Designer Lewis Acids for Selective Organic Synthesis

Hisashi Yamamoto, Keiji Maruoka, Kazuaki Ishihara

School of Engineering, Nagoya University,

Abstract: In view of the ever expanding repertoire of Lewis acids available as proton substitutes in current synthetic organic methodology, our goal was to engineer an artificial proton substitute possessing unique topology; effective as an artificial enzymefor chemical reactions, by harnessing the high reactivity of the metal atom towards oxygen and nitrogen. Such a concept was realized by examining the recognition ability of specially designed metal receptors for various oxygen- and nitrogen-containing substrates.

1. Introduction

An enzyme is typically a large molecule, large enough to support the substrate, whereas chemical reagents are composed of much smaller molecular makeup. Still, the much smaller molecular apparatus of commercial chemical reagents is expected to induce reactions with selectivities comparable to that of a large enzyme. These set of observations therefore beg the question: are enzymatic reactions really an appropriate catalysis system for laboratory chemical reactions? A case in point is the important role of hydrogen bonding during enzymatic reactions. In the course of such processes, the giant template of the enzyme will specify quite accurately the position and direction of a proton for hydrogen bonding, before and after the reaction. However, a proton by itself cannot behave in this fashion. As a perfect sphere, it has no directional selectivity for hydrogen bonding outside the domain of the enzyme, thus it is unable act as a "delicate finger" in an ordinary organic reaction, as it does in the enzymatic transformation. Therefore obliviously one can suspect whether an appropriate substitute for the proton might induce chemical reactions capable of selectivities comparable to those afforded by enzymes. An excellent candidate as a proton substitute is a Lewis acid. The observation that main group organometallic compounds have high reactivity, inspired us to devise a new series of reagents based on these metals, viz.: novel "designer Lewis acids" for organic synthesis. Since an organometallic compound would have multiple ligands around the metal, the structural design of such a catalyst could be quite flexible. Our goal, then, was to engineer an artificial proton of a specific shape, which could be utilized as an effective tool for chemical reactions, by harnessing the high reactivity of the metal atom towards oxygen and nitrogen (ref. 1). Such a concept was initially studied by examining the influence of a specially designed Lewis acids on a typical organic reaction: the nucleophilic addition to a carbonyl substrate. In 1985, we reported enantioselective cyclization of prochiral unsaturated aldehydes using chiral zinc reagent 1 derived from dimethylzinc and enantiomerically pure (R)-binaphthol (ref. 2). Although the reagent 1 is rather unstable, the system is flexible and has important potential for further design in asymmetric metal catalysis.

912 ( 40 ) J. Synth. Org. Chem., Jpn.

From this pioneering work, there is no doubt that the carefully designed chiral Lewis acid may have vast potential for the asymmetric synthesis of carbon skeletons. Choice of an appropriate metal and design of suitable chiral ligands may be the most important factors for effective catalysis. Thus herein is a report on the design of chiral Lewis acids conducted in our laboratory since 1985.

2. Chiral Aluminum Reagent

In view of the well-established capacity of aluminum reagents to enhance the reaction rate and selectivity of various organic reactions (ref. 3), the utilization of a chiral aluminum catalyst should, in principle, lead to asymmetric induction. However, until recently little work has appeared in the area of asymmetric synthesis using chiral aluminum reagents (ref. 4). The first reliable chiral aluminum reagents of types (R)-2 and (S)-2 were devised for enantioselective activation of carbonyl groups based on the concept of diastereoselective activation of carbonyl moieties with the exceptionally bulky organoaluminum reagents, namely MAD and MAT (ref. 5). The sterically hindered, enantiomerically pure (R)-(+)-3,3'-bis(triarylsilyl)binaphthol (R)-3 required for the preparation of (R)-2 can be synthesized in two steps from (R)-(+)-3,3'- dibromobinaphthol by bis-triarylsilylation and subsequent intramolecular 1,3-rearrangement of the triarylsilyl groups as shown in Scheme 1 (ref. 6). Reaction of (R)-3 in toluene with trimethylaluminum

Scheme 1

(R)-2 (S)-2

(R)-3 produced the chiral organoaluminum reagent (R)-2 quantitatively. Its molecular weight, found cryoscopically in benzene, corresponds closely with the value calculated for the monomeric species 2. The modified chiral organoaluminum reagents, (R)-2 and (S)-2 were shown to be highly effective as chiral Lewis acid catalysts in the asymmetric hetero Diels-Alder reaction (ref. 7). Reaction of various aldehydes with activated dienes under the influence of a catalytic amount of 2 (5-10 mol%) at -20 •Ž, after exposure of the resulting hetero Diels-Alder adducts to trifluoroacetic acid, gave predominantly cis- dihydropyrone 5 in high yield with excellent enantioselectivity. The enantioface differentiation of

4 5

Vol.52, No.11 (November 1994) ( 41 ) 913 prochiral aldehydes is controllable by judicious choice of the size of trialkylsilyl moiety in 2, thereby allowing the rational design of the catalyst for asymmetric induction. In fact, switching the triarylsilyl substituent (Ar = Ph or 3,5-Xylyl) to the tert-butyldimethylsilyl or trimethylsilyl group led to a substantial loss of enantio as well as cis selectivity in the hetero Diels-Alder reaction of benzaldehyde and activated diene 4. In marked contrast, the chiral organoaluminum reagent derived from trimethylaluminum and (R)-(+)-3,3'-dialkylbinaphthol (alkyl = H, Me, or Ph) could be utilized, but only as a stoichiometric reagent and results were disappointing both in terms of reactivity and enantioselectivity for this hetero Diels-Alder reaction. An interesting method for the preparation of chiral aluminum reagents has appeared recently. The chiral organoaluminum reagent, (R)-2 or (S)-2 can be generated in situ from the corresponding racemate (±)-2 by diastereoselective complexation with certain chiral ketones (Scheme 2) (ref. 8). Among several terpene-derived chiral ketones, 3-bromocamphor was found to be the most satisfactory. The hetero Diels-Alder reaction of benzaldehyde and 2,4-dimethy1-1-methoxy-3-trimethylsiloxy-1,3- butadiene (4) with 0.1 equiv of (±)-2 (Ar = Ph) and d-bromocamphor at -78 •Ž gave rise to cis-adduct 5 as the major product with 82% ee. Although the level of asymmetric induction attained does not yet match that acquired with the enantiomerically pure 2 (Ar = Ph, 95% ee), one recrystallization of the cis- adduct 5 of 82% ee from hexane gave essentially enantiomerically pure 5, thereby enhancing the practicality of this method. This study demonstrates the potential for broad application of the in situ generated chiral catalyst via diastereoselective complexation in asymmetric synthesis.

Scheme 2

(S)-2

(R)-2 Since the enantioselective activation of carbonyl with the chiral aluminum, (R)-2 or (S)-2, had been demonstrated, the asymmetric ene reaction of electron-deficient aldehydes with various alkenes, by employing the latter reagent, could also be considered a feasible transformation (ref. 9). Indeed in the presence of powdered 4A molecular sieves, the chiral aluminum reagent, (R)-2 or (S)-2 can be utilized as a catalyst without any loss of enantioselectivity.

The concept of the enantioselective activation of carbonyl groups with the bulky, chiral aluminums, (R)-2 or (S)-2 has been further extended to the enantioselective activation of an ether

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oxygen, which gave rise to the first successful example of the asymmetric Claisen rearrangement of allylic vinyl ethers 6 catalyzed by (R)-2 or (S)-2 (Scheme 3) (ref. 10). This method provides a facile asymmetric synthesis of various acylsilanes 7 or 8 (X = SiR3) and acylgermane 7 (X = GeMe3) with high enantiomeric purity (Table 1). Among the various trialkylsilyl substituents of 2, use of a bulkier t- butyldiphenylsilyl group gives rise to the highest enantioselectivity. Conformational analysis of two possible chairlike transition-state structures of an ally! vinyl ether substrate 6 reveals that the chiral organoaluminum reagent 2 can discriminate between these two conformers, A and B, which differ each other only in the orientation of a-methylene groups of ethers.

Scheme 3

A 7

B 8

Table 1. Asymmetric Claisen Rearrangement of Allylic Vinyl Ethers 6

Notably, the asymmetric Claisen rearrangement of cis-allylic a-(trimethylsilyl)vinyl ethers with (R)-2 produced optically active acylsilanes with the same absolute configuration as those produced from trans-allylic a-(trimethylsilyl)vinyl ethers (ref. 11).

Vol.52, No.11 (November 1994) ( 43 ) 915 3. Chiral Titanium Reagent

A new type of chiral helical titanium catalyst of type 9 has been rationally designed with the expectation that a high level of asymmetric induction can be achieved by way of an efficient enantioface recognition of prochiral substrates using a fixed helical conformation of a chiral ligand. Such an idea originates from a characteristic helix conformation found in various naturally occurring substances, such as the secondary structures of DNA, polypeptides (proteins and collagens), and starch.

(P)-9 (M)-9

The requisite chiral helical ligands (R)-10a•`e were conveniently prepared from an enantiomerically pure (R)-binaphthol derivative (R)-12 as outlined in Scheme 4, and transformed to chiral helical titanium catalysts (P)-11a•`e by treatment with Ti(OPri)4 with azeotropic removal of

Scheme 4

a-c d a

(R)-12

isopropanol. These catalysts were found to be highly effective for the asymmetric Diels-Alder reaction. Indeed, asymmetric Diels-Alder reaction of cyclopentadiene and acrolein was effected under the influence of catalytic titanium reagent (P)-11d (10 mol%), producing the major endo adduct in 96% ee with the S.configuration. In addition, the minor exo isomer was obtained in 93% ee with the S configuration. Using other 0-unsaturated aldehydes and dienes in the presence of 10 mol% titanium catalyst (P)-11d, the Diels-Alder adducts were produced with a uniformly high level of enantioselectivity (81•`98% ee) (ref. 12). Such an advantage was not observable for the previously known, effective asymmetric Diels-Alder catalysts. Another interesting feature is that the enantioselectivity in the present Diels-Alder reaction is not greatly influenced by the reaction

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temperature. For example, the endo-adduct 13 (R = H) was obtained in 96% ee at -97•`-78 •ŽC, 92% ee at -20 •Ž, and 88% ee at 0 •Ž, respectively, with the catalyst (P)-11d (10 mol%). Such temperature independence is rather in contrast to most of the known metal-catalyzed asymmetric reactions.

The present titanium reagent (P)-11 appears to play an important role as a reliable chiral template for the conformational fixation of a,f3-unsaturated aldehydes, thereby allowing efficient enantioface recognition of the substrates for achievement of reproducibly high asymmetric induction, regardless of reaction temperature.

4. Chiral Boron Reagent

4.1. Chiral (Acyloxy)borane (CAB) Reagent I 4.1.1. Enantioselective Diels-Alder Reaction We have come to recognize that (acyloxy)boranes (RCO2BR2) behave as a Lewis acid which can be prepared by controlled addition of diborane to a carboxylic acid. The chiral (acyloxy)borane (CAB) complex 14 in situ formed from monoacyl tartaric acid and diborane is an excellent asymmetric catalyst (10 mol%) for the Diels-Alder reaction of cyclopentadiene and acrylic acid (78% ee) (ref. 13) or of cyclopentadiene and methacrolein (96% ee) (ref. 14). The process is quite general for simple diene and aldehydes. For example, acrolein and cyclopentadiene, cyclohexadiene or 2,3-dimethyl-1,3-butadiene gave cycloadducts with 80-84% ee and exo/endo selectivity being 12/88•`•ƒ1/99. The a-substituent on the dienophile induced an increase in the enantioselectivity (acrolein vs. methacrolein). The active boron catalyst is believed to have the structure shown below, with a five-membered ring and a free carboxylic group. The latter seems not to be crucial for the enantioselectivity since comparable results are obtained when the carboxylic group is transformed into an ester.

Further application of the CAB was extended to the asymmetric Diels-Alder reaction of a- bromo-a,13-enalswith dienes (ref. 15). a-Bromo-a,13-enal is a useful dienophile in the Diels-Alder

Vol.52, No.11 (November 1994) ( 45 ) 917 process because of the exceptional synthetic versatility of its resulting adducts: in particular, the cycloadduct is an important intermediate for prostaglandin synthesis (ref. 24a). High enantioselectivity and exo selectivity were obtained for Diels-Alder additions of a-bromoacrolein with cyclopentadiene in the presence of 10 mol% of 14. Our catalyst 14 was also applied to the asymmetric intramolecular Diels-Alder reaction of 2- methyl-(E,E)-2,7,9-decatrienal (ref. 16). 4.1.2. Enantioselective Hetero DieIs-Alder Reaction We recenly developed a new CAB 15 complex prepared in situ by mixing tartaric acid derivative and arylboronic acid at room temperature. In contrast to 14 which is both air and moisture sensitive, the B-alkylated catalyst 15 is self stable. A solution of this catalyst (20 mol%) is effective in catalyzing a hetero Diels-Alder reaction of aldehydes with Danishefsky diene to produce dihydropyrone derivatives of high optical purity (up to 98% ee) (ref. 17). The extent of asymmetric induction is largely dependent on the structure of boronic acid. In general, the bulky phenylboronic acid (Ar=2,4,6- Me3C6H2, o-114e0C6H4) resulted in excellent asymmetric induction.

4.1.3. Enantioselective Aldol Reaction The CAB 14 was shown to be an excellent catalyst (20 mol%) for the Mukaiyama reaction of simple enol silyl ethers of achiral ketones with various aldehydes. Furthermore, the reactivity of aldol- type reactions could be improved without reducing the enantioselectivity by using 10-20 mol% of 15 (Ar=3,5-(CF3)2C6H3). Further improvements in enantioselectivity could be attained without any appreciative drop in chemical yield by using 20 mol% of 15 (Ar=o-PhOC6H4) prepared from o- phenoxyphenylboronic acid and a chiral tartaric acid derivative. The CAB-catalyzed aldol process allows the formation of adducts in a highly diastereo- and enantioselective manner (up to 99% ee) under mild reaction conditions (ref. 18a,18c).

The relative stereochemistry of the major adducts was assigned as erythro, and predominant re- face attack of enol ethers at the aldehyde carbonyl carbon was confirmed in cases where a natural tartaric acid derivative was used as a Lewis acid ligand. It is noteworthy that, in all cases shown above,

918 ( 46 ) J. Synth. Org. Chem., Jpn.

regardless of the stereochemistryof the starting enol silyl ether generated from ethyl ketone, gave in high stereoselectivitythe erythro aldols. The observedunprecedently high erythro selectivitiestogether with their lack of dependenceon the stereoselectivityof silyl enol ethers in the CAB-catalyzedreactions are fully consistent with Noyori's TMSOTf-catalyzedaldol reactions of acetals (ref. 19), and thus may reflect the acyclicextended transition state mechanismpostulated in the latter reactions. In similar work a catalytic asymmetric aldol type reaction of ketene silyl acetals with achiral aldehydes also proceeded smoothly with 14 which could furnish erythro P-hydroxy esters in high optical purities (ref. 18b, 18c). The use of silyl ketene acetals generated from phenyl esters led to good diastereo- and enantioselectivitieswith excellent chemicalyields. The observederythro selectivitiesand re-face attack of nucleophiles on carbonyl carbon of aldehydes are consistent with the aforementionedaldol reactions of ketone enol silyl ethers. 4.1.4. Enantioselective Allylation Reaction In 1991 we reported on the novel application of 15 in a catalytic enantioselective Sakurai- Hosomi allylation protocol (ref. 20a). y-Alkylated allylsilanes exhibited excellent diastereo- and enantioselectivities affording erythro homoallylic alcohols of higher optical purity. The observed preference for relative and absolute configurations of the adduct alcohols from the (2R,3R)-ligb orane reagent was predicted on the basis of an extended transition state model similar to that of the CAB-catalyzedaldol reaction (ref. 18). Next, several arylboronic acids were examined in place of borane-THF in order to improve the Lewis acidity of 15 and the stereoselectivity(ref. 20b). The boron substituentof 15 was found to have both a strong influenceon the chemicalyield and enantiomericexcess of the allylationadduct, and 3,5- bistrifluoromethylbenzeneboronicacid was found to be most effective for enhancingreactivity: when the complex which was easily prepared from tartaric acid derivative and 3,5- bistrifluoromethylbenzeneboronicacid in propionitrile at room temperature was employed, reactivity was improvedwithout reducingenantioselectivity.

4.1.5. The Mechanism of CAB-Catalyzed Enantioselective Diels-Alder Reaction The boron-substituent-dependentenantioselectivity in the CAB-catalyzedDiels-Alder reaction was studied as a first step towards obtaining mechanistic information on the sp2—sp2conformational preferencesin a,13-enals where the possibility of s-cis or s-trans conformersexists in the transition-state assembly of a Diels-Alderreaction catalyzedby Lewis acid. It is noteworthythat the a-substituted a,I3- enal (methacrolein) favored s-trans conmformer in the transition-state assembly was found to be independent of the steric feature of the boron-substituent. On the other hand, the sp2—sp2 conformationalpreference of a-nonsubstituted a43-enals(acrolein and crotonaldehyde)were reversed by altering the structure of the boron-substituent; in other words s-trans conformation was preferred when the boron substituent was small (H, C=CBu), while s-cis conformation was preferred when it was of a bUlky nature (o-PhOC6H4) (ref. 21).

Vol.52, No.11 (November 1994) ( 47 ) 919

We have also resorted to NOE measurements in order to elucidate the conformations of the CAB-complexedmethacrolein and crotonaldehyde. These results are in agreementwith the transition- state preferencefor the s-trans or s-cis conformationof a4-enal, which is assumed to preside over both the enantioselectivitiesof the aldol and Diels-Alder reactions catalyzed by CAB. Finally, it has been established that the effective shielding of the si-face of the CAB-coordinateda,13-enal arises from it- stacking of the 2,6-diisopropoxybenzenering and the coordinated aldehyde (ref. 21).

methacrolein-14 crotonaldehyde-14

4.2. Chiral (Acyloxy)borane Reagent II A characteristic feature of the aforementioned CAB catalyzed system is an a-hydroxy carboxylic acid species as a chiral ligand of the boron reagent. Thus, the five-membered ring system seems to us to be the major structural feature for the active catalyst. Our laboratory and others have reported an efficient but simple chiral boron catalyst for the Diels-Alder reaction (ref. 22). This new catalyst can be prepared from the readily available sulfonamide of amino acids. In practice the starting sulfonamide can be simply be prepared by exposing the appropriate amino acid to sulfonyl chloride in the presence of sodium hydroxide to afford characteristically a white crystalline product. The obtained sulfoamide was treated with an equimolar amount of borane-THF complex. The catalyst 16 thus obtained having a structure as shown below was then used in the Diels- Alder reaction. Although the enantiomeric excess of the products from these reactions is not particularly high, the new catalyst reveals a broader range of applicability; since D-aminobutyric acid is commercially available, the other enantiomer can also be easily prepared. Further improvements in optical yields of the reaction have been noted in accordance with increases in steric bulk of the benzenesulfonyl group. More recently, the similar catalysts were utilized in both aldol (ref. 23) and Diels-Alder reactions (ref. 24) by other groups.

4.3. BrƒÓnsted Acid Assisted Chiral Lewis acid (Chiral BLA) The utility of the chiral boron ate complexes (BLA) as a new catalyst in enantioselective synthesis has encouraged us to seek new members of this class which achieve selectivity through a double effect of intramolecular hydrogen bonding and attractive ƒÎ-ƒÎ donor-acceptor interactions in the transition-state in the vicinity of the hydroxy aromatic group.

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Reaction of (R)-3,3'-bis(2-hydroxyphenyl)-2,2'-dihydroxy-1,1'-binaphthyl with B(OMe)3 in dichloromethane at reflux with removal of methanol gave a white precipitate (R)-17. Much to our delight extremely high enantioselectivities (•„99 to 92% ee) and exo selectivities (•„99 to 97% exo) were obtained for Diels-Alder additions of a-substituted ƒ¿,ƒÀ-enals with dienes in the presence of the catalyst

(R)-17 (ref. 25). The absolute stereopreference in the Diels-Alder reaction can be easily understood in terms of the most favorable transition-state assembly A, in which an attractive donor-acceptor interaction favors coordination of the dienophile at the face of boron which is cis to the 2-hydroxyphenyl substituent. It is during this stage of the reaction that the conformation of the a,f3-enal is of a high s-trans preference. We believe that the coordination of a proton of 2-hydroxyphenyl group with an oxygen of the adjacent B-O bond in complex A plays an important role in asymmetric induction; this hydrogen binding interaction of a BrƒÓnsted acid type would cause Lewis acidity of boron and ƒÎ-basicity of the phenoxy moiety to increase, and thus the transition-state assembly A would be stabilized. Subsequently, the ƒÎ- basic phenoxy moiety and the it-acidic dienophile can assume a parallel orientation at the ideal separation

(3A) for donor-acceptor interaction. In this conformation, the hydroxyphenyl group blocks the si-face of the dienophile, leaving the re-face open to approach by a diene.

Non-Helical Transition-State (A)

References and Notes

(1) (a) Ojima, I. Catalytic Asymmetric Synthesis; VCH Publishers: New York, 1993. (b)

(c) Maruoka, K.; Yamamoto, H. J. Synth. Org. Chem., Jpn. 1993, 51, 1074. (2) (a) Sakane, S.; Maruoka, K.; Yamamoto, H. Tetrahedron Lett. 1985, 26, 5535. (b) Sakane; S.; Maruoka, K.; Yamamoto, H. Tetrahedron 1986, 42, 2203. (3) Reviews: (a) Mole, T.; Jeffery, E. A. Organoaluminum Compounds; Elsevier: Amsterdam, 1972. (b) Bruno, G. The Use of Aluminum Alkyls in Organic Synthesis; Ethyl Corporation: Baton Rouge, USA, 1970, 1973, and 1980. (c) Maruoka, K.; Yamamoto, H. Angew. Chem. Int. Ed. Engl. 1985, 24, 668. (4) (a) Hashimoto, S.-I.; Komeshima, N.; Koga, K. J. Chem. Soc., Chem. Commun. 1979, 437. (b) Takemura, H.; Komeshima, N.; Takahashi , I.; Hashimoto, S.-I.; Ikota, N.; Tomioka, K.; Koga, K. Tetrahedron Lett. 1987, 28, 5687. (5) (a) Maruoka, K.; Itoh, T.; Yamamoto, H. J. Am. Chem. Soc. 1985, 107, 4573. (b) Maruoka, K.; Itoh, T.; Sakurai, M.; Nonoshita, K.; Yamamoto, H. ibid. 1988, 110, 3588.

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(6) Maruoka, K.; Itoh, T.; Araki, Y.; Shirasaka, T.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1988, 61, 2975. (7) Maruoka, K.; Itoh, T.; Shirasaka, T.; Yamamoto, H. J. Am. Chem. Soc. 1988, 110, 310. (8) Maruoka, K.; Yamamoto, H. J. Am. Chem. Soc. 1989, Ill, 789. (9) Maruoka, K.; Hoshino, Y.; Shirasaka, T.; Yamamoto, H. Tetrahedron Lett. 1988, 29, 3967. (10) (a)Maruoka, K.; Banno, H.; Yamamoto, H. J. Am. Chem. Soc. 1990, 112, 7791. (b) Maruoka, K.; Banno, H; Yamamoto, H. Tetrahedron: Asymmetry 1991, 2, 647. (11) Maruoka, K.; Yamamoto, H. Synlett 1991, 793. (12) Maruoka, K.; Murase, N.; Yamamoto, H. J. Org. Chem. 1993, 58, 2938. (13) Furuta, K.; Miwa, Y.; Iwanaga, K.; Yamamoto, H. J. Am. Chem. Soc. 1988, 110, 6254. (14) (a) Furuta, K.; Shimizu, S.; Miwa, Y.; Yamamoto, H. J. Org. Chem. 1989, 54, 1481. (b) Gao, Q.; Yamamoto, H. Org. Synth. 1993, 72, 86. (15) Ishihara, K.; Gao, Q.; Yamamoto, H. Org. Chem. 1993, 58, 6917. (16) Furuta, K.; Kanematsu, A.; Yamamoto, H.; Takaoka, S. Tetrahedron Lett. 1989, 30, 7231. (17) Gao, Q.; Maruyama, T.; Mouri, M.; Yamamoto, H. J. Org. Chem. 1992, 57, 1951. (b) Gao, Q.; Ishihara, K.; Maruyama, T.; Mouri, M.; Yamamoto H. Tetrahedron 1994, 50, 979. (18) (a) Furuta, K.; Maruyama, T.; Yamamoto, H. J. Am. Chem. Soc. 1991, 113, 1041. (b) Furuta, K.; Maruyama, T.; Yamamoto, H. Synlett 1991, 439. (c) Ishihara, K.; Maruyama, T.; Mouri, M.; Gao, Q.; Furuta, K.; Yamamoto, H. Bull. Chem. Soc. Jpn. 1993, 66, 3483. (19) Noyori, R.; Murata, S.; Suzuki, M. Tetrahedron 1981, 37, 3899. (20) (a) Furuta, K.; Mouri, M.; Yamamoto, H. Synlett 1991, 561. (b) Ishihara, K.; Mouri, M.; Gao, Q.; Maruyama, T.; Furuta, K.; Yamamoto, H. J. Am. Chem. Soc. 1993, 115, 11490. (21) Ishihara, K.; Gao, Q.; Yamamoto, H. J. Am. Chem. Soc. 1993, 115, 10412. (22) (a) Takasu, M.; Yamamoto, H. Synlett 1990, 194. (b) Sartor, D.; Saffrich, J.; Helmchen, G. Synlett 1990, 197. (c) Sartor, D.; Saffrich, J.; Helmchen, G.; Richards, C. J.; Lambert, H. Tetrahedron: Asymmetry 1991, 2, 639. (23) (a) Kiyooka, S.; Kaneko, Y.; Komura, M.; Matsuo, H.; Nakano, M. J. Org. Chem. 1991, 56, 2276. (b) Parmee, E. R.; Tempkin, O.; Masamune, S. J. Am. Chem. Soc. 1991, 113, 9365. (c) Parmee, E. R.; Hong, Y.; Tempkin, O.; Masamune, S. Tetrahedron Lett. 1992, 33, 1729. (d) Kiyooka, S.; Kaneko, Y.; Kume, K. Tetrahedron Lett. 1992, 33, 4927. (e) Corey, E. J.; Cywin, C. L.; Roper, T. D. Tetrahedron Lett. 1992, 33, 6907. (24) (a) Corey, E. J.; Loh, T.-P. J. Am. Chem. Soc. 1991, 113, 7794. (b) Corey, E. J.; Loh, T.-P.; Roper, T. D. Azimioara, M. D.; Noe, M. C. J. Am. Chem. Soc. 1992, 114, 8290. (c) Corey, E. J.; Loh, T.-P. Tetrahedron Lett. 1993, 34, 3979. (25) Ishihara, K.; Yamamoto, H. J. Am. Chem. Soc. 1994, 116, 1561. (ReceivedJune 23, 1994)

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