Synthesis of tetracyclic and pentacyclic condensed

In 1951-1953 Rabindran and Tilak51,52 introduced a new dibenzo-thiophene synthesis and extended it to tetracyclic and pentacyclic systems. This method involves a two-step process of condensing a thioarenol with an a-halo ketone or a-haloacetal to form an a-(arylthio) ketone or a-(arylthio)-acetal intermediate, which is subsequently cyclized intramolecularly. The three principal variations introduced by Tilak and co-workers are presented in Schemes 3, 5, and 6 and were summarized in I960.528... 

The synthesis of alnusenone (119) utilized a different method of assembling the pentacyclic skeleton. Condensation of the ketones (120) and (121) led via the tetracyclic ketone (122) to two pentacyclic ethers (123) and (124). The trans,anti,cis-stereochemistvy of (123) was confirmed by X-ray analysis. The desired trans,anti,trans-ether (124) was transformed into alnusenone (119). 

The enantioselective approach to quebrachamine adopted by Fuji and collaborators (Scheme 41) (279) has also been modified to afford a new synthesis of (-)-aspidospermidine (251). Here, the lactone 446 was converted by titanium trichloride into the lactone hemiacetal 485, which, after appropriate reduction and oxidation stages, gave the acetal acid 486. Condensation with tryptamine gave the tetracyclic lactam 487, which was then rearranged by means of trifluoromethanesulfonic acid to the pentacyclic indolenine lactam 488, reduction of which gave (-)-aspidospermidine (251) (Scheme SI). 

Following their recent synthesis of ( )-vincadifformine (Scheme 75) (343,344), Szantay and his collaborators have contributed another s3mthesis of pseudovincadifformine and its epimers (392). Condensation of the trypt-amine derivative 587 with the aldehydoester 698 gave, via an unstable secodine derivative, the epimeric tetracyclic esters 715, which, without separation, were subjected to debenzylation, with partial epimerization and cyclization. The product, a mixture of the two cis C/D-fused pentacyclic... 

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