Dewar pyridone

Photochemically induced ring opening also occurs in the fused aziridine (59) leading to the Dewar pyridone (60)... 

On irradiation in dilute solution, A -alkyl- und A -aryl-2-pyridones are converted into Dewar pyridones (2-azabicyclo[2.2.0]hex-5-en-3-ones). For instance, 4,6-dimethyl-l-phenyl-2-pyridone 18 affords product 19 ... 

Table 1. Selected Examples of Dewar Pyridones 5 and (4+4) Dimers 6...

Figure 2. Isolated yields of Dewar pyridones (outlined shapes) and [4+4] dimers (filled shapes) as a function of the initial concentration of 1 -methyl-2-pyridone. Note the absence of a solvent effect. Data from reference 8.

Both the [4+4] photodimerization reaction (4 — 6) and the isomerization to give Dewar-pyridone products (4 —> 5) tolerates substitution at every position (Table 1), but certain combinations of substituent and position are notable exceptions. The parent 2-pyridone 4a and N-methyl 4b are typical, yielding either 5 or 6 depending on the starting concentration. - A single additional allyl substituent, represented by l,n-dimethyl examples 4c-4f, all yield at least the [4+4] dimer. Even a /-butyl substituent at C4 (4g) which presents a possible steric impediment, is compatible with the dimerization reaction. Fusion of a cyclohexyl at C5/C6 (4k) or a cyclohexadienyl ring at C4/C5 (41) leads to acceptable yields of the photodimer 6. Chlorine substitution... 

The observation that 4-alkoxy-2-pyridones do not photodimer-l2e 21,22 Qjjjy jQ jjjg Dewar-pyridone products, has provided a... 

Productive photodimerization and photoisomerization reactions of 2-pyridones in the solid state have been achieved by Toda and Tanaka using asymmetrical hosts (Figure 11). In many cases, the 2-pyridones isomerize to Dewar-pyridone products with complete enantiomeric purity. The symmetrical [4+4] products are achiral and are formed only in what one might anticipate to be the rare circumstances of a crystal structure with the appropriate orientation and distance between pyridones. Nevertheless, this has been accomplished with the two complexes depicted in Figure 11. 

In contrast to 64, tail-to-tail 65 produces a clean isomerization to the bis-Dewar-pyridone 66. It is likely that this is a mixture of two diastereomers although this could not be ascertained by NMR or chromatography. 

Also, small ring molecules are known to add isocyanates across a single C-C single bond to give 1 1 adducts. For example, reaction of the tetramethylcyclobutane aluminum trichloride complex 242 with alkyl and aryl isocyanates gives the Dewar pyridones 243, which rearrange in the presence of trifluoroacetic acid to give 2-pyridones 244 °. 

The enantioselective preparation of pyridone photoisomers has been described by several laboratories, motivated in part by the importance of 5-lactams as antibiotics. Lipase hydrolysis of racemic 104 and similar Dewar pyridones has led to resolutions with high enantiomeric purities. Isomerization of pyridones when complexed to the chiral scaffold 106 gave the isomerized product with good to excellent levels of enantioselectivity. ... 

FIGURE 103.6 Pyridones that yield Dewar pyridones but do not dimerize. 

FIGURE 103.7 Preparation of enantiomerically pure Dewar pyridones has been accomplished by resolution (105), useofachiralhost (106), in the solid state by CO crystallization (41 + 107) and with asymmetric crystals (108). ... 

The photo-isomerization of certain pyridines to Dewar pyridines is described in Section 3.2.1.2.2, and the formation of bridged ring 6H-1,2-dihydro-3-pyridones in Section 3.2.1.10.4. 

A Dewar isomer of unsubstituted pyridine was identified by Wilzbach, but it was very unstable.63 Other Dewar isomers of six-membered heterocycles, isolated and fairly stable, are substituted with perfluoroalkyl groups. However, 1,2-dihydroheterocyclic compounds such as a-pyrone or a-pyridone derivatives are easily photoisomerized to bicyclic Dewar isomers, which are useful for the synthesis of cyclobutadienes. The photochemistry of 1,2-dihydroheterocyclic compounds will be discussed, followed by those of fully aromatic compounds. 

The photoreaction of concentrated pyridone solutions usually gives dimers whereas in dilute solution, an isomer of the Dewar type is formed (130)139>. 

The photohydration of pyridine to 2-pyridone by irradiation in alkaline aqueous solution proceeds by hydrolysis of a Dewar pyridine intermediate. Some years ago this was proposed as an undergraduate experiment, and improvements to the procedure have now been published. ... 

There appear to be no ERE data for pyridones and related compounds, and although Dewar and co-workers18 have calculated the heats of atomization for a number of structures of this type, the results were used for a discussion of tautomeric equilibria rather than for deriving values for the Dewar resonance energies. Empirical studies on tautomeric equilibria have, however, been used to estimate the difference in ERE of pyridones and pyridine (Section II,A, 4). Beak et al 1 studied the gas-phase 1-methyl-2-pyridone 2-methoxypyridine equilibrium and compared the result with the equilibrium constant for the jV-methylvalerolactam ... 

Dewar pyridine, 2-azabicyclo[2.2.0]hexa-2,5-diene (218), thermally reverted to pyridine at room temperature with a half-life time of 2.5 minutes (Ea = 16 kcal/mol).255 Far more stable were 2-azabicyclo-[2.2.0]hex-5-en-3-ones (225).265-267 The kinetics of the thermal (2 + 2)-cycloreversion of 225 (R = Me) in the temperature range of 130° to 160° have been reported (A//J = 33.2 kcal mol-1 ASt = + 2.7 cal mol-1 deg-1).266 An interesting difference in rate was observed between 225 (R = H) and its methyl homolog 225 (R = Me). At 130° the former reverted ten times as rapidly to 2-pyridone as the latter did to 1-methyl-2-pyridone this difference has been related to the intermediacy of the lactim tautomer of 225 (R = H) in the former reaction. Dewar benzene oxide, 2,3-epoxybicyclo[2.2.0]hex-5-ene (266), isomerized to an equilibrium mixture of benzene oxide/oxepin at 115° with a half-life time of 18 minutes.270 The relatively high thermal stability of such strained bicyclic heterocycles has been attributed to the fact that the symmetry-allowed conrotatory process would give rise to an unfavored cis.trans heterocyclic diene.265... 

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