Preparation of Chlorosilyl Enolates

Denmark, Scott E.; Stavenger, Robert A.; Winter, Stephen B.D.; Wong, Ken‐Tsung; Barsanti, Paul A.

doi:10.1021/jo981740h

Cited by 52 publications

(41 citation statements)

References 33 publications

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“…Conversions of 9a/b in the presence of LiCl or LiI and TMSCl afforded some type 8/10/12 indolizidinones, but the isolated yields were low. [27] Obviously, the removal of the benzyl group by von Braun-type degradation of an intermediate acyl ammonium salt was the crucial step. [28] The competing lactonization (Ǟ 11) was the major reaction path, especially when handling more than 1 mmol quantities of the epoxides 9a/b.…”

Section: Resultsmentioning

confidence: 99%

Synthesis of the Bicyclic Core of Pumiliotoxins

et al. 2002

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Keywords: Medium rings / Ring contractions / Ring expansion / Sigmatropic rearrangements / ZwitterionsThe bicyclic core of the pumiliotoxins was synthesized in nine to eleven steps starting from L-(−)-proline. This chiral pool starting material was initially converted into an optically active 2-vinylpyrrolidine by standard operations. The first key step allowed the generation of a nine-membered ring lactam by means of a zwitterionic aza-Claisen rearrangement. The 1,4 chirality transfer was found to be low, but the double bond of the azoninone was generated with an exclusive trans configuration in a planar-S arrangement. The mixture of diastereomers thus obtained was immediately epoxid-

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Section: Resultsmentioning

confidence: 99%

Synthesis of the Bicyclic Core of Pumiliotoxins

et al. 2002

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“…Previous studies in this laboratory have shown that enol silylation of ketones with SiCl 4 and an amine base is catalyzed by hexamethylphosphoric triamide (HMPA) (43). We were pleased to find that the reaction of isobutyraldehyde with SiCl 4 and triethylamine in the presence of 10 mol % HMPA in methylene chloride proceeded smoothly at room temperature to afford trichlorosilyl enolate 2 (Fig.…”

Section: Preparation Of Trichlorosilyl Enolatementioning

confidence: 94%

“…The synthesis of trichlorosilyl enolates derived from ketones was previously accomplished by metathesis of trimethylsilyl enol ethers with silicon tetrachloride (SiCl 4 ) in the presence of a catalytic amount of mercury(II) acetate or palladium(II) acetate (43). In the presence of 5 mol % of either Hg(OAc) 2 or Pd(OAc) 2 , the metathetical reaction of trimethyl-[2-methyl-(propenyloxy)]silane 1 with SiCl 4 proceeded very slowly at room temperature to afford only trace amounts of trichlorosilyl enolate 2 according to 1 H NMR analysis (Fig.…”

Section: Preparation Of Trichlorosilyl Enolatementioning

confidence: 99%

Chiral phosphoramide-catalyzed, enantioselective, directed cross-aldol reactions of aldehydes

Denmark

Bui

2004

Proc. Natl. Acad. Sci. U.S.A.

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Catalytic, enantioselective, directed cross-aldol reactions of aldehydes are described. The addition of isobutyraldehyde trichlorosilyl enolate 2 to various aldehydes in the presence of 10 mol % bisphosphoramide 4 provides aldol products in high yields with moderate to good enantioselectivities. The reaction works well with a wide range of aromatic, olefinic, and aliphatic aldehydes. Enantioselectivities are highly dependent on the electronic nature of the aldehyde substituent. Hammett studies reveal that enantioselectivity increases as aldehydes become either more electron rich or more electron poor. T he catalytic enantioselective aldol reaction has achieved ''strategy level'' status in organic synthesis (1-6). In this process, a new carbon-carbon bond is formed with the creation of up to two new stereogenic centers. The relative and absolute configurations of the newly created stereocenters are usually controlled through the use of chiral Lewis acids (7) or through enzymatic methods (3,8). Chiral Lewis acid-catalyzed aldol reactions generally afford aldol products from enoxysilane derivatives of ketones or esters [directed aldol reaction (9)] in high yields with good to excellent selectivity. A few notable examples of chiral Lewis acids used in these catalytic aldol processes include bisoxazoline copper(II) complexes (10, 11) and Schiff base titanium(IV) complexes (12, 13), which have been developed by Evans et al. and Carreira et al., respectively. Recent advances also include the development of direct catalytic enantioselective aldol reactions that employ the two unmodified carbonyl components. Early reports on this type of aldol reaction using LaLi 3 tris(binaphthoxide) as a catalyst were disclosed by . Moderate to good yields and selectivities were obtained in these studies, albeit at long reaction times. Further, Trost et al. (17,18) have developed dinuclear zinc catalysts for direct catalytic enantioselective aldol reactions. These catalysts, which mimic class II aldolases, have been shown to catalyze aldol reactions of unmodified ketones with aldehydes in moderate to good yields with excellent levels of stereocontrol. Parallel to these successful developments is the finding of proline and its derivatives as effective catalysts for direct enantioselective aldol reactions (19-21). These simple cyclic amine catalysts are capable of mimicking class I aldolases, and they provide another approach to asymmetric aldol reactions (22,23).Besides these methods, the use of enzymes (3, 24) and monoclonal antibodies (8) in enantioselective aldol reactions has also been described with success. Wong et al. (24) have demonstrated the use of natural aldolase-catalyzed aldol reactions as a means to effectively and selectively synthesize highly oxygenated carbon chains, allowing a rapid access to various carbohydrates. However, broadening enzyme specificities and being able to perform enzymatic transformations on a preparative scale remain challenges for further development.For the directed aldol reaction, silyl enol et...

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“…Recently, Denmark et al [5] examined the effectiveness of chiral derivatives of HMPA as Lewis base promoters 2 ) 3 ) in several enantioselective reactions, including the so-called desymmetrization of meso-epoxides and aldol additions (Scheme 1, Eqns. 1 and 2).…”

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confidence: 99%

“…4 ) For recent reviews on applications of 1-phenylethylamine in the preparation of enantiomerically pure compounds, see [8]. 5 ) For N-monoalkylations of 1-phenylethylamine in DMPU solution, see [9]. 6 ) The moderate yield encountered in the cyclization reaction is a consequence of oligomer formation [12].…”

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confidence: 99%

Synthesis of New Chiral Derivatives of N,N′-Dimethylpropyleneurea (DMPU) and Examination of Their Influence on the Regio- and Enantioselectivity of Addition of 2-(1,3-Dithianyl)lithium to Cyclohex-2-en-1-one

Juaristi¹,

Hernández‐Rodríguez²,

López-Ruíz³

et al. 2002

HCA

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The preparation of three new chiral derivatives of DMPU (N,N'-dimethylpropyleneurea) is described (Schemes 2 ± 4); one type of derivative carries 1-phenylethyl or 1-cyclohexylethyl groups at the N-atoms of the tetrahydropyrimidin-2(1H)-one ring (2 and 4), another type of derivative is substituted at C(4) and C(6) of the heterocyclic ring (7). The potential of these chiral Lewis bases as promoters in the regio-and/or enantioselective addition of 2-(1,3-dithianyl)lithium to cyclohex-2-en-1-one was explored; they are all unable to effect enantioselective addition; the derivatives with branched substituents at the N-atoms do not shift the addition mode from 1,2 to 1,4, while the 3,4,5,6-tetrahydro-1,3,4,6-tetramethylpyrimidin-2(1H)-one does (Scheme 5). The results provide useful information regarding the nature of the nucleophilic organolithium reagent: obviously, the steric hindrance to Li complexation on the CO O-atom of the tetrahydropyrimidin-2(1H)-one by branched substituents at N-atoms (cf. X-ray crystal structure of 2 in the Fig.) prevents solvent-separated-ionpair (SSIP) formation; this was confirmed by PM3 and B3LYP/3-21-G(d)//PM3 calculations (Scheme 6).

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Preparation of Chlorosilyl Enolates

Cited by 52 publications

References 33 publications

Synthesis of the Bicyclic Core of Pumiliotoxins

Synthesis of the Bicyclic Core of Pumiliotoxins

Chiral phosphoramide-catalyzed, enantioselective, directed cross-aldol reactions of aldehydes

Synthesis of New Chiral Derivatives of N,N′-Dimethylpropyleneurea (DMPU) and Examination of Their Influence on the Regio- and Enantioselectivity of Addition of 2-(1,3-Dithianyl)lithium to Cyclohex-2-en-1-one

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