Diphenyl methanol enantioselective reactions

This enolate can then react with a plethora of electrophiles, setting a new stereocenter by a diastereoface-selective reaction. The simplest electrophile to trap enolate 71 is H" ", which can, for example, originate from methanol [89] or diphenyl acetaldehyde (as a readily enolizable aldehyde) [90] leading to the acy-lated catalyst species (Fig. 38). The free catalyst is regenerated by acyl-group transfer to methanol(ate) or the aldehyde-derived enolate, producing methyl or enolesters 72/73 in good yields and enantioselectivities. 

In the course of the continuing study [9a,b] on the enantioselective addition of dialkylzincs to aldehydes by using chiral amino alcohols such as diphenyl(l-methyl-2-pyrrolidinyl)methanol (45) (DPMPM) [48] A. A -dibutylnorephedrine 46 (DBNE) [49], and 2-pyrrolidinyl-l-phenyl-1-propanol (47) [50] as chiral catalysts, Soai et al. reacted pyridine-3-carbaldehyde (48) with dialkylzincs using (lS,2/ )-DBNE 46, which gave the corresponding chiral pyridyl alkanols 49 with 74-86% ee (Scheme 9.24) [51]. The reaction with aldehyde 48 proceeded more rapidly (1 h) than that with benzaldehyde (16 h), which indicates that the product (zinc alkoxide of pyridyl alkanol) also catalyzes the reaction to produce itself. This observation led them to search for an asymmetric autocatalysis by using chiral pyridyl alkanol. 

R,R-diphenyl ethylene carbonate CR,R-DPEC)) with a racemic zirconaaziridine. (C2-symmetric, cyclic carbonates are attractive as optically active synthons for C02 because optically active diols are readily available through Sharpless asymmetric dihydroxylations [67].) Reaction through diastereomeric transition states affords the two diastereomers of the spirocyclic insertion product protonolysis and Zr-mediated transesterification in methanol yield a-amino acid esters. As above, the stereochemistry of the new chiral center is determined by the competition between the rate of interconversion of the zirconaaziridine enantiomers and the rate of insertion of the carbonate. As the ratio of zirconaaziridine enantiomers (S)-2/(R)-2 is initially 1 1, a kinetic quench of their equilibrium will result in no selectivity (see Eq. 32). Maximum diastereoselec-tivity (and, therefore, maximum enantioselectivity for the preparation of the... 

The asymmetric catalytic hydrogenation of 2-(6 -methoxy-2 -naphthyl)propenoic acid to (S)-naproxen was monitored in both methanol- and C02-expanded methanol.f The catalyst used was [dichloro-(S)-(-)-2,2 -bis(diphenyl-phosphino)-1, l -binapthyl]ruthenium(II). Addition of CO2 to the methanol produced strong retardation of the reaction rate. An average reduction in enantioselectivity of 6%i was observed compared to that in methanol. The enantioselectivity was found to increase as the temperature decreased in both neat methanol and C02-expanded methanol systems, indicating that the underlying mechanisms were similar. Insufficient oxygen removal may have prevented the... 

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