Chair-like six-membered transition state

Following the general rules (Section 1.3.3.1.2.), the racemic ( )-2-butenyl derivative 1 exhibits good anti diastereoselectivity on reaction with benzaldehyde2. This is explained as passing through a six-membered chair-like transition state. 

In contrast to ordinary chiral aldehydes (having no ability to be chelated), the reaction of 9-allyl-9-borabicyclo[3.3.1]nonane(allyl-9-BBN) with the corresponding chiral imines 4 produces the isomer syn-6 either exclusively or predominantly (Cram selectivity Table 8)5,6. The very high 1,2-asymmetric induction is explained by a six-membered. chair-like transition state, in which the inline R group occupies an axial position due to the stereoelectronic effect of imines (R CH = NR). 

Mechanistically, a-methylenecyclopentenone (2-391) reacts with ester enolate 2-392 in a Michael addition to give the enolate 2-393, which is then trapped with an aldehyde 2-394 generating the alcoholate 2-396. This eventually cyclizes through lactonization to afford 2-397 in good yield. The products 2-397 are obtained as single diastereomer thus, it can be assumed that the aldol reaction proceeds via the six-membered chair-like transition state 2-395. 

RCp2Ti(crotyl), R = Me, iPr) [13]. Both the regio- and the diastereoselectivity in the allyl-titanation reactions of aldehydes can be explained in terms of a six-membered chair-like transition state (Figure 13.1). 

A reaction of dibromoacetic acid with different aldehydes promoted by Sml2, followed by an elimination reaction also promoted by samarium diiodide, affords ( > ,/l-unsaturalcd carboxylic acids with total stereoselectivity (Scheme 8).42 A mecha- (g) nism that involves chelation of the Sm(III) centre with the oxygen atom of the alcohol group through a six-membered chair-like transition state has been described. 

The addition of a hydride donor to a /i-hydroxyketo ne can also be conducted in such a way that the opposite diastereoselectivity is observed. However, the possibility previously discussed for additions to a-chiral carbonyl compounds is not applicable here. One must therefore use a different strategy as is shown in Figure 10.22, in which the OH group at the stereocenter C-/i of the substrate is used to bind the hydride donor before it reacts with the C=0 double bond. Thus, the hydridoborate A reacts intramolecularly. This species transfers a hydride ion to the carbonyl carbon after the latter has been protonated and thereby made more electrophilic. The hydride transfer takes place via a six-membered chair-like transition state,... 

The predominant formation of the endo-isomers 50-A and 50-B can be explained as follows. Based on the concept of "concerted but asynchronous" cycloaddition, Roush stated that a 1,7,9-decatriene system possessing an electron-withdrawing group at the terminal position of the dienophile part can generally adopt a six-membered chair-like transition state while a six-membered boat-like transition state may be occasionally favored depending on the kind of substituents [45, 46]. It seems reasonable to apply the six-member transition state model to our substrate 48. Therefore, four chair-like transition states TS-Chair-A to TS-Chair-D and four boat-like transition states TS-Boat-A to TS-Boat-D are possible (Fig. (5)). All boat-like transition... 

The cis stereoselectivity can be explained by a six-membered chair-like transition state model (19) resulting from the interaction of an ( )-enolate with an imine in its trans configuration (Scheme 20). Conditions favoring ( )-enolate formation (LDA, THF) predominantly yield cis -lac-tams. 55,156,158 Addition of HMPA or reactions at higher temperature favor the forma-... 

This undergoes a vQtro-Claisen rearrangement under the reaction conditions. The mechanism of the [3,3]-sigmatropic rearrangement is shown above. The concerted reaction proceeds via a six-membered, chair-like transition state (36). The driving force of the reaction in this case is the reduced ring strain of the product molecule 24. 

For comparison purposes, the methyl-substituted analog was examined. Under the same experimental conditions, the cyclic organozinc iodide 50 was obtained and, after iodinolysis, the diastereomeric ratio (tmns/cis, 93 7) as well as the chemical yield (80%) was in good correlation with the carbolithiation reaction (see Scheme 7-38). This result indicates clearly that the stereocontrol is attributed to steric interactions where the methyl group now occupies a pseudo-equatorial position in the six-membered chair-like transition state [49] (Scheme 7-41). 

The reduction presumably involves complexation of BH3 with the ring nitrogen, followed by coordination of the ketone oxygen to the ring boron and finally transfer of the hydride to the carbonyl group via a six-member chair-like transition state. 

ATBN-F may selectively activate (bind) one of the two possible six-member chair-like transition states in which the (R)-substituent is equatorial. 

Cerium enolates are generated by the reaction of lithium enolates with anhydrous cerium chloride in THF. The cerium enolates react readily with various aldehydes and ketones at -78 °C (Scheme 24). The yields are generally higher than in reactions of lithium enolates. This is presumably due to the relative stabilities of the adducts, that of the cerium reagent being greater by virtue of coordination to the more oxophilic cerium atom. The stereochemistry of the products is almost the same as in the case of lithium enolates, as shown in Table 4. The reaction is assumed to proceed through a six-membered, chair-like transition state, as with lithium enolates. 

By combining the utility of Cu hydride catalysis with the ability of C=N containing azaarenes to activate adjacent alkenes toward nucleophilic additions, the enantioselective reductive coupling of alkenyl azaarene 154 with ketone 155 has been developed by Lam (Scheme 11.34). This process is tolerant to a wide variety of azaarenes and ketones, and provides aromatic heterocycles 157 bearing tertiary-alcohol-containing side chains with high levels of diastereo- and enantioselection via a six-membered chair-like transition state 156 [53]. 

Sol 3. (d) Initially dimethyl malonate (active methylene compound) undergoes Knoevenagel condensation with aldehyde group of citronellal to give the diene, which on heating undergoes intramolecular ene reaction via the formation of a stable six-membered chair-like transition state to give I. 

Scheme 11.18 Schematic presentation ofthe putative cyclic six-membered chair-like transition state of the Ireland-Claisen rearrangement. 8 should be destabilized by 1,3-diaxial interactions between the methyl group and R.

A rationale for this stereochemical outcome is outlined in Scheme 20, which involves oxidative addition of the lactone to Pd(0) with inversion of configuration, transmetallation with diethylzinc with retention of configuration, and allylation of benzaldehyde through a six-membered chair-like transition state, placing the phenyl group in an equatorial position. The generality of this reaction pattern is demonstrated with other cyclohexenyl and cyclopen-tenyl benzoate derivatives. 

Y-Alkoxyallylaluminium compounds (32), generated in situ from allylic ethers, react at the y-position with aldehydes or ketones to give 5 n-allylic 1,2-diol derivatives as the major products (Scheme 22 R bulkier than R ), in accord with a six-membered chair-like transition state. In a closely related... 

The BINAP/AgF system catalyzes the reaction of allyltrimethoxy-silanes (Table 13). H NMR studies suggest a fast transmetallation and equilibration to the -crotylsilver species, followed presumably by a six-membered, chair-like transition state. The reaction is limited to aromatic aldehydes. ... 

This mechanistic analysis suggested that enhancement of the electron density at the phenyl substituents of the guanidinium ion would be beneficial for the reaction, and led to the development of a new p-(dimethylamino)phenyl-substituted catalyst 25, which afforded better yields and stereoselectivities than the parent guanidinium 24 in a subset of substrates (Scheme 40.37) [45]. Catalyst 24 was also tested in the Claisen rearrangement of substituted 0-allyl (i-ketoesters, with excellent results (Scheme 40.38) [46]. Again, di- and tri-substituted compounds rearranged with high selectivity (from 5 1 dr to >20 1 dr) for the major diastereomer predicted by a six-membered chair-like transition state. 

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