The Chemistry of Phytoplankton

Abstract

Phytoplankton have a high potential for CO₂ capture and conversion. Besides being a vital food source at the base of oceanic and freshwater food webs, microalgae provide a critical platform for producing chemicals and consumer products. Enhanced nutrient levels, elevated CO₂, and rising temperatures increase the frequency of algal blooms, which often have negative effects such as fish mortalities, loss of flora and fauna, and the production of algal toxins. Harmful algal blooms (HABs) produce toxins that pose major challenges to water quality, ecosystem function, human health, tourism, and the food web. These toxins have complex chemical structures and possess a wide range of biological properties with potential applications as new therapeutics. This review presents a balanced and comprehensive assessment of the roles of algal blooms in generating fixed carbon for the food chain, sequestering carbon, and their unique secondary metabolites. The structural complexity of these metabolites has had an unprecedented impact on structure elucidation technologies and total synthesis, which are highlighted throughout this review. In addition, the influence of biogeochemical environmental perturbations on algal blooms and their influence on biospheric environments is discussed. Lastly, we summarize work on management strategies and technologies for the control and treatment of HABs.

Figure 1

Venus, Earth, and Mars represent the three rocky inner planets in our Solar System. The dramatic transformation of Earth’s atmosphere to a highly oxygenated system was initiated and sustained by phytoplankton, thus exemplifying the tremendous impact they have on the environment. (Images sourced from NASA).

Figure 2

a: Satellite images of large-scale phytoplankton blooms. a1: Algal bloom in Western Lake Erie on September 26, 2017. The Microcystis sp bloom shown is near downtown Toledo and stretches all the way to Lake Ontario. a2: Red tides in Benguela upwelling in 2008. a3: Cyanobacteria bloom in Lake Erie. (Photo courtesy of Tim Davis). a4: Algal bloom of Emiliania huxleyi off the southern coast of Devon and Cornwall in England in 1999. b: The taxonomic composition of HABs species (b1: Prymnesiophytes; b2: Diatoms; b3: Cyanobacteria; b4: Dinoflagellates). c: Total number of recorded HAB events (Y-axis) per year (X-axis) around the globe showing a steady upward trend.

Figure 3

CO₂ emission and absorption.

Figure 4

A Mediterranean outflow of highly alkaline water results in a nonskeletal carbonate factory that mitigates the effects of climate change by sequestering CO₂.

Figure 5

Summary of major approaches for mitigating HABs. Reduction of nutrient and greenhouse gas input is the most effective way to prevent HABs, but it is difficult to implement. The physical control of clay application to HABs has been widely used for a long time, but it is not suitable for large bodies of water. Biological controls, including parasites and viruses infecting dinoflagellates, are limited and can have consequences on the surrounding land.

Figure 6

Geographical distributions of ASP, PSP, DSP, NSP and CP.

Scheme 1. Synthesis of the C-1-C-29 Fragment

a. acrolein, Hoveyda-Grubbs II, DCM, 72% yield; b. acetaldehyde, Bi(NO₃)₃·5H₂O, DCM; c. NaBH₄, MeOH, 87% yield (2 steps); d. NapBr, NaH, DMF, 82% yield; e. TBAF, THF, 96% yield; f. DMP, DCM, 89% yield; g. 71, THF, 69% yield; h. TsOH·H₂O, 89% yield BRSM; i. TBSOTf, 2,6-lutidine, quant.; j. BH₃·SMe₂, CyH; NaOH, H₂O₂, 91% yield; k. 1-phenyl-1H-tetrazole-5-thiol (PTSH); PPh₃, DIAD, 91% yield; l. m-CPBA, 86% yield; m. HC≡CC₄H₈OPMB, n-BuLi, 81% yield; n. Ru cat. 76, IPA, 90% yield, 94% ee; o. H₂, Pd/C(en), 89% yield; p. TBSCl, imidazole, DMF, quant.; q. 78, n-BuLi, THF, quant. r. (S)-(−)-2-methyl-CBS-oxazaborolidine, BH₃·SMe₂, PhMe quant. dr 95/5; s. TBSCl, imidazole, DMF, quant.; t. Hoveyda-Grubbs II, DCM, 70% yield, E/Z = 13/1; u. TBSOTf, 2,6-lutidine, quant.; v. DDQ, pH 7 buffer, DCM; w. SO₃·Py, DMSO, NEt₃, 67% yield (2 steps); x. 74, KHMDS, THF, 86% yield; y. K₂OsO₄·2H₂O, DHQ-MEQ, K₂CO₃, K₃Fe(CN)₆, MeSO₂NH₂, t-BuOH/t-BuOMe/H₂O, 65% yield, dr 13/1; z. TBSOTf, 2,6-lutidine, DCM, 93% yield; aa. DDQ, pH 7 buffer, DCM, then NaBH₄, MeOH/THF, 83% yield; ab. o-O₂NC₆H₄SeCN, PMe₃, 4 ÅMS, THF, then aq. H₂O₂, 97% yield.

Scheme 2. Synthesis of the C-30-C-52 Fragment

a. ethyl acrylate, Hoveyda-Grubbs II, 72% yield; b. DIBAL, DCM, 95% yield; c. NaH, BnBr, DMF, 87% yield; d. K₂OsO₄·2H₂O, DHQ-MEQ, K₂CO₃, K₃Fe(CN)₆, MeSO₂NH₂, t-BuOH/t-BuOMe/H₂O,, 83% yield, dr 7.5/1; e. Ac₂O, DMAP, pyridine, 93% yield; f. 89, PdCl₂(dppf), aq. Cs₂CO₃, DMF, 87% yield; g. HF·pyr, THF, 82% yield; h. Ti(O-i-Pr)₄, TBHP, L-(+)-DET, 4 ÅMS, DCM; i. K₂CO₃, MeOH; j. PPTS, DCM; 64% yield (3 steps); k. PPTS, 1,1-dimethoxycyclopentane, DCM, 97% yield; l. K₂OsO₄·2H₂O, DHQ-MEQ, K₂CO₃, K₃Fe(CN)₆, MeSO₂NH₂, t-BuOH/H₂O,; 96% yield; dr 10/1; m. PPTS, 1,1-dimethoxycyclopentane, DCM, 82% yield; n. MsCl, NEt₃, DCM, 96%; o. Raney Ni, H₂, EtOH, 88% yield; p. K₂CO₃, MeOH, 78% yield; q. 95, Et₂O/hexane, 85% yield; r. TBSOTf, 2,6-lutidine, DCM, 91% yield; s. DIBAL, Ni(dppp)Cl₂, THF, then NIS; 76% yield; t. DDQ, pH 7 buffer, DCM, 91% yield; u. TESCl, NEt₃, DCM, 95% yield; v. t-BuLi, Et₂O, 71% yield, C-43 epimer ratio 1.7/1; w. TBSOTF, 2,6-lutidine, DCM, 93% yield; x. TBAF/AcOH, THF, 94% yield; y. (COCl)₂, NEt₃, DMSO; z. Ohira-Bestman reagent, MeOH, Cs₂CO₃, 93% yield (2 steps); aa. LHMDS, MeI, THF, quant.; ab. PdCl₂(P-o-tol₃)₂, Bu₃SnH, THF; ac. I₂, DCM, 73% yield (2 steps).

Scheme 3. Synthesis of the C-53-C-67 Fragment

a. PTSH, PPh₃, DIAD, THF, 99% yield; b. (NH₄)₆Mo₇O₂₄·4H₂O, aq. H₂O₂, EtOH/THF, quant. c. 105, Pd(PPh₃)₄, THF, 81% yield; d. 107, Pd(PPh₃)₄, CuCl, DMSO/THF, 75% yield.

Scheme 4. AM3 Prepared from Compounds 85 and 102

a. 85: 9-BBN, THF, then 1M aq. Cs₂CO₃ combined with 102, Pd(PPh₃)₄, DMF, 77% yield; b. DDQ, pH 7 buffer, DCM, 75% yield (2 cycles); c. (COCl)₂, NEt₃, DMSO, DCM, quant.; d. KHMDS, THF/HMPA, 75% yield, E/Z = 10/1; e. HF·pyr, MeOH, (CH₂OH)₂, THF, 58% yield (2 cycles).

Scheme 5. Nishikawa’s Synthesis of OTX-E

a. m-CPBA, DCM; b. H₅IO₆, Et₂O; c. 3-methoxyphenylmagnesium bromide, THF, 52% yield (3 steps), dr 1/1; (COCl)₂, NEt₃, DMSO, 92% yield; e. RuCl[(S,S)-Tsden(p-cymene), HCO₂H, NEt₃, 78% yield; f. NaH, MeI, DMF, 98% yield; g. O₃, pyridine, MeOH then PPh₃, 88% yield; h. BF₃-Et₂O, THF then H₂O₂, NaOH; i. HCl-Et₂N(CH₂)₂SH, t-BuOK, DMF, 73% yield (2 steps); j. TESOTf, 2.6-lutidine, DCM, 84% yield; k. O₃, pyridine, MeOH/DCM then PPh₃, 93% yield; l. n-BuLi, – 78 °C THF, then add 206 and warm to −40 °C, 71% yield; m. DMP, DCM, 84% yield; n. Amberlyst-15, DCM, 64% yield; o. CDI, MeCN then CH₂(CO₂Me)(CO₂K), MgCl₂, NEt₃, THF; p. K₂CO₃, MeOH, 72% yield (2 steps); q. TIPSCl, DBU, DCM, 84% yield; r. LiBH₄, Et₂O, 94% yield; s. BF₃-Et₂O, DCM, MS4 Å, 59% yield; t. TBAF, HOAc, THF, 97% yield.

Scheme 6. Synthesis of Nhatrangin from Compound 211

a. TCBCl, NEt₃, DMAP, toluene, 90% yield; b. O₃, DCM/MeOH, – 78 °C then PPh₃, 93% yield; c. NaClO₂, NaH₂PO₄, 2-methyl-2-butene, t-BuOH/H₂O, 92% yield; d. TBAF, THF, 92% yield.

Scheme 7. Synthesis of Lyngbyatoxin A (265)

a. CsF. MeCN, 0 °C to rt, 75% yield; b. H₂, Pd/C, NEt₃, EtOAc; c. Ac₂O, AcOH, rt (87% yield, 2 steps); d. K₂CO₃, DMF, 65 °C, 96% yield; e. ZrCl₄, DCM, 34 °C, 90% yield; f. NaHCO₃, MeOH, 40 °C, 50% yield; g. LiBH₄, THF 0 °C to rt; h. TBSCl, TBAI, imidazole, DMF, rt, 90% yield (2 steps); i. NBS, – 78 °C to −15 °C, THF, 87% yield; j. [P(t-Bu)₃PdBr]₂ (15 mol %), LiBr, THF/PhMe, 80 °C, 75% yield, dr 1/1; k. Cp₂Zr(H)Cl, THF, 50 °C l. BrPPh₃CH₃, t-BuOK, THF, 0 °C to rt, 64% yield (2 steps); m. LiBF₄, CSA, THF, rt, 63% yield.

Scheme 8. Synthesis of the G-Ring Fragment 301

a. NEt₃, DMAP, PhH, 84% yield; b. THF, – 78 °C then TMSCl; c. rt, 12 h then aq. work-up; d. LAH, Et₂O, 81-83% yield (2 steps); e. benzoyl chloride, pyridine; f. DDQ, DCM/H₂O; g. O₃, N-methylmorpholine N-oxide, DCM; h. NaBH₄, EtOH, 77% yield (4 steps); i. (COCl)₂, DMSO, NEt₃, DCM; j. Bn₂NH₂⁺TFA^–, PhMe, 50 °C, 89% yield; k. NaBH₄, EtOH; l. MOMCl, TBAI, i-Pr₂NEt; m. TBAF, THF; n. (COCl)₂, DMSO, NEt₃; o. Ph₃PCH₃Br, KHMDS, 87% yield (5 steps); p. LAH, Et₂O; q. (COCl)₂, DMSO, NEt₃, 87% yield (2 steps).

Scheme 9. Preparation of the BCD-Fragment (311)

a. Pd(dppf)Cl₂, AsPh₃, Cs₂CO₃; b. AD-mix β; c. TBAF; d. (COCl)₂, DMSO; e. CSA, MeOH, then solvent swap to cyclohexane, rt, 48 h, 78% yield; f. TIPSCl, imidazole; g. TESCl, imidazole; h. DIBAL; i. PMBO(CH₂)₃Li; j. DMP, pyridine; k. TMSCH₂Li; l. KHMDS, 76% yield (8 steps); m. DDQ, DCM/H₂O; n. I₂, PPh₃, imidizole, 87% yield (2 steps).

Scheme 10. Final Stages of the Pinnatoxin A Synthesis

a. t-BuLi, Et₂O, 1 h, 78% yield; b. TBAF; c. DMP; d. CH₂CHMgBr; e. second-generation Hoveyda-Grubbs (20 mol %); f. DMP, pyridine, DCM; g. MeCu(CN)Li, BF₃-Et₂O, 73% yield (2 steps); h. DDQ, DCM/H₂O; i. Ts₂O, pyridine, DCM; j. NaN₃, DMF, 80 °C, 66% yield (3 steps); k. LiBF₄, IPA/H₂O, 71% yield; l. TEMPO, PhI(OAc)₂; m. NaClO₂, NaH₂PO₄; n.TMSCHN₂; o.H₂, Pd/CaCO₃; p. triethylammonium mesitoate, PhMe, 85 °C, 60 h; q. LiOH THF/H₂O 37-43% (6 steps).

Scheme 11. Synthesis of the ABCD Ring System

a. 1-phenyl-1H-tetrazole-5-thiol, NaH, THF/DMF; b. H₂O₂, (NH₄)₆Mo₇O₂₄·H₂O, NaH₂PO₄ buffer, EtOH, 71% yield (2 steps); c. KHMDS, DMPU, THF, 85% yield, 10:1 E:Z; d. Shi ketone, Bu₄NHSO₄, oxone, K₂CO₃, Na₂B₄O₇, DMM/MeCN/H₂O then TBAF, THF, 65% yield, dr 4/1; e. pH 11 phosphate buffer, 54% yield; f. VO(acac)₂, TBHP, DCM, 68% yield; g. PMPBr, NaH, DMF, 92% yield; h. (E)-1-iodo-2-methylpenta-1,4-diene, n-BuLi, Et₂O, CuCN; i. BnBr, NaH, DMF, 77% yield (2 steps); j. Shi ketone, Bu₄NHSO₄, oxone, K₂CO₃, Na₂B₄O₇, DMM/MeCN/H₂O; k. DDQ, pH 11 phosphate buffer, DCM, 65% yield, dr 6/1; l. NBS, 4 ÅMS, HFIP, 68% yield; m. t-BuOK, THF; n. 9-BBN, THF, NaOH, H₂O₂, 93% yield (2 steps); o. BnBr, NaH, DMF, 99% yield; p. TsOH·H₂O, MeOH/DCM, 89% yield; q. I₂, PPh₃, imidazole, THF, 95% yield; r. TESCl, imidazole, DCM, 97% yield; s. t-BuOK, THF, 95% yield.

Scheme 12. Synthesis of the FGH Ring System

a. O₃, DCM then PPh₃, 78% yield; b. KHMDS, DMPU, THF, 86% yield, 9:1 E:Z; c. Shi ketone, K₂CO₃, Na₂B₄O₇, DMM/MeCN/H₂O, 76% yield, dr 3/1; d. BF₃·Et₂O, DCM; e. TBSCl, imidazole, DMAP, DMF, 24% yield (2 steps); f. K₂CO₃, MeOH, 88% yield; g. SO₃·pyr, NEt₃, Ph₃PCHCO₂Me, DMSO, DCM, 82% yield; h. [(PPh₃)CuH]₆, THF; i. DIBAL, DCM; j. TEMPO, PIDA, DCM, 79% yield (3 steps); k. O₃, DCM then NaBH₄, MeOH; l. TBDPSCl, imidazole, DCM/DMF, 86% yield (2 steps); m. KHMDS, Comins’ reagent, HMPA, THF, 77% yield.

Scheme 13. Synthesis of the KLM Ring Fragment

a. (COCl)₂, DMSO, NEt₃, DCM; b. MeMgBr, Et₂O; c. TPAP, NMO, 4 ÅMS, DCM, 42% yield (3 steps); d. KHMDS, DMPU, THF; e. H₂O₂, (NH₄)₆Mo₇O₂₄·H₂O, NaH₂PO₄ buffer, EtOH, 41% yield (2 steps); f. Compound 363, KHMDS, DMPU, THF, 85% yield; g. Shi ketone, Bu₄NHSO₄, Oxone, NaHCO₃, MeCN; h. TBAF, THF, 71% yield (2 steps), dr 2/1; i. pH 8 phosphate buffer, 23% yield; j. DIBAL, PhMe; k. La(OTf)₃, 416, PhMe, 73% yield (2 steps); l. TsOH·H₂O, MeOH, DCM, 94% yield; m. I₂, PPh₃, imidazole, THF, 97% yield; n. TESCl, imidazole, DCM, 99% yield; o. t-BuOK, THF, 97% yield.

Scheme 14. Preparation of the ABCDEFGH Ring System

a. 399: 9-BBN, THF, Cs₂CO₃, H₂O; then combine with 408, Pd(PPh₃)₄, DMF, 83% yield; b. BH₃, THF, NaOH, H₂O₂, 93% yield, dr >10/1; c. TPAP, NMO, 4 ÅMS, DCM; d. DBU, DCM, 82% yield (2 steps); e. TMSOTf, Et₃SiH, DCM, 78% yield; f. I₂, PPh₃, imidazole, PhH/THF; g. TESCl, imidazole, DMAP, DCM; h. t-BuOK, THF, 89% yield (3 steps); i. 418: 9-BBN, THF, Cs₂CO₃, H₂O; then combine with 423, PdCl₂(dppf)·DCM, DMF, 85% yield; j. DMDO, DCM then MeMgBr; k. Ac₂O, DMAP, pyridine, 77% yield, (2 steps); l. TBAF, THF; m. TPAP, NMO, 4 ÅMS, DCM; n. K₂CO₃, MeOH, 83% yield (3 steps); o. TsOH·H₂O, MeOH/DCM; p. TESOTf, TESH, MeCN; q. TsOH·H₂O, acetone, 65% yield (3 steps); r. DMP, DCM; s. CH₂I₂, CrCl₂, DMF/THF, 76% yield (2 steps); t. 9-BBN, THF, NaOH, H₂O₂; u. TEMPO, PIDA, DCM, 89% yield (2 steps); v. KHMDS, (PhO)₂P(O)Cl, HMPA/THF, 99% yield.

Scheme 15. Final Stages of the Gymnocin B Synthesis

a. 422: 9-BBN, THF; Cs₂CO₃, H₂O; then combine with 430. Pd(PPh₃)₄, DMF, 78% yield; b. BH₃, THF, NaOH, H₂O₂; c. TPAP, NMO, 4 ÅMS, DCM; d. DBU, DCM, 72% yield (3 steps); e. EtSH, Zn(OTf)₂, MeNO₂; f. TESOTf, 2,6-lutidine, DCM; g. Ph₃SnH, AIBN, PhMe, 57% yield (3 steps); h. H₂, Pd/C, THF; i. I₂, PPh₃, imidazole, THF; j. TESCl, imidazole, DMAP, DCM/DMF, 65% yield (3 steps); k. vinylmagnesium bromide, CuI, HMPA/THF; l. TASF, THF/DMF; m. methacrolein, Hoveyda-Grubbs II, DCE, 44% yield (3 steps).

Scheme 16. Synthesis of the C-1-C-9 Fragment

a. LAH, THF, 53% yield; b. (Bu₃Sn)₂, n-BuLi, CuCN, THF/H₂O, 78% yield; c. TBSCl, imidazole, DMAP, 95% yield; d. (E)-(3-iodoallyl)(phenyl)sulfane, Pd(PPh₃)₄, CuTC, Ph₂PO₂NBu₄, DMF, E/Z = ca. 10/1; e. Na₂WO₄, aq. H₂O₂, MeOH/PhH, 63% yield (2 steps).

Scheme 17. Synthesis of the C-10-C-23 Fragment

a. CH₃C≡CMgBr, THF, 81% yield, dr 2/1; b. MnO₂, DCM, 61% yield; c. 463 cat., HCO₂H/NEt₃, DCM, 91% yield, dr >20/1; d. TBSCl, imidazole, DCM, 87% yield; e. 464, EDC, DMAP, DCM, 97% yield; f. H₂ (1 atm), Lindlar cat., quinoline, EtOAc, 98% yield; g. Tebbe reagent, THF/PhMe, 47% yield; h. H₂ (1 bar), Pt/C, EtOH, 74-83% yield, dr >20/1; i. TBAF, THF, 81% yield; j. DMP, NaHCO₃, DCM; k. Ph₃P = CHCO₂Me, DCM, 86% yield (2 steps); l. DIBAL, THF, 83% yield; m. TBSOTf, 2,6-lutidine, DCM, 95% yield; n. DIBAL, DCM, 87-93% yield; o. (COCl)₂, NEt₃, DMSO, DCM; p. 470, (R)-3,3′-diBr-BINOL, PhMe, 44-59% yield (2 steps), dr >20/1; q. Ac₂O, pyridine, DMAP, DCM, 90% yield; r. DDQ, pH 7 buffer, DCM, 77% yield.

Scheme 18. Synthesis of the C-24-C-36 Fragment

a. [Ir(cod)Cl]₂, 474, allyl acetate, Cs₂CO₃, 4-Cl-3-NO₂-benzoic acid, THF, 81% yield; b. Co cat. 476, t-BuOOH, iPrOH, O₂, 72% yield, dr >20/1; c. PIDA, TEMPO, MeCN/H₂O, 86% yield; d. MeN(H)OMe-HCl, CDI, DCM, 89% yield; e. 2-methyl-1-propenylmagnesium bromide, THF then DBU cat., DCM, 98% yield; f. NaBH₄, CeCl₃·7H₂O, MeOH, 94% yield, dr >20/1; g. 2-(bromomethyl)naphthalene, NaH, TBAI, THF/DMF, 90% yield; h. Pt(dba)₃ cat., 479 R = mesityl, B₂pin₂, THF, then NaBO₃·4H₂O, THF/H₂O, 76% yield, dr >20/1; i. 1-((2,4,6-triisopropylphenyl)sulfonyl)-1H-imidazole, NaH, THF; j. (1-(trimethylsilyl)vinyl)magnesium bromide, CuCN, THF, 74% yield (2 steps); k. TBSOTf, 2,6-lutidine, DCM, quant.; l. NIS, 2,6-lutidine, HFIP, THF, 80% yield; m. TBAF, HOAc, THF, 83% yield; n. DDQ, pH 7 buffer, DCM, 98% yield.

Scheme 19. End Stages of the Synthesis

a. Pd₂(dba)₃, CuI, PPh₃, iPr₂NH, 91% yield; b. JohnPhosAu(MeCN)SbF₆ cat., PPTS, DCM, 69% yield; c. TBSOTf, 2,6-lutidine, DCM, 92% yield; d. HF·pyr, pyridine, 92% yield; e. Ti(OiPr)₄, L-(+)-DIPT, cumene hydroperoxide, DCM, 84% yield, dr >20/1; f. I₂, PPh₃, imidazole, DCM, 88% yield; g. 459, n-BuLi, DMPU, THF, 86% yield, dr ca. 3/2; h. LiBHEt₃, THF then [(dppp)PdCl₂], LiBHEt₃, 49% yield; i. HF·pyr, pyridine, THF, 50% yield.

Scheme 20. Alternative Route to the Central Fragment

a. 2-(dimethoxymethyl)naphthalene, TsOH·H₂O, DMF, 86% yield; b. Ag₂CO₃/Celite, PhMe; c. TBSCl, imidazole, DCM, 57% yield (2 steps); d. TBDPSCl, imidazole, DCM; e. DIBAL, pentane; f. Ph₃PCHCO₂Et, PhMe, 59% yield (3 steps); g. DIBAL, THF, 91% yield; h. NapBr, NaH, TBAI, THF, then TBAF, 87% yield; i. I₂, PPh₃, imidazole, THF, 90% yield; j. t-BuLi, THF, 81% yield; k. TBSOTf, Et₃SiH, then PhBCl₂, DCM, MS4 Å, 72% yield; l. (COCl)₂, NEt₃, DMSO, DCM; m. 461, (R)-3,3′-Br-BINOL, PhMe, 68% yield (2 steps); n. Ac₂O, pyridine, DMAP, DCM, 90% yield; o. DDQ, pH 7 buffer, DCM, 67% yield (21% diastereomer).

Scheme 21. Synthesis of the C-1-C-11 Fragment

a. methyl acrylate, DABCO; b. DIBAL, THF, 57% yield (2 steps); c. TBSCl, NaH, THF, 87% yield; d. MsCl, NEt₃, THF, 88% yield; e. LiCl, THF, 98% yield; f. vinylmagnesium bromide, CuI, THF; g. TBDPSCl, imdiazole, DCM, 81% yield (2 steps); h. Hoveyda-Grubbs II, methyl acrylate, DCM, 86% yield; i. TMS-SEt, AlCl₃, THF, 86% yield; j. MeMgBr, CuBr·SMe₂, (S)-(+)-1-[(R)-2-(diphenylphosphino)ferrocenyl]ethyldicyclohexylphosphine, t-BuOMe, 90% yield, dr >20/1; k. TESH, Pd/C, DCM, 85% yield; l. Bestmann-Ohira reagent, K₂CO₃, MeOH, 94% yield; m. 9-I-9-BBN, hexane, then AcOH, quant.; n. Zn, LiCl, THF; o. 500, Pd(PPh₃)₄, THF then TBAF, 76% yield (2 steps); p. Ac₂O, pyridine, DMAP, 96% yield.

Scheme 22. Synthesis of the C-12-C-27 Fragment

a. Allyltrimethylsilane, BF₃·Et₂O, MeCN, 56% yield; b. NaOMe, MeOH; c. p-methoxybenzaldehyde dimethylacetal, TsOH cat., DMF, 79% yield; d. TBSOTf, 2,6-lutidine, DCM, 86% yield; e. DIBAL, DCM, quant.; f. (COCl)₂, DMSO, NEt₃, DCM, 87% yield; g. 509, (R)-3,3′-diBr-BINOL, PhMe, 96% yield; h. TBSOTf, 2,6-lutidine, DCM, quant.; i. DDQ, DCM/H₂O, 99% yield; j. 511, Pd₂(dba)₃, PPh₃, CuI, iPr₂NH, 93% yield; k. JohnPhos-Au(MeCN)SbF₆., PPTS, DCM, 65-78% yield; o. OsO₄, NaIO₄, 2,6-lutidine, 1,4-dioxane/H₂O, 87-93% yield.

Scheme 23. Synthesis of the C-28-C-40 Fragment 520

a. Allyltrimethylsilane, TMSOTf, MeCN, 57% yield, dr >10/1; b. K₂CO₃, MeOH; c. TBSOTf, 2,6-lutidine, DCM, 95% yield (2 steps); d. Modified Grubbs catalyst, 1-buten-3-ol, DCM, 75% yield; e. TBDPSCl, imidazole, DCM, quant.; f. CSA (cat.), MeOH/DCM, 77% yield; g. Pb(OAc)₄, THF, 61% yield; h. SnCl₄, DCM, 83% yield, dr 5/1; i. Bu₃SnLi, THF, 91% yield.

Scheme 24. Final Stages of the Synthesis

a. MgBr₂·Et₂O, DCM, 88% yield; b. PPh₃, 4-nitrobenzoic acid, DEAD, PhMe, 67% yield; c. NaOH, MeOH/THF, 91% yield; d. TBSOTf, 2,6-lutidine, DCM, 84% yield; e. Ph₃CK, PhNTf₂, THF; f. (Bu₃Sn)₂CuCNLi₂, THF, 63% yield (3/1 isomers); g. 506, Pd(PPh₃)₄, CuTC, [Bu₄N][Ph₂P(O)O], DMF/THF, 60% yield; h. HF·pyr, THF/pyridine, 32% yield.

Scheme 25. New Approach Used in the Second-Generation Route

a. JohnPhos-Au(MeCN)SbF₆ (2 mol %), PPTS, DCM, 79% yield; b. AgF, MeCN, 90% yield; c. OsO₄, NaIO₄, dioxane/H₂O, 79% yield; d. 528, 520, DCM, 84% yield (12% C-27-epimer); e. (Bu₃Sn)₂CuCNLi, THF/MeOH, 80% yield; f. 506, Pd₂(dba)₃ cat., LiCl, DMF; g. NaOH, THF/MeOH/H₂O, 70% yield (2 steps); h. TBAF, THF, 99% yield.

Figure 7

Numbering system for each unit of cryptophycins C (R₁=C1) and D (R₁=H).