Efficient strategy for the synthesis of stereopentad subunits of scytophycin, rifamycin S, and discodermolide

Chemistry and biology of spiroacetals from myxobacteria

This review details the isolation, biosynthesis, biological activity and synthesis of spiroacetals from the myxobacterium Sorangium cellulosum. The strategies utilised to access the challenging structures and stereochemistry of these natural products are highlighted.

Diastereo- and enantioselective synthesis of 1,3,5,7-tetraol structural units using a Prins cyclisation–reductive cleavage sequence

Stereocontrolled access to all the diastereomers of 1,3,5,7-tetraol structural units was developed using a Prins cyclisation–reductive cleavage sequence.

Use of sultines in the asymmetric synthesis of polypropionate antibiotics

At low temperature and in the presence of an acid catalyst, SO2 adds to 1,3-dienes equilibrating with the corresponding 3,6-dihydro-1,2-oxathiin-2-oxides (sultines). These compounds are unstable above -60 °C and equilibrate with the more stable 2,5-dihydrothiophene 1,1-dioxides (sulfolenes). The hetero-Diels-Alder additions of SO2 are suprafacial and follow the Alder endo rule. The sultines derived from 1-oxy-substituted and 1,3-dioxy-disubstituted 1,3-dienes cannot be observed at -100 °C but are believed to be formed faster than the corresponding sulfolenes. In the presence of acid catalysts, the 6-oxy-substituted sultines equilibrate with zwitterionic species that react with electron-rich alkenes such as enoxysilanes and allylsilanes, generating β,γ-unsaturated silyl sulfinates that can be desilylated and desulfinylated to generate polypropionate fragments containing up to three contiguous stereogenic centers and an (E)-alkene unit. Alternatively, the silyl sulfinates can be reacted with electrophiles to generate polyfunctional sulfones (one-pot, four-component synthesis of sulfones), or oxidized into sulfonyl chlorides and reacted with amines, then realizing a one-pot, four-component synthesis of polyfunctional sulfonamides. Using enantiomerically enriched dienes such as 1-[(R)- or 1-(S)-phenylethyloxy]-2-methyl-(E,E)-penta-1,3-dien-3-yl isobutyrate, derived from inexpensive (R)- or (S)-1-phenylethanol, enantiomerically enriched stereotriads are obtained in one-pot operations. The latter are ready for further chain elongation. This has permitted the development of expeditious total asymmetric syntheses of important natural products of biological interest such as the baconipyrones, rifamycin S, and apoptolidin A.

Total synthesis of the marine natural product (+)-discodermolide in multigram quantities

The novel polyketide (+)-discodermolide was isolated in very small quantities from sponge extracts. This compound is one of several microtubule stabilizers showing promise as novel chemotherapeutic agents for the treatment of cancer. The clinical evaluation of this and similar compounds is hampered by lack of material, and at present, the only way to obtain the necessary quantities is total chemical synthesis.

Diastereoselective dihydroxylation and regioselective deoxygenation of dihydropyranones: a novel protocol for the stereoselective synthesis of C1-C8 and C15-C21 subunits of (+)-discodermolide

Diastereoselective dihydroxylation of dihydropyranones and subsequent regioselective alpha-deoxygenation provides 1,3-trans-beta-hydroxy-delta-lactones stereoselectively. This protocol has been applied for the synthesis of C(1)-C(8) and C(15)-C(21) subunits of (+)-discodermolide.

A second-generation total synthesis of (+)-discodermolide: the development of a practical route using solely substrate-based stereocontrol

A novel total synthesis of the complex polyketide (+)-discodermolide, a promising anticancer agent of sponge origin, has been completed in 7.8% overall yield over 24 linear steps, with 35 steps altogether. This second-generation approach was designed to rely solely on substrate control for introduction of the required stereochemistry, eliminating the use of all chiral reagents or auxiliaries. The common 1,2-anti-2,3-syn stereotriad found in each of three subunits, aldehyde 9 (C(1)-C(5)), ester 40 (C(9)-C(16)), and aldehyde 13 (C(17)-C(24)), was established via a boron-mediated aldol reaction of ethyl ketone 15 and formaldehyde, followed by hydroxyl-directed reduction to give 1,3-diol 14. Alternatively, a surrogate aldehyde 22 was employed for formaldehyde in this aldol reaction, leading to the beta-hydroxy aldehyde 20 as a common building block, corresponding to the discodermolide stereotriad. Key fragment unions were achieved by a lithium-mediated anti aldol reaction of ester 40 and aldehyde 13 under Felkin-Anh control to provide (16S,17S)-adduct 51 and a boron-mediated aldol reaction between enone 10 and aldehyde 9, exploiting unprecedented remote 1,6-stereoinduction, to give the (5S)-adduct 57.

Expeditious Asymmetric Synthesis of a Stereoheptad Corresponding to the C(19)-C(27)-Ansa Chain of Rifamycins: Formal Total Synthesis of Rifamycin S

In the presence of sulfur dioxide and an acid promoter, (-)-(1E,3Z)-2-methyl-1-((1S)-1-phenylethoxy)penta-1,3-dien-3-yl isobutyrate reacts with (Z)-3-(trimethylsilyloxy)pent-2-ene giving a silyl sulfinate intermediate that undergoes, in the presence of palladium catalyst, a desilylation and retro-ene elimination of SO(2) with formation of (-)-(1Z,2S,3R,4S)-1-ethylidene-2,4-dimethyl-5-oxo-3-((1S)-1-phenylethoxy)-heptyl isobutyrate as major product. This ethyl ketone undergoes cross-aldol reaction with (2S)-2-methyl-3-[(tert-butyldimethylsilyl)oxy]propanal giving an aldol that is reduced into a stereoheptad corresponding to the C(19)-C(27)-segment of Rifamycins with high diastereoselectivity and enantiomeric excess.

Total Synthesis of Zincophorin and Its Methyl Ester

A total synthesis of the naturally occurring ionophore zincophorin has been realized. The route features an intramolecular oxymercuration of a cyclopropanemethanol and a Carroll-Claisen rearrangement for the respective elaboration of the C1-C12 and C13-C25 subunits, which have been assembled by using a highly diastereoselective titanium-mediated aldol condensation.

Total synthesis of scytophycin C. 1. Stereoselective syntheses of the C(1)-C(18) segment and the C(19)-C(31) segment

[structure: see text] Stereoselective total synthesis of scytophycin C, a marine 22-membered macrolide displaying potent activity against a variety of human carcinoma cell lines, has been reported in which the polypropionate structure bearing contiguous asymmetric centers was stereospecifically constructed by using new acyclic stereocontrol. This paper describes stereoselective syntheses of the C(1)-C(18) segment (Segment A) including a trans-disubstituted dihydropyran ring and the C(19)-C(31) segment (Segment B) having eight stereogenic centers.

Synthesis of Long-Chain Polyketide Fragments by Reaction of 1,3-Dioxy-1,3-dienes with Allylsilanes: Umpolung with Sulfur Dioxide

[reaction: see text] In the presence of a Lewis or protic acid and at low temperature, 1,3-dioxy-1,3-dienes add to sulfur dioxide generating zwitterionic intermediates that can react with carbon nucleophiles such as allylsilanes. After a retro-ene elimination of SO(2), valuable polyketide precursors are obtained.

Total Synthesis of Scytophycin C. 2. Coupling Reaction of the C(1)−C(18) Segment and the C(19)−C(31) Segment, a Key Macrolactonization, and the Crucial Terminal Amidation Reaction

[reaction: see text] Stereoselective syntheses of the C(1)-C(18) segment (segment A) and the C(19)-C(31) segment (segment B) are described in the preceding paper. This paper reports the key coupling reaction of both segments, 22-membered lactonization, and the crucial terminal amidation reaction culminating in the total synthesis of scytophycin C.

Stereoselective synthesis of the C1-C13 fragment of (+)-discodermolide using asymmetric allyltitanations

[reaction: see text] The synthesis of the C1-C13 fragment of (+)-discodermolide has been achieved. The configurations of the stereogenic centers have been controlled by enantioselective allyl- and crotyltitanations of aldehydes, and the Z configuration of the olefin at C8-C9 was controlled by a ring-closing metathesis.