Syntheses of 2,5-dimethyl-4-naphth-2′-yldioxolanes and their stereoselective isomerization to naphtho[1,2-c]pyrans, angular analogues of glucoside B, a cleavage product of the aphid insect pigments the protoaphins

Benzynes were generated selectively through loss of ortho-bromotosylate from 1,2-dibromo-3- tosylates. Thus when treated with bu tyl lithium in the presence of furan rel -(2 R ,4 S ,5 R )-4-(2',3'- dibromo-5'-methoxy-4'-toluene- p -sulfonyloxyphenyl)-2,5-d imethyl-1,3-dioxolane 21 was converted in two steps into rel -(2 R ,4 S ,5 R )-4-(1'-bromo-4'-methoxynaphthalen-2'-yl)-2,5- dimethyl-1,3-dioxolane 8 in good yield. Attempted stereoselective isomerization of dioxolane 8 with titanium(IV) chloride at low temperature led to the recovery, almost exclusively, of starting material. The debrominated analogue rel -(2 R ,4 S ,5 R )-4-(1'-methoxynaphthalen-3'-yl)-2,5- dimethyl-1,3-dioxolane 31 , on the other hand, isomerized readily to give rel -(1 R ,3 R ,4 S )- and rel - (1 S ,3 R ,4 S )-3,4-dihydro-4-hydroxy-6-met hoxy-1,3-dimethylnaphtho[1,2- c ]pyrans 32 and 34 in a ratio (~1:3) that did not vary with reaction temperature