Asymmetric hetero-Diels–Alder reactions of alkenyldihydrooxazoles. Synthesis of oxazolo[3,2-c]pyrimidines and related compounds

Alkenyldihydrooxazoles undergo a highly diastereoselective formal aza-Diels–Alder reaction with aryl and arenesulfonyl isocyanates to give oxazolo[3,2-c]pyrimidines. Depending on the substitution pattern of the alkenyldihydrooxazole, these compounds may then undergo addition of a second equivalent of the isocyanate to give either tetrahydrooxazolo[3,2-c]pyrimidine-8-carboxamides or octahydroazeto[2,3-d]oxazolo[3,2-c]pyrimidines. The second addition is sensitive to steric and electronic factors, and can be prevented in some cases.