Asymmetric hetero-Diels-Alder reactions: X-ray structures of three novel chiral dihydropyrimidines

Three novel dihydropyrimidine compounds N8,6-di(4-nitrophenyl)-(3R)-ethyl-(7R)-methyl-5-oxo-2,3,6,7-tetrahydrooxazolo[3,2,c] pyrimidine-8-carboxamide (2), N8,6-di((4-methylphenyl)-sulfonyl)-(3R)-ethyl-5-oxo-(7R)-phenyl-2,3,6,7-tetrahydrooxazolo [3,2,c]pyrimidine-8-carboxamide (3) and N8,6-di ((4-methylphenyl)sulfonyl)-(3R)-ethyl-(7R)-methyl-5-oxo-2,3,6,7-tetrahydrooxazolo[3,2,c] pyrimidine-8-carboxamide (4) have been prepared (from 2-amino-1-butanol of 64.4% e.e.) and structurally characterized by X-ray crystallography. All three compounds contain stereogenic centers, but the crystal of (2) chosen was found to be racemic whilst those of (3) and (4) were found to be homochiral. Compound (2) crystallizes in the monoclinic space group P2₁/c, with a = 17.958(4), b = 12.431(2), c = 9.653(2) Å, = 96.20(3)°, U = 2142.3(7) ų, Z = 4, and D_c = 1.449 g cm^–3. Compounds (3) and (4) both crystallize in the monoclinic space group P2₁, with a = 9.349(2), b = 5.824(5), c = 26.513(8) Å, = 99.43(2)°, U = 1424.1(13) ų, Z = 2 and D_c = 1.389 g cm^–3 for (3), and a = 5.9526(9), b = 16.3521(10), c = 13.2263(11) Å, = 92.81(12)°, U = 1285.9(2) ų, Z = 2 and D_c = 1.378 g cm^–3 for (4).