Acyloxy chiral

In peptide syntheses, where partial racemization of the chiral a-carbon centers is a serious problem, the application of 1-hydroxy-1 H-benzotriazole ( HBT") and DCC has been very successful in increasing yields and decreasing racemization (W. Kdnig, 1970 G.C. Windridge, 1971 H.R. Bosshard, 1973), l-(Acyloxy)-lif-benzotriazoles or l-acyl-17f-benzo-triazole 3-oxides are formed as reactive intermediates. If carboxylic or phosphoric esters are to be formed from the acids and alcohols using DCC, 4-(pyrrolidin-l -yl)pyridine ( PPY A. Hassner, 1978 K.M. Patel, 1979) and HBT are efficient catalysts even with tert-alkyl, choles-teryl, aryl, and other unreactive alcohols as well as with highly bulky or labile acids. 

In 1989 Yamamoto et al. reported that the chiral (acyloxy)borane (CAB) complex 3 is effective in catalyzing the Diels-AIder reaction of a number of a,/ -unsaturated aldehydes [5]. The catalyst was prepared from monoacylated tartaric acid and bo-... 

The strategy based on tandem cycloaddition leads to a short and efficient asymmetric synthesis of the pyrrolizidine necine base (-)-hastanecine, as shown in Scheme 8.32.163 Pyrrolizidine alkaloids have a long history for attracting the interest of synthetic chemists because of their physiological properties. The method of Denmark shown in this scheme is very simple and applied to synthesis of various alkaloids. The Lewis acid-promoted [4+2] cycloaddition between 2-acyloxy nitroalkene and chiral vinyl ether gives a nitronate that... 

The palladium-catalyzed asymmetric allylic substitution using seven different phosphano-oxazoline ligands at various ligand-to-metal ratios was also studied.112 An aluminum block containing 27 wells was placed in a dry box in which the reactions were carried out in parallel. Analyses were performed by conventional chiral GC equipped with an autosampler. Such a setup allowed about 33 catalyst evaluations per day. Apparently, only a few dozen were carried out in the study, resulting in the identification of a catalyst showing an ee-value of 74% in the reaction of 4-acyloxy-2-pentene with malonate.112 It is not clear whether further ligand diversification would lead to catalysts more selective than the record set in this case by the Trost-catalyst (92% ee).113... 

Chiral (Acyloxy)borane Complex Catalyzed Asymmetric Diels-Alder Reaction (1 R)-1,3,4-Trimethyl-3-cyclohexene-1-carboxaldehyde. 

Stable aryl boronates derived from tartaric acid catalyze the reaction of cyclo-pentadiene with vinyl aldehyde with high selectivity. Chiral acyloxy borane (CAB), derived from tartaric acid, has proved to be a very powerful catalyst for the enantioselective Diels-Alder reaction and hetero Diels-Alder reaction. Scheme 5 23 presents an example of a CAB 73 (R = H) catalyzed Diels-Alder reaction of a-bromo-a,/i-cnal 74 with cyclopentadiene. The reaction product is another important intermediate for prostaglandin synthesis. In the presence of... 

Apart from its application in intermolecular Diels-Alder reactions, chiral acyloxy boron (CAB) can also be used to effect intramolecular Diels-Alder reactions with excellent stereoselectivity (Scheme 5-45).73... 

Acyloxy-l-cyanoalkanes [45, 46], which can be used as precursors for ketones [47], a-hydroxy ketones [48] and 1,4-dicarbonyl compounds [47], are prepared in one pot from the appropriate aldehyde, sodium or potassium cyanide, and the acylating agent under phase-transfer catalytic conditions [47-49]. Attempts to synthesize chiral cyanhydrins using chiral phase-transfer catalysts have been unsuccessful (see Section 12.3). 

CHIRAL (ACYLOXY)BORANE COMPLEX-CATALYZED ASYMMETRIC DIELS-ALDER REACTION (1R)-1,3,4-TRIMETHYL-3-CYCLOHEXENE-1-CAR BOX ALDEHYDE (3-Cyclohexene-1-carboxaldehyda, 1,3,4-trlmethyl-, (-)-)... 

Although enol esters have a similar structure to enamides, they have proven more difficult substrates for asymmetric hydrogenation, which is evident from the significantly fewer number of examples. One possible explanation is the weaker coordinating ability of the enol ester to the metal center, as compared to the corresponding enamide. Some rhodium complexes associated with chiral phosphorous ligands such as DIPAMP [100, 101] and DuPhos [102] are effective for asymmetric hydrogenation of a-(acyloxy)acrylates. 

The chiral acrylate 164 was used in a 1,3-dipolar cycloaddition with a nitrile imine. Bis(trityl)nitrile imine was found to undergo a diastereoselective 1,3-dipolar cycloaddition with (f )-a-(acyloxy)-p,p-dimethyl-y-butyrolactone 164 to give the 2-pyrazoline product with a de of 50% (Scheme 12.51) (275). 

The aspects of the preparation and reactions of chiral Grignard reagents were reviewed . Exchange of alkyl hahdes is synthetically useful mostly if a-halogen or a-acyloxy substituents are present. The resulting Grignard reagents react smoothly with various electrophiles (equation 43) . ... 

S. Hanessian, P. C. Tyler, and Y. Chapleur, Reaction of lithium dimethylcuprate with confor-mationally biased acyloxy enol esters. Regio and stereocontrolled access to functionalized six-carbon chiral synthons. Tetrahedron Lett. 22 4583 (1981). 

Enantioselective condensation of aldehydes and enol silyl ethers is promoted by addition of chiral Lewis acids. Through coordination of aldehyde oxygen to the Lewis acids containing an Al, Eu, or Rh atom (286), the prochiral substrates are endowed with high electrophilicity and chiral environments. Although the optical yields in the early works remained poor to moderate, the use of a chiral (acyloxy)borane complex as catalyst allowed the erythro-selective condensation with high enan-tioselectivity (Scheme 119) (287). This aldol-type reaction may proceed via an extended acyclic transition state rather than a six-membered pericyclic structure (288). Not only ketone enolates but ester enolates... 

The combination of Ru complex-catalyzed stereomutation of secondary alcohols with enzyme-catalyzed enantioselective acylation is an efficient procedure to obtain chiral acyloxy compounds with excellent optical purity from a variety of racemic secondary alcohols via dynamic kinetic resolution [112]. 

The characteristic feature of the aforementioned oxazaborolidine catalyst system consists of a-sulfonamide carboxylic acid ligand for boron reagent, where the five-membered ring system seems to be the major structural feature for the active catalyst. Accordingly, tartaric acid-derived chiral (acyloxy)borane (CAB) complexes can also catalyze the asymmetric Diels-Alder reaction of a,P-unsaturated aldehydes with a high level of asymmetric induction [10] (Eq. 8A.4). Similarly, a chiral tartrate-derived dioxaborolidine has been introduced as a catalyst for enantioselective Diels-Alder reaction of 2-bromoacrolein [11] (Eq. 8A.5). 

Addition to (l-acyloxy enol esters. Addition of lithium dimethylcuprate to chiral enol esters derived from carbohydrates provides access to C6 chiral synthons with four alternating and/or consecutive methyl and hydroxyl groups.21 Example ... 

Intramolecular oxidative cyclizations in the appropriately substituted phenols and phenol ethers provide a powerful tool for the construction of various practically important polycyclic systems. Especially interesting and synthetically useful is the oxidation of the p-substituted phenols 12 with [bis(acyloxy)iodo]-arenes in the presence of an appropriate external or internal nucleophile (Nu) leading to the respective spiro dienones 15 according to Scheme 6. It is assumed that this reaction proceeds via concerted addition-elimination in the intermediate product 13, or via phenoxenium ions 14 [18 - 21]. The recently reported lack of chirality induction in the phenolic oxidation in the presence of dibenzoyltar-taric acid supports the hypothesis that of mechanism proceeding via phenoxenium ions 14 [18]. The o-substituted phenols can be oxidized similarly with the formation of the respective 2,4-cyclohexadienone derivatives. 

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