Stereoselectivity chair-like transition state

The smooth cyclizations of ( )- and (Z)-/V-( 3-alkenyl/ethoxy lac tarns with formation of a six-membered ring (via a chair-like transition state) occur completely stereoselectively in quantitative yield with retention of the geometry of the double bond913. 

Imine 214 (R = H) gave a 3 1 mixture of ot/(3 chloro epimers of 215 (R = H) when a mixture of Lewis acids, TiCl4, and Ti(OiPr)4 was used. The stereoselectivity of the formation of 215 is rationalized by a chair-like transition state 217 with equatorial attack of chloride ion <1998TL7239, 2000JOC655>. 

On the other hand, Beckwith rules [22] are useful guidelines for predicting the stereoselectivity in the cyclisation of substituted hexenyl radicals i) 1- or 3-substituted radicals give preferentially cij-disubstituted cyclopentane derivatives and ii) 2- or 4-substituted radicals give mainly rrans-disubstituted derivatives. These rules can be explained in terms of 1,3-diaxial interactions present in a chair-like transition state. Some examples which demonstrate such an effect are [19b] ... 

The synthesis of modhephene (54), which proceeds according to plan in an overall yield of >16%, with complete control of relative stereochemistry, demonstrates the ability of radical cyclisations to form propellane systems and generate highly crowded neopentyl quaternary centres. The accepted pathway for the cyclisation of the vinyl trimethylstannane is shown in Scheme 7.25. The chair-like transition state in which the methyl substituent on the radical is pseudoequatorial, accounts for the observed endo stereoselectivity. 

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. 

Closer examination of tetrahydropyrans 173 clearly reveals that two molecules of aldehyde 174 have been appended onto allylsilane 171 via a novel three-component coupling reaction. Marko et al. proposed the mechanism depicted in Scheme 13.61 [65], Formation of heterocycles 173 is described as a sequence of two processes an initial ene-type reaction [80] which leads to alcohol 177 via the chair-like transition state 176, in which both the aldehydic R-group and the OTMS substituent assume an equatorial position. The high regio- and stereoselectivity observed in this ene-reaction can be nicely explained by considering the stabilizing /(-silicon effect and the repulsive 1,3-diaxial interactions. Transition state 176 contains no 1,3-diaxial interactions and benefits fully from the stabilizing /(-silicon effect [81, 82] (for more detailed transition-state discussion see ref. [63]). 

In each case, stereoselectivity can be explained by assuming that the reaction proceeds through a chair-like transition state, in which there are interactions between the two ends of the C-Li and C=C bonds, and in which the substituents all occupy pseudo-equatorial positions (structures 338, 339 and 340). The same transition state model suffices to explain the stereoselectivities of the furan and pyrrolidine forming reactions above. Stereoselectivity in the cyclisations of the selenide-derived tertiary organolithiums would arise from a conformation with a precedented pseudo-axial phenyl ring. 

Chamberlin proposed similar chair-like transition states 342 and 345 to account for the stereoselectivity of the cyclisations of vinyllithiums such as 341 and 344,137 and again a... 

Chamberlin and coworkers proposed a similar chair-like transition state 29 to account for the stereoselectivity of the carbolithiation reaction of vinyllithium 10 to 1224. The observed diastereoselectivity is consistent with a four-centre transition state where a preferred coplanar approach of the carbon-lithium bond to the double bond would give the obtained major product (Scheme 8). 

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-... 

Allylation of allylsilacyclobutane with carbonyl substrates also proceeds bimole-cularly (Sch. 66) [107]. In contrast with the aldol reaction, a mechanism with penta-coordinate silicon intermediates in chair-like transition states resulted in high stereoselectivity. 

Initial efforts in this area involved the addition of BuaSnLi to fratw-crotonaldehyde and conversion of the racemic hydroxy stannane adduct to diastereomeric (-)-menthyloxy-methyl ethers by reaction with (-)-menthyloxymethyl chloride (Eq. 32) [52]. These dia-stereomers could be separated by careful chromatography. They formed diastereomeric anti, (Z) adducts with aldehydes upon heating to 130 °C. The results parallel those seen for the racemic OMOM allylic stannanes (Table 25). Formation of the (Z) double bond in these adducts is attributed to steric interactions between the allylic OR substituent and the adjacent stannane butyl groups in a chair-like transition state as pictured in Eq. (9). The excellent stereoselectivity of these additions is suggestive of a highly ordered transition state. 

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