Synthesis of Naphtho[2,3‐b]pyrandiones: (−)‐Cryptosporin

A new method for the synthesis of naphtho[2,3‐ b ]pyrandiones from sulfonyllactones and 1‐nitroglycals is presented. Thus, the sulfonyllactone 6 reacted with the 1‐nitroglycal 7 in the presence of LDA at room temperature to give the naphthopyrandione 9 in high yields ( Scheme 1 ). Reaction at a lower temperature led to the (intermediate) Michael ‐addition products 8 . The sulfonyllactone 4 and the 1‐nitroglycal 5 were prepared for the synthesis of the naphthopyrandione 18 ( Scheme 2 ). The base‐promoted condensation of 4 and 5 gave 17 , which was deprotected to give 18 , the enantiomer of the fungal metabolite (+)‐cryptosporin 1 .