Unexpected transformation of 1,1"-methylenebis(2-naphthol) into 1,2,1",2"-tetrahydrospiro(naphthalene-1,3"-naphtho[2,1-b]pyran)-2-one, a spirodimer of 1,2-naphthoquinone 1-methide, in the presence of boric acid and paraformaldehyde