Lewis acids menthol-aluminum

Better enantiomeric excess was obtained by using (-)-menthol-aluminum Lewis acids supported on silica gel and alumina through the aluminum atom [17]. 

The first report of an asymmetric Diels-Alder reaction with chiral Lewis acids (252) was made by Russian chemists in 1976 (253). Koga was probably the first to report a meaningful enantioselective Diels-Alder reaction (Scheme 105) in which the cyclopentadiene-methacro-lein exo adduct was obtained in 72% ee with the aid of 15 mol % of a menthol-modified aluminum chloride (254). The ee is highly dependent on the structures of the substrates, and asymmetric induction has not been observed with methyl acrylate as dienophile. Disproportionation... 

The earliest report of a reaction mediated by a chiral three coordinate aluminum species describes an asymmetric Meerwein-Poimdorf-Verley reduction of ketones with chiral aluminum alkoxides which resulted in low induction in the alcohol products [1]. Subsequent developments in the area were sparse until over a decade later when chiral aluminum Lewis acids began to be explored in polymerization reactions, with the first report describing the polymerization of benzofuran with catalysts prepared from and ethylaluminum dichloride and a variety of chiral compounds including /5-phenylalanine [2]. Curiously, these reports did not precipitate further studies at the time because the next development in the field did not occur until nearly two decades later when Hashimoto, Komeshima and Koga reported that a catalyst derived from ethylaluminum dichloride and menthol catalyzed the asymmetric Diels-Alder reaction shown in Sch. 1 [3,4]. This is especially curious because the discovery that a Diels-Alder reaction could be accelerated by aluminum chloride was known at the time the polymerization work appeared [5], Perhaps it was because of this long delay, that the report of this asymmetric catalytic Diels-Alder reaction was to become the inspiration for the dramatic increase in activity in this field that we have witnessed in the twenty years since its appearance. It is the intent of this review to present the development of the field of asymmetric catalytic synthesis with chiral aluminum Lewis acids that includes those reports that have appeared in the literature up to the end of 1998. This review will not cover polymerization reactions or supported reactions. The latter will appear in a separate chapter in this handbook. 

A Lewis acid prepared from (-)-menthol and ethyl aluminum dichloride has also been examined as a catalyst for the ene reaction [62], As would be expected, this catalyst is apparently faster than the dialkoxy methyl aluminum catalyst 98 and results in 75 % yield of the homoallylic alcohol 305 with 10 mol % catalyst at -78 °C. It is sur-... 

The first successful examples of enantioselective Diels-Alder reactions catalyzed by chirally modified Lewis acids were reported by Koga [85]. The catalysts were prepared from menthol and AlEt2Cl [86]. Alumina-supported chiral menthoxy aluminum derivatives (64, 65, 66, 67) have been prepared by simple mixing of (-)-menthol, AlEt2Cl, and alumina in toluene under reflux. The reaction of methacrolein with cyclopentadiene (Eq. 20) was conducted with 67 as catalyst at -50 °C and afforded 81 % conversion with 31 % ee [87] Koga reported 57 % ee at -78 °C by use of an homogeneous catalyst [85]. Solid catalyst 69, prepared from silica gel-supported proli-nol 68 and AlEt2Cl (Eq. 21) is also an active catalyst in the same reaction, but with low enantioselectivity [87]. When the same catalyst was attached to crosslinked polystyrene (70) the ee in the reaction was lower [88]. 

Few examples have been reported demonstrating enantioselective cyclization methodology. One known example, however, is similar to the diastereoselective cyclization of 175, which uses a menthol-derived chiral auxiliary and a bulky aluminum Lewis acid (see Eq. (13.55)). The enantioselective variant simply utilizes an achiral template 188 in conjunction with a bulky chiral binol-derived aluminum Lewis acid 189 (Eq. (13.59)) [75]. Once again the steric bulk of the chiral aluminum Lewis acid complex favors the s-trans rotamer of the acceptor olefin. Facial selectivity of the radical addition can then be controlled by the chiral Lewis acid. The highest selectivity (48% ee) was achieved with 4 equivalents of chiral Lewis acid, providing a yield of 63%. 

Few examples have been reported demonstrating enantioselective cyclization methodology. One known example, however, is similar to the diastereoselective cyclization which uses a menthol-derived chiral auxiliary and a bulky aluminum Lewis acid to impart selective cyclization [30[. The enantioselective variant simply... 

Menthol [(—)-l] has been used as a chiral ligand for aluminum in Lewis acid catalyzed Diels-Alder reactions with surprising success2 (Section D.l.6.1.1.1.2.2.1). The major part of its application is as a chiral auxiliary, by the formation of esters or ethers. Esters with carboxylic acids may be formed by any convenient esterification technique. Esters with saturated carboxylic acids have been used for the formation of enolates by deprotonation and subsequent addition or alkylation reactions (Sections D.l.1.1.3.1. and D.l.5.2.3.), and with unsaturated acids as chiral dienes or dienophiles in Diels-Alder reactions (Section D. 1.6.1.1.1.), as chiral dipolarophiles in 1,3-dipolar cycloadditions (Section D.l.6.1.2.1.), as chiral partners in /(-lactam formation by [2 + 2] cycloaddition with chlorosulfonyl isocyanate (SectionD.l.6.1.3.), as sources for chiral alkenes in cyclopropanations (Section D.l.6.1.5.). and in the synthesis of chiral allenes (Section B.I.). Several esters have also been prepared by indirect techniques, e.g.,... 

One of the most common auxiliaries is menthyl, where an acrylic acid derivative is attached to menthol to form a menthyl ester, 255. Morrison and Mosher22l showed that asymmetric induction is possible with 255 when it reacts with cyclopentadiene to give diastereomers 256 and 257, 22 as shown in Table 11.13.221 in the absence of a Lewis acid, however, the % ee is rather poor. Similarly, (-)-dimenthyl fumarate (258) reacted with butadiene to give 259 and 260, after reduction of the ester products with lithium aluminum hydride. Hydride reduction of esters (sec. 4.2.B) is a common method for removing ester auxiliaries. The work of... 

The first example of appreciable enantioselectivity in a catalytic asymmetric Diels-Alder reaction was reported in 1979 by Koga (Equation 14) [73]. This was achieved by use of 14 mol % of an aluminum complex resulting from reaction of menthol and EtAlCl2, as formulated in the proposed catalyst structure 133. In the presence of this catalyst, cycloadduct 134 was obtained in 57 % ee and 69 % yield. This result provided an important proof of principle that catalytic enantioselective Diels-Alder reactions can indeed be achieved with chiral Lewis acids [73, 74). 

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