E, Z-stereochemistry

E/Z stereochemistry of a,/ -ethylenic phosphonium derivatives is essentially governed by steric factors apart from when there is extra stabilization, as is the case with betaines87. Z->E isomerization may be performed using sterically crowded amines (so that any Michael-type addition would be prevented)192,193. Spectrometry allows the progress of the reaction to be observed (Section I.C). 

Z)-vinyl halide ( )-alkenylboronate ester diene with (E, Z) stereochemistry... 

Oxathiacrown ethers containing a diene unit 40 and 41 were prepared by the reaction of l,3,5,5-tetrachloro-2-nitrobutadiene derivatives with bis(2-mercaptoethyl) ether (Scheme 7) <2002PS2529>. The authors reported that 40 formed by the cyclization of an intermediate (Cl2C=CCl-C(N02)=C(SR)-S-(CH2)2-0-(CH2)2-S-Na+). Compound 41 was prepared by the cyclization of the intermediate (Na+S-(CH2)2-0-(CH2)2-S-ClC=CCl-C(N02)2=C(SR)-S(CH2)2-0-(CH2)2-S Na+ or Cl2C=CCl-C(N02)2=C(SR)-S(CH2)2-0-(CH2)2-S-S-(CH2)2-0-(CH2)2-S Na+), which were obtained by the reaction of l,3,5,5-tetrachloro-2-nitrobutadiene derivatives with bis(2-mercaptoethyl) ether. In this report, the E/Z stereochemistry of these compounds was not reported. 

An additional problem that has been partially overcome with the newer catalysts is the control of the E Z stereochemistry of the resulting products. For example, in the synthesis of epotholone, the Danischefsky group examined the stereochemistry as a function of the substituents around the ring [24]. With Ru-2, the Z stereochemistry of the product was found to be controlled by subtle conformational changes induced by substituents. That the Ru-2 and other bisphosphine catalysts gave the kinetically controlled product was demonstrated in a simple system (Figure 6.9). 

Unlike polyethylene and other simple alkene polymers, natural rubber is a polymer of a diene, isoprene (2-methyl-l,3-butadiene). The polymerization takes place by addition of each isoprene monomer unit to the growing chain, leading to formation of a polymer that still contains double bonds spaced regularly at four-carbon intervals. As the following structure shows, these double bonds ha e Z stereochemistry ... 

In molecules with two or more double bonds the configuration of each is determined independently, and the E/Z symbols are incorporated into the name as for 7 and 8. In some dienes, one double bond has E/Z stereochemistry and the other does not because two groups on one carbon are the same. Such an example is 9, which therefore has the name (Z)-3-chloropenta-1,3-diene. 

J Phosphorus-31 NMR. - Positive chemical shifts, 63 ip, are downheld of the external reference 85% phosphoric acid, and are usually given without the appellation ppm. One-coordinate compounds studied include phosphaalkynes and their cyclotetramerisation. Two-coordinate compounds. The NMR data of the simple phosphaalkenes R CH=PR (R/R = H/H, H/Me, H/Et, Me/H, Me/ Me, Ph/H, and Ph/Me) have been reported for the first time, with 6 in the range + 179 to +285, and found to be consistent with the proposed structures. Stereochemistry of the (Z)- and (E)-isomers was established according to the cis-rule. The P and NMR data of a series of 40 iminophosphines, R-P=N-R, revealed that their E/Z stereochemistry can be predicted on the basis of a simultaneous comparison of the values of 831 p and Jpn- ... 

Double Stille couplings are valuable for symmetrical compounds like p-carotene 226. The symmetrical bis-tributylstannyl-pentaene 224 is coupled at both ends to the iodo-triene 223 in one step with catalyst 225 to give an excellent yield of P-carotene 226 with full control over E/Z stereochemistry.35... 

B.iii.b. Stereochemistry. The stereochemistry of Pd-catalyzed allylation of organometals is considerably more complicated than the regiochemistry discussed above, since it involves both E,Z stereochemistry of the C=C bond and / ,S-configuration of the allyfic Cjp3 center. 

Streptirnidone is an antibiotic and has the structure shown How many diastereomers of streptimidone are possible" How many enantiomers" Using the E Z and R S descnptors specify all essential elements of stereochemistry of streptimidone... 

Double bonds m the mam chain are signaled by the ending enow acid and their position IS designated by a numerical prefix Entries 6 and 7 are representative carboxylic acids that contain double bonds Double bond stereochemistry is specified by using either the cis-trans or the E-Z notation... 

As noted in the Chapter 7 Focus On, rubber is a naturally occurring polymer of isoprene, or 2-methyl-l,3-butadiene. The double bonds of rubber have Z stereochemistry, but gutta-percha, the E isomer of rubber, also occurs naturally. Harder and more brittle than rubber, gutta-percha has a variety of minor applications, including occasional use as the covering on golf balls. 

The (E/Z) ratio of the product can often be changed by a change in solvent or by the addition of salts. Another way of controlling the stereochemistry of the product is by use of the aforementioned phosphonic acid bisamides. In this case the betaine (42) does form and when treated with water gives the (3-hydroxyphosphonic... 

The reaction shows a dependence on the E- or Z-stereochemistry of the enolate. Z-enolates favor anti adducts and E-enolates favor syn adducts. These tendencies can be understood in terms of an eight-membered chelated TS.299 The enone in this TS is in an s-cis conformation. The stereochemistry is influenced by the s-cis/s-trans equilibria. Bulky R4 groups favor the s-cis con former and enhance the stereoselectivity of the reaction. A computational study on the reaction also suggested an eight-membered TS.300... 

A Mitsunobu process simultaneously coupled the enyne acid fragment 4 to /J-lactam 10 and inverted the CIO stereochemistry to the required (S)-configured ester 11 in 93% yield. A deprotection provided alcohol 12, the key /J-lactam-based macrolactonization substrate, which, under conditions similar to those reported by Palomo for intermolecular alcoholysis of /J-lactams (Ojima et al, 1992, 1993 Palomo et al, 1995), provided the desired core macrocycle 13 of PatA 13 (Hesse, 1991 Manhas et al, 1988 Wasserman, 1987). Subsequent Lindlar hydrogenation gave the required E, Z-dienoate. A Stille reaction and final deprotection cleanly provided (-)-PatA that was identical in all respects to the natural product (Romo etal, 1998 Rzasaef al, 1998). This first total synthesis confirmed the relative and absolute configuration of the natural product and paved the way for synthesis of derivatives for probing the mode of action of this natural product. 

Benzoylation of D-g/ycero-D-gw/o-heptono-1,4-lactone with an excess of benzoyl chloride and pyridine afforded the hept-2-enono-1,4-lactone as the main product (198). The di- and triunsaturated compounds were isolated in very low yield from the mother liquors (199). Higher yields of the di- and triunsaturated derivatives 153 and 154 were obtained when the /5-elimination reaction was performed with triethylamine on the previously synthesized per-O-benzoyl D-g/ycero-D-gw/o-heptono-1,4-lactone. Employing 10% triethylamine in chloroform, the lactone 153 was obtained as an E, Z dias-tereomeric mixture in 9 11 ratio as determined by H n.m.r. When 20% triethylamine was used, the furanone 154 was obtained in 59% yield (200). Its structure was assigned, on the basis of H and 13C n.m.r. spectra, as 3 -benzoyloxy - (5Z)-[(Z)-3 - benzoyloxy - 2 - propenyliden] -2(5 H)- furanone. The stereochemistry of the exocyclic double bonds was established (201) by nuclear Overhauser effect spectroscopy (NOESY). 

The effect of catalysts on the stereochemistry of the Horner-Wittig reaction under liquid-liquid conditions has been studied by Mikolajczyk et al. (1975). The E/Z isomer ratios in the product [162] proved to depend on the catalyst... 

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