Isomeric purity

DPXC ndDPXD. The economic pressure to control dimer costs has had an important effect on what is in use today (ca 1997). Attaching substituents to the ring positions of a [2.2]paracyclophane does not proceed with isomeric exclusivity. Indeed, isomeric purity in the dimer is not an essential requirement for the obtaining of isomeric purity, eg, monosubstituted monomer, in the pyrolysis. Any mixture of the four possible heteronucleady disubstituted dichloro[2.2]paracyclophanes, will, after all, if pyrolyzed produce the same monomer molecule, chloro- -xyljIene [10366-09-3] (16) (Fig. 4). 

Fig. 2. 3-AIkoxycyclohexene yields different alcohols of varying isomeric purities and yields, as shown under the various conditions noted. The catalyst is Rh[P(C, H J Cl. Rin the first two reactions is Si(CH )2(/-C Hg). In the last case R = P(CgH5)2.

It is preferable to prepare tbe 1,2,3,4-tettacblotobutane [3405-32-1] from /n r-l,4-dicbloto-2-butene [110-57-6] wbicb gives mainly tbe meso tetracbloride [28507-96-2] wbicb in turn gives a better yield and bigber isomeric purity of tbe resulting monomer (13). 

Z)-2-(2-hulenyl)-4,4,5,5-tetramethyl-i,3,2-dioxaborolane-, yield 78% isomeric purity via fluorodimethoxyborane route4 97% isomeric purity via the triisopropyl borate route16 >99%. 

E)-4,4,5t5-tetnimethyl-2-[3-(2-trimethylsiIyIethoxy)-2-propenyl -l,3,2-dioxahorolane yield 69% isomeric purity 90% E. 

Alkylthio-substituted allylboronates have also been prepared27. The ethylthio -isomer, however, was prepared with only 70% isomeric purity. 

The procedure for reactions involving the /.-isomeric starting material (ca. 90% isomeric purity) is similar with the exception that 0.9 equiv of aldehyde is used to take advantage of the greater reactivity of the... 

It is noteworthy that reaction diastereoselectivity closely parallels the isomeric purity of the allyiboronates, thus underscoring the requirement that the method of reagent synthesis be highly stereoselective. The data presented in Table 1 also provide strong evidence for the involvement of chair-like, cyclic transition states, analogous to the transition states previously invoked for aldol reactions46. 

E)-2-(4-benzyloxy-l-chloro-2-butenyl)-4.4,5,5-tetramethyl-l, 3,2-dioxaborolane yield 93% 90% isomeric purity... 

Pure (A)-1 -chloropropene was obtained by careful distillation of a mixture of (E)- and ( )-l -chloropropene (available from Columbia Organic Chemicals Company Inc.) using a Nester-Faust Teflon annular spinning band column [(Z)-l-chloropropene has b.p. 33° (A)-l-chloropropene has b.p. 37°]. Small quantities of powdered sodium bicarbonate and hydroquinone (1,4-benzenediol) placed in the distillation flask inhibit acid-catalyzed isomerization and polymerization. Gas chromatographic analysis of the material used in these experiments on a 4-m., 15% l,2,3-tris(2-cyanoethoxy)propane (TCEP) on Chromosorb P column, operated at room temperature, typically indicated that it had isomeric purity >99.9%. (A)- 1-Chloropropene is stable for several months at room temperature, but it should be stored in a cool place. 

In this method, one alkyne is treated with Schwartz s reagent (see 15-17) to produce a vinylic zirconium intermediate. Addition of MeLi or MeMgBr, followed by the second alkyne, gives another intermediate, which, when treated with aqueous acid, gives the diene in moderate-to-good yields. The stereoisomer shown is the one formed in usually close to 100% purity. If the second intermediate is treated with I2 instead of aqueous acid, the 1,4-diiodo-1,3-diene is obtained instead, in comparable yield and isomeric purity. This reaction can also be done intramolecularly Diynes 56 can be cyclized to ( , E) exocychc dienes 57 by treatment with a zirconium complex. 

The proton magnetic resonance spectrum of the product in trifluoroacetic acid shows that the isomeric purity is greater than 90%. The proton magnetic resonance spectral properties for the isomeric amino ketones in both trifluoroacetic acid and chloroform-d are as follows (solvent) 8 (multiplicity, number of protons, assignment, coupling constant J in Hz.) 1-... 

The less highly substituted Mannich bases can also be prepared directly from ketones and dimethyl(methylene)ammonium trifluoroacetate by the procedure reported here, which takes advantage of the isomerization of Mannich bases in trifluoroacetic acid. (In acetic acid the Mannich bases undergo elimination of dimethylamine to give a-methylene ketones.) This method is rapid and affords products having an isomeric purity of at least 90% without difficult separations. The 49-57% yield of l-(di-methylamino)-4-methyl-3-pentanone obtained with this procedure compares favorably with the overall yields of amino ketones prepared by the indirect routes mentioned previously. 

The isomeric N,N-diethylnerylamine could be obtained in 99% isomeric purity (77-86% yield) by telomerization of isoprene with diethylamine in the presence of n-BuLi [215]. 

A corollary to the above argument is that enantioselectivities depend on alkene geometry. Indeed, isomeric enolsilanes provide enantiomeric products. Because obtaining enolsilanes such as 344 in high isomeric purity is difficult, enantioselectivities with these nucleophiles are reflective, Eqs. 214 and 215. Pyrrole-derived enolsilanes are accessible in very high isomeric purity (>99 1) thus providing a convenient solution to this problem. Their use in the catalytic amination reaction provides access to a-hydrazino acid derivatives in high enantioselectivity. 

Dimethyl allylboronates (e.g., 17,18) are sensitive to hydrolysis, and the allylboronic acids are unstable in the presence of O2. The pinacoi and tartrate esters, however, are quite stable and many have been purified by distillation. We routinely monitor the isomeric purity of the crotyl reagents by capillary GC analysis (ref 3e). 

The submitters report that Z-4-(tr1methy1silyl)-3-buten-l-ol of >98% isomeric purity can be obtained in ca. 60% overall yield by a more lengthy... 

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