Transform stereoselectivity

Fluorinated dicarbonyl triphenylphosphoranes, easy to prepare (by trifluoro-acetylation of the stabilised ylides), have been transformed stereoselectively into / -substituted jS-trifluoromethyl alkenoates by treatment with organolithium reagents (Eq. 115) [306]. The stereoselectivity could be reversed by O-methyla-tion of the initial adduct followed by protonation [307]. 

Vinyltributylstannanes are smoothly transformed, stereoselectively, to vinyl chlorides (equation 150)972. 

Asymmetric Desymmetrization. Desymmetrization of an achiral, symmetrical molecule is a potentially powerful but relatively unexplored concept for the asymmetric catalysis of carbon-carbon bond formation. While the ability of enzymes to differentiate between enantiotopic functional groups is well known, little is known about the similar ability of nonenzymatic catalysts to effect carbon-carbon bond formation. The desymmetrization by the enantiofacial selective carbonyl-ene reaction of prochiral ene substrates with planar symmetry provides an efficient access to remote internal asymmetric induction which is otherwise difficult to attain (eq 6). The (2R,5S)-xyn product is obtained in >99% ee along with more than 99% diastereoselectivity. The desymmetrized product thus obtained can be transformed stereoselectively by a more classical diastereoselective reaction (e.g., hydroboration). 

The Weinreb group has reported the only examples of intramolecular [4 + 2] cycloadditions of iV-sulfi-nyl compounds. - For example, diene carbamate (127) could be converted to an -sulfmyl compound which cyclized intramolecularly to dihydrothiazine oxide (128) (Scheme 16). Using the chemistry outlined in equation (54), (128) was transformed stereoselectively to tAreo-sphingosine (129). Similarly, ( ,Z)-diene carbamate (130) was transformed via adduct (131) to erytAro-sphingosine (132). 

Addition of titanium tetraisopropoxide to the anion (61 R = R = H) produces a 3-(alkylthio)allyltita-nium reagent that condenses with aldehydes to give a/ih-P-hydroxy sulfides in a highly regio- and stereo-selective manner (Scheme 34). A/ih-3-hydroxy sulfides are transformed stereoselectively to /ra/u-vinyloxiranes or 1,3-alkadienes. In contrast, the titanium and lithium reagents from crotyl ethyl sulfide react with aldehydes affording 8-hydroxyvinyl sulfides exclusively. 

Very often, enzymes are known (for instance esterases) which are able to transform stereoselectively only the required enantiomer of a precursor into the product. Several schemes based on this technique - especially those using immobilized enzymes and microorganisms that also work in organic media - have been established in industrial-scale processes. 

Stoichiometric studies of M-H additions to alkynes also show mixed stereochemical results. The more common cw-addition is typified by the CO-promoted transformation of Cp2Nb(H)(RCECR) to ds-Cp2Nb(CO)(Ti -CR=CHR) [90]. In mononuclear systems where frans-additions have been found, radical-type mechanisms have been implicated [91] or cis/trans isomerization of the intermediate vinyl species [92] has been found. Although the intermediacy of alkyne complexes has not been established, Schwartz s hydrozirconation of alkynes [93] by Cp2ZrHCl represents a general entry to vinyl-metal species which can be transformed stereoselectively to alkenes, vinyl halides, and/or carboxylic acids. 

The synthetic route can be shortened, if a sterically demanding silyl ketone is used as the synthetic equivalent of acetaldehyde, according to Bouffard and Salzmann. The lithium alkoxide is then transformed stereoselectively in a Brook rearrangement into the silyl ether. This is of advantage, since during the down-stream-processing no separate protection step is needed. [59]... 

Epoxidation of l,3-bis(trimethylsilyl)propene creates a versatile building block for the stereoselective construction of both E-and Z-2-alkenyltrimethylsilanes via a magnesium-mediated rearrangement to bis(trimethylsilyl)propanal. It was found that treatment of the resulting aldehyde with a variety of Clrignard reagents then leads to an intermediate alcohol species, which can be transformed stereoselectively into an alkene depending on the conditions chosen. This allows for the synthesis of valuable substituted allylsilanes for further transformations. ... 

Reaction of certain sulfur-substituted allylic anions with Ti(0-i-Pr)4 produces a 3-(alkylthio)allyltitanium reagent that condenses, through its a-terminus, with aldehydes to give onri-p-hydroxy sulfides in a highly stereo- and regioselective manner (eq 18). These latter compounds can be transformed, stereoselectively, into froar-vinyloxiranes or 1,3-dienes. ... 

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