Synthesis of the C29-C44 portion of spongistatin 1 (altohyrtin A)

Two synthetic approaches to the C29-C44 portion of spongistatin 1 (altohyrtin A) have been developed. The key step of the first approach relies on the Claisen rearrangement of glucal 18 to provide ester 20a. This intermediate was advanced to silyl enol ether 30, which was coupled under Mukaiyama aldol conditions with aldehyde 3. Cyclization of this aldol adduct completed our first synthesis of the C29-C44 portion of spongistatin 1, requiring 25 total steps and occurring in 2.4% yield over the longest linear sequence (21 steps). We have also developed a second-generation approach based on the C-glycosidation of glucal 43. Through equilibration of the corresponding C-glycosides 49a/b and 50a/b the desired C-glycoside (50a) was obtained in good yield. Aldol condensation of this ketone provided cyclization precursor 67, which undergoes acid-catalyzed ketalization to close the E-ring of the spongistatins. An oxidation/reduction protocol was employed to set the C37 stereocenter. Protection of the C37 carbonol and selective unmasking of the C44 carbonol completed our second generation synthesis. This approach requires 27 steps and occurred in 13.2% yield over the longest linear sequence (18 steps).     

Synthesis of the C16-C28 spiroketal subunit of spongistatin 1 (altohyrtin A): the pyrone approach

The synthesis of the CD spiroketal fragment of spongistatin 1 (altohyrtin A) has been accomplished utilizing the addition of a metalated pyrone to an aldehyde and subsequent acid-catalyzed spirocyclization. A stereoselective hydrogenation and subsequent conformational inversion establish the C19 stereocenter and the axial-equatorial spiroketal center.     

Synthesis and biological evaluation of analogs of altohyrtin C (spongistatin 2)

Several structural analogs that contain only part of the altohyrtin structure have been prepared and compared with synthetic altohyrtin C (2) for in vitro cytotoxicity against human colon (HCT116) and ovarian (A2780) cell lines. Whereas altohyrtin C was found to be exceedingly potent against these lines (IC₅₀=0.0003 μM), analogs 3-5 were >27,000-fold less potent (IC₅₀>8 μM). Analogs 6 and 7 also demonstrated weak cytotoxicity with IC₅₀ values for the HCT116 and A2780 cells of 4.8 μM and 2.4 μM, respectively, for 6.     

Formal total synthesis of altohyrtin C (spongistatin 2). Part 2: Construction of fully elaborated ABCD and EF fragments

Coupling of the C1C14 (AB) crotylstannane with the C15C28 (CD) aldehyde followed by stereochemical arrangements gave the C1C28 (ABCD) fragment of altohyrtin C. The C29C44 (EF) fragment was also prepared. The syntheses of these two fragments, both of which were identical with those prepared by the Smith group, constitute a formal total synthesis of altohyrtin C.     

Synthesis of a C(29)-C(51) subunit of spongistatin 1 (Altohyrtin A) starting from (R)-3-benzyloxy-2-methylpropan-1-ol

A protected C(29)-C(51) subunit ((+)-38) of spongistatin 1 has been obtained. Key steps involve the aldol condensation of (3S, 4R)-3-methyl-7-[(p-methoxybenzyl)oxy]-4-[(triethylsilyl)oxy]octan- 2-o ne ((-)-6) with (tert-butyl)dimethylsilyl 4-deoxy-2, 3-di-O-(methoxymethyl)-4-methyl-6-O-(tert-butyl)dimethylsilyl)-bet a-D -glycero-L-gluco-heptodialdo-1,5-pyranoside ((+)-7) and a C-glycosidation of (4R,7R&S,E)-7, 8-dichloro-2-methylidene-1-(trimethylsilyl)oct-5-en-4-yl p-methoxybenzoate (16). Aldehyde (+)-7 was derived from (R)-3-benzyloxy-2-methylpropan-1-ol ((+)-10) in 13 formal steps but requiring the isolation of five intermediate products only. The longest linear synthetic scheme converts (+)-10 into (+)-38 in 2% overall yield (isolation of 11 intermediate products).     

A modular approach to marine macrolide construction. 4. Assembly of C36-C51 and C29-C44 building blocks and evaluation of key coupling reactions targeting spongistatin 1 (altohyrtin A)

Routes have been developed for the stereocontrolled elaboration of two highly functionalized sectors of spongistatin 1. The approach to ring F takes advantage of B-alkyl Suzuki-Miyaura coupling to install the C44-C45 bond. The E-ring pyran moiety was generated by acylation of an alpha-sulfonyl carbanion, the stereogenic centers of which were incorporated by sequential asymmetric aldol reactions. [structure: see text].     

Synthesis of the C(29)-C(45) bis-pyran subunit (E-F) of spongistatin 1 (Altohyrtin A)

A synthesis of the C(29)-C(45) bis-pyran subunit 2 of spongistatin 1 (1a) is described. The synthesis proceeds in 19 steps from the chiral aldehyde ent-7, and features highly diastereoselective alpha-alkoxyallylation reactions using the gamma-alkoxy substituted allylstannanes 17 and 19, as well as a thermodynamically controlled intramolecular Michael addition to close the F-ring pyran. The E ring was assembled via the Mukaiyama aldol reaction of F-ring methyl ketone 3 and the 2,3-syn aldehyde 4.     

Formal total synthesis of altohyrtin C (spongistatin 2). Part 1: Aldol approach to unite AB and CD spiroacetals

The Mukaiyama aldol coupling of the second-generation C1C14 (AB) fragment of altohyrtins (spongistatins) with the model α-methyl-β-alkoxyaldehydes revealed that the stereochemistry at the newly formed carbon centers was controlled by the β-alkoxy chiral center of the model aldehydes. The union of the AB fragment with the C15C28 (CD) fragment under the same conditions gave the fully elaborated C1C28 (ABCD) subunit in good yield.     

Synthesis of the C(2)-C(13) fragment (the A-B spiroketal unit) of spongistatin 1 (altohyrtin A): use of a common intermediate for the synthesis of both spongistatin spiroketals

[reaction: see text] A convergent synthesis of 14 corresponding to the A-B spiroketal core of spongistatin 1 has been accomplished via an iodo-spiroketalization reaction of glycal 9, which was synthesized in three steps from a late-stage intermediate used in our synthesis of the C-D spiroketal fragment of spongistatin 1. Elaboration of 14 to the A-B spiroketal 15 was accomplished in three steps.     

The stereocontrolled total synthesis of altohyrtin A/spongistatin 1: the CD-spiroacetal segment

Stereocontrolled syntheses of the C16-C28 CD-spiroacetal subunit of altohyrtin A/spongistatin 1 , relying on kinetic and thermodynamic control of the spiroacetal formation, are described. The kinetic control approach resulted in a slight preference (60 : 40) for the desired spiroacetal isomer. The thermodynamic approach allowed ready access to the desired spiroacetal by acid-promoted equilibration, chromatographic separation of the C23 epimers and resubjection of the undesired isomer to the equilibration conditions. This scalable synthetic sequence provided multi-gram quantities of , thus enabling the successful completion of the total synthesis of altohyrtin A/spongistatin 1, as reported in Part 4 of this series.     

The stereocontrolled total synthesis of altohyrtin A/spongistatin 1: fragment couplings, completion of the synthesis, analogue generation and biological evaluation

The antimitotic marine macrolide altohyrtin A/spongistatin 1 has been synthesised in a highly convergent and stereocontrolled manner, thus contributing to the replenishment of the largely exhausted material from the initial isolation work. Coupling of the AB- and CD-spiroacetal subunits by a stereoselective aldol reaction was achieved by using either a lithium (67 : 33 dr) or boron enolate (90 : 10 dr). A highly (Z)-selective Wittig coupling was used to unite the northern hemisphere aldehyde with the southern hemisphere phosphonium salt . Deprotection and subsequent regioselective macrolactonisation on a triol seco-acid completed the synthesis of altohyrtin A. Two structural analogues were also prepared and evaluated as growth inhibitory agents against a range of human tumour cell lines, including Taxol-resistant strains, alongside altohyrtin A and paclitaxel (Taxol), revealing that dehydration in the E-ring is tolerated and results in enhanced cytotoxicity (at the low picomolar level), whereas the presence of the full C44-C51 side-chain appears to be crucial for biological activity.     

The stereocontrolled total synthesis of altohyrtin A/spongistatin 1: the AB-spiroacetal segment

The convergent synthesis of the C1-C15 AB-spiroacetal subunit of altohyrtin A/spongistatin 1 is described. This highly stereocontrolled synthesis relies on matched boron aldol reactions of chiral methyl ketones, under Ipc(2)BCl mediation, to establish the C5, C9 and C11 stereocentres, and formation of the desired thermodynamic spiroacetal under acidic conditions. The scalable synthetic sequence developed provided access to multi-gram quantities of , thus enabling the successful completion of the total synthesis of altohyrtin A/spongistatin 1, as reported in Part 4.     

Diastereoselective synthesis of the C(17)-C(28) fragment (the C-D spiroketal unit) of spongistatin 1 (altohyrtin A) via a kinetically controlled iodo-spiroketalization reaction

[reaction: see text] A convergent and stereocontrolled synthesis of spiroketal 15 corresponding to the C-D fragment of spongistatin 1 has been accomplished by a sequence utilizing a kinetically controlled intramolecular iodo-spiroketalization of glycal 2, which in turn was synthesized via a ring-closing metathesis reaction.     

A method for constructing the C44-C51 side chain of altohyrtin C

[formula: see text] A method to construct the C44-C51 side chain of altohyrtin C has been developed and applied to a model aldehyde derived from D-glucose. The approach relies on a Wittig reaction to couple the side chain to an aldehyde and utilizes an allylic diazene rearrangement to place the C45 double bond in the correct position.     

A modular approach to marine macrolide construction. 3. Enantioselective synthesis of the C1-C28 sector of spongistatin 1 (altohyrtin A)

[structure: see text] A completely stereocontrolled approach to assembly of the major C1-C28 subunit of spongistatin 1 (altohyrtin A) is described. Key steps included the control of two asymmetric aldols by means of Fujita-Nagao (chiral N-acyl-1,3-thiazolidine-2-thione auxiliary) and Mukaiyama (BF3 x OEt2-promoted enolsilane coupling) protocols in complex settings.     

The stereocontrolled total synthesis of altohyrtin A/spongistatin 1: the southern hemisphere EF segment

The fully functionalised C29-C51 southern hemisphere of altohyrtin A/spongistatin 1 , incorporating the E- and F-ring tetrahydropyran rings and the unsaturated side chain, has been synthesised in a highly convergent and stereocontrolled manner. Key steps in the synthesis of this phosphonium salt include four highly diastereoselective, substrate-controlled, boron aldol reactions to establish key C-C bonds and accompanying stereocentres, where the introduction of the chlorodiene side chain and the C47 hydroxyl-bearing centre were realised by exploiting remote stereoinduction from the F-ring tetrahydropyran.     

Studies on the total synthesis of sanglifehrin A: stereoselective synthesis of the C(29)-C(39) fragment

A highly stereoselective synthesis of the C(29)-C(39) fragment of the potent immunosuppressant sanglifehrin A has been accomplished by a sequence involving 16 steps (18% overall yield) from N-propionyloxazolidinone 9. Key steps are a diastereoselective hydroboration, and a diastereoselective epoxidation of an allylic alcohol followed by a 1,5-anti boron-mediated aldol reaction of methyl ketone 4 with chiral aldehyde 5.     

Short diastereoselective synthesis of the C1-C13 (AB spiroacetal) and C17-C28 fragments (CD spiroacetal) of spongistatin 1 and 2 through double chain-elongation reactions

A unique and practical synthetic sequence for rapid access to polyketides and to further the spiroacetals derived from them, which utilizes a bidirectional Hosomi-Sakurai allylation approach around key allylsilanes in the synthesis of the AB and CD ring systems of spongistatin 1 and 2, is reported. The synthesis of the AB spiroacetal 9 requires 13 steps, with a longest linear sequence of seven steps in an overall yield of 27%. The synthesis of the CD spiroacetal 13 requires 15 steps, with a longest linear sequence of 11 steps in an overall yield of 30%. Both syntheses start from but-3-enol.     

Halichondrin B: synthesis of the C(1)-C(15) subunit

A short and efficient synthesis of the C(1)-C(15) subunit of halichondrin B in its natural configuration is described. The polycyclic caged ketal 3, containing nine asymmetric centers, is prepared in 14 steps from alpha-D-glucoheptonic acid gamma-lactone (7). Key steps in the two similar routes described include EtMgBr-promoted pinacol ring expansions of hydroxy mesylates 23 and 34, intramolecular Michael additions of 29 and 37, and a one-pot, HF-induced conversion of 4 to 3involving in situ silyl ether cleavage, acetal hydrolysis, Michael addition, and caged ketal formation. Alternative protocols for carbinol inversion at C(11), one early and one late in the synthetic sequence, are also described.