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Linchpins, reactions

Multicomponent Linchpin Reactions. 2-Lithio-2-TMS-1,3-dithiane has also been employed as a bi-directional nucleophile or linchpin in epoxide opening reactions where both openings occur in an intermolecular fashion. The dithiane anion first attacks the less hindered carbon of the epoxide then, in the presence of THF and 12-crown-4,1,4-Brook rearrangement occurs with transfer of the TMS group to reveal a new dithiane anion that can react with an additional equivalent of epoxide (eq 27). This reaction, first reported by Tietze et al. in 1994, suffered from a lack of control over the rearrangement step, thus limiting the use to the formation of symmetric 1,5-diols. ... [Pg.607]

After becoming engrossed with this chemistry, we had an epiphany. Could the Tietze-Smith linchpin reaction be used twice If this was to be executed the previously constructed right-hand fragment (i.e., bromide 22) could not... [Pg.262]

The Z- or -configuration of the vinyl groups of linchpins (106,107) is preserved when the stannyl groups are displaced by triflate esters, as shown in reactions 49 and 50301. [Pg.414]

Scheme 10.5 The stereospecific allylic substitution approach to a stereotetrad using a linchpin cross-coupling reaction. Scheme 10.5 The stereospecific allylic substitution approach to a stereotetrad using a linchpin cross-coupling reaction.
Sunay, U. Fraser-Reid, B. Synthetic studies relating to the C1-C9 eastern" half of rosara-micin. Tetrahedron Lett. 1986, 27, 5335-5338. Smith, A. B. Pitram, S. M. Boldi, A. M. Gaunt, M. J. Sfouggatakis, C. Moser, W. H. Multicomponent linchpin couplings. Reaction of dithiane anions with terminal epoxides, epichlorohydrin, and vinyl epoxides efficient, rapid, and stereocontrolled assembly of advanced fragments for complex molecule synthesis./. Am. Chem. Soc. 2003, 125, 14435— 14445. [Pg.137]

Multicomponent linchpin couplings can be carried out with 2-trialkylsilyl substituted 1,3-dithianes 207 and epoxides and was successfully used in the synthesis of natural products182. Tietze and coworkers303 found out that 2-lithio-2-trimethylsilyl-1,3-dithiane 208 reacted with two equivalents of a chiral epoxides in the presence of a crown ether to give first the monoadduct 209, which suffered 1,4-Brook rearrangement304 generating a new dithiane anion 210. Final reaction with an epoxide afforded products 211, which are equivalents of acetone aldol products (Scheme 60). [Pg.174]

Smith et al. adapted Sheehan and Izzo s synthesis of 2-aryl-4(5//)-oxazolones and developed a general synthesis of 2-alkyl-4(5//)-oxazolones. The reaction between acid halides and AgNCO followed by treatment with ethanol-free diazomethane produced oxazolones 252, which served as precursors to triflyloxyoxazoles 253 (Scheme 74). Triflyloxyoxazole 253 (R = CH2Br) was utilized as a difunctional linchpin for the bidirectional assembly of the natural product phorboxazole <2001JA10942>. [Pg.527]

A one-pot five-component dithiane linchpin coupiing was applied as the key synthetic transformation in A.B. Smith s approach to Schreiber s C16-C28 trisacetonide subtarget for mycoticins A and B. To prevent a premature Brook rearrangement, ether was used instead of THF as a solvent for the initial deprotonation of 2-TBS-1,3-dithiane. The third component in the iinchpin coupiing was (S,S)-diepoxypentane that was added to the reaction mixture along with HMPA in THF. [Pg.419]

A facile assembly of 2,4-orthogonally-functionalised oxazoles as useful bidirectional linchpins was achieved by treatment with diazomethane of acyl isocyanates, generated in situ from acyl halides 132 oxazolones 133 were converted to the oxazole triflates 134, useful intermediates for further synthetic elaborations as highlighted by the synthesis of 135 via Wittig and Stille reactions <01SL1739>. This linchpin tactic was used in the stereocontrol led total synthesis of the potent cytostatic agent (+)-phorboxazole A <01JA10942>. [Pg.246]

The viabihty of this new Type II ARC involving threefold domino reactions was demonstrated by developing a one-flask three component coupling protocol using 1,3-dithiane epoxy compound 176 as linchpin (Scheme 4.33) [59]. Thus,... [Pg.132]

Smith and coworkers next extended Type II ARC domino protocol beyond dithiane arena by designing a variety of effective ARC linchpins with different ASGs capable of three and four component couplings. In their first effort toward this goal, they developed readily available 2-bromoallylsilane [62a], and subsequently, allyltrimethylsilane 185 [63], and recently o-TMS (trimethylsilyl) benzaldehyde 188 as promising linchpins for Type II ARC process [64, 65]. However, domino anionic relay reactions with these linchpins 185 and 188 are not discussed in this chapter, since they are not initiated by nucleophilic substitution as the first step. [Pg.134]

In a further effort for search of other viable ASGs that would extend the scope and general utihty of the ARC tactic. Smith and coworkers have developed synthesis and reactions of novel achiral and chiral linchpins, with other ASGs such as nitrile, phenyl, and phenylthio functionalities (191-193) (Scheme 4.38) [63, 66-68]. An evaluation of these newly synthesized compounds as linchpins for Type II ARC tactics, reveals that both phenyl and phenylthio moieties are viable as ASGs for... [Pg.135]

Scheme 4.38 New bifunctional linchpins for Type II ARC domino reactions. Scheme 4.38 New bifunctional linchpins for Type II ARC domino reactions.
ARC process [67]. The initiating nucleophiles included either lithiated 2-methyl-1,3-dithiane or lithium di-n-butylcuprate, which on reaction with these linchpins (191-193), followed by trapping of the rearranged anion 172 by allyl bromide as electrophile, provide three-component adducts such as 194-196 in 58-68% overall yields (Scheme 4.38) [63, 67]. Of particular note is the stereochemical outcome of these reactions. High diastereoselectivity was observed with linchpins possessing a methyl substituent a to the silyl group, when a phenyl moiety serve as ASG (Scheme 4.38, Eq. (2)), while poor diastereoselectivity is observed when a phenylthio moiety is employed as the ASG irrespective of the presence of a-substituent (Scheme 4.38, Eq. (3)) [67, 68]. [Pg.136]

One of the most used sequential component reactions in the asymmetric construction of chiral compounds is the so-called anion relay chemistry (ARC) [2], This linchpin coupling protocol consists in the alkylation of an anion, generally a silyl lithium dithiane derivative, by an epoxide or an aziridine, resulting in an oxy- or aza-anion, which in the presence of hexamethylphosphoramide (HMPA) or other polar solvent gives a 1,4-Brook rearrangement, thereby leading to a new reactive dithiane anion that is capable of reacting with a second electrophile E+ (Scheme 11.1). [Pg.310]

A short synthesis of (5 ,9 )-(—)-indolizidine 223AB (1806) by Smith and Kim used the silylated dithiane 1847 as a linchpin for the one-pot tandem alkylation with epoxide (- -)-1848 and the N-tosylaziridine (—)-1849 (Scheme 233). The first intermediate is presumably alkoxide 1850, which undergoes a 1,4-Brook rearrangement to 1851 before reaction with the aziridine. The bis-alkylated dithiane (—)-1852 was isolated in... [Pg.343]


See other pages where Linchpins, reactions is mentioned: [Pg.256]    [Pg.259]    [Pg.262]    [Pg.262]    [Pg.263]    [Pg.256]    [Pg.259]    [Pg.262]    [Pg.262]    [Pg.263]    [Pg.142]    [Pg.196]    [Pg.364]    [Pg.366]    [Pg.104]    [Pg.24]    [Pg.418]    [Pg.419]    [Pg.582]    [Pg.97]    [Pg.29]    [Pg.268]    [Pg.70]    [Pg.130]    [Pg.133]    [Pg.465]    [Pg.358]    [Pg.1289]    [Pg.1123]    [Pg.1289]   
See also in sourсe #XX -- [ Pg.414 ]




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Tietze-Smith linchpin reaction

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