Acetals in Asymmetric Synthesis

The (5.S)-(+)-pentane-2,4-diol and (/ ./ )-(-)-butane-2,3-dioI (and their enantiomers) used in the foregoing examples are expensive and their water solubility 

Silver(I) is not necessary to provoke the 1 -shift a commercial process accomplished the same task using aqueous acid at elevated temperature but details are sparse. 

For further examples of the use of acetals in asymmetric synthesis see the reviews in section 2.9.3. 

---

A review of chiral acetals in asymmetric synthesis is available.102... 

Chiral Acetals in Asymmetric Synthesis, Alexakis, A, Mangeney. P. Tetrahedron Asymmetry 1990, /, 477. 

The complete transformation of a racemic mixture into a single enantiomer is one of the challenging goals in asymmetric synthesis. We have developed metal-enzyme combinations for the dynamic kinetic resolution (DKR) of racemic primary amines. This procedure employs a heterogeneous palladium catalyst, Pd/A10(0H), as the racemization catalyst, Candida antarctica lipase B immobilized on acrylic resin (CAL-B) as the resolution catalyst and ethyl acetate or methoxymethylacetate as the acyl donor. Benzylic and aliphatic primary amines and one amino acid amide have been efficiently resolved with good yields (85—99 %) and high optical purities (97—99 %). The racemization catalyst was recyclable and could be reused for the DKR without activity loss at least 10 times. 

Diols such as the optically active 1,1 -binaphthyl-2-2 -diol (BINOL) have been used as versatile templates and chiral auxiliaries in catalysts employed successfully in asymmetric synthesis. The application of enzymes in the enantioselective access to axially dissymmetric compounds was first reported by Fujimoto and coworkers.83 In aqueous media, the asymmetric hydrolysis of the racemic binaphthyl dibutyrate (the ester) using whole cells from bacteria species afforded the (A)-diol with 96%ee and the unreacted substrate (A)-ester with 94% ee at 50 % conversion. Recently, in non-aqueous media, lipases from Pseudomonas cepacia and Ps. fluorescens have been employed in the enantioselective resolution and desymmetrization of racemic 6,6 -disubstituted BINOL derivatives using vinyl acetate.84 The monoacetate (K)-73 (product) was obtained in 32-44 % chemical yields and 78-96% ee depending on the derivatives used. The unreacted BINOL (S)-72 was obtained in 30-52 % chemical yield and 55-80% ee. 

A wide variety of substituted y-butyrolactones can be prepared directly from olefins and aliphatic carboxylic acids by treatment with manganic acetate. This procedure is illustrated in the preparation of 7-( -OCTYL)-y-BUTYROLACTONE. Methods for the synthesis of chiral molecules are presently the target of intensive investigation. One such general method developed recently is the employment of certain chiral solvents as auxiliary agents in asymmetric synthesis. The preparation of (S.SM+H, 4-BIS(DIMETHYLAMINO)-2,3-DIMETHOXY-BUTANE FROM TARTARIC ACID DIETHYL ESTER provides a detailed procedure for the production of this useful chiral media an example of its utility in the synthesis of (+)-(/ )-l-PHENYL-l-PEN-TANOL from benzaldehyde and butyllithium is provided. 

Reviews Concerning the Synthesis and Reactions of NfO-Acetals and Their Use In Asymmetric Synthesis... 

Cyclic acetals and dithioacetals as protecting groups 89OPP705. Cyclic acetals of chiral diols in asymmetric synthesis 87YGK944. Cyclic nitrosonium salts as oxidants 88H(27)509. 

Reactions with Sulfur Nucleophiles. The use of sulfur nucleophiles in palladium-catalyzed allylic substitution reactions is less well documented than that of carbon, nitrogen and oxygen nucleophiles. The asymmetric synthesis of allylic sulfones utilizing a catalytic phase transfer system has been used to produce (35)-(phenylsulfonyl)cyclohex-l-ene on a 45 g scale (eq 10). In many cases, it has been reported that allylic carbonates are more reactive than allylic acetates in asymmetric allylic substitution... 

The lipase catalyzes the kinetic resolution of racemic amines, e.g. 1-phenyl-ethylamine (Fig. 19-21)[11. Products are intermediates for pharmaceuticals and pesticides. They can also be used as chiral synthons in asymmetric synthesis. As acylating agent ethylmethoxyacetate is used, because the reaction rate is more than 100 times faster than that with butyl acetate. Probably an enhanced carbonyl activity induced by the electronegative a-substituents accounts for the activating effect of the methoxy group. The lipase is immobilized on polyacrylate. The lowered activity caused by use of in organic solvent (tert-methylbutylether = MTBE) can be increased... 

As for the diols, the symmetric compounds have found most uses for nonsymmetric diols, a versatile synthesis via silyl ketones using the SAMP/RAMP methodology has been developedl5. Both enantiomers of the simplest symmetric diol, 2,3-butanediol (11), are often used in asymmetric synthesis, mostly for the formation of acetals and ketals with carbonyl compounds and subsequent reactions with acidic catalysts (Section D. 1.1.2.2.), Grignard reagents (Section D. 1.3.1.4.) and other carbanions (Sections D. 1.5.1., D. 1.5.2.4.), and diastereoselective reductions (Section D.2.3.3.). Precursors of chiral alkenes for cycloprotonations (Section D.1.6.1.5.) and for chiral allenes (Section B.I.), and chiral haloboronic acids (Section D. 1.1.2.1.) are other applications. The free diol has been employed as a chiral ligand in molybdenum peroxo complexes used for enantioselective epoxidation of alkenes (Section D.4.5.2.2.). 

Substrate-controlled routes to optically enriched materials via diastereoselective oxidative dearomatizations constitute a second strategy for harnessing this process in asymmetric synthesis. Best results are obtained in intramolecular dearomatizations in which a preexisting stereogenic center is present on the side-chain nucleophile of a prochiral arene substrate [50]. For example, intramolecular oxidative dearomatization of 54 proceeds diastereoselectively as a consequence of conformational effects operative in the course of acetal formation (Scheme 15.20) [51]. 

Enamines of cyclohexylamine have been enantioselectively cyclized to bicyclo[3.3.1] nonanedione systems, using acryloyl chloride and chiral pyrrolidine catalysis. Enantio-pure A-sulflnylimines have been used in asymmetric synthesis of isoquinolone alkaloids, and a stereocontrolled synthesis of 3,4,5,6-tetrahydropyrimidine-based amino acids from imino ethers has been reported. Diastereoselective additions of chiral acetals of (2-lithiophenyl)acetaldehyde to arylimines have been used in an asymmetric synthesis of 1-aryltetrahydroisoquinolines. " Organolithiums react with chiral imines, in the presence of Lewis acids or bases, to give amines in up to 100% de. Diastereoselective additions of copper reagents to imines derived from (5)-l-phenylethylamine have been reported. 

One of the pervasive problems in asymmetric synthesis has been the development of stereoselective acetate ester aldol reactions. Although a number of chiral auxiliaries perform superbly well in diastereoselective propionate aldol additions, these have, with rare exceptions, been unsuccessful in the corresponding additions of unsubstituted acetate-derived enolates [19, 63, 64). Braun s disclosure of a stereoselective acetate aldol addition reaction with 103 was an important milestone in the development of the field (Scheme 4.11) [63, 65]. The diol auxiliary can easily be prepared from mandelic acid esterification of the secondary alcohol is obsei ved, without interference from the tertiary counterpart. Its use has been showcased in a number of syntheses [53]. The high yield and diastereoselectivity generally obtained with 103 were highlighted by investigators at Merck in the construction of the chiral lactone fragment that is common in a number of HMG-CoA reductase inhibitors, such as compactin (105) [66]. 

An early demonstration, reported by Hoppe, of the use of sparteine for the generation of dipole-stabilized carbanions and their subsequent application in asymmetric synthesis involves carbamates such as 37 (Scheme 13.5) [48], The synthesis of the Drosophila mulleri pheromone (S)-l-methyldodecyl acetate (39) illustrates the features of this process. This approach was also employed with substrate 40 (> 95 % ee), which could be sequentially metalated and functionalized to give 42 (dr >97 3) as shown in Scheme 13.6 [49]. 

Simple esters cannot be allylated with allyl acetates, but the Schiff base 109 derived from o -amino acid esters such as glycine or alanine is allylated with allyl acetate. In this way. the o-allyl-a-amino acid 110 can be prepared after hydrolysis[34]. The Q-allyl-o-aminophosphonate 112 is prepared by allylation of the Schiff base 111 of diethyl aminomethylphosphonates. [35,36]. Asymmetric synthesis in this reaction using the (+ )-A, jV-dicyclohex-ylsulfamoylisobornyl alcohol ester of glycine and DIOP as a chiral ligand achieved 99% ec[72]. 

The primary disadvantage of the conjugate addition approach is the necessity of performing two chiral operations (resolution or asymmetric synthesis) ia order to obtain exclusively the stereochemicaHy desired end product. However, the advent of enzymatic resolutions and stereoselective reduciag agents has resulted ia new methods to efficiently produce chiral enones and CO-chain synthons, respectively (see Enzymes, industrial Enzymes in ORGANIC synthesis). Eor example, treatment of the racemic hydroxy enone (70) with commercially available porciae pancreatic Hpase (PPL) ia vinyl acetate gave a separable mixture of (5)-hydroxyenone (71) and (R)-acetate (72) with enantiomeric excess (ee) of 90% or better (204). 

In an extension of this work, the Shibasaki group developed the novel transformation 48—>51 shown in Scheme 10.25c To rationalize this interesting structural change, it was proposed that oxidative addition of the vinyl triflate moiety in 48 to an asymmetric palladium ) catalyst generated under the indicated conditions affords the 16-electron Pd+ complex 49. Since the weakly bound triflate ligand can easily dissociate from the metal center, a silver salt is not needed. Insertion of the coordinated alkene into the vinyl C-Pd bond then affords a transitory 7t-allylpalladium complex 50 which is captured in a regio- and stereocontrolled fashion by acetate ion to give the optically active bicyclic diene 51 in 80% ee (89% yield). This catalytic asymmetric synthesis by a Heck cyclization/ anion capture process is the first of its kind. 

---