Synthetic studies toward amphidinolide B1: synthesis of the C9-C26 fragment

[reaction: see text] The synthesis of the C9-C26 portion of amphidinolide B1 is described. A Fleming allylation followed by elimination was employed for the construction of the C13-C15 diene portion. Sharpless asymmetric dihydroxylation was utilized for regioselective functionalization of a styrene-derived alkene, in the presence of the C13-C15 diene functionality. A highly diastereoselective aldol reaction was developed to establish the C18 stereochemistry.     

Synthetic studies on amphidinolides C and F: synthesis of the C18-C29 segment of amphidinolide F

The C18-C29 segment of amphidinolide F is synthesised in 12 steps from 1,4-butanediol. Key steps include a mono-Sharpless dihydroxylation of a dienoate, iodocyclisation to construct the trans-THF ring and an E-selective Wittig reaction to introduce the C25-C26 olefin.     

An expedient synthesis of 2,5-disubstituted-3-oxygenated tetrahydrofurans

Single enantiomer 2,5-disubstituted-3-oxygenated tetrahydrofurans are synthesized in as little as four steps from a commercially available epoxide. The key steps are homoallylic alcohol epoxidation, palladium-catalysed alkoxy-carbonylation-lactonisation and Mitsunobu inversion. The protocol is applied to the formal total syntheses of (+)-kumausallene, (6S,7S,9R,10R)-6,9-epoxynonadec-18-ene-7,10-diol and the core of lytophilippine A.     

Towards the total synthesis of amphidinolide E: an enantioselective synthesis of C12-C29 fragment

An enantioselective synthesis of the C12-C29 fragment of amphidinolide E is described. Key transformations include an intramolecular mercuriocyclization reaction, stereoselective introduction of methyl group at the C2 position, and Stille coupling for the introduction of the diene side chain.     

Total synthesis of C19 lipid diols containing a 2,5-disubstituted-3-oxygenated tetrahydrofuran

The total synthesis of the C(19) lipid diols 5 and 6, the enantiomers of the anthelmintic marine natural products 1 and 3, is described. Key steps in the divergent syntheses include a syn selective epoxidation of a homoallylic alcohol, a one-pot alkoxypalladation-carbonylation-lactonisation reaction sequence and a DMEAD promoted Mitsunobu inversion.     

Halichondrin B: synthesis of the C1-C22 subunit

[Reaction: see text]. Two efficient routes to the C1-C22 subunit of halichondrin B are described. The cage ketal 7, which contains 11 asymmetric centers embedded within the ABCDEF-ring framework, was assembled from (+)-conduritol E (27) in 18 steps and 4% overall yield. In a separate route, 7 was also synthesized in 18 steps and 2% overall yield from a derivative of alpha-d-glucoheptonic acid gamma-lactone (62). While the former route installs the fully elaborated C-ring endowed with the correct C12 stereochemistry early in the synthesis, the latter features a late-stage introduction of the C12 stereocenter during the ultimate one-pot Michael addition/ketalization cascade to form the CDE-ring system of the cage. The importance of the C12 stereocenter to the crucial ketalization event is discussed through comparison of these two strategies.     

Gram scale synthesis of the C(18)-C(34) fragment of amphidinolide C

The synthesis of the C(18)-C(34) fragment of amphidinolide C has been achieved via two routes, culminating in both the shortest (11 steps) and highest yielding (26% overall yield) approaches to this segment. The highly convergent approach will facilitate the synthesis of analogues, including the C(18)-C(29) fragment of amphidinolide F. Synthetic highlights include the selective methylation of a diyne, and the highly efficient use of a second generation cobalt catalyst in the Mukaiyama oxidative cyclization to form the trans-THF ring.     

Toward the synthesis of reidispongiolide a: stereocontrolled synthesis of the C17-C22 and C23-C35 degradation fragments

[structure: see text] By relying on the asymmetric aldol reactions of chiral ketones, a highly stereocontrolled synthesis of each of the C(17)-C(22) and C(23)-C(35) degradation fragments of reidispongiolide A has been achieved. This permits a configurational assignment of the complete C(17)-C(36) region of this antimitotic macrolide, along with providing advanced intermediates for a projected total synthesis.     

Synthetic studies on reidispongiolide A, an actin-depolymerizing marine macrolide: synthesis of C11-C22 and C23-C35 segments

The C11-C22 and C23-C35 segments 2 and 3 of reidispongiolide A (1), an actin-depolymerizing marine macrolide, were synthesized enantioselectively in 12 steps from (R)-glycidyl trityl ether and in 12 steps from chiral ketone 15, respectively.     

Synthesis of the C1-C12 fragment of amphidinolide T1

A synthesis of the C1-C12 fragment of amphidinolide T1 utilising Evans’ aldol, oxy-Michael and cross metathesis reactions as the key steps is described.     

A highly diastereoselective synthesis of 3-carbethoxy-2,5-disubstituted-3-pyrrolines by phosphine catalysis

Tributylphosphine was used as catalyst to facilitate a [3+2] cycloaddition between γ-substituted allenoates and N-sulfonylimines. The resulting adducts, 3-carbethoxy-2,5-disubstituted-3-pyrrolines, were formed in excellent yields with high diastereoselectivity. The reaction went to completion in several hours at room temperature, and the starting materials were easily prepared with one step from commercially available compounds via known procedures.     

Toward a total synthesis of amphidinolide X and Y. The tetrahydrofuran-containing fragment C12-C21

The key THF derivative (9a) for an enantioselective synthesis of amphidinolide X/Y was obtained from 1a via a selenoetherification reaction. In fact, among the cyclization methods investigated, the highest yield and stereocontrol were achieved at -78 degrees C with PhSeCl/EtiPr2N from diols 1a (anti-Z) and 1b (anti-E) and with PhSeCl/ZnBr2 from diols 1c (syn-Z) and 1d (syn-E). Also, surprisingly, use of protecting groups (on the allylic OH) was detrimental in the cases studied. The diverse THF-tetrasubstituted stereoisomers will provide a series of amphidinolide X/Y analogues. [structure: see text]     

Stereocontrolled synthesis of the C8-C22 fragment of rhizopodin

A convergent synthesis of the central C8-C22 core of the potent macrolide antibiotic rhizopodin is reported. Notable features of the stereocontrolled approach include an asymmetric reverse prenylation of an alcohol using a method of Krische, a thiazolium catalyzed transformation of an epoxyaldehyde as described by Bode, and a late-stage oxazole formation from advanced intermediates. This route demonstrates the applicability of these methodologies in complex natural product synthesis.     

Studies towards the total synthesis of cruentaren A and B: Stereoselective synthesis of fragments C1-C11, C12-C22 and C23-C28

A convergent and stereoselective approach for the synthesis of C1-C11, C12-C22, and C23-C28 fragments of cytotoxic natural products cruentaren A and B are accomplished. Highlights of the strategy include a Sharpless epoxidation followed by a regioselective opening of epoxide to generate anti and syn-stereochemistry at C9-C10 and C15-C16, an Alder-Rickert reaction between a 1,5-dimethoxy-1,4-cyclohexadiene and dienophile to construct the aromatic ring, and a lithium-mediated aldol reaction to install the C17-C18 anti-stereochemistry. The synthesis of C1-C11 and C12-C22 fragments proceed with a longest linear sequence of 10 and 17 steps from commercially available 2-butyne-1,4-diol and cis-2-butene-1,4-diol respectively.     

New method for the synthesis of cis-2,5-disubstituted sulfolane derivatives

A new method for the preparation of functional derivatives of sulfolane by ozonolysis of 13-thiabicyclo[8.2.1]-cis-5-tridecene and its derivatives was developed.Communication III of the series Reaction of Sulfur Halides with Unsaturated Compounds.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 3, pp. 360–364, March, 1974.     

Synthesis of C13-C22 of amphidinolide T2 via nickel-catalyzed reductive coupling of an alkyne and a terminal epoxide

Several stereoselective routes to the synthesis of (1S,3R)-t-butyldimethyl-(1-methyl-3-oxiranyl-propoxy)-silane (13a) were explored, and the use of Jacobsen's hydrolytic kinetic resolution to separate a mixture of diastereomeric epoxides was a key step in the shortest of these. As part of an approach to the total synthesis of amphidinolide T2 (2), this epoxide, corresponding to C17-C22 of the natural product, was successfully joined with an alkyne (C13-C16) by way of a nickel-catalyzed reductive coupling reaction.     

Stereoselective syntheses of the C(1)-C(9) fragment of amphidinolide C

Stereoselective syntheses of the C(1)-C(9) fragments 18 and 28 of amphidinolide C have been developed. The first-generation sequence involves a diastereoselective chelate-controlled [3 + 2]-annulation reaction of 6 and 7, while the second-generation synthesis involves an intramolecular hetero-Michael cyclization of 8.     

Synthesis of (-)-amphidinolide K fragment C9-C22

[reaction: see text] The key fragment (2a or 2b) in a total synthesis of the cytotoxic macrolide (-)-amphidinolide K (1) has been achieved from synthons C9-C14 (3) and C15-C22 (4), which have both been prepared from glutamic acid in good overall yields.     

Amphidinolide B: asymmetric synthesis of a C7-C20 synthon

[reaction: see text] An asymmetric synthesis of a C(7)-C(20) synthon of amphidinolide B is described. The synthesis entails the construction of C(7)-C(13) and C(14)-C(20) fragments and makes extensive use of catalytic asymmetric bond constructions to establish the requisite stereochemical relationships. Fragment coupling proceeds by Suzuki cross-coupling and installs the trisubstituted diene unit that is among amphidinolide B's defining structural features.     

Synthetic studies on borrelidin: enantioselective synthesis of the C1-C12 fragment

[structure: see text] An efficient, enantioselective synthesis of the C1-C12 fragment 2 of borrelidin is presented. Construction of the "skipped" polymethylene chain of 2 was accomplished by iteration of Myers' alkylation, while formation of the C3 stereocenter was achieved by Roush's asymmetric allylboration methodology.