Synthesis of 2,6-bridged piperazine-3-ones by N-acyliminium ion chemistry

Beyond the Diketopiperazine Family with Alternatively Bridged Brevianamide F Analogues

A method for the preparation of 3,5-bridged piperazin-2-ones from a tryptophan-proline-based diketopiperazine is described using diphosgene to induce the ring closure. Density functional theory calculations were conducted to study the mechanism of this C-C bond formation. Several derivatives of the thus obtained α-chloroamine were synthesized by substitution of the chlorine atom using a range of O-, N-, S-, and C-nucleophiles. This novel class of brevianamide F analogues possess interesting breast cancer resistance protein inhibitory activity.

Amino acid chirons: a tool for asymmetric synthesis of heterocycles

As a result of their easy availability in enantiomerically enriched form and their possession of synthetically transformable diverse functional groups, amino acids have been extensively used by synthetic organic and medicinal chemists as a chiral pool for access to heterocycles (monocycles, bicycles or polycycles, either bridged or fused). This review describes the syntheses of diverse asymmetric heterocycles with various membered rings (n = 3-9) followed by benzo or heteroannulated ones, for the period from 1996 to Dec. 2013. It details solution phase synthetic methodologies in which the naturally occurring α-amino acid is incorporated, totally or partially, into the final product.

Recent synthetic approaches to 6,15-iminoisoquino[3,2-b]3-benzazocine compounds

Saframycins, safracins, renieramycins, cribrostatins, and esteinascidins are 6,15-iminoisoquino[3,2-b]3-benzazocine compounds that constitute the largest subgroup among the antitumor antibiotics belonging to the tetrahydroisoquinoline family. Their structural complexity has led to widespread synthetic attention to obtain them in both racemic and enantiopure forms. Publication in 1996 of the first total synthesis of ecteinascidin 743 by Corey's group was an important milestone, but the development of preparative protocols for these structures has continued, offering new possibilities to exploit the biological activity of the above-mentioned natural products and their analogues. This minireview is intended to update this progress following a methodological rather than a chronological organization. Besides of a brief description of the different strategies evolved from retrosynthetic analyses, which have been organized according to the order of bonding events that will link the precursors, semisynthetic approaches and a brief account of the total syntheses of ecteinascidin 743, have been analyzed.

Practical Solid-Phase Parallel Synthesis of Δ5-2-Oxopiperazines via N-Acyliminium Ion Cyclization

A practical solid-phase strategy for the synthesis of Delta(5)-2-oxopiperazines via N-acyliminium ion cyclization has been developed. A key step in the library synthesis is tandem acidolytic cleavage with subsequent in situ iminium formation, followed by stable enamide transformation. This approach is exemplified by the preparation of a 192-member pilot library using bromoacetal resins without further purification.

Solid-Phase Parallel Synthesis of Natural Product-Like Diaza-Bridged Heterocycles through Pictet−Spengler Intramolecular Cyclization

A multistep, practical solid-phase strategy for the synthesis of natural product-like diaza-bridged heterocycles was developed. A key step in the library synthesis is tandem acidolytic cleavage with subsequent in situ iminium formation followed by the Pictet-Spengler intramolecular cyclization. The Pictet-Spengler-type intramolecular cyclization step was regioselective and diastereoselective to give final products as single diastereomers in exceptional yields and purities, which was confirmed by NMR structural study and LC/MS analysis. This approach is exemplified by the preparation of a 384-member library of 3,9-diazabicyclo[3.3.1]non-6-en-2-one skeletons, fused with indole and dihydroxybenzene and diversified at two bridging nitrogen atoms, using the solid-phase parallel synthetic methodology without further purification. In this pilot library, two diastereomerically enriched diaza-bridged core skeletons were modified by amide and urea bond formation on bridging nitrogen atoms, and this scheme exhibits the potential for expansion to obtain further diversification.

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.

Synthetic applications of aliphatic unsaturated α-H-α-amino acids

This article provides an overview of the literature concerning synthetic applications of unsaturated aliphatic amino acids in the period May 2000 to December 2004.

Synthetic studies on (-)-lemonomycin: stereocontrolled construction of the 3,8-diazabicyclo[3.2.1] skeleton

Stereoselective synthesis of the pentacyclic key intermediate 22 for (-)-lemonomycin (1) has been accomplished using the Ugi 4-CC reaction with our novel isocyanide 8, a cross-metathesis of 13 and allylsilane and a subsequent intramolecular Hosomi-Sakurai type reaction.