A new, simple route to tetracyclic intermediates for the synthesis of diterpenoid alkaloids

Studies on the syntheses of heterocyclic compounds. 675. A facile regiospecific and stereocontrolled synthesis of a diterpene alkaloid intermediate from benzocyclobutenes

“…After continuous stirring for 2 h, the mixture was concentrated to give 50 mg (98%) of 24 as a pale-reddish gum, which was used in the following reaction without further purification: ir (CHCI3) 2225, 1725, 1685 cm-1; m/e 407, 405. 4aa-Cyano-1,2,3,4,4a,9-hexahydro-13-methoxy-1 a-methoxycarbonyl-l¡8-methyl-9-oxophenanthrene (25). A solution of 820 mg (4.2 mmol) of N-phenylbenzamidine and 480 mg (1.19 mmol) of the bromide 24 in 20 ml of o-xylene was refluxed for 3 h. Removal of the solvent gave a reddish gum, which was acidified with 20 ml of 5% hydrochloric acid.…”

“…The stereochemistry of the hydrogen at C-5 in the lactam 20 also could not be determined in this series, but this was tentatively assigned as the a configuration because the protons at the C-17 position of the second lactam 21, described later, resonated at 3.35 and 3.80 as AB type doublets which were very similar to those of 20, but the third lactam 276 having a /3-hydrogen showed the proton resonance at 3.36 as a broad singlet. 25 The stereocontrolled rearrangement of the benzocyclobutene 16 into the hydrophenanthrene 18 can be explained as follows.…”