Total Synthesis of Bio-Active Macrolide Natural Products and Sulfinamide Based Ligands in Asymmetric Catalysis

Abstract

The thesis entitled “Total synthesis of bio-active macrolide natural products and sulphonamide based ligands in asymmetric catalysis” is divided into two chapters. First chapter of the thesis describes the total synthesis of bio-active macrolide natural products cladospolide A 1, seimatopolide A 2 and synthetic studies towards aetheramides A 3 and B 4 (Figure 1). Figure 1: Bio-active macrolide natural products. Section A of chapter 1 describes the enantiospecific total synthesis of cladospolide A (ent-1). Cladospolide A was isolated from three different sources such as culture filtrate of cladosporium fulvam FI-113, Fungus cladosporium tenuissimum and Fermentation broath of cladosporium sp. FT-0012. Cladospolide A is shown to inhibit the root growth of lettuce seedlings. Enantiospecific total synthesis of cladospolide A ent-1 was accomplished in 9% overall yield in 11 linear steps using D-ribose as a chiral pool precursor. Key reactions in the present approach include olefin cross metathesis and Yamaguchi macrolactonization reactions (Scheme 1). Scheme 1: Total synthesis of cladospolide A (ent-1). Section B of chapter 1 describes the use of furan as a surrogate for the E-but-2-ene-1, 4-dione unit in the total synthesis of seimatopolide A 2. Seimatopolide A 2 was isolated by Heip and co-workers from the fungus Seimatosporium discosioides in 2012 and is shown to activate the γ-subtype peroxysome proliferator-activated receptors (PPAR-γ), which is a pivotal process in the type-2 diabetes. Total synthesis of ent-seimatopolide A was accomplished in 7.8% overall yield in 14 linear steps from furfural. Nagao acetate aldol and Shiina macrolactonization reactions were employed as key reactions for the synthesis of ent-seimatopolide A (ent-2) (Scheme 2). Scheme 2: Stereoselective total synthesis of seimatopolide A (ent-2). In section C of Chapter 1, studies towards the synthesis of aetheramides A 3 and B 4 are described. Aetheramides A 3 and B 4 are isolated by Müller’s group in 2012 from the novel myxobacterial genus “Aetherobacter”. Aetheramides are cyclic depsipeptides, which are shown to inhibit the HIV-I infection with IC50 values of ∼0.015 μM and cytostatic activity against human colon carcinoma (HCT-116) cells with IC50 values of 0.11 μM. Stereochemistry at two chiral centers present in the molecules is unassigned. The first approach (Scheme 3) relied on macrolactonization as the key step while the second approach (Scheme 4) relied on RCM to accomplish the macrolactonization. The required precursors were synthesized from elaboration of chiral furyl carbinol, while synthesis of the RCM precursor was accomplished employing the aldol reaction. Scheme 3: Macrolactonization strategy for synthesis of 3 from chiral furyl carbinol. Scheme 4: RCM strategy for synthesis of 3 from chiral furyl carbinol. The successful synthesis of the macrolactone core of aetheramide A 1 is accomplished by employing the ring closing metathesis reaction to construct the C18-C19 bond. RCM precursor has been synthesized by the amidation of the amine derived from R-mandelic acid, while the acid fragment is synthesized from allyl trityl ether (Scheme 5). Scheme 5: RCM strategy for synthesis of 3 from R-mandelic acid. Second chapter of the thesis describes the synthesis and application of novel sulfinamide ligands in asymmetric catalysis. In section A of chapter 2, chiral 2-pyridylsulfinamides are shown to be effective catalysts in the alkylation of aryl and alkyl aldehydes with diethylzinc providing the corresponding alcohols in excellent enantioselectivity. It was found that the chirality present at sulfur in the ligand is pivotal for the asymmetric induction (Scheme 6). Scheme 6: Asymmetric alkylation of benzaldehyde with some of the 2-Pyridyl sulfinamide catalysts. Second section of chapter 2 describes the synthesis and application of C2-symmetric bis-sulfinamides in Rh (I) catalyzed conjugate addition of PhB(OH)2 to enones. Chirality present at sulphur in sulfonamide as well as symmetry present in the ligand plays crucial role in the outcome of the reaction (Scheme 7). Scheme 7: Asymmetric arylation of enones using C2-symmetric bis-sulfinamide/olefin ligands.

The thesis entitled “Total synthesis of bio-active macrolide natural products and sulphonamide based ligands in asymmetric catalysis” is divided into two chapters. First chapter of the thesis describes the total synthesis of bio-active macrolide natural products cladospolide A 1, seimatopolide A 2 and synthetic studies towards aetheramides A 3 and B 4 (Figure 1).

Figure 1: Bio-active macrolide natural products.

Section A of chapter 1 describes the enantiospecific total synthesis of cladospolide A (ent-1). Cladospolide A was isolated from three different sources such as culture filtrate of cladosporium fulvam FI-113, Fungus cladosporium tenuissimum and Fermentation broath of cladosporium sp. FT-0012. Cladospolide A is shown to inhibit the root growth of lettuce seedlings. Enantiospecific total synthesis of cladospolide A ent-1 was accomplished in 9% overall yield in 11 linear steps using D-ribose as a chiral pool precursor. Key reactions in the present approach include olefin cross metathesis and Yamaguchi macrolactonization reactions (Scheme 1).

Scheme 1: Total synthesis of cladospolide A (ent-1).

Section B of chapter 1 describes the use of furan as a surrogate for the E-but-2-ene-1, 4-dione unit in the total synthesis of seimatopolide A 2. Seimatopolide A 2 was isolated by Heip and co-workers from the fungus Seimatosporium discosioides in 2012 and is shown to activate the γ-subtype peroxysome proliferator-activated receptors (PPAR-γ), which is a pivotal process in the type-2 diabetes. Total synthesis of ent-seimatopolide A was accomplished in 7.8% overall yield in 14 linear steps from furfural. Nagao acetate aldol and Shiina macrolactonization reactions were employed as key reactions for the synthesis of ent-seimatopolide A (ent-2) (Scheme 2).

Scheme 2: Stereoselective total synthesis of seimatopolide A (ent-2). In section C of Chapter 1, studies towards the synthesis of aetheramides A 3 and B 4 are described. Aetheramides A 3 and B 4 are isolated by Müller’s group in 2012 from the novel myxobacterial genus “Aetherobacter”. Aetheramides are cyclic depsipeptides, which are shown to inhibit the HIV-I infection with IC50 values of ∼0.015 μM and cytostatic activity against human colon carcinoma (HCT-116) cells with IC50 values of 0.11 μM. Stereochemistry at two chiral centers present in the molecules is unassigned. The first approach (Scheme 3) relied on macrolactonization as the key step while the second approach (Scheme 4) relied on RCM to accomplish the macrolactonization. The required precursors were synthesized from elaboration of chiral furyl carbinol, while synthesis of the RCM precursor was accomplished employing the aldol reaction.

Scheme 3: Macrolactonization strategy for synthesis of 3 from chiral furyl carbinol.

Scheme 4: RCM strategy for synthesis of 3 from chiral furyl carbinol.

The successful synthesis of the macrolactone core of aetheramide A 1 is accomplished by employing the ring closing metathesis reaction to construct the C18-C19 bond. RCM precursor has been synthesized by the amidation of the amine derived from R-mandelic acid, while the acid fragment is synthesized from allyl trityl ether (Scheme 5).

Scheme 5: RCM strategy for synthesis of 3 from R-mandelic acid. Second chapter of the thesis describes the synthesis and application of novel sulfinamide ligands in asymmetric catalysis. In section A of chapter 2, chiral 2-pyridylsulfinamides are shown to be effective catalysts in the alkylation of aryl and alkyl aldehydes with diethylzinc providing the corresponding alcohols in excellent enantioselectivity. It was found that the chirality present at sulfur in the ligand is pivotal for the asymmetric induction (Scheme 6).

Scheme 6: Asymmetric alkylation of benzaldehyde with some of the 2-Pyridyl sulfinamide catalysts.

Second section of chapter 2 describes the synthesis and application of C2-symmetric bis-sulfinamides in Rh (I) catalyzed conjugate addition of PhB(OH)2 to enones. Chirality present at sulphur in sulfonamide as well as symmetry present in the ligand plays crucial role in the outcome of the reaction (Scheme 7).

Scheme 7: Asymmetric arylation of enones using C2-symmetric bis-sulfinamide/olefin ligands.