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 alkenyldihydro oxazole, these compounds may then undergo addition of a second equivalent o f the isocyanate to give either tetrahydrooxazolo[3,2-c]pyrimidine-8-carbox amides or octahydroazeto[2,3-d]oxazolo[3,2-c]pyrimidines. The second additi on is sensitive to steric and electronic factors, and can be prevented in s ome cases.