Synthesis of the C1-C17 segment of phorboxazole B

Wavelength dependent photoextrusion and tandem photo-extrusion reactions of ninhydrin bis-acetals for the synthesis of 8-ring lactones, benzocyclobutenes and orthoanhydrides

Ninhydrin bis-acetals give access to 8-ring lactones, benzocyclo-butenes and spirocyclic orthoanhydrides through photoextrusion and tandem photoextrusion reactions. Syntheses of fimbricalyxlactone B, isoshihunine and numerous biologically-relevant heterocycles show the value of the methods, while TA-spectroscopy and TD-DFT studies provide mechanistic insights on their wavelength dependence.

Total Synthesis of Tetraketide and Cryptorigidifoliol I via a Sequential Allylation Strategy

A unified and efficient synthetic route for both tetraketide (1) and cryptorigidifoliol I (2) has been devised successfully from commercially available starting materials in 11 and 17 steps, with 16% and 11% overall yields, respectively. Highlights of the syntheses involved sequential Lewis acid-catalyzed highly regio- and diastereoselective allylations and intramolecular Mitsunobu lactonization.

Total synthesis of the marine toxin phorboxazole A using palladium(II)-mediated intramolecular alkoxycarbonylation for tetrahydropyran synthesis

The potent antitumor agent phorboxazole A was synthesized from six subunits comprising C1-C2 (115), C3-C8 (98), C9-C19 (74), C20-C32 (52), C33-C41 (84) and C42-C46 (85). Tetrahydropyrans B and C containing cis-2,6-disubstitution were fabricated via palladium(II)-mediated intramolecular alkoxycarbonylation which, in the case of tetrahydropyran C, was carried out with catalytic palladium(II) and p-benzoquinone as the stoichiometric re-oxidant. Tetrahydropyran D was obtained by a stereoselective tin(IV)-catalyzed coupling of a C9 aldehyde with an allylsilane, and the C19-C20 connection was made using a completely stereoselective Wittig-Schlosser (E) olefination. Coupling of the oxazole C32 methyl substituent with the intact C33-C46 δ-lactone 3was accompanied by elimination of the vinyl bromide to a terminal alkyne, but the C32-C33 linkage was implemented successfully with 83 and C33-C41 lactone 84. The C42-C46 segment of the side chain was then appended via Julia-Kocienski olefination. The macrolide portion of phorboxazole A was completed by means of an Ando-Still-Gennari intramolecular (Z)-selective olefination at C2-C3 which required placement of a (dimethoxyphosphinyl)acetate moiety at C24. Final deprotection led to phorboxazole A via a route in which the longest linear sequence is 37 steps and the overall yield is 0.36%.

Phosphate tether-mediated approach to the formal total synthesis of (-)-salicylihalamides A and B

A concise formal synthesis of the cytotoxic macrolides (-)-salicylihalamides A and B is reported. Key features of the synthetic strategy include a chemoselective hydroboration, highly regio- and diastereoselective methyl cuprate addition, Pd-catalyzed formate reduction, and an E-selective ring-closing metathesis to construct the 12-membered macrocycle subunit. Overall, two routes have been developed from a readily prepared bicyclic phosphate (4 steps), a 13-step route and a more efficient 9-step sequence relying on regioselective esterification of a key diol.

Total synthesis of dolabelide C: a phosphate-mediated approach

The first synthesis of dolabelide C (1), a cytotoxic marine macrolide, is reported utilizing a phosphate tether-mediated approach. Bicyclic phosphates (S,S,S(P))-5 and (R,R,R(P))-5 serve as the central building blocks for the construction of two major 1,3-anti-diol subunits in 1 through selective cleavage pathways, regioselective olefin reduction, and cross-metathesis. Overall, phosphate-mediated processes provided copious amounts of both major subunits allowing for a detailed RCM macrocyclization study to the 24-membered macrolactone 1.

Divalent and Multivalent Activation in Phosphate Triesters: A Versatile Method for the Synthesis of Advanced Polyol Synthons

The construction of mono- and bicyclic phosphate trimesters possessing divalent and multivalent activation and their subsequent use in the production of advanced polyol synthons is presented. The method highlights efforts to employ phosphate tethers as removable, functionally active tethers capable of multipositional activation and their subsequent role as leaving groups in selective cleavage reactions. The development of phosphate tethers represents an integrated platform for a new and versatile tether for natural product synthesis and sheds light on new approaches to the facile construction of small molecules.

A multifaceted phosphate tether: application to the C15-C30 subunit of dolabelides A-D

Construction of the C15-C30 subunit of dolabelide utilizing a temporary phosphate tether is described. Two routes are reported that make use of the orthogonal protecting- and leaving-group properties innate to phosphate esters. One route relies on a selective terminal oxidation, while a second utilizes a CM/selective hydrogenation sequence. Both routes depend on a highly regio- and diastereoselective cuprate addition to set the requisite stereochemistry at C22.

Total Synthesis of Phorboxazole A. 1. Preparation of Four Subunits

[Structure: see text] Four subunits of the potent antitumor agent phorboxazole A were constructed; fragments C20-C32 and C9-C19 containing tetrahydropyrans A and B, respectively, were assembled using palladium-catalyzed intramolecular alkoxycarbonylation.

Stereochemically General Approach to Adjacent Bis(tetrahydrofuran) Cores of Annonaceous Acetogenins

A series of six 2,5-disubstituted adjacent bis(tetrahydrofuran) stereoisomers with trans/erythro/cis, trans/threo/trans, or cis/threo/cis relative stereochemistry have been synthesized from known dihydroxycyclooctenes via ring opening/cross metathesis and Pd(0)-mediated asymmetric double cycloetherification. The stereochemistry of four of these isomers has been found in the biologically active annonaceous acetogenin natural products. [reaction: see text].

Total Synthesis of Phorboxazole B

An efficient and highly convergent total synthesis of the potent antitumor agent phorboxazole B has been achieved. The synthetic strategy of this synthesis features: 1) a highly efficient substrate-controlled hydrogenation to construct the functionalized cis-tetrahydropyrane unit; 2) iterative crotyl addition to synthesize the segment that contains alternating hydroxyl and methyl substituents; 3) Hg(OAc)2/I2-induced cyclization to establish the cis-tetrahydropyrane moiety; 4) 1,3-asymmetric induction in the Mukaiyama aldol reaction to afford the stereogenic centers at C9 and C3; and 5) the exploration of the Still-Gennari olefination reaction to complete the macrolide ring of phorboxazoloe B.

A Catalytic Enantioselective Hetero Diels−Alder Approach to the C20−C32 Segment of the Phorboxazoles

[reaction: see text] An efficient synthesis of the C20-C32 segment of the phorboxazoles has been achieved using an enantioselective hetero Diels-Alder reaction catalyzed by Jacobsen's Cr(III) amino indanol Schiff base catalyst.

Synthetic Efforts toward the C22−C36 Subunit of Halichondrin B Utilizing Local and Imposed Symmetry

The C22-C34 portion (2) of halichondrin B was synthesized from meso-symmetric bis-silyl protected cyclopentenediol (7) in 20 steps and 7% overall yield. This was accomplished through a two-directional synthesis/terminus differentiation strategy that proceeded via achiral, meso-symmetric intermediates for eight steps and employed a Pd(0)-mediated asymmetric double cycloetherification to establish both tetrahydropyran rings. [Structure: see text]

Toward the Total Synthesis of Phorboxazole B: An Efficient Synthesis of the C20−C46 Segment

An efficient synthesis of the C20-C46 segment of phorboxazole B is described. The key steps involved Hg(OAc)(2)/I(2)-induced cyclization to construct the cis-tetrahydropyran moiety, the coupling of the metalated 2-methyloxazole 7 with lactone 6, and Julia olefination to furnish the conjugated diene moiety.

Phorboxazole B synthetic studies: construction of C(1–32) and C(33–46) subtargets

The convergent syntheses of the C(1-32) and C(33-46) domains of phorboxazole B are described. An iterative cyclocondensation strategy exploited the Jacobsen hetero-Diels-Alder (HDA) reaction as a platform for the synthesis of both the C(5-9) and C(11-15) tetrahydropyran rings. The use of 2-silyloxydiene coupling partners bearing an increasing resemblance to the phorboxazole skeleton was found to lead to a reduction in diastereoselectivity, however, in the case of the C(11-15) ring. The coupling of aldehyde and 2-silyloxydiene by this route provided a C(1-32) fragment which was elaborated to the macrolide core of phorboxazole B. The synthesis of the C(33-46) domain involved a Nozaki-Kishi coupling of aldehyde 31 and vinyl iodide 39. The syntheses of 31 and 39 were highly diastereoselective: an Evans [Cu(Ph-pybox)](SbF6)2-catalysed Mukaiyama aldol reaction formed the cornerstone of the synthesis of 31 whilst a Nagao-Fujita acetate aldol reaction provided a convenient means of installing the sole stereogenic centre of 39.