Six-membered chair-like

The stereoselectivity of these reactions, and the observed 1,3-allylic transposition, are consistent with the participation of chair-like, six-membered ring transition states37. 

The stereoselectivity of these reactions has been interpreted in terms of chair-like six-membered ring transition states in which the substituents a to tin adopt an axial position, possibly because of steric and anomeric effects. The cc-substituted (Z)-isomers are less reactive because the axial preference of the a-substituent would lead to severe 1,3-diaxial interactions17. 

An intermediate allyltin trichloride, possibly stabilized by an oxygen-tin hypervalent interaction, which then reacts with the aldehyde via a chair-like six-membered ring transition state with the substituent a to tin in an axial position, may be involved94. On heating with aldehydes, the 4-benzyloxypentenylstannane shows only modest diastereofacial selectivity22. 

As with other noncatalyzed reactions of allylstannanes with aldehydes (see Section 1.3.3.3.6.1.3.2.) these reactions are believed to proceed via a chair-like six-membered ring transition state in which the a-substituent, in this case methoxymethoxy, adopts the axial position103. 

The diastereoselectivity observed on carbonyl addition of ( )-alkenyltitanium derivatives is in accordance with the reaction proceeding via a chair-like six-membered Zimmerman Traxler transition state. The following facts are diagnostic ... 

In this case, /6 -attack at the carbonyl group is induced by the stereogenic center at the metal. This fact is best explained by an approach of the aldehyde trans to the nitroso group, forming a chair-like six-membered transition state in the lower half-sphere which is not shielded by the cyclopentadiene residue. 

The Raman spectra of solid a- and PS O show characteristic differences but both spectra can be assigned assuming a chair-like six-membered homocycle with an exocyclic oxygen atom (molecular symmetry Cj see Scheme 4). 

It can be assumed that in the domino process of, for example 5-3, a reactive radical cation intermediate 5-5 is initially formed [5]. The intramolecular cyclization then proceeds almost exclusively through a stable, chair-like, six-membered transition state 5-8 to give a distonic radical cation 5-9, which is trapped by the aromatic... 

Compounds such as y-dimethylvalerophenones (8) and P-ethoxypropiophenone (9), which lack y— H, produce exclusively cyclopentanol 10 and tetrahydrofuranol 11 derivatives, respectively, albeit in low quantum yields (Scheme 8.2). With benzene as solvent, the diastereomeric ratio for 11 was 5 1 (trans/cis), but was 1 1 in acetonitrile. The easy 1,5-H-transfer in these acyclic ketones was rationalized as involving a torsion-free, chair-like six-membered transition state. 

A kinetic study of the Ph2BOH-catalysed reactions of several aldehydes with 2 revealed that the rate of the disappearance of 2 followed first-order kinetics and was independent from the reactivity of the aldehydes used. Taking into account this result, we have proposed the reaction mechanism in which a silyl enol ether is transformed to the corresponding diphenylboryl enolate before the aldol addition step takes place (Scheme 13.1). The high diastereoselectivity is consistent with the mechanism, in which the aldol step proceeds via a chair-like six-membered transition state. The opposite diastereoselectivity in the reaction with the geometrical isomers of the thioketene silyl acetal shown in Table 13.3 also supports the mechanism via the boron enolate, because this trend was also observed in the classical boron enolate-mediated reactions in dry organic solvents. Although we have not yet observed the boron enolates directly under the reaction conditions, this mechanism can explain all of the experimental data obtained and is considered as the most reasonable one. As far as we know, this is the first example of... 

Upon treatment of indoles with allylic borane, [l,3]hydrogen shift from the nitrogen atom to C-3 proceeds, producing the imine complex 8. Further allylboration of the C=N bond formed occurs with rearrangement through the chair-like six-membered transition state 9 allylic group being added mainly or quantitatively in /ram-fashion relative to the 3-R. Hydrolysis of iminoborane 10 formed leads to the 2-allylated indoline 11. 

Brown s crotyl borane 158 (chapter 24) provides reagent control through a chair-like six-membered ring in the formation of 159. Oxidation of the alkene (ozone with oxidative workup) gives the free acid marked in 160 and on removal of the benzylic group the freed amine (circled) cyclises to it to give the pyrrolidone required (147 as its methyl ester). The synthesis can easily be completed from there. 

The organoindium chalcogenides also exhibit extended ring, cage, and cluster-like structures as is commonly found for 111-V derivatives. For example, intra-dative coordination is demonstrated by [Me2lnCH2SMe)2], which adopts conformations where the S-bound Me groups are placed in equatorial positions of the chair-like six-membered ring systems. ... 

The aldol addition reactions are believed to proceed by way of a chair-like six-membered cyclic transition state in which the ligated metal atom is bonded to the oxygen atoms of the aldehyde and the enolate (Zimmerman-Traxler model). For the reaction of a ds-enolate 46 with an aldehyde RCHO, the transition state could be represented as 48 (1.64). This places the R group of the aldehyde in a pseudoequatorial position in the chair-like conformation and leads to the syn aldol product. Likewise, reaction of the tran -enolate proceeds preferentially via the... 

Larger binary chalcogeno phosphate anions are almost exclusively based on similar tetrahedral [PE4] phosphorus (V) building blocks, which can condense to form rings or clusters. This is well illustrated by the isostructural [1,4-P2Eg] anions (E = Se ) (3) which contain chair-like six membered P2E4... 

The present reaction system can be explained by a mechanism involving a boron enolate as a reaction intermediate generated by Si-B exchange (Scheme 10.1). That the diastereoselection is reversed by using the stereoisomers of the silyl enolate supports this hypothesis because this type of reversal is commorJy observed in die traditional boron enolate mechanism, which involves a chair-like six-membered transition-state. Furthermore, the trend that anti selectivity is poorer than syn selectivity in the reactions is also found in traditional boron enolate mediated aldol reactions. The mechanism is based on the hypothesis that PhjBOH can react vnth a silyl enolate to form the corresponding boron enolate under the conditions. When a Z-enolate is used, an aldehyde and the boron enolate react via a chair-like six-membered transi-... 

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