Development of Novel Enantioselective Synthetic Methods

ElectrophiHc substitutions with carbon and hetero electrophiles a to the carbonyl group of aldehydes and ketones are among the most important synthetic operations. Such regio-, diastereo-, and enantioselective substitutions can be carried out efficiently with the SAMP/RAMP hydrazone methodology [3]. For cases where virtually complete asymmetric inductions could not be attained, an alternative approach based on a-silylated ketones 2 was developed [4]. They can be prepared easily from ketones 1 in high enantiomeric purity (ee 98%) by asymmetric carbon silylation employing the SAMP/RAMP hydrazone method (Fig. 1.1.1). After the introduction of various electrophiles via classical enolate chemistry with excellent asymmetric inductions, the desired product ketones 3 

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Other examples of shown effectiveness of PTCs were reported by Park, Jew, and co-workers [98], They developed enantioselective synthetic method for the preparation of (/ )-a-alkylserines and (S)-a-alkylserines based on asymmetric alkylation of phenyl or o-biphenyl oxazoUne derivatives using CBPTC XXXXVIa and binaphthol-derived Maruoka s catalyst XXV, respectively (Scheme 8.16). The same authors [99] also expanded this methodology to enantioselective synthesis of (/ )-a-alkylcysteines and (5)-a-alkylcysteines using thiazoline-4-carboxylates in PTC alkylation. In 2006, Maruoka and co-workers [100] reported first example of the catalytic asymmetric alkylation of glycolates that establishes stereogenic quaternary carbon centers (Scheme 8.17). Their approach utilized 3,5-diaryloxazolidin-2,4-diones 48 as novel... 

This chapter has discussed the transition metal-catalyzed synthesis of allenes. Because allenes have attracted considerable attention as useful synthons for synthetic organic chemistry, effective synthetic methods for their preparation are desirable. Some recent reports have demonstrated the potential usefulness of optically active axially chiral allenes as chiral synthons however, methods for supplying the enantiomerically enriched allenes are still limited. Apparently, transition metal-catalyzed reactions can provide solutions to these problems. From the economics point of view, the enantioselective synthesis of axially chiral allenes from achiral precursors using catalytic amounts of chiral transition metal catalysts is especially attractive. Considering these facts, further novel metal-catalyzed reactions for the preparation of allenes will certainly be developed in the future. 

As another extension of this process, Davies et al. have developed highly regio-, diastereo- and enantioselective C-H insertions of methyl aryldiazoace-tates into cyclic A-Boc-protected amines catalysed by rhodium(II) S)-N- p-dodecylphenyl)sulfonylprolinate. The best results were obtained in the case of the C-H insertion of methyl aryldiazoacetates into A-Boc-pyrrolidine, which gave, in all cases, a diastereoselectivity and an enantioselectivity greater than 90% de and 90% ee respectively (Scheme 10.77). The synthetic utility of this method was demonstrated by means of a two-step asymmetric synthesis of a novel class of C2-symmetric amines. 

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