines 337 and ent-337. Using conventional chemistry, elaboration of the functional groups within tropane 334... 

General Synthesis.- The principle of steric shielding has been applied by a number of research groups to the elaboration of 3-substituted carboxylic acids, with enantiomeric enrichments of 94-98% one of the variants of these asymmetric Michael additions is outlined in Scheme 1. Overall yields are generally excellent, the chiral auxiliary can easily be recovered, and the sense of the chirality in the final product can in principle be varied simply by changing the order of introduction of the two substituents, given that both organo-copper species are available. An entirely different approach to chiral carboxylic... 

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