. . "is protected hydroxy, or a salt thereof; (e) reacting a compound of the formula: ##STR15## wherein R1, R3 and n are each as defined above, and Rb 2 is a leaving group, or a salt thereof, with a base, to give a compound of the formula: ##STR16## wherein R1, R3 and n are each as defined above, or a salt thereof; (f) oxidizing a compound of the formula: ##STR17## wherein R1, R3 and n are each as defi" . .