Mechanistic considerations diastereoselectivity

Treatment of amino acid 156, imine and 2-chloro-l-methylpyridinium iodide (Mukaiyama s reagent) in the presence of triethylamine in refluxing dichloro-methane afforded spiro-(3-lactams 157,158. These were obtained as a 1.8 1 mixture of diastereoisomers and separated by column chromatography. The reaction of 159 and imine under the usual experimental conditions resulted in the formation of a single diastereoisomer 160. The absolute (3 S, 4 S, 7 -configuration was assigned on the basis of mechanistic considerations and XH NMR spectra. The presence of the stereocenter affords complete diastereoselectivity (only trans diastereoisomers 157, 158) and enantioselectivity (160). 

One of the foremost methods for oxidation of olefins is stereospecific syn-di-hydroxylation by treatment with OSO4. Because of the toxicity and cost of osmium, however, the need for methods that would employ substoichiometric quantities of this reagent was identified early on [180], A number of stoichiometric oxidants for catalytic dihydroxylations were examined, including metal chlorates, HjOa, NMO [181], f-BuOOH [182], and KjFeiCN) [40, 183], The mechanism of 0s04-catalyzed dihydroxylation reactions has been the subject of much debate details of these mechanistic considerations are beyond the focus of this book and are amply discussed elsewhere [40-42,47,49,184-186]. A number of interesting stereoselectivity trends of general importance in substrate-controlled diastereoselective dihydroxylations [40-43] and catalytic asymmetric dihydroxylations are discussed below (Section 9.8) [42, 44-49]. 

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