Auxiliary, Sugar

Asymmetric synthesis Chiral auxiliary Sugar template Chiral ligand Transition-metal catalyst... 

The chiral auxiliary sugar can be isolated by pentane extraction of the crude hydrolysis reaction mixture. The amino acids can then be purified by ion exchange chromatography. The shortcoming of this methodology is that simple amino acids such as phenylalanine are difficult to separate from the hydrolyzed by-product 2-aminobenzyl alcohol (155). According to Linderman and coworkers, the size of the... 

Textile motors Crane motors Determining the size of motor Sugar centrifuge motors Motors for deep-well pumps Motors for agricultural application Surface-cooled motors Torque motors or actuator motors Vibration and noise level Service factors Motors for hazardous locations Specification of motors for Zone 0 locations Specification of motors for Zone I locations Motors for Zone 2 locations Motors for mines, collieries and quarries Intrinsically safe circuits, type Ex. f Testing and certifying authorities Additional requirements for ciritical installations Motors for thermal power station auxiliaries Selection of a special-purpose motor... 

Hultin P. G., Earle M. A. and Sudharshan M. Synthetic Studies with Carbohydrate-Derived Chiral Auxiliaries Tetrahedron 1997 53 14823-14870 Keywords sugar-linked dienes, sugar-linked dienophiles... 

Nitroalkenes with Chiral Auxiliaries The use of carbohydrates as chiral auxiliary in Diels-Alder reactions for the stereoselective preparation of carbocyclic and heterocyclic chiral rings is well documented.48 For example, D-manno-nitroalkene reacts with 2,3-dimethyl-1,3-butadiene to give a 65 35 mixture of adducts, as shown in Eq. 8.29. The configurations at C-4 and C-5 have been determined to be (4R,5R) and (45,55), respectively. Hydrolysis of the product followed by degradative oxidation of the sugar side chains leads to enantiomerically... 

However, most asymmetric 1,3-dipolar cycloaddition reactions of nitrile oxides with alkenes are carried out without Lewis acids as catalysts using either chiral alkenes or chiral auxiliary compounds (with achiral alkenes). Diverse chiral alkenes are in use, such as camphor-derived chiral N-acryloylhydrazide (195), C2-symmetric l,3-diacryloyl-2,2-dimethyl-4,5-diphenylimidazolidine, chiral 3-acryloyl-2,2-dimethyl-4-phenyloxazolidine (196, 197), sugar-based ethenyl ethers (198), acrylic esters (199, 200), C-bonded vinyl-substituted sugar (201), chirally modified vinylboronic ester derived from D-( + )-mannitol (202), (l/ )-menthyl vinyl ether (203), chiral derivatives of vinylacetic acid (204), ( )-l-ethoxy-3-fluoroalkyl-3-hydroxy-4-(4-methylphenylsulfinyl)but-1 -enes (205), enantiopure Y-oxygenated-a,P-unsaturated phenyl sulfones (206), chiral (a-oxyallyl)silanes (207), and (S )-but-3-ene-1,2-diol derivatives (208). As a chiral auxiliary, diisopropyl (i ,i )-tartrate (209, 210) has been very popular. 

The highly stereoselective 1,3-dipolar cycloaddition of C-phenyl-iV-glycosylnitrones (336) and (679) to dimethyl maleate D14, with the sugar moiety acting as a chiral auxiliary, has been used in enantioselective syntheses of isoxazolidines (678) and (678 ent) (Scheme 2.292) (118). 

Synthesis of p-lactam antibiotics from sugars (either as chiral auxiliary or chiron), describing the general methodology developed in 1990s by Chmielewski and based on [2 + 2] cycloaddition of isothiocyanides to sugar olefins, was also comprehensively reviewed.5... 

There are some problems associated with the use of sugar-derived auxiliaries 16 and 18. The nudeophilicity is much lower than for 1, possibly because of the presence of multiple ether functions that can complex lithium ion. It was necessary to include 4 equiv. of LiCl in the addition reactions of 16 and 19 to enamides, otherwise yields were low. A more serious problem associated with 16 is the erosion of the ee of products that was observed when the reactions were scaled up from 0.2 to 4 mmol. Fortunately, a chiral auxiliary that is prepared from camphor does not have these shortcomings [10]. 

Auxiliary 20 was prepared in four steps. Addition of 17, followed by cyclization in situ as before, led to cyclopentenone 18 in 78% yield and 86% ee (Eq. 13.7). Unlike the sugar-derived auxiliaries, the reactions of 20 scaled up without any erosion of... 

The attachment of chiral sugar-derived auxiliaries to the uracil unit generally induced low diastereoselectivities. Only in the reaction between diyne 582 and uracil 583 was a high syn and diastereoselectivity observed, complex 584 being obtained as the major diastereomer (equation 167). 

Sugar fermentation Lactic acid Acidulant, electroplating additive, textile/leather auxiliary... 

MgBr2-mediated asymmetric nucleophilic addition of Grignard reagents and allyl-tributyltin to aldehydes bearing sugar-derived jS- or y-tetrahydropyranyloxy chiral auxiliaries designed to complex with MgBt2 has been achieved. ... 

TABLE 2.53. FACIAL SELECTIVITIES WITH A SUGAR-BASED AUXILIARY... 

Stoichiometric enantioselective addition of lithium aluminum hydride, modified with ( — )-men-thol or sugar derivatives as chiral auxiliaries to enynols provides an entry to /Thydroxyallenes of low enantiomeric purity117 119. This method has mainly been used to establish absolute configurations of naturally occurring allenes, e.g., (—)-marasin119, and has not been optimized. 

An alternate approach comprises replacing the pendant sugar by either a carbo-cyclic or a heterocyclic ring. The enantioselective synthesis starts by formation of the imide (45-3) by reaction of the aion from the chiral auxiliary (45-2), derived from S-phenylalaninol and the pentene ester (45-1). Treatment of the product with triethyl amine and the trifalate from dibutylboronic acid leads to the transient enol borate (45-4). Aldol addition of that enol to acrolein proceeds stereospecifically to the alcohol (45-5) due to the transfer of chirality from the chiral auxiliary. 

Following this concept, the 1,4-addition of phenylmagnesium bromide to (-)-menthyl crotonate was investigated, however with quite disappointing results.7 By using sugar-derived chiral auxiliaries, some-... 

Compound 32 may be removed, after the Ugi reaction, under particularly mild conditions, thanks to sulfur activation by soft electrophiles, such as mercury salts. The yields obtained in zinc-mediated Ugi reactions are excellent and the diastereo-meric ratios are in line with those obtained with 27. Cleavage of the chiral auxiliary can be performed, after methylamine-promoted deacylation of the sugar hydroxy groups, by a diluted solution of CF3CO2H in the presence of Hg(OAc)2. Under these conditions the acyl group on nitrogen is retained. However, the enantiomer of 32 is not easily accessible. 

---