Fructose-derived catalyst

Asymmetric epoxidation of prochiral olefins is a powerful strategy for the synthesis of enantiomericaUy enriched epoxidesJ Previously, we reported a fructose-derived catalyst (1) that gives high ee for a wide variety of trans- and trisubstituted olefins (Figure 6.5). " Recently, we discovered a new catalyst (2) derived from D-glucose that can epoxidize many c -olefins with high enantioselectivity and no c w/rran -isomerization. ... 

In the Shi epoxidation, an oxone (potassium persulfate, KOSO2OOH) in the presence of a fructose-derived catalyst, 7.57, generates epoxides with high enantiomeric excess oxone is best used to oxidize aldehydes to carboxylic acids in the presence of DMF. 

D-fructose derived catalyst 629 to obtain initially intermediate 635 in 83% yield. Later in the sequence, the advanced intermediate 636 was epoxidized in the presence of the enantiomeric L-fructose-derived catalyst ent-629 to furnish oxirane 637 (Scheme 131). After further transformations the first total synthesis and the complete assignment of the stereochemistry of this complex natural product was achieved. 

Shi has also recently reported asymmetric epoxidation mediated by alkahne hydrogen peroxide [44,45]. High yields and ees were obtained under these reaction conditions with up to 95% ee for 1-phenylcyclohexene oxide using the original fructose derived catalyst 10. Peroxyimidic acid 11 is postulated to be the active oxidant (Scheme 1.15). 

Scheme 1.16 Shi s fructose-derived catalyst for a,)3-unsaturated esters...

Subsequently, high chemoselectivity and enantioselectivity have been observed in the asymmetric epoxidation of a variety of conjugated enynes using fructose-derived chiral ketone as the catalyst and Oxone as the oxidant. Reported enantioselectivities range from 89% to 97%, and epoxidation occurs chemoselectively at the olefins. In contrast to certain isolated trisubstituted olefins, high enantioselectivity for trisubstituted enynes is noticeable. This may indicate that the alkyne group is beneficial for these substrates due to both electronic and steric effects. 

Among many other methods for epoxidation of disubstituted E-alkenes, chiral dioxiranes generated in situ from potassium peroxomonosulfate and chiral ketones have appeared to be one of the most efficient. Recently, Wang et /. 2J reported a highly enantioselective epoxidation for disubstituted E-alkenes and trisubstituted alkenes using a d- or L-fructose derived ketone as catalyst and oxone as oxidant (Figure 6.3). 

Cadogan and coworkers160 developed a fructose-derived l,3-oxazin-2-one chiral auxiliary which they applied in the Diels-Alder reactions of its iV-enoyl derivatives 246 with cyclopentadiene using diethylaluminum chloride as the Lewis acid catalyst. The reactions afforded mixtures of endo 247 and exo 248 (equation 68). The catalyst binds to the chiral dienophile in a bidentate fashion (co-ordination to both carbonyl groups). As a consequence, the dienophile is constrained to a rigid conformation which accounts for the almost complete diastereofacial selectivities observed. 

The breakthrough came already in 1996, one year after Curd s prediction, when Yang and coworkers reported the C2-symmetric binaphthalene-derived ketone catalyst 6, with which ee values of up to 87% were achieved. A few months later, Shi and coworkers reported the fructose-derived ketone 7, which is to date still one of the best and most widely employed chiral ketone catalysts for the asymmetric epoxidation of nonactivated alkenes. Routinely, epoxide products with ee values of over 90% may be obtained for trans- and trisubstituted alkenes. Later on, a catalytic version of this oxygen-transfer reaction was developed by increasing the pH value of the buffer. The shortcoming of such fructose-based dioxirane precursors is that they are prone to undergo oxidative decomposition, which curtails their catalytic activity. 

Many other variations of the basic structure 10 have been explored, including an-hydro sugars and carbocyclic analogs, the latter derived from quinic acid 13 [23-26]. In summary, the preparation of these materials (e.g. 14-16) requires more synthetic effort than the fructose-derived ketone 10. Occasionally, e.g. when using 14, catalyst loadings can be reduced to 5% relative to the substrate olefin, and epoxide yields and selectivity remain comparable with those obtained by use of the fructose-derived ketone 10. Alternative ex-chiral pool ketone catalysts were reported by Adam et al. The ketones 17 and 18 are derived from D-mannitol and tartaric acid, respectively [27]. Enantiomeric excesses up to 81% were achieved in the epox-idation of l,2-(E)-disubstituted and trisubstituted olefins. 

The epoxidation procedure described herein utilizes the fructose-derived ketone (1) as catalyst and Oxone" j or H,0, as oxidant. The procedure provides a valuable method for... 

The ability of non-C2 symmetric ketones to promote a highly enantioselective dioxirane-mediated epoxidation was first effectively demonstrated by Shi in 1996 [114]. The fructose-derived ketone 44 was discovered to be particularly effective for the epoxidation of frans-olefins (Scheme 17 ). frans-Stilbene, for instance, was epoxidized in 95% ee using stoichiometric amounts of ketone 44, and even more impressive was the epoxidation of dialkyl-substituted substrates. This method was rendered catalytic (30 mol %) upon the discovery of a dramatic pH effect, whereby higher pH led to improved substrate conversion [115]. Higher pH was proposed to suppress decomposition pathways for ketone 44 while simultaneously increasing the nucleophilicity of Oxone. Shi s ketone system has recently been applied to the AE of enol esters and silyl enol ethers to provide access to enantio-enriched enol ester epoxides and a-hydroxy ketones [116]. Another recent improvement of Shi s fructose-derived epoxidation reaction is the development of inexpensive synthetic routes to access both enantiomers of this very promising ketone catalyst [117]. 

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