Twist boat-like transition

These results are interpreted by the attack of the aldehyde on the least hindered side of the enolate 6.104, through a chair or twist-boat-like transition state [408] (see above). Both (S)- and (R)-chromium tricarbonyl complexes of 2-me-thoxyacetophenone have been prepared, and their boron enolates react selectively with MeCH=CHCHO, although further decomplexation was not carried out [1280]. Chiral aminonitrile 1.71 bearing the ephedrine skeleton [301] or men-thone-derived ketals [374] have been used as chiral auxiliaries in aldol reactions, but modest selectivities were obtained. 

Addition of All-MgBr to 120 yielded piperidine 121 as practically single stereo-isomer. We assumed, that the attack of the hydride anion on the imine moiety occurred from the re face and proceeded through a preferred chair-like transition state, rather than through a twisted boat-like transition state (Fig. 2) Similar high stereoselectivity in the... 

Fig. 2 Twisted boat-like transition state is disfavored and, therefore, 121 is the only observed diastereoisomer.

Subsequently, certain authors (126-128) have supported this interpretation, but alternative explanations have also been proposed (129-132). Another early discussion of the stereochemistry of the enolization process was that of Vails and Toromanoff (133). They proposed that if stereoelectronic effects are an important parameter, the cyclohexanone enolate should react by two different pathways, one involving a chair-like transition state (443 444) and the other a boat-like transition state (443 445 446) Thus, both of these reactions proceed by perpendicular attack (134) of the electrophile. Their energy difference results from the difference in strain between the chair (444) and the twist-boat (445) forms. 

According to the Woodward-Hoffmann rales, five concerted transition states are possible for the Claisen as well as the closely related Cope rearrangements chair, boat, twist, cross and plane (Table 6). Only the chair and boat TS have to be considered, as twist, cross and plane are antarafacial-anta-rafacial processes and require highly elevated temperatures. - For the correct prediction of product stereochemistry it is nevertheless crucial to know the preference for chair- or boat-like transition state in the actual 3,3-sigmatropic shift. 

The transition state for the degenerate [3,3]sigmatropic rearrangement of the spirodienones (267) is postulated to be of the form (268). This is substantiated by the increased activation energy observed for compounds whose substituents impose substantial twisting constraints on this classical boat-like transition state. 

A cyclohexenyl system can exist in the two conformations 81 and 82. A syn attack on ffl to give 84 should take place via a transition state having the chair-like stereochemistry 83 where the electron pair is oriented anti-periplanar to the C - X bond. An anti attack on 81 should occur via the twist-boat transition state 85 where the electron pair is syn to the C-X bond, yielding the product in the twist-boat conformation 86. A syn attack on conformer 82 should give 88 via the twist-boat 87 where the electron pair is antiperiplanar to the C —X bond, whereas an anti attack should give 90 via the chair form transition state 89 where the electron pair is syn to the C -X bond. 

The twist-boat can also interconvert with the boat form itself. The effect is to make all the protons isochronous if the twist-boat is also in rapid equihbrium with the boat-boat form, as is extremely likely from the previous discussion. The boat is probably the transition state in this pathway (TB B - TB) and thus the activation energy calculated is 9.4 kcal/mole. This pathway has a sufficient barrier for study by nmr hne shape methods if cyclooctane actually exists in these forms. At high temperatures e.g. room temperature) all the protons should be isochronous and should give... 

Another explanation takes into account that boat- and twist-shaped six-membered, closed transition states can successfully compete with the chair model. " Evans et al. pointed out that in a-unsubstituted enolate reactions, missing allyl strain interactions lead to lower selectivity in diastereoselective aldol reactions.Calculations indicate that a twist-boat can easily be formed from the U-configuration of a-unsubstituted enolates. The possible transition state in this case has a geometry like 34 and is favored by the chelating character of the complexation mode for the zinc cation and the outward-pointing substituents of the oxazolidinone moiety. This twist-boat transition state correctly predicts the stereochemical outcome of the reaction. 

For more conformationally-constrained chiral substrates, however, diastereoselectivity can be expected to be good to excellent. Lithium enolates derived from sterically unencumbered cyclohexanones undergo preferential axial acylation as illustrated by the reductive acylation of (R)-(-)-carvone 4 to afford a 3 1 mixture of esters 5 and 6. whereas equatorial acylation is favored in compounds that possess an alkyl substituent in a 1,3-syn-axial relationship to the reacting center, as in the conversion of tricyclic enone 7 to ester 8 (epimeric with the product from the more traditional sequence of acylation followed by alkylation). (In substrates of this kind it is assumed that the transition state structure is based on a twist-boat conformation which permits the reagent to approach along an axial-like trajectory on the less encumbered, lower face of the substrate.) ... 

First, the stereochemistry at C-8 is set by the Z olefin geometry, which was obtained via a standard Wittig reaction. Conformational restrictions placed on dibenzocyclooctadiene ring systems are generally limited to the twist-boat-chair (TBC) and the twist-boat (TB) limiting conformations. The stereodefmed Z-alkene excludes the TB structure. We therefore expected carbons 6, 7, 8, and 9 to assume the chair-like transition state geometry of the TBC conformation. 

H) Regioselectivity of limonene-1,2-epoxide hydrolysis For limonene-1,2-epoxide, the natural substrate of LEH, the situation is less complex than for 1-methylcyclohexene oxide. The isopropyl substituent at C4 of limonene-1,2-epoxide determines the preferred helicity of the half-chair, and for each stereoisomer, only the helicity with the substituent in an equatorial position will be observed (Scheme 4). For this helicity, attack is preferred at the carbon that leads to a chair-like transition state. The transition states for attack at either Cl or C2 of 5 are shown in Figure 3. Attack at Cl of 5 leads to a chair-like transition state and exhibits a barrier of 14.9 kcal mol. Attack at C2 of 5 results in a twist-boat transition state and exhibits a barrier of... 

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