Ring selective

Compound Ref. Ri Space group Z Dihedral angle between the phenyl and the pyrimidine rings/° Selected intermolecular distances [A]... 

The proposed catalytic cycle for reduction of acetophenone is illustrated in Figure 1.28. The (5)-oxazaborolidine catalyst (5)-28A has both Lewis acidic and basic sites, and its borane adduct 28B acts as a chiral Lewis acid. The B center in the borolidine ring selectively interacts with a stericaUy more accessible electron... 

The cobalt-catalyzed pyridine synthesis is the only known one-step process for the selective preparation of the industrially significant 2-substituted-pyridine derivatives. Moreover, the method is applicable to a broad variety of substituted alkynes and nitriles, thereby giving access to a whole range of pyridine derivatives having 1,2,3 or 5 substituents in the ring. Selected examples follow and are compared to the prior state of the art. 

The ring strain associated with the three-membered ring facilitates the generation of reactive species such as biradicals (cyclopropanes) and ylides (oxiranes and azir-idines) upon photoexcitation. These intermediates can undergo cycloaddition reaction with olefins to give five-membered rings. Selected examples from each reactive intermediate are presented in this section. 

Saito, I., Morii, T., Okumura, Y., Mori, S., Yamaguchi, K. and Matsuura, T. (1986) Ring-selective photorearrangement of bithiazoles. Tetrahedron Letters, 27 (52), 6385-6388. 

Substituents in the pyrrole ring affected drastically the ratio of pyrrole-1- to pyrrole-2-carbodithioate isomers. With unsubstituted pyrrole, only the 1-isomer 135 (R = R = R = H, R = Et) was formed (63%) and practically no 2-isomer was detected. When just one methyl group was introduced into pyrrole s a-position, the pyrrole-2-carbo-dithioates 134 became the only product (46%) (Equation (38)). Any combinations of alkyl substituents on the pyrrole ring selectively gave pyrrole-2-carbodithioates 134 in a yield of up to 71%. With an aryl substituent at the a-position, along with the major pyrrole-2-carbodithio-ates 134 (44-59%), the 1-isomers 135 were also formed in 24—33% yields (Equation (38)). 

Catalytic hydrogenation can be used for the selective reduction of a carbon-carbon double bond in the presence of other functional groups such as a carbonyl group or an aromatic ring. Selective reduction of one double bond in (R)-limonene (6.7), which contains two double bonds, gives (R)-carvomenthene (6.8) by hydrogenation over Ni metal. 

Nicolaou, K.C., Duggan. M.E.. Hwang. C.K., and Somers, P.K., Activation of 6-endo over 5-exo epoxide openings. Ring-selective formation of tetrahydropyran systems and stereocontrolled synthesis of the ABC ring framework of brevetoxin B, J. Chem. Soc.. Chem. Commun.. 1359. 1985. 

Both Novikov [58] and Du Bois [59] have developed systems involving sulfones (39) and sulfonates (41) that form six-membered rings selectively (Scheme 7). Novikov takes the traditional approach of preforming the diazo compound (such as 39), then treating it with Rh2(OAc)4 (25a) at room temperature to generate the cyclized products 40 in 53-80% yield. Du Bois, in addition to this protocol, also forms the carbenoid in situ from the methylene group through the use of Phl=0 and cesium carbonate. 

Once deprotonated by a strong base, the carbanion will react with electrophilic carbon in an alkyl halide or sulfonate ester to elongate the side-chain alkyl group, or with carbonyl carbon in an aldol or Claisen type reaction. The aldol readily loses water to form an alkenyl derivative, where the double bond is conjugated with the pyrimidine ring. Selective reactivity is demonstrated by the reaction of 2,5-dimethylpyrimidine with benzaldehyde, where only the electrophilic 2-methyl group reacts, to yield the 2-styryl derivative (389) (Scheme 64). Several examples are known . 

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