Synthesis of 5-aza-analogues of angucyclines: manipulation of the 2-deoxy-C-glycoside subunit

Structural and pharmacological diversity of 1,4-naphthoquinone glycosides in recent 20 years

1,4-naphthoquinones are the most widespread naphthoquinone compounds. Recently, many 1,4-naphthoquinone glycosides with different structural features have been obtained from both nature and synthesis, which has led to an increasing variety of naphthoquinone glycosides. In this paper, the structure variety and biological activity in recent 20 years are reviewed, and classified them according to the source and structure characteristics. Meanwhile the synthetic methods of O-, S-, C- and N-naphthoquinone glycosides and their structure activity relationships are also described. It was referred that the presence of polar groups of C2 and C5 and non-polar groups attached to C3 on the naphthoquinone ring are beneficial for their biological activities. It will provide more comprehensive literature resources for the future research of 1, 4-naphthoquinone glycosides and lay a theoretical foundation.

(-)-Shikimic Acid as a Chiral Building Block for the Synthesis of New Cytotoxic 6-Aza-Analogues of Angucyclinones

We describe the syntheses of nine new angucyclinone 6-aza-analogues, achieved through a hetero Diels-Alder reaction between the shikimic acid derivative-azadiene 13, with different naphthoquinones. The cytotoxic activity of the new synthesized compounds and five angucyclinones, previously reported, was evaluated in vitro against three cancer cell lines: PC-3 (prostate cancer), HT-29 (colon cancer), MCF-7 (breast cancer), and one non-tumoral cell line, human colon epithelial cells (CCD841 CoN). Our results showed that most 6-azadiene derivatives exhibited significant cytotoxic activities, which was demonstrated by their IC₅₀ values (less than 10 μM), especially for the most sensitive cells, PC-3 and HT-29. From a chemical point of view, depending on the protected group of ring A and the pattern of substitution on ring D, cytotoxicity elicited these compounds, in terms of their potency and selectivity. Therefore, according to these chemical features, the most promising agents for every cancer cell line were 7a, 17, and 19c for PC-3 cells; 7a, 17, and 20 for HT-29 cells, and 19a for MCF-7 cells.

Angucyclines: Biosynthesis, mode-of-action, new natural products, and synthesis

Covering: 1997 to 2010. The angucycline group is the largest group of type II PKS-engineered natural products, rich in biological activities and chemical scaffolds. This stimulated synthetic creativity and biosynthetic inquisitiveness. The synthetic studies used five different strategies, involving Diels-Alder reactions, nucleophilic additions, electrophilic additions, transition-metal mediated cross-couplings and intramolecular cyclizations to generate the angucycline frames. Biosynthetic studies were particularly intriguing when unusual framework rearrangements by post-PKS tailoring oxidoreductases occurred, or when unusual glycosylation reactions were involved in decorating the benz[a]anthracene-derived cores. This review follows our previous reviews, which were published in 1992 and 1997, and covers new angucycline group antibiotics published between 1997 and 2010. However, in contrast to the previous reviews, the main focus of this article is on new synthetic approaches and biosynthetic investigations, most of which were published between 1997 and 2010, but go beyond, e.g. for some biosyntheses all the way back to the 1980s, to provide the necessary context of information.

Concise and efficient synthesis of calothrixin B

A convergent synthesis of the naturally occurring alkaloid Calothrixin B is presented, which used a regioselective hetero-Diels-Alder reaction between a "push-pull" 2-aza-diene and a N-protected 3-bromo-9H-carbazole-1,4-dione to construct the five-ring skeleton of the molecule. Protection of the indole motif with a benzyl group was unattractive for delivery of sufficient target material because the removal of the protecting group had not been high yielding. We therefore elected to temporarily protect the indole motif with a more labile benzyloxycarbonyl group. Accordingly, the synthesis of calothrixin B proceeded in 17% overall yield over 9 steps from the commercially available 1,2,3,9-tetrahydro-4H-carbazol-4-one.

Asymmetric Synthesis of Rubiginones A2 and C2 and Their 11-Methoxy Regioisomers

Convergent enantioselective syntheses of angucyclinone-type natural products rubiginones A(2) (2) and C(2) (1) and their 11-methoxy regioisomers 3 a and 3 b have been achieved by using two domino processes from a common enantiomerically pure 1-vinylcyclohexene 4. Key steps in the synthesis of this diene were the stereoselective conjugate addition of AlMe(3) on (SS)-[(p-tolylsulfinyl)methyl]-p-quinol (9) and the elimination of the beta-hydroxy sulfoxide fragment, after oxidation to sulfone, to recover a carbonyl group. The first domino sequence comprised Diels-Alder reaction with a sulfinyl naphthoquinone followed by sulfoxide elimination. An efficient opposite regioselection in the cycloaddition step was achieved in the convergent construction of the tetracyclic skeleton using a sulfoxide at C-2 or C-3 of the dienophiles 5 or 6, derived from 5-methoxy-1,4-naphthoquinone. The second domino process, triggered by oxygen and sunlight, allowed the transformation of the initial tetracyclic adducts into the final products after B ring aromatization, silyl deprotection and C-1 oxidation.