Dewar pyrimidinones

Taylor, J.S., Garrett, D.S., Cohrs, M.P. Solution-state structure of the Dewar pyrimidinone photoproduct of thymidylyl-(3 -5 )-thymidine. Biochemistry 1988, 27, 7206-7215. 

The reaction of Dewar pyrimidinone 220 with water in 1 9 mixture of water and acetone at 20°C for 1 hour afforded 4//-pyrido[ 1,2-a]pyrimidin-4-one 108 and enamine 107 in 34% and 64% yields, respectively [89JCS(P1)1231]. Enamine 107 was formed by the cleavage of bond a of 4-hydroxyazetidin-2-one 221, and ring closure of enamine 107 by elimination of water gave 4//-pyrido[l, 2-a]pyrimidin-4-one 108 (Scheme 15). When the solvent polarity was increased, the cleavage of bond b of 4-hydroxyazetidin-2-one 221 also occurred. The reaction in aqueous acetoni-trile-d3 at 35°C for 37 hours gave 4//-pyrido[ 1,2-a]pyrimidin-4-one 109 and lactam 222 in 85% and 10% yields, respectively. The reaction of Dewar... 

The irradiation of 2-methyltetrahydropyridopyrimidin-4-one 643 (R = Me) in a mixture of acetic acid and acetonitrile under argon at 25°C afforded betaine 651 (R = Me) in 57% yield (83TL5237 87JOC2455). Upon the formation of betaine 651 after the protonation of the imino nitrogen of the Dewar pyrimidinone 646, ring opening yielded azetidinyl cation... 

The Dewar pyrimidinone 644 (R = /Bu) could be isolated in 16% yield when 2-/m-butyl-6,7,8,9-tetrahydro-4//-pyrido[ 1,2-a]pyrimidin-4-one 643... 

When Dewar pyrimidinone 644 (R = /Bu) was left to stand in acetonitrile containing hydrogen sulfide at 0°C for 15-19 hours, hexahydro-4//-pyrido[2,l-fe][l,3]thiazin-4-one 223 and tetrahydro-4f/-pyrido[ 1,2-a] pyrimidin-4-one 643 (R = /Bu) were obtained in 52% and 32% yields, respectively [89JCS(P1)1231]. When pyrido[2,l-/ ][l,3]thiazine 223 was set aside for 64 hours at 35°C in CDC13, pyrido[ 1,2-a]pyrimidin-4-one 643 (R = /Bu) was obtained in 95% yield. The hydrated enamine 107 in methanol at 16-23°C reverted to the bicyclic pyrido[ 1,2-a]pyrimidin-4-one 643 (R = /Bu) after 2 days in almost quantitative yield [85JOC166, 85TL3247 89JCS(P1)1231]. 

Dewar pyrimidinone 657 was obtained in 92% yield when a solution of pyrido[l, 2-a]pyrimidin-4-one 114 (R = Ph) in acetonitrile was irradiated with a medium-pressure mercury lamp at 20°C (86CC687). 

Ab initio calculations on 1-azetine 2 have been carried out at the HF and MP2/6-31G(d,p) levels of theory to compare with the structures, orbital hybridization, bond orders, and charge distributions of the Dewar pyrimidinones 569, supporting the abnormally elongated C-N bond distance in pyrimidinones observed by X-ray analysis. The bond distances and angles in azetine 2 have been obtained by theoretical calculations <1997JOC2711>. 

Earlier reports that pyridinium cation (75) can follow an excited state electrocyclisation pathway have been developed into a versatile method for synthesis of 6-alkyl-6-azabicyclo[3.1.0]hex-2-en-4-yl alcohols and ethers (77). Irradiation of (75) under basic conditions in hydoxylic solvents results in capture of cation (76) to yield (77). These may be converted regio- and stereoselectively to rra/i, tra 5-3,5-disubstituted-4-alkylaminocyclopentenes or to cis-fused cyclo-penteno-P-lactams. The structures of a series of valence bond photoisomers of 4(3H)-pyrimidinones (78) have been confirmed by X-ray crystallography to be the corresponding Dewar pyrimidinones. Photolysis of chloro-iminoenamines... 

Many exeunples of photorearrangement arising by 4tt- and 6ir-electrocyclic pathways have been reported. Full experimental details for the photohydration of the pyrimidin-4-ones (10) to the enamides (11) via the Dewar pyrimidinones (12) have been published. 6fr-Electrocyclizatlon followed by elimination of benzoic acid is observed in the conversion of the 4-aryl-N-... 

The X-ray crystal structures of a series of Dewar pyrimidones (151), obtained by the irradiation of the pyrimidones (152), have been determined. The photochemical reactivity of l-methyl-4,6-diaryl-2(///)pyrimidinone derivatives in the presence of various thiols has been studied. ... 

The Dewar isomer, 123, of 3-methyl-4(3H)-pyrimidinone (122) was generated in argon matrices by 308 nm irradiation of the parent molecule and identified by comparison of the experimental matrix IR spectrum with spectra computed for 123 and other possible products. " For several 4(3H)-pyrimidinones not methylated at N3, two other types of matrix photoreactions were observed in addition to the isomerization to the Dewar isomer phototautomerism and ring opening. Somewhat later, matrix isolated Dewar pyridine (124) was produced by UV-irradiation of pyridine in solid argon and was identified with the aid of DFT computations. On further photolysis, 124 produced cyclobutadiene (118) and HCN. 

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