Synthesis of (+)-bullatacin via the highly diastereoselective [3+2] annulation reaction of a racemic aldehyde and a nonracemic allylsilane

Access to Stereodefined (E)-2-Silylallylboronates via Regioselective Chloroboration of Allenylsilanes

A catalyst-free method for the highly regioselective chloroboration of allenylsilanes is described. In the presence of BCl₃ and 2,6-lutidine, chloroboration of allenylsilanes proceeds smoothly without any catalyst, and the product could be treated with pinacol to afford the corresponding pinacol borates in one-pot reaction. This reaction provides a direct approach to construct valuable 2-silylallylboronate frameworks with operational simplicity and high atom-economy.

Stereodivergent Catalysis

This review covers diastereo- and enantiodivergent catalyzed reactions in acyclic and cyclic systems using metal complexes or organocatalysts. Among them, nucleophilic addition to carbon-carbon and carbon-nitrogen double bonds, α-functionalization of carbonyl compounds, allylic substitutions, and ring opening of oxiranes and aziridines are considered. The diastereodivergent synthesis of alkenes from alkynes is also included. Finally, stereodivergent intramolecular and intermolecular cycloadditions and other cyclizations are also reported.

Catalytic Asymmetric Roskamp Reaction of Silyl Diazoalkane: Synthesis of Enantioenriched α-Silyl Ketone

A catalytic enantioselective Roskamp reaction of silyl diazoalkane to synthesize a highly optically active α-silyl ketone from aldehydes is described. In the presence of a chiral oxazaborolidinium ion catalyst, the reaction provides α-chiral silyl ketones with good yields (up to 97%) and high enantioselectivities (up to >99% ee). In addition, a one-pot procedure using an asymmetric Roskamp/reduction strategy gives highly optically active syn-β-hydroxysilane in good yields (up to 94%) with high enantioselectivities (up to 99% ee) and syn stereoselectivities (>20:1).

Acetogenins from Annonaceae

In recent decades, annonaceous acetogenins have become highly studied plant secondary metabolites in terms of their isolation, structure elucidation, synthesis, biological evaluation, mechanism of action, and toxicity. The aim of the present contribution is to summarize chemical and biological reports published since 1997 on annonaceous acetogenins and synthetic acetogenin mimics. The compounds are considered biologically in terms of their cytotoxicity for cancer cell lines, neurotoxicity, pesticidal effects, and miscellaneous activities.

Tandem reactions involving 1-silyl-3-boryl-2-alkenes. New access to (Z)-1-fluoro-1-alkenes, allyl fluorides, and diversely α-substituted allylboronates

The reactions of 1-silyl-3-boryl-2-alkenes with various electrophilic reagents (Selectfluor, N-halosuccinimides, benzhydryl, and propargylic alcohols in the presence of a Lewis acid, N-alkoxycarbonyliminium ion) have been investigated as new routes to α-substituted allylboronates. Further functional transformations, including allylboration, Suzuki coupling, protodeboronation, and cycloisomerization, have been carried out to illustrate the synthetic potential of these γ-borylallylsilanes.

Divergent synthesis of functionalized carbocycles through organosilane-directed asymmetric alkyne-alkene reductive coupling and annulation sequence

An organosilane-directed alkyne-alkene reductive coupling of readily available propargylsilanes is used to access densely functionalized chiral allylsilanes. The divergent reactivity of the allylsilanes can be controlled to afford a range of novel carbocyclic ring systems through an intramolecular allylation, [3+2] annulation, and Sakurai-like homodimerization.

Enantioselective syntheses of syn- and anti-β-hydroxyallylsilanes via allene hydroboration-aldehyde allylboration reactions

The kinetic hydroboration of allenylsilane 5 with ((d)Ipc)(2)BH at -40 °C provides allylborane 9Z with ≥12:1 selectivity. When the hydroboration is performed at temperatures above -40 °C, 9Z isomerizes to the thermodynamically more stable allylborane 9E with >20:1 selectivity. Subsequent treatment of 9Z or 9E with aldehydes at -78 °C provides syn- or anti-β-hydroxyallylsilanes, 7 or 8, respectively.

Stereoselective synthesis of gamma-substituted (Z)-allylic boranes via kinetically controlled hydroboration of allenes with 10-TMS-9-borabicyclo[3.3.2]decane

Kinetically controlled hydroboration of allenes 8 and 14a-d with the readily accessible Soderquist borane 7, which is generated in situ from borohydride 6, constitutes a convenient and preparatively useful method for synthesis of (Z)-gamma-(substituted)allylboranes 9 and 15a-d. These allylboranes undergo highly diastereo- (> or = 90: 10) and enantioselective (typically 89-96% e.e.) allylboration reactions with representative aldehydes to give syn-beta-functionalized homoallylic alcohols.

Recent progress on the total synthesis of acetogenins from Annonaceae

An overview of recent progress on the total synthesis of acetogenins from Annonaceae during the past 12 years is provided. These include mono-tetrahydrofurans, adjacent bis-tetrahydrofurans, nonadjacent bis-tetrahydrofurans, tri-tetrahydrofurans, adjacent tetrahydrofuran-tetrahydropyrans, nonadjacent tetrahydrofuran-tetrahydropyrans, mono-tetrahydropyrans, and acetogenins containing only gamma-lactone. This review emphasizes only the first total synthesis of molecules of contemporary interest and syntheses that have helped to correct structures. In addition, some significant results on the novel synthesis and structure-activity relationship studies of annonaceous acetogenins are also introduced.

Tuning the acyclic ether moiety of anticancer agent AA005 with conformationally constrained fragments

A new series of anticancer annonaceous acetogenin mimetics were designed, synthesized, and evaluated based on our previously developed compound AA005, in which a variety of conformationally constrained fragments were introduced. Parallel syntheses of all new compounds were accomplished by replacement of the acyclic bis-ether functionality of AA005 with certain conformationally constrained fragments. Slight effects to the anticancer activity were exerted by altering stereochemistries in the middle modification region. Similar to AA005, most newly synthesized mimetics were found to exhibit potent activities against breast cancer cells, and showed satisfactory selectivities between cancerous and non-cancerous cells. An N,N'-dimethyl bis-amide compound 67 exhibits 30 times more potency against MDA-MB-468 cells than its parent molecule AA005. This study indicates that the introduction of appropriate conformational constraints is a useful optimizing tool for this class of anticancer agents. Successes in the bis-amide analogues of AA005 make this unique class of anticancer agents much simpler and more flexible for future further developments.

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.

Diastereoselective synthesis of tetrahydrofurans via mead reductive cyclization of keto-beta-lactones derived from the tandem Mukaiyama aldol lactonization (TMAL) process

The development of a diastereoselective, three-step strategy for the construction of substituted tetrahydrofurans from alkenyl aldehydes based on the tandem Mukaiyama aldol-lactonization process and Mead reductive cyclization of keto beta-lactones is reported. Stereochemical outcomes of the TMAL process are consistent with models established for Lewis acid-mediated additions to alpha-benzyloxy and beta-silyloxy aldehydes while reductions of the five-membered oxocarbenium ions are consistent with Woerpel's models. Further rationalization for observed high diastereoselectivity in reductions of alpha-silyloxy 5-membered oxocarbenium ions based on stereoelectronic effects are posited. A diagnostic trend for coupling constants of gamma-benzyloxy beta-lactones was observed that should enable assignment of the relative configuration of these systems.

Enantio- and Diastereoselective Synthesis of syn-β-Hydroxyallylsilanes via a Chiral (Z)-γ-Silylallylboronate

syn-beta-Hydroxyallylsilanes of general structure 11 and 28 are prepared in 50-86% yield and 91-95% ee (for aliphatic aldehydes; 50% ee for benzaldehyde) via the BF(3).Et(2)O-promoted gamma-silylallylboration reactions, using reagents 14 and 15.

Cascade Triple-Aldehyde Addition of 1,2,3,4-Tetrakis(pinacolatoboryl)but- 2-ene: Stereoselective Synthesis of 2,3-Bis(alkylidene)alkane-1,5-diols

Treatment of (Z)-1,2,3,4-tetrakis(pinacolatoboryl)but-2-ene, prepared from 2,3-bis(pinacolatoboryl)buta-1,3-diene and bis(pinacolato)diboron, with three molar equivalents of aldehyde in toluene at 100 degrees C gave the 2,3-bis(alkylidene)alkane-1,5-anti-diol as a single stereoisomer. The reaction is applicable to both aromatic and alpha-unbranched aliphatic aldehydes. The 1,5-anti-diols were also synthesized by the one-pot preparation/triple-aldehyde addition of the tetraborylated butene. Experimental results for the stepwise treatment of the butene with two types of aldehydes suggest that the rate-determining step of the triple-aldehyde addition is the third allylation.

Total Synthesis of Amphidinolide E and Amphidinolide E Stereoisomers

Four amphidinolide E stereoisomers, amphidinolide E (1), 2-epi-amphidinolide E (2), 19-epi-amphidinolide E (3), and 2-epi-19-epi-amphidinolide E (4), have been synthesized via the judicious union of aldehyde 5, allylsilanes 7 or 8, acids 9 or 10, and vinylstannane 6. The C19 stereocenters of the C19 epimeric allylsilanes 7 and 8 were introduced via crotylboration reactions early in the synthesis. [3+2]-Annulation reactions of aldehyde 5 with allylsilanes 7 and 8 were employed to set the core tetrahydrofuran units of 1-4. Finally, the C2 stereocenter was installed by esterification using acid 9, without incident, or with acid 10, in which case an unexpected and completely stereoselective inversion of C2 occurs.

Annonaceous acetogenin mimics bearing a terminal lactam and their cytotoxicity against cancer cells

Annonaceous acetogenins are a large class of naturally occurring polyketides exhibiting potent anticancer activities. Based on our previous discovery of AA005, a multi-ether mimic of natural acetogenins having potent antitumor activities and significant selectivity between normal cells and cancer cells, a new series of mimics containing a terminal lactam were designed, synthesized and evaluated. Bioactivity study against cancer cells shows that the N-methylated lactam-containing compounds 3, 4, and 5 exhibit comparable potencies to that of AA005, as well as the similar selectivity to cancer cells. Hydrocarbon-length effects of N-alkyl were further explored through synthesizing derivatives 24-26, and application of this derivation protocol to the fluorescent labeling was also investigated.

Total synthesis of amphidinolide E

A convergent and highly stereocontrolled synthesis of amphidinolide E (1) has been accomplished. The synthesis features a highly diastereoselective (>20:1) BF3.Et2O promoted [3+2] annulation reaction between aldehyde 3 and allylsilane 4 to afford substituted tetrahydrofuran 2.

Advances in Lewis Acid Controlled Carbon−Carbon Bond-Forming Reactions Enable a Concise and Convergent Total Synthesis of Bullatacin

Lewis acids control regioselectivity in the alkylation of epi-chlorohydrin and the stereochemistry of an alkyne addition to set the C24/C23 anti relationship. These advances facilitate an efficient total synthesis of bullatacin in 13.3% overall yield from commercial starting materials.

A modular approach to the synthesis of 2,3,4-trisubstituted tetrahydrofurans

A highly diastereoselective Lewis acid-mediated [1,3] rearrangement of 1,3-dioxepins is the key step along a modular route to 2,3,4-trisubstituted tetrahydrofurans.