Synthetic Studies toward Ecteinascidin 743

An efficient synthesis of a fully functionalized tetracycle (A-B-C-H) 7 containing a 1,4-bridged 10-membered lactone was developed. Phenolic aldol condensation between 2-methylsesamol (15) and
Garner's aldehyde provided the protected amino diol 16, which was converted to free amine 11 in
excellent yield. A Pictet−Spengler reaction between 11 and ethyl glyoxylate under carefully
controlled conditions (LiCl, toluene, 1,1,1,3,3,3-hexafluoro-2-propanol, room temperature) provided
the acid-sensitive tetrahydroisoquinoline (18) in high yield, which was converted to the amino alcohol
9. Enantioselective alkylation of a glycine template in the presence of a catalytic amount of chiral
cinchonidium salt was the key step for the access of enantiomerically pure amino aldehyde 10.
Union of the two fragments 9 and 10 via oxazolidine intermediate afforded amino nitrile 39, which
upon esterification of the primary alcohol with (R)-N-(S-4,4‘,4‘ ‘-trimethoxyltrityl) Cys (42) afforded
43. Cyclization of 43 (1% trifluoroacetic acid in trifluoroethanol) provided compound 44 by a domino
process involving (a) unmasking of the S-trimethoxytrityl group, (b) fragmentation of dioxane
assisted by an electron-rich aromatic ring, and (c) formation of a 1,4-bridged 10-membered lactone
via formation of a sulfide linkage. Treatment of 7, obtained in two steps from 44b, under acidic
conditions (0.5% methyl sulfonic acid in acetonitrile) afforded the pentacyclic compound 51 via
fragmentation of the 10-membered cyclic sulfide followed by an intramolecular Pictet−Spengler
reaction.