Oxazolo pyrimidine-6-carboxylate

Aminooxazole was reacted with EMME by heating in boiling trichlorobenzene for 2 hr to yield ethyl oxazolo[3,2-a]pyrimidine-6-carboxylate (73BRP1331059). 

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. °... 

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). 

Tetrahydro-7H-oxazolo[3,2-a]pyrimidin-7-ones (275) can be prepared from 2-amino-2-oxazolines and a,/8-unsaturated carboxylic esters. The orientation of the substituents in the reaction product corresponds to Michael addition of the oxazoline nitrogen as the first step (74LA593). 

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