Bicyclo[3.1.0]hexane conformation. Crystal structure of N-exo-6-bicyclo[3.1.0]hexyl-p-bromosulfonamide

Grostic, Marvin F.; Duchamp, D. J.; Chidester, C. G.

doi:10.1021/jo00819a004

Cited by 20 publications

(6 citation statements)

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“…The carba analogues, on the other hand, favor the boat forms regardless of fluoro substitution. The computed preferences for the boat conformation in structures analogous to 4c , where C(4) bears an electronegative atom [22], and in the bicyclo[3.1.0]cyclohexanes [23–26] are supported by several X-ray crystallographic structures, some of which are illustrated in Fig. 2.…”

Section: Resultsmentioning

confidence: 92%

Fluorine as a Regiocontrol Element in the Ring Opening of Bicyclic Aziridiniums

Lam

Houk²,

Cossy

et al. 2012

Helvetica Chimica Acta

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The origin of the variation in the regioselectivity of the nucleophilic ring-opening of a series of bicyclic aziridinium ions derived from N-alkylprolinols was investigated by quantum chemical computations (M06-2X/6-31+G(d,p)—SMD). These aziridiniums differ only in the degree and the stereochemistry of fluoro substitution at C(4). With the azide ion as nucleophile, the ratio of the piperidine to the pyrrolidine product was computed. An electrostatic gauche effect influences the conformation of the adjoining five-membered ring in the fluorinated bicyclic aziridinium. This controls the regioselectivity of the aziridinium ring-opening.

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Section: Resultsmentioning

confidence: 92%

Fluorine as a Regiocontrol Element in the Ring Opening of Bicyclic Aziridiniums

Lam

Houk²,

Cossy

et al. 2012

Helvetica Chimica Acta

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“…The chair product ( S , S )-4 , which arises from the α-epoxidation, is also higher in energy by 3 kcal/mol than the boat product ( R , R )-4 , from β-epoxidation. Bicyclo[3.1.0]hexane systems are known to prefer the boat conformation over the chair due to torsional interactions with the bridgeheads . In the case of parent 6-oxabicyclo[3.1.0]hexane, this preference is computed to be 4.0 kcal/mol 5d.…”

mentioning

confidence: 99%

Torsional Steering Controls the Stereoselectivity of Epoxidation in the Guanacastepene A Synthesis

2006

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The stereoselectivity of the key epoxidation step in the synthesis of Guanacastepene A is shown to be controlled by torsional steering. In this particular epoxidation reaction, the transition structure energetic difference is enhanced by the great asynchronicity of the forming C-O bonds that intensifies the torsional interactions.The stereoselective epoxidation of the Guanacastepene A precursor 1 constitutes a key step which provides the correct stereochemistry of the β-acetoxy ketone, as shown above. 1 Cursory inspection of the structure suggests that attack should occur on the α-face, yet exclusive formation of the β-epoxide was observed experimentally. The crystal structure of the hydroxyketone formed from 2 shows that the isopropyl group is in equatorial position, imparting little steric differentiation between the α-and β-faces. 1c The computational study described here reveals that the origin of stereoselective β-attack in this epoxidation is a strong torsional control of stereoselectivity that we have named torsional steering. 2 The computed epoxidation transition structures of a model alkene containing all the substituents of the cyclopentene ring of guanacastepene precursor 1, computed with density functional theory (B3LYP/6-31G*), 3 are shown in Figure 1. 4 The α-epoxidation transition structure TS-(S,S)-3, with the dimethyldioxirane (DMDO) approaching from the sterically less hindered face of the cyclopentene ring is 4 kcal/mol higher in energy than the analogous β-epoxidation process TS-(R,R)-3, in agreement with experimental observations that only the β-epoxide is formed.Both transition structures are very asynchronous; one of the forming C-O bond is ~1.8 Å, while the other forming C-O bond length is ~2.4 Å. This high asynchronicity is the result of electron donation from the acetoxy group and the conjugated alkene.The inherent stability difference between the transition structures is due to the difference in torsional strain between bonds linking the forming epoxide to the rest of the skeleton, as shown in the insets Newman projections in Figure 1. The α-epoxidation transition structure TS-(S,S)-3 exhibits substantial eclipsing, in contrast to the substantially more staggered β-epoxidation transition structure TS-(R,R)-3. Such torsional control of electrophilic or nucleophilic reactions has been observed in many reactions. 2The chair product (S,S)-4, which arises from the α-epoxidation, is also higher in energy by 3 kcal/mol than the boat product (R,R)-4, from β-epoxidation. Bicyclo[3.1.0]hexane systems are known to prefer the boat conformation over the chair due to torsional interactions with the bridgeheads. 5 In the case of parent 6-oxabicyclo[3.1.0]hexane, this preference is computed to be 4.0 kcal/mol. 5d As shown in the corresponding insets, the difference between the eclipsing in (S,S)-4 and (R,R)-4 is reduced as compared to the transition state.It is noteworthy that the energy difference between the transition structures is greater than the energy difference between the products by ...

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“…A five-membered ring fused with one or two cyclopropane rings forms a highly strained skeleton in either case. Our knowledge of the structural properties of such systems originates mainly from studies on bicyclo[3.1.0]hexane (4) and its derivatives, using techniques such as microwave spectroscopy (Cook & Malloy,1974), electron diffraction (Mastryukov, Osina, Vilkov & Hilderbrandt, 1977) or X-ray diffraction (Grostic, Duchamp & Chidester, 1971;Morris, Murray-Rust & Murray-Rust, 1977). In molecules containing the framework (4) the five-membered ring adopts an envelope conformation such that a global boat form ensues with the fused cyclopropane.…”

Section: H(4) (3)mentioning

confidence: 99%