Methanol, achiral additives

High reactivity was observed for 21b, and 21a was found to be the most selective. In the presence of 10 mol% 21a selectivity factors as high as 6.5 were observed with racemic 1-(1-naphthyl)ethanol as substrate (Scheme 12.6) [18]. The TBS analog of 21a was found to be good catalyst for asymmetric addition of methanol to a variety of prochiral aryl alkyl ketenes [18]. The catalytic asymmetric addition of achiral alcohols to prochiral ketenes is discussed in Section 13.2. 

The separation of chiral compounds will be discussed in Chapter 22. However, the separation of diastereomers can be accomplished using achiral stationary phases. Another alternative is the use of chiral columns for the separation of diastereomers in either the reversed-phase or normal-phase mode. The use of achiral bonded phases without chiral additives, such as phenyl and alkyl bonded phases for the separation of diastereomeric pharmaceutical compounds, is acceptable. Different selectivities can be obtained by employing stationary phases containing varying functionalities (phenyl, polar embedded moieties). The effect of aqueous mobile-phase pH, temperature, and type of organic eluent (acetonitrile versus methanol) can also play a dramatic role on the separation selectivity of diastereomeric compounds. 

The achiral 14-membered trans-diimine macrocycle (f , S )-102, in the presence of trifluoroacetic acid, rearranges quantitatively into the chiral seven-membered monoimine ( )-103 (Section Ill.C.l.h)". If the rearrangement of (i , S )-102 is carried out in methanol containing a suspension of ( )-(—)-78, an orange solution is obtained from which pure [I ,(Sas,I as)] ( )-1 6 can be isolated by the addition of ammonium hexafluorophosphate. The yield of the complex was ca 50%. The addition of more acid and halide in an attempt to facilitate racemization of the free arsine and thereby promote the further crystallization of the complex by second-order asymmetric transformation was unsuccessful. Nevertheless, this highly stereoselective synthesis of [H,(Sas,IIas)] ( ) 106 is a more expedient route to (R,I )-(—)-102 than the one involving resolution of the benzyl alcohol complex (R, SA.)-(-)-92a. 

Examination of A. qfficinarum by Itokawa and his group provided besides the known compound 6 (8) and the ketol 22 (25) two new enones, 28 and 29 (26). Circular dichroism studies revealed the interesting fact that, in contrast to the known components of Alms firma and A. sieboldiana, which contained (S)-6 and (5)-22, in Alpinia officinarum the same alcohols were present as the R enantiomers. On further investigation the same plant also yielded a total of eight new achiral or racemic diarylheptanoids 23, 24, 31 (27), 25, 26, and 30, (28), 33 (29), 27 and 32 (30). Since the highly susceptible P-hydroxyketones can readily undergo dehydration and subsequent addition of methanol, the enones 28, 29, and 30, as well as the P-methoxyketones 23, 25, and 26 may be artefacts. This assumption is also supported by the racemic nature of 23, 25, and 26. 

Furukawa et al. explored the use of methanol and ethanol as additives for diethylzinc-based epoxide polymerization systems, and found that both the yield and crystallinity of the resulting polymers were inferior to those for polymers synthesized with the ZnEt2/H20 system. The use of achiral alcohols as cocatalysts was revisited in 1994 when Kuran and Listos reported the polymerization of propylene oxide and cyclohexene oxide (a meso molecule) with ZnEt2/polyhydric phenol (such as 4-tert-butyl-catechol), phenol, or l-phenoxy-2-propanol. The poly(propylene oxide) formed from these systems contained mostly isotactic dyads (72% m), whereas the poly(cyclohexene oxide) contained mostly syndiotactic dyads (80% r) (Scheme 24.8). 

Fumkawa and co-workers explored catalysts derived from the addition of methanol or ethanol to diethylzinc as epoxide polymerization systems, and found that both the yield and crystallinity of the resulting polymers were inferior to those for polymers synthesized with the ZnEt2/H20 system. The use of achiral alcohols as cocatalysts was revisited in 1994 when... 

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