Diastereoselective synthesis glycolates

Claisen rearrangement of glycolates. Two laboratories 2 have reported that allylic glycolate esters undergo Claisen-Ireland rearrangement (6, 276-277) with useful diastereoselectivity. This rearrangement was used in a synthesis of 1, the aggregation pheromone of the European elm bark beetle.1... 

Ponzo, V. L. Kaufman, T. S. Diastereoselective alkoxymethylation of aromatic aldehydes with chiral lithiomefhyl ethers. Synthesis of optically active monoprotected glycols. Can. J. Chem. 1998, 76,1338-1343. 

T. Nakata et al. developed a simple and efficient synthetic approach to prepare (+)-methyl-7-benzoylpederate, a key intermediate toward the synthesis of mycalamides. The key steps were the Evans asymmetric aldol reaction, stereoselective Claisen condensation and the Takai-Nozaki olefination. The diastereoselective Claisen condensation took place between a 5-lactone and the lithium enolate of a glycolate ester. 

West and Naidu found that the diazoketone 358, prepared by alkylating the benzyl ester of L-proline with 5-bromo-l-diazopentan-2-one, cyclized to give a transient spirobicyclic ammonium ylide 359 when heated with coppeifll) acetylacetonate in toluene (Scheme 44) (355,356). This unstable ylide underwent a diastereoselective [1,2]-Stevens rearrangement to give the quinolizidinone 360 and its bridgehead epimer in a ratio of 95 5. However, some racemization (possibly through an achiral diradical intermediate) must have occurred, since 360 had an ee of only 75%. Reduction of the ester and defimctionalization of thioketal 361 with the unusual combination of sodium and hydrazine in hot ethylene glycol completed a synthesis of the unnatural (- )-enantiomer of epilupinine (ent-331). 

Kocienski et al. have used a vinyl silane as a masked vinyl alcohol in the synthesis of the C26 to C32 fragment of rapamycin (Scheme 4.82) [77]. Ireland-Claisen rearrangement of the vinylsilane glycolate ester via the Z-silyl ketene acetal yielded the corresponding anti pentenoic acid with high diastereoselectivity. Oxidative cleavage of the furyl substituent and Tamao oxidation then afforded the allyhc alcohol. 

Since the vast majority of the substrates employed in diastereoselective syntheses are attached via a carboxyl function to the respective auxihary, this chapter deals with the diastereoselective a-fiinctionalization of chiral carboximides and esters, as well as with the homologation and aldolization alternatives of chiral glycinates, glycolates and haloacetates. Also discussed is the use of chiral a,/3-unsaturated representatives in the synthesis of e.p. linear /3- and a,/3-substituted carboxyhc acid derivatives and cyclic analogs (Diels-Alder reactions). Other methods, in which substrates are coupled, for example, via a carbonyl function in order to facilitate the diastereoselective a-aUcylation of aldehydes or... 

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