Stereoselective ring opening

One of the key steps in the synthesis of actinomycin D (241 Scheme 3.88) and its serine analogue involved the regio- and stereoselective ring-opening of aziridine 238 with the acid 239 [138, 139]. This transformation took place in methylene... 

Enantioselective alcoholysis of racemic, prochiral, or meso cyclic anhydrides can be catalyzed by hydrolases, yielding the corresponding monoesters (Eigure 6.25). In most cases, the enantioselectivity was moderate ]75-77]. Organometallic catalysts or organocatalysts such as cinchona alkaloids are often more efficient than enzymes for the stereoselective ring opening of cyclic anhydrides. 

Regio- and stereoselective ring opening of epoxides is facilitated by TiIV complexes. The existence of hydroxy groups at the n-position of epoxides is quite important to the reactivity of... 

More recent reports from Cordova [155] and Wang [156] have described the cyclopropanation of a, P-unsaturated aldehydes 99 with diethyl bromomalonates 100 and 2-bromo ethyl acetoacetate catalysed by a series of diaryIprolinol derivatives. Both describe 30 as being the most efficient catalyst in many cases and optimal reaction conditions are similar. Some representative examples of this cyclopropanation are shown in Scheme 40. The transformation results in the formation of two new C-C bonds, a new quaternary carbon centre and a densely functionalised product ripe for further synthetic manipulation. Triethylamine or 2,6-lutidine are required as a stoichiometric additive in order to remove the HBr produced during the reaction sequence. The use of sodium acetate (4.0 equivalents) as an additive led to subsequent stereoselective ring opening of the cyclopropane to give a,P-unsaturated aldehydes 101. It can be envisioned that these highly functionalised materials may prove useful substrates in a variety of imin-ium ion or metal catalysed transformations. 

Fig. 42 Stereoselective ring-opening of epoxides according to Jacobsen...

Ajellal N, Thomas CM, Carpentier J-F (2009) Functional syndiotacticpoly(P-hydroxyalkanoate)s via stereoselective ring-opening copolymerization of rac-P-butyrolactone and rac-allyl-P-butyrolactone. J Polym Sci A Polym Chem 47 3177-3189... 

Ajellal N, Bouyahyi M, Amgoune A, Thomas CM, Bondon A, Pillin I, Grohens Y, Carpentier J-F (2009) Syndiotactic-eirriched Poly(3-hydroxybutyrate)s via stereoselective ring-opening polymerization of racemic (3-butyrolactone with discrete yttrium catalysts. Macromolecules 42 987-993... 

The preparation of the required t/ireo-3-methylcysteine component in this synthesis, used the stereoselective ring opening of aziridinesJ62-63 It was shown to be possible to maintain stereospecificity of the reaction using the aziridine ring as an intermediate. 

Furthermore, simple and cheap IV-nucleophiles, such as metalated (S)-l-phenylethanami-ne, may also be used as efficient stereoselective ring-opening reagents9. Very good yields for 1 (d.r. 95 5) are obtained with the corresponding sodium amide as AT-nucleophile14. 

Synthesis of chiral monosaccharides. Masamune, Sharpless, and coworkers2 have developed a general, iterative sequence for addition of two chiral hydroxymethy-lene units to an aldose. The key step involves regio- and stereoselective ring opening... 

J. G. Buchanan, Migration of epoxide rings and stereoselective ring opening of acetoxyepoxides, Methods Carbohydr. Chem., 6 (1972) 135-141. 

Suzuki, T. Saimoto, H. Tomioka, H. Oshima, K. Nozaki, H. Regio- and stereoselective ring opening of epoxy alcohols with organoalumi-num compounds leading to 1,2-diols. Tetrahedron Lett. 1982, 23, 3597-3600. 

In this context it is also of interest that the parent cyclobutene radical cation can be observed at 77 K and, upon warming to 110 K, undergoes stereoselective ring opening to transoid butadiene radical cation [270]. A cyclopropylcarbindiyl radical cation has been proposed as an intermediate for this transformation [271]. 

Epoxides are versatile intermediates in organic synthesis owing to the possibility of controlled stereoselective ring opening. The ability of various chlorosilanes to serve as a source of chloride ion for the opening of epoxides was first recognized... 

Bertolini, F., Crotti, S., Bussolo, V.D., Macchia, F., and Pineschi, M. (2008) Regio- and stereoselective ring opening of enantiomerically enriched 2-aryl oxetanes... 

In addition to the above hydrolysis reactions, dinuclear approach to providing joint Lewis acid activation and nucleophile activation has been applied to other organic reactions (Figure 6.14) including stereoselective ring-opening of epoxides (18) [65, 66], stereoselective aldol condensation (19) [67, 68], and stereoselective reduction (20) reactions [69]. 

Wang, M.X., Wang, D.X., Zheng, Q.Y. et al. 2007. Nitrile biotransformations for the efficient synthesis of highly enantiopure l-arylaziridine-2-carboxylic acid derivatives and their stereoselective ring-opening reactions. Journal of Organic Chemistry, 72 2040-5. 

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