Regioselectivity synthon formation

Chapter I first describes some common synthons and corresponding reagents. Emphasis is on regioselective carbanion formation. In the second part some typical synthetic procedures in the following order of "arrangements of functionality in the target molecule" are given ... 

CHC(=0)Me, BuNH2, MeCN, reflux, 24 h 16% change in regioselectivity and preservation from deacylation may perhaps be explained by the possibility for Schiff base formation of the synthon and product, respectively, in... 

Recent developments in the enzymatic synthesis of carbohydrates can be classified into four approaches 1) asymmetric C-C bond formation catalyzed by aldolases (1-10 2) enzymatic synthesis of carbohydrate synthons (loll) 3) asymmetric glycosidic formation catalyzed by glycosidases (12.-17) and glycosyl transferases (18-23.) and 4) regioselective transformations of sugars and derivatives (24-25). These enzymatic transformations are stereoselective and carried out under mild conditions with minimum protection of functional groups. They hold promise in preparative carbohydrate synthesis. In connection with this book, we focus on the first two approaches. 

Over the past decades, radical chemistry has been developed into an important and integral part of organic chemistry. Radical cyclization becomes a facile and useful strategy for stereo-and regioselective C-C bond formation, affording useful chiral synthons for the synthesis of C-branched sugar derivatives [56,57]. The reactions in this section are divided into intramolecular and intermolecular free radical cyclization. 

The 2,4,6-triisopropylbenzophenone chromophore has been incorporated into a polymer in order to examine whether the photocyclisation to a benzocyclobutenol could be used as the basis for a photoresponsive polymer material. Benzocyclobutenol formation has also been used to prepare a potential synthon for the synthesis of podophyllotoxin thus benzophenones (384) have been converted to benzocyclobutenols (385). The reaction is highly regioselective and yields the stereochemistry expected from thermal electrocyclic ring closure of an E,E enol (386), which is in agreement with the conclusions of Wagner described above. 

The final enzyme discussed in the chapter, haloalcohol dehalogenase, catalyzes the formation of epoxides from halohydrins. However, the reverse reaction, the ring opening of epoxides with nonhalide nucleophiles such as cyanide, azide, and nitrite, has been exploited commercially to generate /3-substituted alcohols that are potential synthons. For this reason, the regioselectivity of the enzymatic ring opening reaction is of... 

Dong developed the Rh(I)-catalyzed 4 -1-1 synthesis of tryptamines andtrypto-phols 245 based on a regioselective hydroformylation of orfho-aminostyrenes 244, wherein carbon monoxide served as a one-carbon synthon (Scheme 9.87) [240]. This hydroformylation-cyclocondensation sequence, occurring via the formation of the key o-aminophenyl aldehyde intermediate 246, allows the preparation of a variety of substituted indoles with unfunctionalized N- and C2-sites in modest to good yields (Scheme 9.87). However, indoles possessing substituents at the C4 position could not be prepared via this protocol. 

The following conclusions can be drawn from this investigation (i) Allenic and acetylenic phosphonates 53c, 53c, 55c and 55e are not stable under the conditions of Mi-chaelis-Arbuzov and Michaelis-Becker reactions (ii) chloroadenallene 45, chloiobutyne 14c and 14e can serve as synthons for stereo- and regioselective syntheses of phosphonates 47,50,60c and 60e (iii) the scope and limitations of allene formation (compound 61) during Michaelis-Arbuzov reaction certainly deserve attention. 

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