Stereospecificity, isomerization

Optical resolution of the dithiirane 1-oxides 2 and 3 was accomplished by HPLC equipped with a chiral column (97T12203). Absolute configurations of 2a and 2b were determined by X-ray crystallography. Tire stereospecific isomerization (epimerrzation) of 2a to 3b and 2b to 3a was observed during the resolution study. 

By a sequence of thermal and photochemical steps in the course of a simple sulfolene reaction, stereospecific isomerizations are possible429-431 (equation 71). On the other... 

In a reaction that establishes the flavonoid heterocyclic C-ring, chalcone isomerase (CHI) catalyzes the stereospecific isomerization of chalcones to their corresponding (2S)-flavanones, via an acid base catalysis mechanism. Almost 40 years ago, the first flavonoid enzyme to be described was CHI (in the adopted hometown of the authors of this chapter). Since then CHI has been analyzed in great detail, and surprisingly, it shows little similarity to other known protein sequences, although CHI-like sequences have recently been reported from plants and other organisms. ... 

The stereospecific isomerization of 3,4-epoxyalcohols under acidic or basic conditions <2002T6199, 1998TL8259> has been the method of choice for the final step in syntheses of merrilactone (Equation 48) <2006AGE4843>. An analogous acid-promoted closure of benzyl ethers of epoxy alcohols has also been observed <2000H(52)171>. An unusual 4-/ro/o-isomerization of a hydroxy epoxide apparently reflects the inability of a primary alcohol to cyclize via 4-exo- or 5-endo-modes transesterification liberates the secondary alcohol, which undergoes a 4-< //6>-cyclization (Equation 49). An X-ray structure of the product has been reported <2003JOC4422>. Section... 

A novel stereospecific isomerization of 2-(l-bromoalkyl)-l-sulfonylaziridines 111 into 2-(bromomethyl)-3-alkyl-1-sulfonylaziridines 113 was reported to occur by carrying out the reaction in the presence of MgBr2- The isomerization reaction was found to be considerably dependent on the nature of the solvent used. Polar solvents such as MeOH or dimethylformamide (DMF) are able to solvate the magnesium bromide and give 113 as the thermodynamically most stable regioisomer (Scheme 29) <2005TL6541>. 

The ramifications of Se transmetalations for chiral allenylstannanes are briefly summarized in Scheme 5.2.72 in which the common electrophile, (2f )-3-benzyloxy-2-methylpropanal, serves to selectively generate two distinct diastereoisomers 336 and 337. The reaction with (M)-286 and SnCU proceeds via anti-SE transmetalation followed by stereospecific isomerization to give the (P)-trichloroallenylstannane... 

The reactivity and nutritional value of a particular TAG depend on stereospecific isomerism. If a particular TAG composition is undesirable, it is possible to randomize the product, most often by treatment with alkaline catalysts. In a randomized fat or oil, the distribution of FA among TAG species is quite random. The randomization is particularly important in solid fats because the melting interval and the texture are changed for example, the melting point of cocoa butter increases from 34.8 to 54.2°C during the randomization. 

Gounder R, Davis ME (2013) Titanium-beta zeolites catalyze the stereospecific isomerization of D-glucose to L-sorbose via intramolecular C5-C1 hydride shift. ACS Catal 3(7) 1469-1476... 

Chalcone isomaraso (EC 5.5.1.6) a plant enzyme, catalysing the stereospecific isomerization of chal-cones to the corresponding (-)(2S)flavanones (Fig.),... 

The further biosynthetic pathway to anthocyanins involves the formation of L-phenylalanine from chorismic acid. L-phenylalanine is then converted to trans-cmrL mic acid through fra 5-elimination of ammonia. In a second step fra 5-cinnamic acid is hydroxylated and activated to yield 4-coumaroyl-CoA. In the next step, 4-coumaroyl-CoA is condensed with 3 molecules of malonyl-CoA to yield naringenin chalcone. naringenin chalcone is rapidly and stereospecifically isomerized to naringenin. 

A more stereospecific course of glycoside formation by reaction of the polymer-bound C-1 bromide with the second monosaccharide hydroxyl component involves the initial formation of a c -l 2-orthoester. This undergoes an acid-catalyzed stereospecific isomerization (Kochetkov et al., 1967) to the corresponding glycoside (disaccharide). 

Stereospecific isomerization. An ethereal soln. of (lRS,2SR)-2-[(SR)-(p-chlorophenyl)sulfinyl]-N,N,3,3-tetramethylcyclopropylamine treated with a slight excess of ethereal HCl, the ether removed immediately, the resulting hydrochloride dissolved in chloroform, and allowed to stand 25 min. at room temp. 

Both the Sfj- and the S j -substitution occur with complete chirality transfer and there are good reasons to suggest that the R-BINAP ligand is a matching enantiomer for the stereospecific isomerization of 160 to 161. 

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