Ketones stereoselectivity

Regio- and stereo-selectivity in the intramolecular cyclization of enolates derived from 4,5-, 5,6-, and 6,7-epoxy-1-phenylalkan-l-ones (18a,b-2 a,b) has been explored. Competition between C- and O-alkylation routes have been discussed. The LHMDS-Sc(OTf)3 protocol seems to be a valuable tool for obtaining the corresponding y-hydroxy ketones stereoselectively from (18a,b) and (20a,b) whereas (19a,b) reacted cleanly only under alternative basic conditions, and yielded products of O-alkylation. 

The Ti(II)-mediated stereoselective reaction of allenyne with CO is reported to give bicyclic ketone (Scheme 11.18) [20[. The formation of the titanabicycle occurs from the less hindered side of the allene moiety, resulting in the production of bicyclic ketone stereoselectively. This reaction requires slightly excess amounts of the Ti reagent (not a catalytic reaction), but these results show the capability of stereoselective carbonylation of allenes. 

Lhommet and coworkers prepared the chiral bicyclic carbamate 1189 from L-lysine by a method that involved an electrochemical methoxylation (Scheme 150). ° This precursor, in effect a disguised acyliminium ion, reacted with the sdylated enol 1190 and trimethylsilyl triflate to give a diaste-reomeric mixture of p-keto esters (+)-1191, hydrolysis and decarboxylation of which produced ketone (—)-1192 as the only discernible product. It was essential to reduce this ketone stereoselectively in order to set up the configuration of the target alkaloid s side chain correctly. Fortunately, this was achieved with either K-Selectride or with lithium triethylborohydride. 

Roush, W.R. and Dilley, G.J. (1999) Studies of fiagment assembly aldol reactions of chiral aldehydes and chiral methyl ketones stereoselective synthesis of the C(13)-C(25) segment of scytophycin C. Tetrahedron Lett., 40,4955-4959. 

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