BINAP prostaglandin synthesis

Scheme 21). Scheme 22 illustrates an example of kinetic resolution of a racemic allylic alcohol with a 1,3-hydrogen shift. When racemic 4-hydroxy-2-cyclopentenone is exposed to a cationic (/ )-BINAP-Rh complex in THF, the S enantiomer is consumed five times faster than the R isomer (32). The slow-reacting stereoisomer purified as the crystalline ferf-butyldimethylsilyl ether is an intermediate in prostaglandin synthesis (33). These isomerizations may occur via initial Rh-olefinic bond interaction (34). 

The cationic BINAP-Rh complexes catalyze asymmetric 1,3-hydrogen shifts of certain alkenes. Diethylgeranylamine can be quantitatively isomerized in THF or acetone to citronellal di-ethylenamine in 96-99% ee (eq 17). This process is the key step in the industrial production of (-)-menthol. In the presence of a cationic (R)-BINAP-Rh complex, (5)-4-hydroxy-2-cyclopentenone is isomerized five times faster than the (R) enantiomer, giving a chiral intermediate of prostaglandin synthesis. ... 

The isomerization of cyclic allyl alcohols to produce ketones proceeds more cleanly [17]. Effective kinetic resolution of rac nic cyclic allylic alcohols has been reported [18]. The isomerization of racemic 4-hydroxy-2-<7clopentanone (29) in the presence of 0.5 mol % of [Rh[(/ )-BINAP](MeOH)2 in THF proceeded with 5 1 enantiomeric discrimination at 0°C to give 13-cyclopentadione (31) via enol ketone 30, leaving the E-staiting allylic alcohol (91% ee and 27% recovery yield) at 72% conversion after 14 days (eq 3.12). (E)-4-Hydioxy-2-building block for prostaglandin synthesis [19]. 

Enantioselective isomerization can be advantageously used for the kinetic resolution of racemic allyl alcohols. For example treatment of 4-hydroxy-2-cyclopente-none (rac-28) in the presence of Rh[(R)-BINAP](MeOH)2 + gives rise to the enan-tiomerically enriched allyl alcohol (R)-29 (Scheme9) [13]. This unsaturated hydroxy ketone is an important building block for the synthesis of prostaglandins... 

Excellent enantiomeric excesses have been obtained by Zhang et al. also in the synthesis of cyclopentanes and cyclopentanones via cycloisomerization of carbon-tethered enynes promoted by the same cationic Rh/BINAP catalysts (Fig. 10.23) [35]. These stmctures had been targeted most particularly since they are useful building blocks for the construction of biologically active molecules and industrially relevant compounds such as prostaglandins and jasmonates. Actually, enantiomerically pure (i5,25)-dihydrojasmonate could be prepared indeed by this elegant methodology. 

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