Reversible -sigmatropic intramolecular

A particularly instructive example is the thermolysis of (Z)-l,3,8-nonatriene in which an intramolecular Diels-Alder reaction competes with a sigmatropic [1,5] hydrogen shift (Scheme 24). The use of high pressure here enables a reversal of the selectivity. At 150°C and 1 bar the [1,5] hydrogen shift passing through a monocyclic transition state is preferred. At 7.7 kbar the intramolecular Diels-Alder reaction is preferred due to its bicyclic transition state. 

Fivefold degenerate reversible [3,3]-sigmatropic shifts were first reported in 1988116,117 in the CPD-amidine system 257, where AG g = 117 to 120 kJmol-1 (equation 88) (for aza-Cope rearrangements see Section IV.E.2). In addition, a slow accumulation of a colored by-product was observed at elevated temperatures. This was identified as a product of a novel intramolecular carbon to nitrogen 1,4-shift of the methoxycarbonyl... 

The process shown in Equation 12.112, in the forward direction termed the ene reaction and in the reverse the retro-ene reaction, bears the same relationship to the [3,3]-sigmatropic reactions as does the [2,3]-elimination (p. 668) to the [2,3]-sigmatropic shift. It is also closely related to the [l,5]-rearrangement of hydrogen. The reaction can occur either intermolecularly, as in, for example, Equations 12.113 and 12.114, or intramolecularly, as in Equations 12.115 and 12.116. 

The intramolecular nitrone-alkene cycloaddition reaction of monocyclic 2-azetidinone-tethered alkenyl(alkynyl) aldehydes 211, 214, and 216 with Ar-aIkylhydroxylamincs has been developed as an efficient route to prepare carbacepham derivatives 212, 215, and 217, respectively (Scheme 40). Bridged cycloadducts 212 were further transformed into l-amino-3-hydroxy carbacephams 213 by treatment with Zn in aqueous acetic acid at 75 °C. The aziridine carbaldehyde 217 may arise from thermal sigmatropic rearrangement. However, formation of compound 215 should be explained as the result of a formal reverse-Cope elimination reaction of the intermediate ct-hydroxy-hydroxylamine C1999TL5391, 2000TL1647, 2005EJ01680>. 

The aluminum-mediated Petasis-Ferrier rearrangement is a stepwise [1,3]-sigmatropic process. The first step is the coordination of the Lewis-acid to the 0-atom of the enol. Coordination to the ether O-atom is reversible and nonproductive. Cleavage of the adjacent C-O-bond, assisted by the antiperiplanar lone pair of the etheral O-atom, stereospecifically gives rise to an oxocarbenium enolate species, which cyclizes to the desired oxacycle. The rate difference in the rearrangement for the five- versus six-membered series can be explained by the more facile 6-(enolendo)-endo-trig cyclization. The last step is the intramolecular equatorial hydride delivery. 

A different influence of pressure on an intramolecular [4 + 2] cycloaddition and a 1,5-sigmatropic rearrangement is responsible for a pressure-induced increase in selectivity in the thermolysis of (Z)-l,3,8-nonatriene 35 to give 36 and 37 as shown by Klamer et al. (Scheme 8.11) [28]. At 0.1 MPa the rearrangement is favored and the products 36 and 37 are formed in a 31 69 ratio. Applying 770 MPa of pressure, the selectivity is reversed favoring the Diels-Alder product 36 in a ratio of 73 27. It can be assumed that 36 is formed via the bicyclic transition structure 38, whereas 37 evolves through the monocyclic transition structure 39 (cf. Chapter 2). 

The borane 28 isomerizes at elevated temperatures producing a vinyl bo-rane 29 via a [1,7]-H sigmatropic shift in the Z,Z-isomer 28d (Scheme 2.9, Figure 2.2). The content of 28d in the equilibrium mixture is only 1 %. Nevertheless due to reversible interconversions of 28a-d taking place via 1,3-B shifts and intramolecular rotations, the 1,7-H shift proceeds until a new equilibrium is established. At room temperature, 3 weeks are required for this transformation, and at 373 K it occurs within several minutes. However, the complete... 

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