The oxa-Pictet-Spengler reaction of 1-(3-furyl)alkan-2-ols
with aldehydes catalyzed by p-toluenesulfonic
acid gives good yields of the corresponding cis-5,7-disubstituted
4,5-dihydro-7H-furano[2,3-c]pyrans.
oxa-Pictet-Spengler reaction - furans - cyclizations - Lewis acids - ene reactions