Synthese von Pyrano[3,2‐ b ][1]benzopyranen aus 1‐Nitroglycalen

Synthesis of Pyranol [3,2‐ b ][1]benzopyrans from 1‐Nitroglycals A synthesis of pyrano[3,2‐ b ][l]benzopyrans β‐addition of salicylic aldehyde to 1‐nitroglycals, followed by a Henry reaction, is described. This sequence gave the cis ‐annelated pyranobenzopyranes 9 and 11 from 5 in 54 and 9% yield, respectively, demonstrating the pronounced diastereoselectivity of the β‐addition. Reductive denitration of the crude product mixture yielded 14 (75%), 15 (15%), and 16 (7%), which were oxidized to the aryl ketones 20 and 21 , respectively. Reduction of 20 (NaBH 4 ) gave exclusively 15 . The ketone 20 was deprotected to 22 and 23 .