Transition synclinal

The diastereoselectivity of these reactions was initially explained in terms of acyclic, an-tiperiplanar transition states23, although a synclinal process has since been proposed on the basis of the stereoselectivity of Lewis acid induced intramolecular allylsilane- aldehyde cycliza-tions56. 

An open-chain antiperiplanar transition state was initially proposed for this reaction74, although a synclinal alternative has since been suggested56. 

The stereoselectivity of the boron trifluoride induced reactions was initially discussed in terms of open-chain, antiperiplanar transition states66. However studies of Lewis acid induced intramolecular allylstannane-aldehyde reactions are supportive of a synclinal process56,67. 

The stereoselectivity of these intermolecular reactions between 1-alkoxyallylstannanes and aldehydes induced by boron trifluoride-diethyl ether complex is consistent with an open-chain, antiperiplanar transition state. However, for intramolecular reactions, this transition state is inaccessible, and either (Z)-.yyn-products are formed, possibly from a synclinal process105, or 1,3-isomerization competes113. Remote substituents can influence the stereoselectivity of the intramolecular reaction114. 

The selectivity observed can be explained by a synclinal transition state in which the Lewis acid is chelated to two acceptor groups32. 

The diastereoselectivity observed can be explained by a synclinal transition state, probably influenced by chelation and/or stereoelectronic effects of the developing cation38. The minor product is formed via an antiperiplanar transition state. All compounds obtained are useful precursors for several spirocyclic natural products, such as terpenes like lubimine or acoradi-ene. 

A resident stereocenter in the enone part can control the formation of two new stereocenters in one step, guided by a synclinal transition state and an axial cyclization mode. The major product on cyclization of 4-methyl-3-[6-(trimethylsilyl)-4-(Z)-hexenyl]-2-cyclohexenone was formed in a ratio of 7.5 13S. 

In the Lewis acid mediated reaction the developing carbenium ion in C is stabilized by the nearby 7t-electrons of the titanium or aluminum enolate. This generates as the major diastereomer the 3,3a-/r .v-relationship between the substitution at the ring junction and the vinyl group at C-3 via a synclinal transition state. 

Fig. 9.2. Most favorable transition structures for reaction of allylsilane with acetaldehyde-fluoroborane (left) anti synclinal (right) syn synclinal. Reproduced from J. Am. Chem. Soc., 119, 12131 (1997), by permission of the American Chemical Society.

The results suggest that competition between antiperiplanar and synclinal transitions... 

Indeed in the transition state leading to ketol 19 (cf. 21), the enolate double-bond is oriented anti peri planar to the carbonyl group. This stereochemical approach must therefore be electronically favored over that leading to the isomeric ketol 20. In the last case, the carbonyl group of the side chain is gauche (synclinal) with the enolate double-bond (cf. 22). 

These results indicate that the cyclization via a chair-like transition state having the iminium cation in a pseudo-equatorial orientation (cf 33) is more facile than that with an axial orientation (cf. 34). Note again the relative orientation of the iminium cation and the olefinic double-bond which is antiperiplanar in 33 and synclinal in 34. 

Finally, the acceptor (activated olefin) may approach the enamine by two different ways, as depicted in Figure 2.11. Stabilizing interactions such as hydrogen bonding, or electrostatic interactions between the catalyst and the olefin, may offset repulsive (steric) interactions preferring transition state I. It is considered, that the acceptor approaches the enamine via an acyclic synclinal transition state, as suggested by Seebach and Golinski [7]. 

Daniel Guillon, Michael A. Osipov, Stephane Mery, Michel Siffert, Jean-Fran ois Nicoud, Cyril Bourgogne, P. Sebastiao Synclinic-anticlinicphase transition in tilted organosiloxane liquid crystals,]. Mater. Chem. 11(11),2700-2708 (2001)... 

As in ethane, the eclipsed conformations are not stable since any rotation leads to a more stable conformation. The staggered conformations are stable since they each lie in a potential energy well. The anti-periplanar conformation, with the two methyl groups opposite each other, is the most stable of all. We can therefore think of a butane molecule as rapidly interconverting between synclinal and anti-periplanar conformations, passing quickly through the eclipsed conformations on the way. The eclipsed conformations are energy maxima, and therefore represent the transition states for in ter conversion between conformers. 

The stereochemical outcome of this new annulation process can be rationalized by assuming the transition state structure 10. The synclinal arrangement of the respective hydrogen atoms exclusively gives the all-cw-annulation product 9a. 

In contrast with the thermal reactions, jyn-homoallylic alcohols were found to be the major adducts of both ( )- and (Z)-allylic stannanes (Table 5). These findings were interpreted by assuming an acychc transition state for the addition in which steric interactions between the aldehyde substituent R and the Me substituent of the stannane were the controlling factor (Fig. 1) [14]. An antiperiplanar arrangement was initially proposed, but later work has implicated synclinal arrangements in certain cases. It has also been found that anti products may result as the major or exclusive isomers from such additions. The issue is a complex one and multiple factors, including orbital overlap, may be operative. 

Figure 1. Possible acyclic transition-state arrangements for crotylstannane aldehyde additions leading to the syn adduct. Only one of two possible synclinal arrays is depicted.

Interestingly, anti adducts predominate in MgBr2-promoted additions of -methyl-crotyl tributyltin to a-oxygenated aldehydes (Eq. 11) [17]. Evidently the -methyl substituent causes the synclinal transition state to be favored in the chelation-con-trolled addition. The effect is not present in BF3 OEt2-promoted additions, which proceed as expected to afford the syn adducts. The two pathways are shown for enantiomeric aldehydes to assist in direct comparison of the two transition states. The actual additions were conducted with achiral (R = H) or racemic (R = Me) aldehydes)... 

Comparison of reaction rates and selectivities for BF3-promoted additions of crotyl tributylstannanes to aldehydes revealed that the trans crotyl isomer reacts faster and is more yn-selective than the cis isomer (Table 8) [18]. It is proposed that the synclinal transition state arrangement for the frans-crotylstannane is stabilized by a favorable interaction between the LUMO of the carbonyl oxygen and the allylic tin sigma HOMO. The analogous transition state for addition of the dx-crotylstannane is destabilized by unfavorable steric interactions (Fig. 3). 

It is suggested that these findings are best accommodated by a synclinal arrangement in the transition state which is favored by HOMO-LUMO orbital interactions, as previously suggested for certain intramolecular additions (Fig. 5). It should be noted that antiperiplanar or the alternative synclinal orientation for these transition states would result in unfavorable steric interactions between the aldehyde substituent, R, and the vinyl Me or (especially) R. ... 

Addition of cis- and trans-crotyl tributyltin to benzaldehyde could also be achieved with this catalyst system (Table 21) [37]. Interestingly, the anti product was predominant and the anttsyn ratio was unaffected by the cis.trans composition of the crotyl-stannane. These results are suggestive of an acyclic synclinal transition state for the reaction. 

It is suggested that steric effects tend to destabilize the antiperiplanar transition state normally associated with the formation of syn adducts in such reactions (Fig. 9). The alternative synclinal arrangement might benefit from favorable HOMO-LUMO interactions (see Fig. 3). 

In all examples examined the syn adduct was strongly favored. It should be noted that the thermal reaction affords the syn adduct which must be formed via a synclinal arrangement because of the cyclic nature of the transition state. Two arguments were advanced to explain the preference for a synclinal transition state in Lewis acid-promoted additions. The first of these entailed Coulombic attractive forces between the developing positive charge at the /3-position of the allylic stannane and the electron-... 

A more recent study reached a similar conclusion [78]. It was found that cycliza-tions of (Z)- and ( )-3-phenyl-8-tributylstannyl-6-octenal were highly diastereoselec-tive (Fig. 17). The (Z) isomer yielded cis, fra/js-3-phenyl-2-vinylcyclohexanol as the major product (96 4) whereas the (E) isomer afforded the trans, trans isomer (95 5). A favorable HOMO-LUMO interaction was proposed as a decisive factor in stabilizing the favored synclinal transition states. This stabilization is lacking in the alternative synclinal and antiperiplanar transition states, neither of which has the correct geometry for orbital overlap. As in the previous study, the aldehyde and double bond substituents (asterisked carbons in Fig. 17) are unable to attain an anti orientation in the antiperiplanar transition states, as has been proposed for the intermolecular additions. 

The MgBr2-promoted additions to the (5)-a-benzyloxy aldehyde proceed via a chelation-controlled transition state (Fig. 18). Approach to the carbonyl face of the nearly planar five-membered magnesium chelate is directed by the methyl substituent resulting in re attack by the allenylstannane through the antiperiplanar geometry for the (P) stannane and the synclinal geometry for the (M) stannane. An antiperiplanar orientation of the (A/) stannane would place the vinylic methyl substituent in close proximity to the chelate ring. 

An alternate model using the transition states (V)-(XII) can be used to rationalize the selectivities in these reactions (Figure 16). This model requires synclinal orientation of the reacting double bonds. The transition states (V) or (VI) and (K) or (X) are favored over (VII), (VIII), (XI) and (XII) because steric interactions between the Lewis acid bonded to the oxygen lone pair anti to the R substituent of the aldehyde and the vinyl methyl group are minimized. 

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