Enolates, aluminum chiral

Compared to the lithium enolates of l and 5, the higher stereoselectivity obtained by the Mukaiyama variation is, in general, accompanied by reduced chemical yields. The chiral alcoholic moieties of the esters 3 and 7 can be removed either by reduction with lithium aluminum hydride (after protection of the earbinol group) or by aqueous alkaline hydrolysis with lithium hydroxide to afford the corresponding carboxylic acid. In both cases, the chiral auxiliary reagent can be recovered. 

The diastereomeric a-alkoxy complexes (1 )-15 and (S)-15, separable by chromatography, were each converted to the corresponding aluminum enolates and reacted with 2-methylpropanal (17)49. Enolate (/ )-16 selectively provided a mixture of two diastereomers with the (Fe/ ,2, 3 i )-complex (/ )-18 identified as the major constituent of a 94 6 mixture. The two chiral auxiliaries of complex (S j-lS exerted antagonistic effects and an undefined mixture of all four possible diastereomers was obtained. 

This finding is also in agreement with another three-component Michael/aldol addition reaction reported by Shibasaki and coworkers [14]. Here, as a catalyst the chiral AlLibis[(S)-binaphthoxide] complex (ALB) (2-37) was used. Such hetero-bimetallic compounds show both Bronsted basicity and Lewis acidity, and can catalyze aldol [15] and Michael/aldol [14, 16] processes. Reaction of cyclopentenone 2-29b, aldehyde 2-35, and dibenzyl methylmalonate (2-36) at r.t. in the presence of 5 mol% of 2-37 led to 3-hydroxy ketones 2-38 as a mixture of diastereomers in 84% yield. Transformation of 2-38 by a mesylation/elimination sequence afforded 2-39 with 92 % ee recrystallization gave enantiopure 2-39, which was used in the synthesis of ll-deoxy-PGFla (2-40) (Scheme 2.8). The transition states 2-41 and 2-42 illustrate the stereochemical result (Scheme 2.9). The coordination of the enone to the aluminum not only results in its activation, but also fixes its position for the Michael addition, as demonstrated in TS-2-41. It is of importance that the following aldol reaction of 2-42 is faster than a protonation of the enolate moiety. 

The methyl group was introduced by a two-step procedure. Thus, the hydrazone Michael adducts 52 were converted into the enol pivaloates 53 in excellent yields and diastereomeric excesses de > 96%) by treatment with pivaloyl chloride and triethylamine. After treatment with lithium dimethylcuprate the chiral auxiliary was removed by addition of 6n HCl in order to obtain the 5-substituted 2-methylcyclopentene carboxylate 54 in good yields and with excellent stereoselectivity (de, ee > 96%). Finally, the asymmetric synthesis of dehydroiridodiol (55, R = Me, = H) and its analogues was accomplished by reduction of 54 with lithium aluminum hydride or L-selectride leading to the desired products in excellent yields, diastereo- and enantiomeric excesses (de, ee > 96%). 

Reviews on stoichiometric asymmetric syntheses M. M. Midland, Reductions with Chiral Boron Reagents, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 2, Academic Press, New York, 1983 E. R. Grandbois, S. I. Howard, and J. D. Morrison, Reductions with Chiral Modifications of Lithium Aluminum Hydride, in J. D. Morrison, ed.. Asymmetric Synthesis, Vol. 2, Chap. 3, Academic Press, New York, 1983 Y. Inouye, J. Oda, and N. Baba, Reductions with Chiral Dihydropyridine Reagents, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 4, Academic Press, New York, 1983 T. Oishi and T. Nakata, Acc. Chem. Res., 17, 338 (1984) G. Solladie, Addition of Chiral Nucleophiles to Aldehydes and Ketones, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 2, Chap. 6, Academic Press, New York, 1983 D. A. Evans, Stereoselective Alkylation Reactions of Chiral Metal Enolates, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 1, Academic Press, New York, 1984. C. H. Heathcock, The Aldol Addition Reaction, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 2, Academic Press, New York, 1984 K. A. Lutomski and A. I. Meyers, Asymmetric Synthesis via Chiral Oxazolines, in J. D. Morrison, ed., Asymmetric Synthesis, Vol. 3, Chap. 

Next, the mechanism of the Type II reactions is discussed. To discriminate one of the enantiofaces of the acceptor it is desirable to place and to activate the electrophiles in a chiral environment. At the same time, effective activation of the Michael donor is required. In Shibasaki s ALB-catalyzed reaction (Scheme 3), it was proposed that the aluminum cation functioned as a Lewis acid to activate enones at the center of the catalyst, and that the Li-naphthoxide moiety deproton-ated the a-hydrogen of malonate to form the Li enolate (Scheme 9). Such simultaneous activation of both reactants at precisely defined positions became feasible by using multifunctional heterobimetallic complexes the mechanism is reminiscent of that which is operative in the active sites of enzymes. The observed absolute stereochemistry can be understood in terms of the proposed transition state model 19. Importantly, addition of a catalytic amount of KOt-Bu (0.9equiv. to ALB) was effective in acceleration of the reaction rate with no deterioration of the... 

Examples of aldol condensations involving chiral aldehydes have also been reported. Condensation of the aluminum enolate derived from (2) with Boc-L-prolinal has been shown to proceed in a highly stereoselective manner and the iron chirality overpowered the latent stereoselectivity inherent in Boc-L-prolinal. ... 

Aldol-Type Addition. Aldol-type addition of the magnesium enolate of (R)-(+)-7-butyl 2-(p-tolylsulfinyl)acetate, prepared with 7-butylmagnesium bromide, with aldehydes and ketones afforded, after desulfurization with Aluminum Amalgam, p-hydroxy esters in very high diastereoselectivity (eq Two chiral centers are created in the first step with very high diastereoselectivity (mainly one diastereomer is formed). A model M based on the structure of the sulfinyl ester enolate (determined by C NMR) and on electrophilic assistance of magnesium to the carbonyl approach, was proposed to explain and predict the absolute configuration of the two created chiral centers. ... 

An approach to lactone [12] similar in concept to that just described, but not requiring a resolution, involved asymmetric Diels-Alder reaction of (benzyloxymethyl)cyclopentadiene [21] with the chiral ester of acrylic add and 8-phenylmenthoI (22), The adduct [22] was obtained in undetermined but apparently quite high e.e. Oxidation of the ester enolate of [22], followed by lithium aluminum hydride reduction, gave diol [23] as an... 

Substituted 2-oxazolidones 165 are useful chiral auxiliaries for diastereoselective functionalization at the a-carbon of their amide carbonyl group. The a-fluoroaldehydes 166 were prepared by a series of reactions electrophilic fluorination of the corresponding oxazolidinone sodium enolates with AMluorobenzenesulfonimine reductive removal of the auxiliary with LiBH4 and Dess-Martin oxidation. The aldehydes are so unstable for isolation that they are converted with (R)-/ -toluenesulfinamide to /7-toluenesul(inimines 167, which are isol-able and satisfactorily enantio-enriched. Chiral sulfinimine-mediated diastereoselective Strecker cyanation with aluminum cyanide provided cyanides 168 in excellent diastereose-lectivity, which were finally derived to 3-fluoroamino acids 169 (see Scheme 9.37) [63]. 

Addition of diethyl aluminum chloride at — 78 °C to a,/ -unsaturated oxazolidinone (154) affords an aluminum enolate that, on hydroxylation with (63a), gives the / -ethyl-a-hydroxy amide (155) with high anti selectivity (Equation (38)) <91AG(E)694>. Formation of the enolate of oxazoline thiol ester (156) under chelation (NaHMDS) and stereoelectronic (NaHMDS/HMPA) control gives the syn and anti alcohols (157), respectively, on hydroxylation with (63a) in good to excellent yield and better than 95% diastereoselectivity (Scheme 28) <93JOC6180>. A counterion dependent reversal in stereochemistry has also been reported for the hydroxylation of chiral amide enolates where the auxiliary was 2-pyrrolidinemethanol <85TL3539>. 

In sharp contrast to the utility of chiral boron Lewis acids, chiral aluminum Lewis acids have been little used for asymmetric aldol reactions of silyl enolates since the first example reported by Reetz et al. [115]. Fujisawa et al. have reported that an equimolar amount of a chiral Lewis acid prepared from Et2AlCl and a bor-nane-2,3-diol promotes the aldol reaction of 48 in moderate yields with good enantioselectivity [127]. 

The only main Group III metal, other than boron, that has been utilized in the aldol reaction is aluminum, the enolates of which behave rather capriciously in terms of stereochemistry. The A1—C bond is relatively weak. However, aldol reactions with aluminum enolates derived from chiral acyl-iron complexes proceed with high asymmetric induction. 

Davies and Liebeskind independently prepared chiral aluminum enolates from enantiomerically homogeneous acyl-iron complexes (137) and recorded the first aluminum-mediated asymmetric aldol reactions. Although the lithium enolate of the chiral iron complex (CHIRAC) provides aldol products with... 

Chiral enolates in which the auxiliary is in the ester portion provide still another route to optically active lactams. Early results indicated that little asymmetric induction was obtained with menthyl enolates. Use of the enolate obtained from 24 did lead to high levels of asymmetric induction. Treatment of 24 with lithium diisopropylamide in tetrahydrofuran, followed by addition of imine 25, gives cf -/(-lactam 26 in 79% yield and 91%ee98. Optically active /3-lactams can be prepared by addition of chiral iron enolates (see Section D.l. 1.1.3.2.) to imines99-101. Addition of aluminum enolate 27 to imine 28, followed by oxidative cyclization with iodine and an amine, affords /(-lactam 29 in 54% yield and >95% ee. 

Stereoselective epoxidation of enoates. The final step in the synthesis of (+)-aphidicolin (4) requires a stereoselective conversion of the cyclic norketone (I) to a Wol,2-diol, >C(0H)-CH20H. Methylcnation of the ketone followed by a Sharpless asymmetric dihydroxylation provides a 1 1 mixture of epimcric 1,2-diols. Reaction with a chiral oxaziridinc also provides a 1 1 mixture of cpimcric epoxides. The transformation is effected successfully by conversion of the ketone to the enol triflate, which is converted to the enoate (2) by Pd-catalyzed carbonylation in methanol (13,234). Epoxidation of 2 with m-CPBA in buffered CH2CI2 with a radical scavenger (4,85-86) results in a single epoxy ester (3) in 90% yield. This product is reduced with lithium aluminum hydride (excess) to aphidicolin (4) in 67% overall yield from the ketone 1. 

Hence, monoprotection of 1,4-butanediol with Nall and trapping with allyl bromide led to 4-allyloxybutan-l-ol (Scheme 21). The crude product was treated with Jones reagent to furnish 4-allyloxybutyric acid (73). Treatment with oxalyl chloride in hexane at room temperature gave the corresponding acid chloride, which reacted with A-oxazoiidinone enolate to give a substrate for asymmetric alkylation upon treatment with NaHMDS and Mel. Reduction of 74 with lithium aluminum hydride led to a primary alcohol and recovered chiral auxiliary. Swern oxidation and treatment with Ph3PCHCOOMe afforded the a,P-unsaturatcd ester 75. 

Chiral thiols 59 were derived from 10-camphorsulfonic add by reduction of the sulfony] chloride with lithium aluminum hydride, yielding a 1 4 mixture of enrfo/exo-diastereomers, which can be separated by chromatography60. Containing a hydroxy and a thiol functionality, 59 can be used for the formation of sulfur-containing heterocycles such as oxathianes which can react with carbanions (Section D.l.3.4.4.) or form enolates (Section D. 1.5.3.4.2.1.) and can direct osmium tetroxide catalyzed dihydroxylations (Section D.4.4.). 

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.,... 

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