Stereoselectivity regioselectivity

The discovery that Lewis acids can promote Diels-Alder reactions has become a powerful tool in synthetic organic chemistry. Yates and Eaton [4] first reported the remarkable acceleration of the reactions of anthracene with maleic anhydride, 1,4-benzoquinone and dimethyl fumarate catalyzed by aluminum chloride. The presence of the Lewis-acid catalyst allows the cycloadditions to be carried out under mild conditions, reactions with low reactive dienes and dienophiles are made possible, and the stereoselectivity, regioselectivity and site selectivity of the cycloaddition reaction can be modified [5]. Consequently, increasing attention has been given to these catalysts in order to develop new regio- and stereoselective synthetic routes based on the Diels-Alder reaction. 

VI. AMIDOESTERS. STEREOSELECTIVE, REGIOSELECTIVE AND ELECTROPHILIC REACTIONS AT THE a-CARBON... 

When R and/or in (5) possess a chiral center, both the initial attack of hydroxide ion and the subsequent migration can be stereoselective, regioselective and stereospecific. Steroidal a-diketones are suitable vehicles for such studies, and their chemistry is considered later (see Section 3.6.3.5). 

Principles of 4-Penten-l-oxyI Radical Cyclizations -Stereoselectivity, Regioselectivity, and Theoretical Considerations... 

In connection with the activation of saturated hydrocarbons via homogeneous catalysis, we have examined transition metal catalyzed reactions of various strained hydrocarbon systems that have unique steric and electronic properties. Strained carbon-to-carbon single bonds have considerable TT-bonding character. The chemistry of these substrates should be intermediate between well-documented transition metal chemistry of alkenes and rather unclarified alkane chemistry (1, 2, 3). Our attention has been focused particularly on the stereoselectivity, regioselectivity, and periselec-tivity of the Ni(0)-catalyzed reactions (4-14). 

Stereospecificity, just like stereoselectivity, regioselectivity, and facioselectivity, becomes nontransparent and experimentally determinable only with the proper substrates. The... 

Chiroselectivity and chirospecificity vide infra), just like stereospecificity, stereoselectivity, regioselectivity, and facioselectivity, become non-transparent, and experimentally verifiable, with the proper substrates (see p. 74). The chiroselectivity or chirospecificity of an 5 2 process cannot be established using CH3I or iodocyclohexane as substrates. The chirospecificity becomes nontransparent and is established experimentally when one uses 4-t-butyliodocyclohexane or enantiopure 2-octyl iodide. Surely, the inversions occur with CH3I or iodocyclohexane as well however, with the latter two substrates, they are experimentally unverifiable. 

Table 4. Comparison of stereoselectivity, regioselectivity, and competitive reactivities in iron cyclam complex-catalyzed epoxidations by various oxidants. 

It is worthy noting that while oxidation of the tetracyclic ring system 7.11 provided the desired p-epoxide 7.12 (see also compound 7.3), Heathcock 145) found that epoxidation of the tricyclic hydrindane derivative 6.4 (Scheme 11) occurred from the a-face either with mCPBA or with VO(acac)2 and BuOOH, or with dimethyldioxirane. It has been suggested that in the case of 6.4 a stereoelectronic effect is responsible for the observed a-stereoselectivity. Regioselective oxidation of diol 25.1 then provided 25.2, a diastereomer of ( )velutinal. 

Selectivity improvement is a critical requirement for industrial applications of lipase. This includes substrate selectivity, stereoselectivity, regioselectivity, and enantio-selectivity. Adsorption of Candida rugosa on celite was reported to enhance the stability of lipase and improve its enantioselectivity up to 3-fold (Ogino, 1970). Entrapment in cellulose acetate-Ti02 gel fiber improved the selectivity of Rhizomucor miehei lipase in the hydrolysis of 1,2-diacetoxypropane (Ikeda and Kurokawa, 2001). Also the enantioselectivity of pegylated P. cepacea lipase was increased 3-fold by... 

An extensive review with 178 references has been published on the cycloadditions of methylene- and alkylidene-cyclopropane derivatives. The stereoselectivity, regioselectivity, and efficiency of the intramolecular transition metal-catalysed 5-1-2-cycloadditions of vinyl-substituted cyclopropanes (192) producing bicyclic adducts (193) have been investigated (Scheme 75) " ... 

Once again, we are not trying to simply cite all the cases where certain manipulations of reagents or reaction conditions have caused changes in stereoselectivity, regioselectivity, or reaction rates. We rather want to demonstrate that various inherent obstacles in cycloaddition reactions or problems with the two reaction partners can be overcome by guiding these molecules into the desired transition state and that this can be done over the whole breadth of cycloaddition reactions. 

The Michael reaction is of central importance here. This reaction is a vinylogous aldol addition, and most facts, which have been discussed in section 1.10, also apply here the reaction is catalyzed by acids and by bases, and it may be made regioselective by the choice of appropriate enol derivatives. Stereoselectivity is also observed in reactions with cyclic educts. An important difference to the aldol addition is, that the Michael addition is usually less prone to sterical hindrance. This is evidenced by the two examples given below, in which cyclic 1,3-diketones add to o, -unsaturated carbonyl compounds (K. Hiroi, 1975 H, Smith, 1964). 

The stereospedfic and regioselective hydrobromination of alkynes with chlorobis(T -cyclopentadienyl)hydrozirconium and NBS produces ( )-vinylic bromides in good yields. The bromine atom usually adds regioselectively to the carbon atom that bears the smaller substituent and stereoselectively trans to the larger substituent (D.W. Hart, 1975 M. Nakatsuka,... 

Unique chemistry is associated with the cyclopentenone all five carbon atoms can be functionalized, and the endo-methyl groups of the acetonide assure clean stereoselective addition of the alkenylcopper reagent from the convex side. The use of the acetonide group to control enolate regioselectivity and to mask alcohols should be generally applicable. 

The Pd-catalyzed hydrogenolysis of vinyloxiranes with formate affords homoallyl alcohols, rather than allylic alcohols regioselectively. The reaction is stereospecific and proceeds by inversion of the stereochemistry of the C—O bond[394,395]. The stereochemistry of the products is controlled by the geometry of the alkene group in vinyloxiranes. The stereoselective formation of stereoisomers of the syn hydroxy group in 630 and the ami in 632 from the ( )-epoxide 629 and the (Z)-epoxide 631 respectively is an example. 

In addition to being regioselective alcohol dehydrations are stereoselective A stereo selective reaction is one m which a single starting material can yield two or more stereoisomeric products but gives one of them m greater amounts than any other Alcohol dehydrations tend to produce the more stable stereoisomer of an alkene Dehydration of 3 pentanol for example yields a mixture of trans 2 pentene and cis 2 pentene m which the more stable trans stereoisomer predominates... 

Regioselective hydrolysis of diesters is a challenging problem ia synthetic chemistry because the side reactions always reduce the yield of desired product. Some Upases are well suited to perform this task. Lipase OF-360 (Meito Sangyo) hydrolyzes diester (55) ia 74% theoretical yield and 93% ee (70). Lipase from Pseudomonas cepacia suspended ia diisopropyl ether saturated with water hydrolyzes triester (56) with a remarkable efficiency and regio- and stereoselectivity (71). 

This procedure illustrates a general method for the preparation of alkenes from the pal 1 adium(Q)-cata1yzed reaction of vinyl halides with organo-lithium compounds, which can be prepared by various methods, including direct regioselective lithiation of hydrocarbons. The method is simple and has been used to prepare a variety of alkenes stereoselectively. Similar stoichiometric organocopper reactions sometimes proceed in a nonstereoselective... 

Cycloaddition involves the combination of two molecules in such a way that a new ring is formed. The principles of conservation of orbital symmetry also apply to concerted cycloaddition reactions and to the reverse, concerted fragmentation of one molecule into two or more smaller components (cycloreversion). The most important cycloaddition reaction from the point of view of synthesis is the Diels-Alder reaction. This reaction has been the object of extensive theoretical and mechanistic study, as well as synthetic application. The Diels-Alder reaction is the addition of an alkene to a diene to form a cyclohexene. It is called a [47t + 27c]-cycloaddition reaction because four tc electrons from the diene and the two n electrons from the alkene (which is called the dienophile) are directly involved in the bonding change. For most systems, the reactivity pattern, regioselectivity, and stereoselectivity are consistent with describing the reaction as a concerted process. In particular, the reaction is a stereospecific syn (suprafacial) addition with respect to both the alkene and the diene. This stereospecificity has been demonstrated with many substituted dienes and alkenes and also holds for the simplest possible example of the reaction, that of ethylene with butadiene ... 

Fluorine and selenium, in the form of a phenylselenenyl group, add to nucleophilic olefins with Markovnikov regioselectivity and anti stereoselectivity on reaction with several reagents that may form phenylselenenyl fluoride in situ [792, 193, 194] (equation 30)... 

Both stereoselectivity and regioselectivity occur in the reaction of steroid vinyl esters, ethers, and related compounds with A -fluoropyridinium salts [75, 7d] (equation 45). 

Examples of the use of lithium aluminum hydride in stereoselective [75] (equation 9), regioselective [16] (equation 10), and product-selective [77](equation 11) displacements of fluonne are available... 

The well-known reduction of carbonyl groups to alcohols has been refined in recent studies to render the reaction more regioselective and more stereoselective Per-fluorodiketones are reduced by lithium aluminum hydride to the corresponding diols, but the use of potassium or sodium borohydride allows isolation of the ketoalcohol Similarly, a perfluoroketo acid fluonde yields diol with lithium aluminum hydnde, but the related hydroxy acid is obtainable with potassium borohydnde [i f] (equations 46 and 47)... 

---