Facile synthesis and desulfurization of 5-(phenylthio)pyrano[3,2-c][1]benzopyrans starting from 5-phenylthio-4-penten-1-ols and salicylaldehyde via in situ intramolecular cycloaddition of substituted o-quinonemethides

5-(Phenylthio)pyrano[3,2-c][1]benzopyrans (6) were successfully synthesized in high yield by the reaction of 5-phenylthio-4-penten-1-ol (5) and salicylaldehyde (1) in benzene in the presence of p-toluenesulfonic acid and trimethyl orthoformate. trans,trans-Isomer 6a and cis,cis-isomer 6d were produced as major products from (E)-5 and (Z)-5, respectively. Treatment of 6a with lithium 4,4′-di-tert-butylbiphenyl or Raney Ni (W-4) lead to trans-pyrano[3,2-c][1]benzopyran 4b which is very difficult to make by direct cycloaddition using unsubstituted 4-penten-1-ol (2b).