Studies on the Syntheses of Heterocyclic Compounds. CDLXXIV. Syntheses of Camptothecin and Related Compounds. III. A Synthesis of 12-Ethyl-6, 7, 10, 12, 13, 13a-hexahydro-12-hydroxy-1H, 9H-furano [3, 4-g] indolo [3, 2-d] quinolizine