Reversal, of stereoselectivity

Comparison of INOC and ISOC reactions for the construction of six-mem-bered ethers 157-158J and thioethers 157-158k showed a reversal of stereoselectivity (see entries j and k). 

The Diels-Alder reaction of /V-acryloyloxazolidinone catalyzed by Cu(f-Bu)BOX shows a reversal of stereoselectivity between 1-acetoxybutadiene and 1-acetoxy-3-methylbutadiene. The former gives a 85 15 endo exo ratio, whereas the latter is 27 73 endo. exo. Explain this reversal in terms of the transition stmcture model given on p. 509. 

Although generic 3-hydroxyaldehydes are usually converted by the aldolases ste-reospecifically [1, 4], an interesting, virtually complete reversal of stereoselectivity had been observed upon RhuA-catalyzed reaction of the confermationally restricted galactodialdose derivatives 21 and 23 (Scheme 2.2.5.9) [30]. While several... 

The ratio of the three products depends on the reacting silane and alkyne, the catalyst, and the reaction conditions. Platinum catalysts afford the anti-Markovnikov adduct as the main product formed via syn addition.442- 146 Rhodium usually is a nonselective catalyst404 and generally forms products of anti addition.447 151 Minor amounts of the Markovnikov adduct may be detected. Complete reversal of stereoselectivity has been observed.452 [Rh(COD)Cl]2-catalyzed hydrosilylation with Et3SiH of 1-hexyne is highly selective for the formation of the Z-vinylsilane in EtOH or DMF (94-97%). In contrast, the E-vinylsilane is formed with similar selectivity in the presence of [Rh(COD)Cl]2-PPh3 in nitrile solvents. 

When palladium ions are bound to poly(ethyleneimine), a soluble polychelatogen, selective conversion of 2-pentyne to ds-2-pentene is achieved under mild conditions (20°C, 1 atm H2, 98% stereoselectivity).170 A dramatic reversal of stereoselectivity (selective formation of frans-2-pentene) is observed, however, if a spacious ligand such as benzonitrile is bound to palladium. 

In our group, we have explored the radical addition of alkyl radicals [97] and anomeric radicals [115] to sugar enones 134 having an exo double bond (see Schemes 46 and 47). Comparison of radical and anionic 1,4-addition, shown here, yields strictly equivalent stereoselectivities. The selectivity also depends on the nature of the entering radical, and reversal of stereoselectivity is observed on going from primary alkyl radicals to the larger t-butyl radical [for analogous observations see also Ref. 116]. 

A reversal of stereoselectivity, with a ratio usually in the range 2 1, was observed when 5-substituted-2-pyrrolines were used [55], 2,5-Pyrrolidines 48 and 49 (Scheme... 

Strikingly, some changes regarding the substitution pattern dramatically influenced the stereoselectivity of this hetero Diels-Alder reaction. Upon attachment of two methyl groups to the dienophilic terminus, the stereoselectivity was almost lost entirely, and additional substitution of the benzene moiety with two chlorine atoms at C-2 and C-4 resulted in a complete reversal of stereoselectivity. 

Similarly, malonate anions add to aldimines with reasonably high enantioselectivity. The new asymmetric center, however, has the opposite absolute configuration to that shown in eq 3. A chelated aldimine of (E) geometry is the proposed intermediate for this reversal of stereoselection (eq 4). 

Whereas ethylmagnesiation of allyl alcohols affords the syn diol with 95 5 diastereoselectivity, the same reaction on the allyl methyl ether affords predominantly the anti diastereomer with 90 10 selectivity [92] (Scheme 7-80). The reversal of stereoselectivity was attributed to the association of the magnesium alkoxide (and not of ethers) with the zirconium reagent (Scheme 7-80). 

The addition of (219) to ,j8-dialkoxynitrones (220) can be rationalized by assuming a transition state model A (Figure 2) similar to that involved in nucleophilic addition to C= (Houk model) <82JA7162>. The reversal of stereoselectivity induced by diethyl aluminum chloride is consistent with a j8-chelation model B. The scope and the synthetic utility of these reactions have been amply demonstrated <95MI 306-01 >. 

Similarly, selectivity was observed in Weinreb s efforts toward the synthesis of the microbial immunosuppressive agent FR901483.24 In this case, axial addition was favored by reaction of the lithium enolate of amide 35 with racemic 1 to produce 36. An interesting reversal of stereoselectivity was observed when, on slight alteration of the synthetic sequence, the Boc-protected amide was subjected to similar conditions. For reasons not fully understood, equatorial alcohol 37 was produced in a 53% yield, the structure of which was confirmed by X-ray crystal analysis. 

Trans-2,5-disubstituted sulfolenes (e.g., (170aA )) can be prepared by alkylation procedures as described in Section 6.3. Under thermal (neutral) or reductive conditions these extrude SO2 to give the predicted ( ,Z)-dienes (171). However, a dramatic reversal of stereoselectivity was observed when the reactions were carried out under basic conditions [116,117] (Scheme 6.54, Table 6.23). This appears to be because the rate of SO2 extrusion to the (Z)-alkene is relatively low. Thus, under basic conditions, epimerization to the c/s-sulfolene (172) can occur and this will extrude SO2 at a much higher rate than (170). This principle has been used in the synthesis of (173b), a sex pheromone of the coding moth, and (E,E)-11,12-hexadecadienal, a cabbage webworm sex pheromone. 

It was demonstrated that a complexation of trans-stilbene with cyclodextrin led to a decrease in stereoselectivity of additive bromination and to a significant yield of DL-stilbene dibromide in contrast to the formation of meso-stilbene dibromide in nonpolar solvents [98]. The authors suggested that this reversal of stereoselectivity was attributed to the polar environment provided by the secondary hydroxyl groups of... 

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