Oxazolo 3,2-c pyrimidines

Ab initio calculations on aza-Diels-Alder reactions of electron-deficient imines with buta-l,3-diene show that these reactions are HOMO (diene)-LUMO(dienophile)-controlled and that electron-deficient imines should be more reactive than alkyl-or aryl-imines. The Diels-Alder reaction of r-butyl 2//-azirine-3-carboxylate (80) proceeds with high diastereoselectivity with electron-rich dienes (81) (Scheme 28). The hetero-Diels-Alder additions of imines with sterically demanding dienes yield perhydroquinolines bearing an angular methyl group. The asymmetric hetero-Diels-Alder reaction between alkenyloxazolines and isocyanates produces diastereometri-cally pure oxazolo[3,2-c]pyrimidines. °... 

Elliott, M. C., Kruiswijk, E., Willock, D. J. Asymmetric hetero-Diels-Alder reactions. Reactions of oxazolo[3,2-c]pyrimidines. Tetrahedron 2001,57, 10139-10146. 

Isothiocyanato-4-methyl-2-pentanone and ethanolamine in toluene refluxed 10 hrs. with separation of the water formed 2,3,6,7,8,8a-hexahydro-7,7,8a-tri-methyl-5H-oxazolo[3,2-c]pyrimidine-5-thione. Y 90%. F. e. s. G. Zigeuner et al., M. 107, 171 (1976). 

Plant The major degradation products of [2- C]terbacil identified in alfalfa using a mass spectrometer were (% of applied amount) 3-/er/-butyl-5-chloro-6-hydroxymethyluracil (11.9) and 6-chloro-2,3-dihydro-7-(hydroxymethyl)-3,3-dimethyl-5//-oxazolo[3,2-a]pyrimidin-5-one (41.2). Two additional compounds tentatively identified by TLC were 3-/er/-butyl-6-hydroxy-methyluracil and 6-chloro-2,3-dihydro-7-methyl-3,3-dimethyl-5//-oxazolo[3,2-a]pyrimidin-5-one (Rhodes, 1977). 

JOC6700). The 1-methyl derivative of 231 (R1 = Me) was accompanied by 10% of the C-l epimer. Reduction of oxazolo[3,2-a]pyrimidine-5-carboxylates 232 with Zn(BH4)2 gave an epimeric mixture of 4-phenylper-hydropyrido[2,l-c][l,4]oxazin-l-one (233) (94JOC3769). When 232 reacted with BF3-etherate at -10°C in THF, then with Grignaird reagents at -78°C, epimeric mixtures of 4-phenyl-6-substituted perhydropyrido-[2,1 -c] [ 1,4]oxazin-1 -ones (234) were isolated after treatment with 2N HC1. 

The 5-oxo-5//-oxazolo[3,2-a]pyrimidine-6-carboxylate (193) reacts faster than the corresponding thiazolo analogue with hydrazine, which is explicable on the basis of the greater electronegativity of oxygen. The initial attack is at C-2 or more likely at C-8a the ring-opened product is subsequently cyclized to a triazine (71JCS(C)1615). 

In the 2,3-dihydro-5-oxo-5Ff-oxazolo[3,2-c]pyrimidinium salt (207) there are three sites for reactions with nucleophilic reagents, viz. C-2, C-8a and C-7. Products resulting from attack at C-2 are observed with DMSO, water, alcohols, benzoate, chloride, diethylamine and pyridine. Products resulting from attack at C-8a are observed with water, hydroxide, alcohols, alkoxide and isopropylamine. Diethylamine also causes attack at C-7 of the cation, which results in cleavage of the pyrimidine ring (75JOC1713). 

ZaR AR, lS,%aR -Tetrahydro-7-hydroxy-2,2-dimethyl-4,7-methano-l, 3-dioxolo[4,5-c]oxepin-6(4//)-one, Q-10 Tetrahydro-3-hydroxy-5-hydroxymethyl-3-furancarboxylic acid, T-26 2,3,3a, 9a-Tetrahydro-3-hydroxy-6-imino-6/ -furo[2, 3 4,5]oxazolo[3,2-a]-pyrimidine-2-methanol, C-170 Tetrahydro-5-hydroxy-3-methylene-2-furanmethanol, D-629 7,8,9,10-Tetrahydro-8-hydroxy-3-methyl-6,9-epoxy-2ff,6ff-pyrimido[2,l-6]-[l,3]oxazocin-2-one, A-715 Tetrahydro-2-hydroxy-6-methylpyran, H-144 Tetrahydro-2-(hydroxymethyl)pyran, T- ... 

The ring systems reviewed include the perhydropyrazolo[l,2-a]-pyridazines (1), perhydropyrazolo[l,5- ]pyridines (2), perhydro-isoxazolo[2,3-a]pyridines (3), perhydropyrrolo[l, 2-/>]pyridazines(4), perhydro-imidazo[ 1,5-u]pyridines (5), perhydro-oxazolo[3,4-o]pyri-dines (6), perhydrothiazolo[3,4-u]pyridines (7), perhydropyrrolo[l,2-c]pyrimidines (8), perhydropyrrolo[l,2-c][l,3]oxazines (9), perhydro-imidazo[l,2-u]pyridines (10), perhydro-oxazolo[3,2-a]pyridines (11),... 

When the reaction is conducted at 150 °C an oxazolo[2,3-c]pyrimidine is obtained . However, in the presence of a catalytic amount of boron trifluoride etherate, complex heterocyclic spiro compounds are produced (2 2 adducts). ... 

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