AN INVESTIGATION INTO STERIC FACTORS IN THE STEREOSELECTIVE FORMATIONOF NAPHTHOPYRANS FROM 4-NAPHTHYLDIOXOLANES

The titanium tetrachloride-induced isomerisation of rel-(2R,4R,5S)- an d 5-dimethyl-4-(8-bromo-4,5,7-trimethoxy-2-naphthyl) dioxolanes 16, ha ving the C-4 oxygen protected by a methyl instead of the previously em ployed isopropyl group, afforded the desired linear rel-(1R,3R,4R)-6-b romo-3,4-dihydro-4-hydroxy-7,9, 10-trimethoxy-1,3-dimethyl-1H-naphtho[ 2,3-c] pyran 20, identified as its acetate 21, as well as the correspo nding angular analogue, ydroxy-6,7,9-trimethoxy-1,3-dimethylnaphtho[1, 2-c] pyran 22 also identified as its acetate 23. Similar treatment of a dioxolane mixture of 16 and 18 led to the isolation of the angular 1 R,3R,4S)-3,4-dihydro-4-hydroxy-6,7,9-trimethoxy-1 ,3-dimethylnaphtho[1 ,2-c]pyran 26, identified as its acetate 27 and, for the first time th e linear thyl)-6,8,9-trimethoxy-1-methylnaphtho[2,3-c]furan 24 identif ied as its acetate 25. A mechanism is proposed for the formation of th e latter product.