Configuration, predominant

The trans-1,4 configuration predominates but there are also small amounts of the three other isomeric stmctures, ie, cis-1,4, -1,2, and -3,4. It was one of the first synthetic mbbers produced in the United States as early as 1931, by the Du Pont Co., because of its good solvent resistance. Its Du Pont trade name is neoprene. Even though it is not as solvent resistant as nitrile mbber, it has many other advantageous properties (see Elastomers,... 

The configuration of pairs of isomeric 4-aryIidene-5-pyrazoIones, (Z)- and (E)-(117), was determined by H NMR data (72G491). When R is H, the E configuration is preferred when it is a methyl or a phenyl group, the Z configuration predominates. The presence of an exocyclic sulfur atom as in (118) lowers the interconversion barrier and the products... 

FIGURE 8.8 Inversion of configuration predominates in SnI reactions because one face of the car bocation is shielded by the leaving group (red). 

A related palladium(O)-catalyzed epimerization of y-aziridinyl-a,P-enoates 244 was also reported by Ibuka, Ohno, Fujii, and coworkers (Scheme 2.60) [43]. Treatment of either isomer of 244 with a catalytic amount of Pd(PPh3)4 in THF yielded an equilibrated mixture in which the isomer 246 with the desired configuration predominated (246 other isomers = 85 15 to 94 6). In most cases the isomer 246 could be easily separated from the diastereomeric mixture by a simple recrystallization, and the organocopper-mediated ring-opening reaction of 246 directly afforded L,L-type (E)-alkene dipeptide isosteres 243. 

PIB exists either as a low mw (about 12,000) viscous or atactic liq or as a cryst matl of about 1.00,000 mw. In this latter form the iso tactic (chain) type configuration predominates over the atactic and amorph forms (Ref 8). Since the isotactic form is the form designated by mil spec (Ref 2), its parameters are presented below ... 

Moreover, Soai et al.53c found that the enantioselective addition of Reformatsky reagents to prochiral ketones proceeds well when N,N-dialkylnorephedine 59 is used as the chiral ligand. When (15, 2R)-59a is used, the //-hydroxyl ester is obtained in 74% ee and 65% yield with ( -configuration predominant. When (lR,25,)-59a is used, the product is obtained in 74% ee and at 47% yield with (R)-configuration prevailing. 

Thus, conjugate addition of PhB(OH)2 to cyclohexenone proceeds smoothly in the presence of 3 mol% of Rh(acac)(C2H4)2 and (S)-BINAP (1 1), providing the product in 97% ee with ( -configuration predominant (Scheme 8-36).77... 

It depends then further on the nature of the molecule and likewise on the nature of the other reaction partner or of the catalyst (also of the solvent) what sort of configurations predominate in the transition state. 

A similar photocyclization was reported on the corresponding formate 124, which exists in the Z configuration predominantly or sometimes exclusively (107). Irradiation in the presence of hydriodic acid afforded the protoberberine iodide 126 in good yields, which was further converted to ( )-xylopinine by sodium borohydride reduction. 

However, even without standard neighboring group participation, predominant retention may occur, as in the following example. The chiral epoxy alcohol 6 gives a mixture of 7A and B in 78-92% total yield. The isomer 7A, which shows retention of configuration, predominates 3 1. At O C the ratio of 7A to 7B is 2 1. ... 

Some interesting regularities were noted. Derivatives with the D-allro configuration predominate in the products of scission, by bases, of methyl... 

Inversion of configuration is normally observed on rearrangement of cyclic 2-amino alcohol substrates. Collins et alP have examined in detail the stereochemical course of the Tiffeneau-Demjanov rearrangement in acyclic substrates. In a series of 2-amino-1,2-diaryl alcohols studied, variable amounts of inversion were observed, while in some cases retention of configuration predominated (equation 19). This complexity can be attributed to the flexibility of such acyclic substrates where several conformations are possible in the transition states. 

In a second investigation, racemic 2-hydroxy-4-phenylbutanoic acid (42) was reacted with lipase PS (LPS) and vinyl acetate (VA) (ethenyl ethanoate) in /-butyl methyl ether (2-methoxy-2-methylpropane) to give acetate (S)-43 in 35% yield and in 99% ee unreacted alcohol 44 was found to have 99% ee with R configuration predominant (Scheme 3.3). 

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