Tandem bonds

The fourth method also involves a tandem bond-forming strategy of N1-C2 and N4—C5, in which simultaneous formation of both amide bonds of a 2,5-DKP occurs in a one-pot reaction from an a-amino ester derivative. ... 

Cascade Addition-Cyclization Reactions Given the importance of cascade reactions in modem chemical synthesis, the MacMillan group has proposed expansion of the realm of iminium catalysis to include the activation of tandem bond-forming processes, with a view toward the rapid constraction of natural products. In this context, the addition-cyclization of tryptamines with a,p-unsaturated aldehydes in the presence of imidazolidinone catalysts 11 or 15 has been accomplished to provide pyrroloindoline adducts in high yields and with excellent enantioselectivities (Scheme 11.3a). This transformation is successful... 

Detection of either I or VI in HPLC eluent was found to be practical using UV light at 209 nm. Mobile phases used had to be compatible with monitoring at this wavelength to obtain the sensitivity needed for plasma assays. In the assay for I a 0.3% isopropanol in heptane solution as mobile phase met these criteria as did water acetonitrile in the assay for VI. The assay developed for I required the use of two tandem bonded-phase columns to adequately separate drug and endogenous plasma constituents. 

Hydrogen-Bond Configurations TWo- and Three-Center Hydrogen Bonds Bifurcated and Tandem Bonds... 

Moser, W. H. The Brook rearrangement in tandem bond formation strategies. Tetrahedron 2001, 57, 2065-2084. 

The Brook Reairangement In Tandem Bond Formation Strategies, Moser. W,IL Tetrahedron, 2001, 57, 2065. 

The term Brook rearrangement refers to the 1,2-Brook rearrangement throughout this chapter unless otherwise indicated. Also, the use of Brook rearrangements in tandem bond formation strategies up to the year 2000 has been excellendy reviewed by Moser. 

Also employing a tandem bond formation strategy, Tadd et al. reported a synthesis of 2-quinolones using 2-(2-haloalkenyl)aryl halide substrates. These substrates had previously been utilized in the synthesis of indoles (Scheme 24.1, disconnection D-4 and Scheme 24.6). A cascade palladium-catalyzed aminocar-bonylation/intramolecular amidation sequence was employed to access a range of quinolone products. For example, a combination of simple dibromide 48 and p-methoxybenzylamine under a balloon pressure of carbon monoxide delivered the desired heterocycle in good yield (Scheme 24.23) [98]. 

The allenyl moiety (2,3-aikadienyl system) in the carbonylation products is a reactive system and further reactions such as intramolecular Diels-Alder and ene reactions are possible by introducing another double bond at suitable positions of the starting 2-alkynyl carbonates. For example, the propargylic carbonate 33 which has l,8(or 1.9)-diene-3-yne system undergoes tandem carbonylation and intramolecular Diels-Alder reaction to afford the polycyclic compound 34 under mild conditions (60 C, 1 atm). The use of dppp as ligand is important. One of the double bonds of the allenyl ester behaves as part of the dieneflSj. 

The cyclization of the enediynes 110 in AcOH gives the cyclohexadiene derivative 114. The reaction starts by the insertion of the triple bond into Pd—H to give 111, followed by tandem insertion of the triple bond and two double bonds to yield the triene system 113, which is cyclized to give the cyclohexadiene system 114. Another possibility is the direct formation of 114 from 112 by endo-rype. insertion of an exo-methylene double bond[53]. The appropriately structured triyne 115 undergoes Pd-catalyzed cyclization to form an aromatic ring 116 in boiling MeCN, by repeating the intramolecular insertion three times. In this cyclization too, addition of AcOH (5 mol%) is essential to start the reaction[54]. 

However, interpretation of, or even obtaining, the mass spectrum of a peptide can be difficult, and many techniques have been introduced to overcome such difficulties. These techniques include modifying the side chains in the peptide and protecting the N- and C-terminals by special groups. Despite many advances made by these approaches, it is not always easy to read the sequence from the mass spectrum because some amide bond cleavages are less easy than others and give little information. To overcome this problem, tandem mass spectrometry has been applied to this dry approach to peptide sequencing with considerable success. Further, electrospray ionization has been used to determine the molecular masses of proteins and peptides with unprecedented accuracy. 

Nonwoven bonding processes iatedock webs or layers of fibers, filaments, or yams by mechanical, chemical, or thermal means. The extent of bonding is a significant factor ia determining fabric strength, dexibiUty, porosity, density, loft, and thickness. Bonding is normally a sequential operation performed ia tandem with web formation, but it is also carried out as a separate and distinct operation. 

The fabric may also be given one or more of a number of other finishing treatments, either ia tandem with web formation and bonding or off-line as a separate operation, as a means of enhancing fabric performance or aesthetic properties. Performance properties iaclude functional characteristics such as moisture transport, absorbency, or repeUency flame retardancy electrical conductivity or static propensity abrasion resistance and frictional behavior. Aesthetic properties iaclude appearance, surface texture, and smell. 

Cartion-cartxin bond formation via tandem Mannich condensation, cationic aza-Cope rearrangement of homoaHylarrunes, leading to pyrolidmes... 

The selectivity for two-alkyne annulation can be increased by involving an intramolecular tethering of the carbene complex to both alkynes. This was accomplished by the synthesis of aryl-diynecarbene complexes 115 and 116 from the triynylcarbene complexes 113 and 114, respectively, and Danishefsky s diene in a Diels-Alder reaction [70a]. The diene adds chemoselectively to the triple bond next to the electrophilic carbene carbon. The thermally induced two-alkyne annulation of the complexes 115 and 116 was performed in benzene and yielded the steroid ring systems 117 and 118 (Scheme 51). This tandem Diels-Alder/two-alkyne annulation, which could also be applied in a one-pot procedure, offers new strategies for steroid synthesis in the class O—>ABCD. 

The field of alkaloid synthesis via tandem cyclizations favors the application of (TMSlsSiH over other radical-based reagents, due to its very low toxicity and high chemoselectivity. For example, cyclization of the iodoarylazide 102, mediated by (TMSlsSiH under standard experimental conditions, produced the N-Si(TMS)3 protected alkaloid 103 that after washing with dilute acid afforded the amine 104 in an overall 83% yield from 102 (Reaction 81). ° The formation of the labile N-Si(TMS)3 bond was thought to arise from the reaction of the product amine 104 with the by-product (TMSlsSil. The skeletons of ( )-horsfiline, ( )-aspidospermidine and (+ )-vindoline have been achieved by this route. - ... 

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