Reducing agents asymmetric

Boranc complexes of optically active /j -hydroxysulfoximines have been developed as highly reactive asymmetric reducing agents of prostereogenic ketones5. The required /(-hydroxysul-... 

Borohydride Reagent. Treatment of NB-enantrane with t-Butyllithium provides the lithium trialkylborohydride NB-Enantride (eq 4). This reagent is an effective asymmetric reducing agent for acetophenone and alkyl methyl ketones such as 2-octanone (eq 5). Few reagents show selectivity for such alkyl ketones. 

Although -3-pinanyl-9-borabicyclo[3.3.1]nonane and related substances have also been developed as efficient asymmetric reducing agents for carbonyl compounds (Volume 8, Chapter 1.3), we discuss here only asymmetric reductions using chirally modified metal hydride reagents. The asymmetric hydrosilyl-ation of a carbonyl group catalyzed by a chirally modified transition metal is mentioned briefly. 

An asymmetric reducing agent (49), prepared from LAH, (-)-)V-methylephedrine (14) and 3,5-di-methylphenol in a 1 1 2 ratio, reduces acetophenone with 83.8% A series of aromatic and alkynyl ketones are reduced by (49) to the corresponding (f )-alcohols (Scheme 1)P Some of the products are useful intermediates for the synthesis of 7-lactone insect pheromones. " ... 

Brown and coworkers have reported a critical examination of the relative effectiveness of various reducing agents for the asymmetric reduction of different classes of ketones.In Table 1 we present an expanded list of 24 typical C=X compounds and 17 selected asymmetric reducing agents or methods. 

Morrison and Mosher collected data for asymmetric induction in the reduction of several acyclic ketones and this monograph is an excellent source when one must choose an asymmetric reducing agent.236 xhe use... 

Table 4.6. Preferred Asymmetric Reducing Agents Class of Ketones...

Forms an asymmetric reducing agent when complexed with LiAlH4. Syrup. Bpo.o5 200°. Mg -34.8 (c, 4.3 in CHCI3). 

The effective asymmetric reduction of NB-Enantride for aliphatic ketones is highly significant, as most asymmetric reducing agents are effective only for aromatic ketones [2]. A trend of decreasing asymmetric induction is observed with increasing size of the alkyl groups. The reduction products are isolated in 70-80% yields. 

On the basis of data of Table 26.35, and the available data from the literature, the selected preferred asymmetric reducing agents for 10 structurally different classes of ketones are suggested (Table 26.36) for each class of compounds. 

Table 26.36 (continued) Preferred asymmetric reducing agents [ i ] ... 

Figure 6. Asymmetric reduction of 2-chloroacetophenone with various boron based asymmetric reducing agents...

In summary, this chapter shows that organoboron-based asymmetric reducing agents, such as K Glucoride (1), K Xylide (2), Alpine-borane (3), Dip-Chloride (4) and oxazaborolidine-catalyzed boranes are highly effective for the reduction of a variety of a-functionalized ketones. We have established a convenient and simple procedure for the preparation of terminal 1,2-diols, a-hydroxy acetals and epoxides with very high optical purity via oxazaborolidine-catalyzed borane reduction using /V-phenylamine-borane complexes as the hydride source. 

The asymmetric reducing agent (78) has been used to prepare naturally occurring y-lactones and enantiomerically pure secondary propargylic alcohols,whereas the chiral alkoxy(acyloxy)borohydride(79) gives 35-50% enantiomeric excess in ketone reductions. 

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