A concise synthesis of a highly functionalized, orthogonally protected amino triol I (R = CH2Ph, R1 = SiPh2CMe3) was achieved via a Mannich reaction between a corresponding phenol II and a chiral oxazinone III as a key step. The utility of such an approach is illustrated by a concise synthesis of a highly oxygenated tetrahydroisoquinoline IV (R = CH2Ph), a key structural unit of ecteinascidin 743 (Et 743) and related natural products. [on SciFinder (R)]