Transannular shift

Transannular shifts of hydrogen atoms have also been observed. 

As further evidence of the reluctance of alkyl groups to undergo transannular shifts, cyclodecane-1,6-diol does not form any ketone on acid treatment. More distant hydride shifts can also be studied, such as 1,4 and 1,5 hydride shifts. 

A hydrogen from across the ring has migrated to yield the products. This kind of a migration is referred to as a transannular shift. In Chapter 2 transannular strain (Figure 2.13) was discussed. It arises from steric repul-... 

A number of the purported syntheses of dibenzo[f>,/][l,4]diazocines in the literature have to be revised and the structure of the products corrected.1 Instead of the expected 1,4-diazocine derivatives 1, isoindolobenzimidazoles 2 are actually isolated, the formation of which requires a hydride shift after transannular attack of one imine group onto the other. 

The above is a typical example of a transannular hydride shift. The 1,2-diol is formed by a normal epoxide hydrolysis reaction (10-7). For a discussion of 1,3 and... 

As clearly shown, the absorption maxima of 28 and 29 are red-shifted considerably as compared with the corresponding acyclic compounds. The C-C bridged parent thiophenophane (7) also shows a redshift in UV spectra. Thus, 7 gave two absorption maxima at 245 nm (e = 7700) and 275 mm (e = 5720), quite different from that of 2,5-dimethylthiophene (Amax = 238 nm e = 7250). This is attributed to the transannular n-it interaction between two thiophene rings (33). However, furanophane (8) did not show such a redshift and indicated no such iT-ir interaction between two furan rings. At room temperature by NMR studies, it is reported that two furan rings in 8 rotate freely,... 

Also, the transannular interactions between amino and carbonyl groups in aminoke-tones, like 40-44, were studied by PES48. Pronounced stabilization of the n orbital and destabilization of the no orbital was established by comparison of the relevant ionization potentials with those of the corresponding monofunctional compounds. The shift of the no orbital was noticed as the best indicator of transannular n /jiQ=Q interaction and the maximum value was again found for the system with an eight-membered ring (41). 

Assignment of an anti configuration to a [2](2,5)furano[2](l,4)naph-thalenophane (42) synthesized by Wasserman and Keehn 65> followed from a comparison of its 1H—NMR spectrum with that of [2]paracyclo-[2](2,5)furanophane (41 a) 66>. The absorption band assigned to the /3-furanoid proton Ha in the spectrum of 42 (r=4.38) appears in the same region as the corresponding band for 41 a. In the case of syn-42, a chemical shift would be expected due to the transannular shielding effect of the naphthalene nucleus. 

A careful NMR study by the same group has shown that the 1-azaquinolizinium ion in neutral aqueous solution undergoes hydration to yield (241), but the equilibrium is shifted to the aromatic species (234) if the solution is made 0.1 M in acid (Scheme 118). Bicarbonate causes ring opening to afford j8-(2-pyridylamino)acrolein (242), which can be recyclized in acid. Deuteromethanol also adds to the 1-azaquinolizinium ring, in a transannular manner, and triethylamine converts the resulting salt (243) to 4-methoxy-l-aza-4 7-quinolizine (244). 

Derivatives of the linear polyacenes, naphthalene,46 anthracene,41,90,93 naphthacene,91,92 and pentacene,92 form stable photodimers M2 when irradiated in concentrated Oa-free solution or (exceptionally) in the crystalline state.94,95 Transannular a-bonding of the molecular dimer components results in a folding of the aromatic planes about the bonded atoms and a reduced -electron delocalization reflected in a shift of the absorption spectrum to much higher frequencies.46,92,96... 

The transannular interaction in the eight-membered (and also in nine-membered) ring amino ketones decreases the oxygen (170) exchange rate with water, as shown by 170 NMR, which also reveals an unusually shielded 170 chemical shift in such compounds (72HCA907). 

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