Transition chair-boat

A cascade of alkene addition reactions begin through a chair-boat-chair conformation transition state. 

Therefore any flexible acetal will undergo conformational changes to permit 2p(0) <-> 2p(C+) stabilizing interaction to intervene in the transition state of its heterolysis. This is also true for pyranosides for which the free energy difference between chair, boat and sofa conformers rarely surpasses 10 kcal/mol. 

Marszalek PE, Li H, Oberhauser AE, Eemandez JM. Chair-boat transitions in single polysaccharide molecules observed with force-ramp atomic force microscopy. Proc Natl Acad Sci USA 2002 99 4278-4283. 

The analogy between S-exo transition states and the chair/boat cyclohexane is simple to apply and has useful predictive and interpretive value, provided that one recalls the differences as well as the similarities. 

Several indium-mediated intramolecular carbonyl allylation reactions have been investigated, and it has been found that these reactions provide an easy access to a variety of cyclic compounds. The intramolecular cyclization of 49a-c mediated by indium in aqueous media proceeds smoothly to afford carbocyclic systems containing y-hydroxy-Q -methylene esters 50a-c, which either spontaneously or readily cyclize to give fused o -methylene-y-butyrolactones 51a-c (Scheme 52). The same cyclization of 49d is too slow to compete with the side-reaction, in which the bromide is substituted by a hydroxy group. The ring junction stereochemistry of fused lactones 51 has been found to be cis in all cases. Of the two possible transition states, the one leading to the m-fused compounds is preferred, because the chair-chair conformation is favored over the chair-boat conformation.209... 

When the transition state for a ring closure forms a chair then a trans relationship results. This is the case for the black Me and brown Me. When a boat is formed a cis relationship results. This is the case for the green Me and black H. Squalene folds up in a chair-boat-chair conformation and that leads to the observed stereochemistry. 

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. 

In acyclic systems, Claisen rearrangements show a well-established prefoence for chair-like transition states. With crotyl propenyl ether, the chair selectivity amounts to 97-98% at 142 C, which corresponds to an approx. 3 kcal mol difference between the fiee energy of activation (AAG ) of chair and boat TS (equation 26). The preference for a chair-like geometry in the TS is even more pronounced in the Cope reaiT ement 99.7% of the 3,4-dimethylhexa-1,5-diene rearranges at 225 C via a chair-like TS, corresponding to a AAG chair-boat of -5.7 kcal mol" . - The latter result closely parallels the difference in energy of the chair and boat conformations of cyclohexane (5-6 kcal mol" ). ... 

The thermal intramolecular allylation of (Z)-158 occurs spontaneously at 25 C. Conversely, the thermal intramolecular allylation of the ( )-stannane 159 required much higher temperature (110°C) for reaction to occur. Keck rationalized that (Z)-158 must react preferentially through the cyclic chair-chair transition state 162 (Fig. 11-14) to form the major adduct 160, while the minor adduct, 161, must arise through the sterically disfavored chair-boat transition state 163. The major... 

P. E. Marszalek, H. Li, A. F. Oberhauser, and J. M. Fernandez, Chair-boat transitions in single polysaccharide molecules observed by force-ramp AFM, Proc. Natl. Acad. Sci., 99 (2002)4278 1283. 

Marszalek PE, Oberhauser AF, Pang YP and Fernandez JM (1998) Polysaccharide elasticity governed by chair-boat transitions of the glucopyranose ring. Nature, 396 661-4 Marx-Figini M (1969) On the biosynthesis of cellulose in higher and lower plants. In Marchessault RH (ed), Proceedings of 6th cellulose conference. WUey, New York Mason DC (ed) (1975) Sawmill Techniques for Southeast Asia. Miller Freeman, San Francisco... 

It has been proposed that the Ag-catalyzed reaction proceeds via a cyclic transition states (boat form from ( )-enolates and chair form from (Z)-enolates) directed by the Lewis acidity of Ag(I) ion and the high affinity of fluoride ion for silicon. 

In 1,1 j rearrangements the chairlike transition state is strictly preferred, if the transition state is not too crowded. Thus, Z-configurated ally vinyl ether 2, obtained from (Z)-3-trimethvl-silyl-2-propen-1-ol by dehydrative coupling with 2-single diastereomer (chair/boat > 99 1)519. 

For the carvyl propanoate 4. both the /,)- and the (L )-silyl ketene acetal rearrange through a boat/chair transition state 6. The chair/chair transition state 5, however, is destabilized due to strong steric interactions of the isopropyl group and the silyloxy group 515 539. The two other possible transition states (boat/boat, chair/boat, not depicted) are disfavored because of interactions of the trialkylsilyl group with the ring. 

The high x/ -diastereofacial selectivity should arise from the preference for chair/boat transition states C and D in comparison to transition states A and B with the greater steric hindrance. However, the a/b ratios of products 3 indicate only a moderate internal asymmetric induction (via chair/boat) despite the chelation-fixed geometry of the vinyl ether double bond. 

Thus, the rearrangement can in principal lead to four products through transition states 20 A-20D. Flowever, because the relative asymmetric induction is the primary determinant for the synjanti selectivity (vide infra), each rearrangement produces one major Claisen product with 97- 98% ee640,643. Incomplete internal asymmetric induction (incomplete chair/boat selectivity) accounts for the observed 85 15 product ratios. Substrate 19a, however, rearranges with increased chair selectivity (chair/boat 95 5) indicating that when R1 = R4 = alkyl, a nonbonding interaction destabilizes transition states 20C and 20D relative to transition states 20 A and 20B. 

Better results are obtained with cyclic substrates 8 (d.r. 76 24 to 97 3, Table 29)647. Contrary to the face selectivity of compounds 19 (cf. p 3522), the face selectivity for this reaction originates from a chair/boat selectivity with a preference for the chair transition state 9 due to lower steric hindrance as a result of a staggered arrangement of the substituents. 

Computational studies predict that the geometry (chair, half-chair, boat, etc) depends on the nature of Ri, R2, and M. Theory also predicts that Z-enolates prefer a closed chair, but that -enolates may prefer a boat [54-56]. For an empirical rule for predicting aldol topicity, see ref. [57]. For an investigation into the effect of metal and solvent on transition structures, see ref. [58]. 

The chair/boat conversion of the cyclohexyl ring is one of the few examples whereby a molecular interpretation can be given to an observed transition. However, such transitions are very frequent below the glass transition temperature. They can often be recognized as a kink in the curve of DSC diagrams (Figure 10-28). In many cases, these jS transitions appear to be caused by the coupled movement of very short main-chain segments. 

The interpretation of the Doering-Roth experiment lay unquestioned for nearly a decade, a period of time, which saw recognition of the role of Conservation of Orbital Symmetry and increased awareness of the need for stereochemical labeling at every site in a reaction to deduce its three-dimensional course. A complete analysis of the geometric possibilities for the 3,3-shift by Goldstein recognized the possibility of four Orbital Symmetry allowed, that is, concerted pathway via chair, boat, twist (or helix), and plane transition states (Scheme 7.84). ... 

Triterpenes Derived via a Chair-Boat-Chair-Boat Transition State. 

TRITERPENES DERIVED VIA A CHAK-BOAT-CHAIR-BOAT TRANSITION STATE... 

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