Total Synthesis of Macrolides

Structurally complex macrolide natural products, isolated from a variety of marine and other sources, continue to provide a valuable source of targets for the synthetic chemist to embark. In this account, we provide the recent progress and pathways in the total synthesis of macrolides and discussed the synthesis of (+)-neopeltolide, aspergillide D, miyakolide and acutiphycin natural products.


Introduction
Macrolides are a class of antibiotics that consist of a large macrocyclic lactone ring attached to deoxy sugars. These antibiotics are bacteriostatic in nature and act by inhibiting protein synthesis of bacteria. These are obtained mainly from certain actinomycetes genus, such as Streptomyces and related species. The original macrolide complex, erythromycin A, was isolated in 1952 as a natural product of Saccharopolyspora erythraea (formerly Streptomyces erythreus). Other examples include clarithromycin, azithromycin, telithromycin, cethromycin, modithromycin, etc. Macrolides structurally contain three characteristic parts in every molecule, that is, a macrocyclic lactone ring, multiple ketone & hydroxyl group, and two deoxy sugars attached by glycosidic bond. According to the carbon number of lactone ring, macrolides are classified into several types. That is, 12-membered ring, 13-membered ring, 14-membered ring, 15-membered ring, 16-membered rings, etc. (Figure 1). Out of these, most of the antibiotic drugs comprised of 14-membered and 16-membered lactone rings.
The construction of macrocyclic structures is a recurrent and challenging problem in synthetic organic chemistry. Theoretically, macrocyclic systems can be generated by cyclization of open, long chain precursors or by cleavage of internal bonds in polycyclic systems. In the course of synthesis, numerous problems are encountered to achieve target molecules. Despite the several problems, however, recent interest in the chemistry of macrolide antibiotics and other biologically active macrolactones and macrolactams resulted in the discovery and development of several new synthetic methods for macrolide formation. In this chapter, total synthesis of some of the macrolides is discussed with scrupulous emphasis on the key macrolide ring forming reactions.

Synthetic strategy for macrolide synthesis
In the polyoxomacrolide ring, generally we will observe the 1,3-diol systems as a core. There are two synthetic approaches for the edifice of 1,3-diols which are illustrated here. They are asymmetric aldol reaction and the other one is asymmetric epoxide and epoxide ring-opening.

Aldolization
Asymmetric synthesis of β-hydroxy ketones by aldol reactions of ketones with aldehydes is the general and efficient method for the synthesis of 1,3-diol systems and is of great interest in the field of total synthesis. By using the range of chiral ketones, highly diastereoselective syn and anti aldol products are produced using various boron enolates [1][2][3]. Some of the reagents shown below (Figure 2) direct the relative and absolute stereochemistry of C▬C bond formation between various achiral and chiral ketones, thus providing a ubiquitous synthetic tool for macrolide synthesis (Figure 3).
A highly efficient and extensively used method for diastereoselective aldol reactions is the Evans aldol reaction using boron enolate derived from a chiral imide [4,5]. Upon treatment of imide 1 with n-Bu 2 BOTf and i-Pr 2 NEt in CH 2 Cl 2 followed by addition of aldehyde, aldol reaction proceeds smoothly in stereoselective manner through the chelation transition state to attain 1,2-syn-aldol adduct 2 in high yield and with excellent diastereoselectivity. After the reaction, the chiral auxiliary is cleaved by hydrolysis to acid, then reduction to aldehyde or alcohol, conversion to  Weinreb amide, etc. In contrast, addition of a Lewis acid to the boron enolate provides either anti-diol 3 or non-Evans 1,2-syn-aldol 4 with excellent diastereoselectivity [6] (Figure 4).

Asymmetric epoxidation and dihydroxylation
The asymmetric epoxidation of allylic alcohols introduced by Katsuki and Sharpless in 1980 has tremendous applications in the synthesis of various  compounds [7]. The Sharpless asymmetric epoxidation (AE) is the efficacious reagent in the synthetic organic chemistry particularly in the synthesis of variety of natural products. Epoxidation is carried out from allylic alcohols 5 with tert-butyl hydroperoxide in the presence of Ti(OiPr) 4 . The resulting epoxide stereochemistry is determined by the enantiomer of the chiral tartrate ester (usually diethyl tartrate or diisopropyl tartrate) employed in reaction. When (À)-diester is used, β-epoxide 6 is obtained, while (+)-diester produces α-epoxide 7 ( Figure 5).
Stereoselective ring-opening of 2,3-epoxy alcohols 10 is extremely valuable for the synthesis of different functionalized compounds [9]. A wide range of nucleophiles such as secondary amine, alcohol, thiol, azide and carboxylic acid predominantly at C-3 position to give 1,2-diol 11 (Figure 7).  The logic of macrocyclization in natural product synthesis can be investigated by different strategies; some of them are Prins reaction [10], lactonization [11,12], ring closing metathesis [13], Wittig reaction [14], Horner Wadsworth Emmons (HWE) reaction [15], Julia-Kocienski reaction [16], metal-mediated cross coupling reaction [17], etc. However, it is true that there is no universal macrocyclization method is reliable in the total synthesis of natural products.

Macrolactonization
Macrolactonization is the one of the effective and popular methods in the synthesis of macrolactones. The method is based on the lactonization of the corresponding seco-acid. Thus various methods are reported in the literature for the macrolactone synthesis, some of the most commonly used methods are Corey-Nicolaou [18], Shiina [19], Yamaguchi [20], Mitsunobu [21], Keck-Boden [22], and Mukaiyama [23] macrolactonizations (Figure 8).  Hansen et al. [24] reported the synthesis of (À)-aplyolide A from 12 in which they adopted the Corey-Nicolaou macrolactonisation as the key step with 78% yield (Figure 9).
Narasaka et al. [25] used the Mukaiyama method for the effective construction of macrocycle ring from corresponding seco-acid 13 in the synthesis of Prostaglandin F-lactone ( Figure 10).
Enev et al. [26] in his studies towards the total synthesis of laulimalide, crucial Yamaguchi macrolactonization was employed on the ynoic seco-acid 14 and then reducing the triple bond obtained the desired macrolactone 15 (Figure 11).
In the synthetic studies towards the synthesis of colletodial, Keck et al.
[27] effectively used DCC-DMAP protocol for the macrolactonization of 16 to precursor of colletodial 17 (Figure 12).    Mitsunobu macrolactonization protocol based on the activation of the seco-acid alcohol 18 to 19 using diisopropyl azodicarboxylate (DIAD) and triphenylphosphine is used in the total synthesis of natural product (+)-amphidinolide K by Williams and Meyer [28] (Figure 13).
In the total synthesis of iejimalide by Schweitzer et al. [29], Shiina macrolactonization (2-methyl-6-nitro benzoic anhydride/DMAP) is used as the key step for the construction of macrolactone 21 in moderate yield. Even the yield is somewhat low, other methods failed to construct the lactone while Shiina protocol worked successfully from 20 (Figure 14).

Ring-closing olefin metathesis
In recent years, ring closing metathesis (RCM) has become one of the most paramount tools in synthetic organic chemistry especially in the field of total synthesis of macrolide natural products [13,30,31]. Furthermore, RCM is becoming the most popular way to construct large rings and has the advantage of being compatible with a wide range of functional groups such as ketones, ethers, esters, amides, amines, epoxides, silyl ethers, alcohols, thioesters, etc. In view of this, among the several reagents developed by Grubbs, Shrock, and Chauvin, the catalysts A-D represents two generations of ruthenium complexes, while E is the molybdenum Shrock catalyst (Figure 15). A is popularly known as Grubbs first generation catalysts, B and C are Grubbs second generation catalysts and D is   Hoveyda-Grubbs catalyst. The choice of the catalysts can be used in the synthetic organic transformations based on the reactivity of the substrate, and other reaction condition parameters. Substitution in the aromatic ring of D has given rise to a new family of third generation catalyst.

Palladium catalyzed coupling reactions
Palladium-catalyzed coupling reactions have gained more attention in recent years in the field of organic chemistry. In this course, Suzuki reaction using organoboron compounds [37], Heck reaction using alkenes [38], Stille reaction with organostannate [39,40], Sonogashira reaction with terminal alkyne [41] and Tsuji-Trost reaction with π-allylpalladium intermediate [42,43], etc. are the most frequently employed reactions in the total synthesis of macrolide natural products. Some of them are depicted here. Tortosa and co-workers [44] in the synthesis of (+)-superstolide A, Suzuki macrocyclisation approach is used for the construction of 24-membered macrocyclic octane 23 (Figure 17).
The first application of the Heck cyclisation to a macrocyclic substrate was reported by Ziegler and co-workers [45] in 1981 during the synthesis of aglycone of the macrocyclic antibiotic carbomycin B. They achieved the cyclisation to the model substrate 25 in 55% yield, by slow addition to a solution of PdCl 2 (MeCN) 2 , Et 3 N and formic acid in MeCN at ambient temperature (Figure 18).
Stille macrocyclisation as illustrated below used as the key step in the total synthesis of the biselyngbyolide A by Tanabe et al. [46] using Pd 2 (dba) 3 and lithium chloride in DMF (Figure 19).
The utility of Sonogashira macrocyclisation in the first total synthesis of penarolide sulfate A1, an α-glucosidase inhibitor is demonstrated by Mohapatra and co-workers [47]. The macrocyclisation was successfully achieved from compound 28 with catalytic Pd(PPh 3 ) 4 and CuI in Et 2 NH at room temperature (Figure 20).
Towards the total synthesis of antibiotic natural product A26771B, Trost and co-workers [48] effectively constructed the macrolactone 31 (Figure 21) by the use of bidentate phosphine ligand (1,4-bis(diphenylphosphino)butane (DPPB)).   In the end game of total synthesis of macrolides, glycosidation to the aglycon also have more significance. Thus, a wide variety of methods are reported for glycosidation in the literature [49,50].

(+)-Neopeltolide
(+)-Neopeltolide is a 14-membered macrolide isolated from north coast of Jamaica by Wright and coworkers from a deep water sponge [51]. It was tested for in vitro antiproliferative activity against several cancer cell lines comprising A549 human lung adenocarcinoma, NCI/ADR-RES ovarian sarcoma and P388 murine leukemia and shows IC 50 values 1.2, 5.1 and 0.56 nM. Besides this, neopeltolide also exhibits anti-fungal activity against Candida albicans [52]. The complexity of the structure with six chiral centres, tetrahydropyran ring, and an oxazole-bearing unsaturated side chain and its efficacious biological activity led to several total syntheses, few of them are discussed below.  In 2013, Ghosh et al. [53] in their total synthesis adopted the retrosynthetic pathway as follows. Disconnection of O▬C bond of the oxazole side chain would give acid which can undergo Mitsunobu esterification. Yamaguchi macrolactonization of acid would in turn give the desired macrolactone. The tetrahydropyran ring in acid could be constructed via a hetero Diels-Alder reaction between aldehyde and silyloxy diene ether using Jacobsen's chromium catalyst.
The synthesis of the macrolactone ring of (+)-neopeltolide began with commercially available 3-methyl glutaric anhydride as shown in the scheme. 3-methyl glutaric anhydride, 36 was desymmetrized using PS-30 'Amano' lipase to obtain acid. The resulting acid was treated with borane-dimethyl sulfide complex to afford alcohol, 37. Alcohol 37 was oxidized to corresponding aldehyde by Swern oxidation and then protected to its acetal, 38. Ester of 38 was then reduced to alcohol and on Swern oxidation obtained aldehyde and the resulting aldehyde was subjected to Brown's allylation protocol using (+)-Ipc 2 BOMe and allyl magnesium bromide to attain alcohol, 39. Alcohol 39 was methylated with MeI, and on Lemieux-Johnson oxidation gave aldehyde and on Brown's allylation protocol afforded alcohol, 40. Acetal protection was deprotected and the aldehyde was converted to α,β-unsaturated ketone, 41 using standard Horner-Wadsworth-Emmons olefination conditions. Secondary alcohol in 41 was then protected with TESOTf to obtain the silyloxy diene, 42 in excellent yield (Figure 22).
After the completion of requisite silyloxy diene, hetero-Diels Alder reaction of tosyl oxyacetaldehyde, 43 with 42 using chiral chromium catalyst (44) gave tetrahydropyranone, 45 in 83% yield (Figure 23). After protection of ketone group in 45 as ketal and displaced the tosylate to nitrile 46 using NaCN in DMF. Nitrile 46 was hydrolysed to acid and on deprotection of ketone to afford ketone 47. Intramolecular Yamaguchi macrolactonization attained the key macrolactone 48 in 40% yield. Olefin 48 was subjected to hydrogenation with 10% Pd/C to give saturated compound and on reduction with NaBH 4 /EtOH to attain alcohol 49. Next, the synthesis of unsaturated oxazole side chain 50 is started with known alkyne with LDA and Bu 3 SnCl to obtain the alkynyl stannate, which on hydrozirconation gave the carbamate in 38% yield. Crucial Stille cross coupling of carbamate with iodooxazole using Pd(MeCN) 2 Cl 2 in DMF gave oxazole which can be easily converted to desired side chain 50. The target neopeltolide compound was furnished by standard Mitsunobu esterification of 49 with acid 50 (Figure 24).  Paterson and coworkers [54] reported the synthesis of neopeltolide as follows. Aldehyde was synthesized starting from known β-keto ester, 51, which on treatment with (S)-BINAP-Ru (II) catalyst under Noyori asymmetric hydrogenation to afford (13S)-alcohol. The alcohol on TBS protection and DIBAL reduction of the ester produced the enantiopure aldehyde 52. Aldehyde 52 was subjected to Brown's methallylation using 2-methyl propene and (À)-Ipc 2 BOMe furnished the desired C11 alcohol with 94:6 dr, and the alcohol was methylated into the methyl ether 53 by NaH, MeI. Methyl ether 53 was subjected to ozonolysis to obtain methyl ketone and on Horner Wadsworth Emmons reaction with trimethyl phosphonoacetate to attain ester 54 in E/Z isomers (75,25). Esters 54 were reduced to its alcohol by DIBAL-H and on subsequent oxidation with Dess-Martin periodinane produced aldehyde 55. Next, organo catalytic hydride reduction of enal 55 using MacMillan strategy with imidzolidinone catalyst 56. TFA (20 mol%) and Hantzch ester furnished as 1,4-reduction product 57 with 76:24 of epimers at C9 stereocentre ( Figure 25). Further, Jacobsen asymmetric hetero Diels-Alder reaction between 57 and known 2-silyloxy diene 58 produced cis-tetrahydropyranone 60 in 60% yield using chiral tridentate chromium (III) catalyst 59. On PMB deprotection and further oxidation of alcohol to corresponding acid followed by TBS deprotection furnished seco-acid 61. Macrolactonization of 61 under standard Yamaguchi conditions afforded macrolactone 62 in 80% yield. Reduction of macrolactone 62 to the alcohol with NaBH 4 in MeOH followed by Mitsunobu esterification with the oxazole side chain 50 achieved (+)-neopeltolide (32) in 52% yield (Figure 26).

Ulanovskaya strategy
The synthesis of neopeltolide by Ulanovskaya et al. [55] is depicted as follows, Prins desymmetrization of diene 63 followed by benzyl protection and Wacker oxidation of alkene afforded ketone 64. Formation of boron enolate from ketone and on addition of aldehyde 65 gave the anticipated aldol product with >98:2 diastereoselectivity, which was treated with Ph 3 P=CH 2 (Wittig methylenation) followed by cleavage of dioxolone by acidic work-up afforded ketone 66 in 75% yield. Ketone 66 was selectively reduced to syn-alcohol using Et 2 BOMe and NaBH 4 followed by ester hydrolysis gave acid which was subjected to Yamaguchi macrolactonization to furnish desired macrolactone 67. Alkene in 67 was hydrogenated using Pd/C to afford desired alcohol 68 as a major product. Alcohol 68 was subjected to Mitsunobu conditions, followed by hydrolysis with K 2 CO 3 in MeOH to get the inversion product. Subsequent methylation with MeO 3 BF 4 & hydrogenolysis of benzyl ether achieved desired macrolide 49. The final coupling of fragment 49 with oxazole side chain 50 with standard Mitsunobu conditions furnished target (+)-neopeltolide 32 (Figure 27).
The retrosynthetic analysis of aspergillide D was depicted as shown above, macrolactone could be synthesized from seco acid via intramolecular Shiina esterification. For the total synthesis of Aspergillide D, the acid fragment was synthesized from commercially available D-ribose which was transformed to lactol 69 by using three step sequence, that is, catalytic amount of H 2 SO 4 & acetone to form acetonide which on reduction with NaBH 4 and on oxidative cleavage of the diol with NaIO 4 . The lactol was subjected to Wittig type olefination using PPh 3 =CH 2 and the obtained primary alcohol 70 was oxidized to carboxylic acid 71 by using TEMPO/ BAIB conditions (Figure 28). The synthesis of alcohol fragment was started with mono-PMB protection 73 of commercially available 1,8-octane diol 72 and the other alcohol was converted to racemic allyl alcohol 74 by Swern oxidation and subsequent treatment of aldehyde with vinyl magnesium bromide in the presence of CuI. The allylic alcohol 74 was subjected to standard Sharpless kinetic resolution conditions by using (À)-DIPT & Ti(OiPr) 4 to obtain enantiomeric epoxy alcohol 75. Upon MOM protection 76 to the secondary alcohol 75 and PMB deprotection produced 77, which on oxidation with Dess-Martin periodinane to afford aldehyde. Aldehyde was converted to olefin 78 by treating PPh 3 =CH 2 in THF. 78 was cleaved to alcohol 79 by reduction with LAH in THF (Figure 29).
Acid 71 and alcohol 79 fragments were coupled together under Yamaguchi esterification conditions afforded diene ester 80 in 65% yield. Intramolecular RCM was employed on diene ester by using Grubbs' second generation catalyst in refluxing CH 2 Cl 2 to produce the requisite macrolactone 81. Double bond in 81 was hydrogenated by using PtO 2 in MeOH to attain saturated lactone 82. Lactone 82 was reduced with DIBAL-H to afford lactol which on further treatment with Ph 3 P=CHCO 2 Et in C 6 H 6 afforded α,β-unsaturated ester 83. The ester was converted to carboxylic acid 84 by LiOH in THF/H 2 O which on adopting key Shiina's   macrolactonization protocol to provide the desired mactrolactone 85 in 51% yield. On deprotection of acetonide with CuCl 2 .2H 2 O gave diol 86 and removal of MOM group, the synthesis of aspergillide D 33 was achieved (Figure 30).

Miyakolide
Evan's strategy of bond connections & key reactions in the synthesis of 34 is illustrated [58].

Conclusions
A number of new macrolide antibiotics with fascinating biological activities have been isolated everyday with the unique and complex structures have been determined with extensive spectroscopic studies. Toward the total synthesis of such macrolide antibiotics, very efficient synthetic strategies and various new methodologies are also developed. Recent advances in macrolide synthesis based on newly developed strategies and methodologies are noteworthy. Further synthetic studies on macrolide antibiotics will make an immense contribution to progress in both organic and medicinal chemistry.