Prolinol alkylation

Synthesis of 1,3,2 oxazaphospholidines 128a-c derived from (S)-prolinol 127 was based either on the thermal aminoalcoholysis of the latter with prochiral alkyl(aryl) phosphonousdiamides 51a-c or its condensation reaction with /-butylphosphonous dichloride carried out in the presence of triethylamine (Scheme 37) [68], The diastereomeric excesses of the prepared derivatives ranged from 80 to 95%. 

Evans and Takacs23 demonstrated a diastereoselective alkylation based on metal ion chelation of a lithium enolate derived from a prolinol-type chiral auxiliary. This method can provide effective syntheses of a-substituted carbox-... 

As with the above pyrrolidine, proline-type chiral auxiliaries also show different behaviors toward zirconium or lithium enolate mediated aldol reactions. Evans found that lithium enolates derived from prolinol amides exhibit excellent diastereofacial selectivities in alkylation reactions (see Section 2.2.32), while the lithium enolates of proline amides are unsuccessful in aldol condensations. Effective chiral reagents were zirconium enolates, which can be obtained from the corresponding lithium enolates via metal exchange with Cp2ZrCl2. For example, excellent levels of asymmetric induction in the aldol process with synj anti selectivity of 96-98% and diastereofacial selectivity of 50-200 116a can be achieved in the Zr-enolate-mediated aldol reaction (see Scheme 3-10). 

The chiral auxiliary can be recycled, since methanolysis of the 1-alkylated 3-trimethylsilyl-2-propynamines regenerated prolinol ether, a precursor of (S)-l-[(dimethoxy)methyl]-2-(meth-Oxymethyl)pyrrolidine. 

The alkylated pyrrolo[l,2-c]oxazolone 5, obtained from the reaction with 3-bromocyclohexene, can be converted to the trans-prolinol derivative 6 by treatment with lithium aluminum hydride in tetrahydrofuran at reflux temperature, then further reduced with palladium on charcoal to provide the cyclohexane derivative 7. 

Enolates from l-acyl-2-pyrrolidinemethanols (A -acylprolinols) and their ethers are alkylated diastereoselectively, usually providing products in good to excellent diastereomeric ratios1-4. The most commonly employed auxiliaries are (S)-2-pyrrolidinemethanol (L-prolinol, 1), (S)-a,a-dimethyl-2-pyrrolidinemethanol (2) and (S)-2-(alkoxymethyl)pyrrolidines 3. For the preparation of these and the corresponding A -acyl derivatives, see Appendix. 

The combined Birch reduction alkylation of chiral, enantiomerically pure aroyl amides of 2-pyrrolidinemethanol (prolinol) or 2-pyrrolidinecarboxylic acid (proline) gives chiral, non-racemic, 1,1-disubstituted 2,5-cyclohexadienes 1 or 2-cyclohexenes 2, respectively, in high diastereomeric ratios. These reactions are useful for the preparation of valuable chiral synthetic intermediates 3 25 29-31-36. 

The highly nucleophilic (S)-prolinol amide enolate (164) (M = Li) was alkylated employing a range of alkyl halides. The carboxylic acids (167) were obtained in chemical yields of 78-96% and outstanding optical yields 177). 

Recently, both enantiomeric forms of callosobruchusic acid (170), a pheromone of the azuki bean weevil, Callosobruchus chinensis L., which induces the male to extrude his genital organ and to attempt copulation, were synthesized by Mori et al.178), applying Evan s alkylation method in natural product synthesis as the key step. Thus, (S)-prolinol propionamide was converted to its enolate (164) by treatment with LDA. 

Then it was alkylated with iodide (168) in the presence of HMPA. Finally the reaction was quenched to produce (169) in 46% yield with 96.6% diastereomeric purity. The amide (169) was hydrolyzed to produce (R)-(170) with 93% e.e. In the same manner, by alkylating (R)-prolinol propionamide with (168), the amide (169 ) was obtained. Acid hydrolysis of (169 ) produced (S)-( 170) with 92% e.e. 

Enantioselectire alkylation of amides. Two laboratories12 have used (S)-prolinol as the chiral auxiliary for a synthesis of chiral amides. Alkylation of the enolate of the amide 1 (prepared with LDA or f-butyllithium) proceeds with pronounced... 

Enantioselective Birch reduction-alkylation The chiral benzoic acid derivative 1, prepared by condensation of o-hydroxybenzoic acid with L-prolinol followed by cyclization (Mitsunobu reaction), undergoes Birch reduction (K, NH3, THF, t-butyl alcohol) followed by alkylation with C2H5I to give essentially only 2. Acid hydrolysis returns the chiral auxiliary and provides the 2-alkylated cyclo-hexenone 3. 

Bis[l,2-bis(diphenylphosphine)ethane]-palladium(O), 34 Tributyl(iodomethyl)tin, 314 Other alkylations Dichlorodimethyltitanium, 216 Potassium fluoride, 256 (S)-Prolinol, 261... 

Zirconium(IV) isopropoxide, 352 Reductive alkylation of aromatic rings Birch reduction, 32 (S)-Prolinol, 261 of carbonyl groups Trityl perchlorate, 339 of other substrates Lithium-Ammonia, 158 Reductive cleavage (see also Reduction of epoxides)... 

Figure 6. Predominant configurations obtained by reaction of the chiral etiolates of an amide of (S)-(—)-prolinol and its O-alkyl derivatives with ethyl iodide are shown. Final products are acid obtained by hydrolyzing the alkylated amide.

A pictorial representation of the opposing diastereoselectivities of alkyl halides and epoxides is shown in Figure 1 A similar electrophile dependence upon diastereoselectivity was first noted in the alkylation of prolinol amide enolates. ... 

Not only polystyrene supports, also other polymer supports were used in the preparation of polymeric amino alcohol ligands for dialkylzinc alkylation. For example, a vinylferrocene derivative with A,N -disubstituted norephedrine was copolymerized with vinylferrocene [60]. This polymeric chiral ligand (53) was used in the ethylation of aldehydes with moderate activity. Brown has reported that chiral oxazaborolidines have catalytic activity in the addition of diethyl zinc to aldehydes [61]. Polymers bearing chiral oxazaborolidines 37 were also active in the reaction and result on moderate enantioselectivity (<58 % ee) [62]. Enantiopure a,a -diphenyl-L-prolinol coupled to a copolymer prepared from 2-hydroxyethylmethacrylate and octadecyl methacrylate... 

Formation of Chiral Quaternary Carbon. Birch reduction-alkylation of benzoic acids and esters establishes quaternary carbon centers. Neighboring stereocenters will influence the stereochemical outcome of the tandem reaction sequence. The following example illustrates how a chiral auxiliary (derived from prolinol) controls the stereoselection in the Birch reduction-alkylation step. ... 

In 1978, Larcheveque and coworkers reported modest yields and diastereoselectivities in alkylations of enolates of (-)-ephedrine amides. However, two years later, Evans and Takacs and Sonnet and Heath reported simultaneously that amides derived from (S)-prolinol were much more suitable substrates for such reactions. Deprotonations of these amides with LDA in the THF gave (Z)-enolates (due to allylic strain that would be associated with ( )-enolate formation) and the stereochemical outcome of the alkylation step was rationalized by assuming that the reagent approached preferentially from the less-hindered Jt-face of a chelated species such as (133 Scheme 62). When the hydroxy group of the starting prolinol amide was protected by conversion into various ether derivatives, alkylations of the corresponding lithium enolates were re-face selective. Apparently, in these cases steric factors rather than chelation effects controlled the stereoselectivity of the alkylation. It is of interest to note that enolates such as (133) are attached primarily from the 5/-face by terminal epoxides. ... 

More recently Katsuki and coworkers have reported that (Z)-enolates of a-alkyl and a-heterosub-stituted amides such as (134), derived from pyrrolidine derivatives having a C2 axis of symmetry, undergo very diastereoselective alkylations with secondary alkyl and other alkylating agents in good to excellent chemical yields (Scheme 62) As with prolinol ether amide enolates, it appears that the direction of approach of the alkylating agent to the enolate (134) is controlled mainly by steric factors within the chiral auxiliary, i.e. chelation effects seem to be of little importance. 

In order to overcome the problems associated with acid hydrolysis of amides of prolinol, the Evans research group has investigated the diastereoselectivity of the alkylation of imides derived from chiral 2-oxazolidones. Imide enolates are somewhat less nucleophilic than amide enolates, but they have the advantage that their diastereomeric alkylation products are easily separated and the imide linkage is cleaved with a variety of reagents under mild conditions. As shown in Scheme 64, alkylation of the chelated (Z)-enolate of the propionimide derived from (S)-valinol (135) with benzyl bromide occurred in high chemical yield and with high si-face diastereoselectivity. In addition to oxazolidones, imidazoli-diones have proved to be useful chiral auxiliaries for diastereoselective enolate alkylations. ... 

The structure proposed for septicine (20), the dehydroindolizidine constituent of Ficus septica, has recently been confirmed by two independent syntheses. The first synthesis proceeded from veratraldehyde, which was condensed with homoveratric acid to give the unsaturated acid (21) this was then converted into the primary chloride (22) by standard methods. Alkylation of L-prolinol with this chloride gave the amino-alcohol (23), which was converted into its O-methanesulphonate ester. Reaction of this ester with sodium hydride in anhydrous... 

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