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.     

The polyol domain of amphidinol 3. A stereoselective synthesis of the entire C(1)-C(30) sector

[structure: see text] The richly oxygenated C(1)-C(30) polyol segment of amphidinol 3 has been synthesized in protected form. Incorporated in this long chain are 10 of the 25 stereogenic centers housed in the target. The asymmetric pathway that has been developed is based on the efficient union of three independently prepared subunits.     

Synthesis of the C(1)-C(12) segment of peloruside A by an alpha-benzyloxymethyl ketone aldol strategy

The C(1)-C(12) segment of 16-membered antitumor macrolide peloruside A has been prepared by a BF(3).OEt(2)-catalyzed Mukaiyama aldol reaction between a glucose-derived C(1)-C(7) aldehyde and a C(8)-C(12) alpha-benzyloxymethyl ketone. Exclusive 2,3-anti and moderate 3,5-anti/syn facial selectivity (3.5:1) was observed in the aldol reaction. The key C(1)-C(7) aldehyde contains the required stereochemistry at carbons two, three, and five, and has been efficiently prepared on multigram scales from commercial triacetyl D-glucal. [reaction: see text]     

A synthesis of the C(1)-C(15) segment of tsukubaenolide (FK 506).

A synthesis of the C(1)-C(15) segment (4) of Tsukubaenolide (1) from Tri-O-acetyl-D-glucal and (S)-Pipecolinic acid methyl ester is described.     

Stereoselective synthesis of the C(1)-C(12) segment of iriomoteolide 1a: a very potent macrolide antitumor agent

A stereoselective synthesis of the C(1)-C(12) segment of the potent cytotoxic macrolide, iriomoteolide 1a, has been accomplished. The key steps involve an enzymatic kinetic resolution of a β-hydroxy amide, a Pd-catalyzed cross-coupling to a substituted allylsilane, a highly regio- and stereoselective conjugate addition of lithium dimethylcopper to an α, β-acetylenic esters and an elaboration of the C(6)-C(7) trans-olefin geometry by a Julia-Kocienski olefination.     

Stereoselective synthesis of the C(1)-C(11) fragment of peloruside A

A highly stereoselective synthesis of the C(1)-C(11) fragment 4 of peloruside A has been accomplished via a stereoselective double allylboration and an intramolecular epoxide opening to provide the functionally dense C(3)-C(11) segment 14. A glycolate aldol reaction was then employed to introduce the remaining stereocenters at C(2)-C(3). [reaction: see text]     

Catalytic, asymmetric synthesis of the C(1)(')-C(10)(') segment of pamamycin 621A

[reaction--see text] This paper describes a catalytic, asymmetric approach to the C(1)(')-C(10)(') segment of pamamycin 621A. We synthesize this segment in a convergent manner, with each of the coupling partners ultimately deriving from enantiomerically enriched methylketene dimer.     

Synthesis of the C(1)-C(18) segment of lophotoxin and pukalide. Control of 2-alkenylfuran (E/Z)-configuration

[reaction: see text] The convergent synthesis of the fully functionalized C(1)-C(18) segment 24 of the furanocembranes lophotoxin and pukalide was accomplished in 11 steps and 10% overall yield. The key step was a stereoselective conversion of alkynoate 21 to trimethylsilyl 2-alkenylfuran 22.     

Synthesis of the C22-C37 segment of prorocentin

The synthesis of the C22-C37 segment of prorocentin, isolated from the dinoflagellate Prorocentrum lima, was achieved. Because the relative stereochemical relationship between C26 and other stereocenters (C28/C31/C32 established as R*/R*/R*) in the C22-C37 region of natural prorocentin has not yet been determined, both epimers at C26 of the C22-C37 segment were selectively constructed. The synthesis was based on a 5-exo epoxide ring opening reaction to form an oxolane (E-ring), Brown asymmetric methallylation to install the C26-stereocenter, acryloylation of the resulting alcohol, and ring-closing olefin metathesis to establish the Z-olefin at C23/C24.     

Efficient synthesis of the C(1)-C(9) fragment of amphidinolides C, C2, and F

The synthesis of the C(1)-C(9) fragment of amphidinolides C, C2, and F was achieved by using a vinyloguous Mukaiyama aldol reaction on a chiral aldehyde with a silyloxyfuran and by using a C-glycosylation of a lactol derivative with an acetyl oxazolidinethione. From the available chiral acetonide-glyceraldehyde, all the stereogenic centers were perfectly induced along the synthesis. The C(1)-C(9) fragment was synthesized as a vinyl stannane at C(9) in 10 steps, with 16% yield.     

Synthesis of the C(1)-C(25) fragment of amphidinol 3: application of the double-allylboration reaction for synthesis of 1,5-diols

[structure: see text] A synthesis of the C(1)-C(25) fragment of amphidinol 3 is described. The synthesis features two applications of double allylboration reaction methodology for the highly stereoselective synthesis of 1,5-diol units in the C(1)-C(15) segment.     

Synthesis of the C8-C20 and C21-C30 segments of pectenotoxin 2

In this study, we synthesized the C8-C20 and C21-C30 segments of the diarrhetic shellfish toxin pectenotoxin 2. The C8-C20 segment was assembled from a phosphonate corresponding to the C8-C15 segment (prepared from l-malic acid in 19 steps) and an aldehyde corresponding to the C16-C20 segment (synthesized from 3-methyl-3-butenol in nine steps) by a twelve-step process including the Horner-Wadsworth-Emmons reaction, regio- and stereoselective reduction of the resulting enone, diastereoselective epoxidation, and 5-exo epoxide cleavage forming the C-ring. The C21-C30 segment was constructed in 13 steps from (S)-glycidol via a route involving E-ring formation by 5-exo epoxide cleavage and stereoselective methylation at C27 by the Evans method.     

Enantioselective synthesis of the C18-C25 segment of lasonolide A by an oxonia-cope prins cascade

[reaction: see text] A 2-oxonia-Cope Prins cascade was developed that led to a facile and stereoselective synthesis of the C18-C25 segment of lasonolide A. The strategy nicely handles the introduction of the quaternary center in the tetrahydropyran ring, and all of the stereogenic centers in the product arise from a single stereocenter introduced in a catalytic enantioselective reaction.     

A second-generation synthesis of the C1-C28 portion of the altohyrtins (spongistatins)

A practical second-generation synthesis of an advanced intermediate in our total synthesis of altohyrtin C (spongistatin 2) has been developed. A new approach to the C1-C15 (AB) portion features a vinyllithium addition to an aldehyde followed by a palladium-catalyzed allylic reduction to install the troublesome C13-C15 segment. Our general approach to the C16-C28 (CD) spiroketal has been retained, but some improvements have been made. Most notably, the kinetically controlled CD-spiroketalization reaction now proceeds in high yield with excellent diastereoselection. This new strategy uses the anti-aldol coupling used in our first-generation synthesis to join AB and CD fragments. A total of 9.6 g of intermediate 57 has been produced using this improved route.     

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.     

Synthesis of the C3-C19 segment of phorboxazole B

[reaction: see text]. Three segment-coupling Prins approaches to the C3-C19 segment of phorboxazole B have been developed. One successful strategy utilized a novel TMSBr-mediated cyclization that proceeded with complete axial selectivity. Displacement of bromide with cesium acetate provided the C13 hydroxyl stereocenter of 22. Additionally, treatment of alpha-acetoxy ether 20 with TFA enabled a more concise synthesis of the C3-C19 target 13 by allowing direct access to the equatorial alcohol.     

Synthesis of the C21-C28 segment of pectenotoxin-4

The synthesis of the C21-C28 segment of pectenotoxin-4 as the C21 Weinreb amide is described. Feasibility studies for the union of a related Weinreb amide and a functionalized alkyne are also reported.     

Synthetic studies of incednine: synthesis of C1-C13 pentaenoic acid segment

Stereoselective synthesis of the C1-C13 pentaenoic acid segment (4) of the novel antibiotic incednine (1) is described.     

(-)-Lytophilippine A: synthesis of a C1-C18 building block

The convergent enantioselective synthesis of a protected C1-C18 building block for the total synthesis of (-)-lytophilippine A was achieved. A catalytic asymmetric Gosteli-Claisen rearrangement and an Evans aldol reaction served as key C/C-connecting transformations during the assembling of the C1-C7 subunit (10 steps from 4, 29%). The synthesis of the C8-C18 segment was achieved utilizing d-galactose as inexpensive ex-chiral-pool starting material (15 steps, 15%). The merger of the subunits was accomplished by a remarkably efficient sequence consisting of esterification and ring-closing metathesis (five steps, 56%).     

Studies directed toward the total synthesis of azaspiracid: stereoselective construction of C(1)-C(12), C(13)-C(19), and C(21)-C(25) fragments

[reaction: see text] The efficient entry to the C(1)-C(12), C(13)-C(19), and C(21)-C(25) fragments of azaspiracid is outlined. The C(1)-C(12) portion is constructed using a key asymmetric allenyl borane addition to the corresponding alpha,beta-unsaturated aldehyde. The synthesis of the C(13)-C(19) portion utilizes an Evans asymmetric alkylation followed by Sharpless asymmetric dihydroxylation. In addition, a novel solution to the mismatched effects of a neighboring chiral oxazolidinone during a Sharpless dihydroxylation is detailed.