<i>C</i>‐Alkylation of Peptides through Polylithiated and LiCl‐Solvated Derivatives Containing Sarcosine Li‐Enolate Units

Seebàch, Dieter; Bossler, Hans G.; Gründler, Hansjörg; Shoda, Shin Ichiro; Wenger, Roland M.

doi:10.1002/hlca.19910740121

Cited by 87 publications

(26 citation statements)

References 90 publications

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“…fractions washed with brine to neutrality, dried (Na,SO,), and evaporated, and the crude product purified by distillation or FC. 17) Results not described in the Exper. Part.…”

Section: )mentioning

confidence: 87%

Transesterifications with 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene/Lithium Bromide (DBU/LiBr) – Also Applicable to Cleavage of Peptides from Resins in Merrifleld Syntheses

Scebach¹,

Thaler

Blaser

et al. 1991

Helvetica Chimica Acta

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(2 1. V. 9 1)A mixture of the amidine base 1,8-diazabicyclo[5,4.0]undec-7-ene (DBU) and LiBr (preferably 0.5 and 5 equiv., resp.) turns out to be a highly efficient catalyst (at 0-25O) for saponifications (in THF/H20) and transesterifications (in ROH). The scope and limitations of the method are determined using ca. two dozens of different ester/alcohol combinations (Schemes 2 and 3 ) . The investigation is focused on peptides as substrates. Under carefully controlled conditions, no epimerization occurs with N-Boc-and N-Z-protected peptide esters, when methyl, ethyl, isopropyl, or ally1 esters are the products, as shown for peptides containing up to six amino acids, with Ala, Leu, MeLeu, Asp(OEt), or Tyr at the C-terminus (Scheme 3 and Tables 1 and 2). Hydrolytic and transesterifying detachments of Boc-Leu-Ala-Gly-Val-OR and Boc-Leu-Ala-Gly-Phe-OR (R = H, Me) from PAM and Wang resins (1-8 h at 0-25", 2 equiv. of DBU, 5 equiv. of LiBr) can be achieved by this method without epimerization of the C-terminal stereogenic center; a comparison with other methods (HF, Ti(OR),) is given (Schemes 4 and 5 ) . Possible protecting-group strategies involving the DBU/LiBr method are discussed (Table 3 ) . Extensive experimental details are given.In the course of our work on olefinations 'A la Horner-Emmons-Wadsworth' [l] with the phosphonate [2] and the highly efficient base system DBU')/LiBr [3] shown in Scheme 1, we noticed that traces of moisture led to rapid hydrolysis of the phosphonate ester at room temperature. Since such esters are normally not readily hydrolyzed, we embarked on a systematic investigation to test, whether saponifications and transesterifications could be generally effected under these conditions.There are numerous methods of achieving transesterifications and ester hydrolyses4) under acidic and basic conditions, with ester-cleaving enzymes'), with titanate@), with ion-exchange resins [8], with KF/crown ether [9], and with distannoxane [lo] to mention

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“…fractions washed with brine to neutrality, dried (Na,SO,), and evaporated, and the crude product purified by distillation or FC. 17) Results not described in the Exper. Part.…”

Section: )mentioning

confidence: 87%

Transesterifications with 1,8‐Diazabicyclo[5.4.0]undec‐7‐ene/Lithium Bromide (DBU/LiBr) – Also Applicable to Cleavage of Peptides from Resins in Merrifleld Syntheses

Scebach¹,

Thaler

Blaser

et al. 1991

Helvetica Chimica Acta

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“…[18] Ther egio-and stereoselective alkylation of cyclosporine was definitely the most spectacular example. [18] Ther egio-and stereoselective alkylation of cyclosporine was definitely the most spectacular example.…”

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confidence: 99%

Total Syntheses and Biological Evaluation of Miuraenamides

et al. 2015

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The miuraenamides, relatively simple representatives of a class of cyclodepsipeptides with high antitumor activity, can be easily and flexibly obtained by the concept of peptide modification. A reaction sequence consisting of an aldol reaction, oxidation, and methylation of the glycine subunit of the cyclodepsipeptides allows the incorporation of the unusual α,β-unsaturated dehydroamino acid in one of the last steps of the synthesis.

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“…Our efforts to suppress the formation of the regioisomeric alkylation product and enhance the rate of these alkylation reactions with additives such as 1,3-dimethyl-3,4,5,6- tetrahydro-2(1 H )-pyrimidinone (DMPU), hexamethylphosphoramide (HMPA), and metal salts were unsuccessful. [16] …”

Section: Resultsmentioning

confidence: 99%