In stereospecific

The reactions of bromine with E- or Z-1 -fluoropropene under ionic conditions result in stereospecific anti additions to yield the 15,26 and 1R,25 products, respectively [ftl] (equation 4)... 

The first objective was the conversion of L-tryptophan into a derivative that could be converted to pyrroloindoline 3, possessing a cis ring fusion and a syn relationship of the carboxyl and hydroxyl groups. This was achieved by the conversions shown in Scheme 1. A critical step was e. Of many variants tried, the use of the trityl group on the NH2 of tryptophan and the t-butyl group on the carboxyl resulted in stereospecific oxidative cyclization to afford 3 of the desired cis-syn stereochemistry in good yield. 

Halogen atoms can be stereoselectively introduced by ring-opening of y-azir-idinyl-a,P-enoates (Scheme 2.39). Treatment of 149 with diethylaminosulfur tri-fluoride (DAST) results in stereospecific ring-opening to yield fluorinated derivative 150 [59]. A related stereoselective conversion of y-aziridinyl-a,P-enoates 151 into allyl halides 152 by use of lithium halide in the presence of Amberlyst 15 was also reported recently [60]. 

Alkanes, as radiolytic products 907 Alkanesulphinates as radiolytic products 907 in stereospecific synthesis of sulphoxides 298, 299... 

A common feature of the compounds that give extensive syn addition is the presence of a phenyl substituent on the double bond. The presence of a phenyl substituent diminishes the strength of bromine bridging by stabilizing the cationic center. A weakly bridged structure in equilibrium with an open benzylic cation can account for the loss in stereospecificity. 

The iodocyclization products have a potentially nucleophilic oxygen substituent (3 to the iodide, which makes them useful in stereospecific syntheses of epoxides and diols. 

Normal rhombic sulphur has differing degrees of solubility in the different rubber types. In NR and SBR the required proportion for crosslinking dissolves relatively rapidly at room temperature. In stereospecific rubbers such as polybutadiene and nitrile it does not solubilise so readily. As one would expect, the solubility of the sulphur within the rubber increases with temperature increase. 

In general, reductive removal of a sulphonyl group from vinyl sulphones is not a stereospecific reaction. However, two methods, both developed by Julia, result in stereospecific products. The first one involves reaction of the vinyl sulphone with n-BuMgCl in the presence of a transition metal catalyst such as Ni(acac)2. This method was used to synthesize a pheromone having a (Z, )-diene (equation 27)59. Palladium catalysts can also be used for... 

Possible elements of chirality in stereospecific polymerizations will be briefly recalled in order to indicate the used terminology. First of all, upon coordination, a prochiral olefin such as propene gives rise to not superpos-able si and re coordinations.22 According to the mechanism described, the isotactic polymer is generated by a large series of insertions of all si- or all re-coordinated monomers, while the syndiotactic polymer would be generated by alternate insertions of si - and re-coordinated monomers. 

The liver alcohol dehydrogenase mentioned in the preceding section has the same pro-R stereospecificity for NAD and ethanol as yeast alcohol dehydrogenase. Furthermore, the oxidation of ethanol by a microsomal oxidizing system, or by catalase and H2O2, likewise proceeds with pro-R stereospecificity for the ethanol77>. The catalase-H2C>2 system is so very different, however, from the pyridine nucleotide dehydrogenase, that one wonders whether the similarity in stereospecificity for ethanol is fortuitous. 

Compounds 29, 34, 39, and 40 constitute chiral synthons suitable for use in stereospecific aninocyclitol synthesis. Thus, 40 or other, appropriately N protected derivatives of 39 may be employed for stereospecific substitution at OH-6 alternatively, after temporary protection of OH-6 followed by removal of the acetal, the molecule should be amenable to manipulation at OH-4. In 9 and 4, the two unequal nitrogenous functions may be reduced stepwise to amino groups, thus offering possibilities for stereospecific introduction of an N-substituent at either position. In order to demonstrate that such a strategy is feasible, reaction sequences leading to the enantiomers of mono-N-methyl-2-deoxystreptamine were performed, as illustrated in Figure 5. 

In addition to aryl-aryl coupling, the Stille reaction can be used with alkenylstan-nanes and alkenyl halides and triflates.125 The reactions occur with retention of configuration at both the halide and the stannane. These reactions have become very useful in stereospecific construction of dienes and polyenes, as illustrated by some of the examples in Scheme 8.10. 

Fig. 4. Isotactic, syndiotactic and atactic isomerism in stereospecific addition polymers.

In the first half of this century, only a few papers concerning the total synthesis of sugars (mostly DL-tetroses) were published. A substantial increase in the number of papers since 1950 was the consequence of many important developments in stereospecific functionalization of organic compounds, in separation techniques, and in methods for structural determination. 

Fig. 13. In a syncrystallized mixture of A-cis-Cr(en)2(H20)(0H)2+ and A-cis-Co(en)2-(H20)(0H)2 (as the dithionate), the cations in the crystals are positioned with A opposite to A, as shown on the left, resulting in stereospecific formation of the heterodinuclear species (-)D-A,A-(en)2Cr(OH)2Co(en)24+ upon heating of the solid salt.

The reverse reaction of oxidation of the metal or reduction of the moiety is also known to occur in stereospecific catalyst systems. It has been long known that the polymerization of olefine materials can be accomplished in non-alkyl systems. Diem, Tucker and Gibbs (43) have shown that the lithium metal polymerization of isoprene proceeds with the initial reduction by the electron seeking lithium of the nucleophilic diene to produce the corresponding alkyl lithium. Fukui, Schimidzu, Yagi, Fukumoto, Kagiya and Yuosa (127) have studied the polymeriza-... 

Additional evidence of that hypothesis is given In Tables 4 and 5. The catalysts prepared with carbonyl clusters in n-hexane medium must avoid the MgO hydrolysis. The selectivity patterns for such catalysts show notable differences in comparison with the aqueous Impregnated type catalysts. The carvotanacetone formation is largely diminished and the stereospecificity to axial-equatorial carvomenthol is totaly inhibited. However in Rhodium silica supported catalysts the selectivity to carvotanacetone practically does not change. The effects in stereospecifity towards the carvomenthol product may be due to a small silica hydrolysis effect. 

CoA labeled with 14C (radioactive carbon) at the carboxyl group, CH3—14C—SCoA, into the citric acid cycle, the 2-oxopentanedioate acid (2-ketoglutarate) formed in the fourth step of the cycle would have all of the 14C in the carboxylate group farthest away from the ketone carbonyl group. For some years, this result was used to argue that citric acid itself could not be an intermediate in the formation of 2-oxopentanedioate. Review Section 19-8 and explain how, in stereospecific enzyme-induced reactions, citric acid could be an intermediate in the formation of 2-oxopentanedioate even if the 14C would not appear equally in both carboxylic carbons of the product. 

Most of the types of cyclofunctionalization reaction discussed in this chapter have been shown to result in stereospecific anti addition across the ir-bond. This result suggests that the important intermediates are ir-complexes (B) or onium ions (C) rather than carbocations (E). In the case of cyclofunctionalization with some electrophiles, such as phenylselenenyl chloride, it has been shown that the formation of addition products such as (D) occurs faster than the cyclization.4 Stereospecific trans addition in these reactions then requires conversion to intermediates (B) or (C) before nucleophilic attack, since nucleophilic attack on intermediate (D) and substitution of X with inversion would result in syn addition. Thus, in the discussions below, intermediates (B) and/or (C) are considered to be the key... 

Halogen-lithium exchange cf. Section 4.7.3.2) at low temperatures with an alkyllithium followed by quenching with methanol or water results in stereospecific reduction of dihalocyclopropyl compounds.113 Stereospecific reduction has also been achieved by reaction with potassium diphenylphosphide (equation... 

Various organoselenium compounds are able to add in stereospecific anti fashion to alkenes282 [Eq. (4.85)]283 or alkynes282 [Eq. (4.86)] to form stable cyclic selenonium ions. 

Addition of sulfur chlorides and sulfenyl halides to hydrocarbon olefins is a classic example of electrophilic reaction which usually proceeds under mild conditions and results in stereospecific trans-addition via intermediate formation of cyclic episulfonium cation [134]. Ring-opening reactions of episul-fonium cation with nucleophile is responsible for formation of regioisomers when nonsymmetrical olefins are used as substrates. 

Epoxyphospholanes 105 are directly prepared from 2-phospholenes 94. Epoxidation of 94 with sodium peroxide give a single diastereomer of erythro 105E (X-ray analysis gives the ORTEP drawing in Fig. 10) in stereospecific and stereoselective manner (Scheme 34 and Table 6). However, 2,3-epoxidation of... 

---