Design of Thermally Stable Versions of the Burgess Reagent: Stability and Reactivity Study

Total Synthesis and Bioactivity Profile of (+)-Ieodomycins A and B and their Stereoisomers

Herein, we report the first- and second-generation syntheses of (+)-ieodomycins A and B and their stereoisomers via the late-stage elaboration of their conjugated E-diene side chains. Key steps for successful synthesis included Keck asymmetric allylation to introduce a hydroxyl group at the C5 position, consecutive Wipf's carboalumination modification, iodination, Sharpless asymmetric dihydroxylation, one-carbon homologation via cyanation, Mukaiyama lactonization, and Stille cross-coupling to form the conjugated E-diene moiety. Further, the preliminary in vitro bioactivity profile against various disease-related molecular targets and cell lines was investigated. Results indicated that compounds 30b and 30c, diastereoisomers of (+)-ieodomycin B (2), serve as α-glucosidase inhibitors, while compounds 30b and 30d inhibit the angiotensin-converting enzyme.

Catalytic Sulfamoylation of Alcohols with Activated Aryl Sulfamates

We report a new catalytic method for alcohol sulfamoylation that deploys electron-deficient aryl sulfamates as activated group transfer reagents. The reaction utilizes the simple organic base N-methylimidazole, proceeds under mild conditions, and provides intrinsic selectivity for 1° over 2° alcohols (up to >40:1 for certain nucleosides). The requisite aryl sulfamate donors are stable crystalline solids that can be readily prepared on a large scale. Mechanistic considerations support the intermediacy of HNSO₂ "aza-sulfene" in the transfer reaction.

Crystal Structure of Burgess Inner Salts and their Hydrolyzed Ammonium Sulfaminates

The solid-state structures of the Burgess reagent, and its analogous ethyl ester reveal structures indicative of triethylamine solvated sulfonyl imides rather than the more commonly depicted triethylammonium sulfonyl amidate. The existence of a reversibly formed hydrate of Burgess reagent is not supported by present studies, but rather a hydrosylate that does not revert to the Burgess reagent with gentle warming under vacuum was isolated and characterised. Structures of the hydrosylates from both the methyl- and ethyl-amidate esters were determined from X-ray crystallographic analysis and are reported. The crystal structures of the Burgess inner salts exhibit geometries at the sulfur atoms that are intermediate between a planar O2S=NCO2R unit and tetrahedral 4-coordinate sulfur centres that would be expected from a strong single (dative) bond between the triethylamine nitrogen and sulfur. The hydrolysed ammonium sulfaminates are water soluble intermolecular salts composed of triethylammonium ions, Et3NH+, and N-(alkoxycarbonyl)sulfaminate, O(−)SO2NHCO2R {R=CH3 or C2H5}.

Synthesis of 2-Oxazolines by in Situ Desilylation and Cyclodehydration of β-Hydroxyamides

A powerful method for the synthesis of 2-oxazolines from silyl-protected β-hydroxyamides is reported. Using diethylaminosulfur trifluoride (DAST) or its tetrafluoroborate salt (XtalFluor-E), silyl-protected β-amidoalcohols can be in situ deprotected and dehydrated to give 2-oxazolines in good yields. The utility of this approach was demonstrated by preparing the first reported oligomer of [2,4']-coupled 2-oxazoline units. By tuning the stability of the silyl protecting groups (ex. IPDMS < TES < TBS, etc.), the deprotection rate can be optimized so that all reaction intermediates remain soluble, allowing cyclodehydration to occur at all potential sites of ring closure. N-Terminal Ser residues containing an Fmoc carbamate are converted into 2-(9'-fluorenylmethyloxy)-2-oxazoline in high yield, thereby providing a new pathway for the synthesis of peptides capped with an N-terminal 2-alkoxy-2-oxazoline or 2-oxazolidinone unit.

A mild method for the synthesis of carbamate-protected guanidines using the Burgess reagent

A simple method for the synthesis of carbamate-protected guanidines from primary amines is described. A variety of thioureas derived from primary amines and isothiocyanates react with the Burgess reagent to give the corresponding guanidines via either a stepwise or one-pot procedure. By tuning the carbamoyl units of isothiocyanates and the Burgess reagent, differentially N,N'-diprotected guanidines can be obtained. Selective deprotection of the products affords N-monoprotected guanidines.

Novel Burgess reagent mediated C-to-N aryl migration reaction in nitrones

Nitrones undergo useful transformations with Burgess reagent, ostensibly involving a [3 + 2] annulation across a σ-bond followed by rearrangement involving C-to-N aryl migration.