Synthesis of the atropurpuran A-ring via an organocatalytic asymmetric intramolecular Michael addition

The asymmetric synthesis of the A-ring fragment 2a-b of atropurpuran 1 has been achieved in 15 steps with up to 70% ee via an organocatalytic intramolecular Michael addition of nitroalkene 10. The absolute configuration of the two contiguous carbon centers in 2a and 2b was assigned to be (4S, 5R) and (4R, 5R), respectively, using the modified Mosher's method.