Synthesis of oxepines and 2-branched pyranosides from a d-glucal-derived gem-dibromo-1,2-cyclopropanated sugar.
TL;DR: The conversion of cyclopropane-fused carbohydrates into oxepines is an attractive method for accessing diverse members of the septanoside family of carbohydrate mimetics, and 2-Bromooxepines are obtained through silver(I)-promoted thermal ring expansion of a d-glucal-derived gem-dihalocyclopropanated sugar.
Abstract: The conversion of cyclopropane-fused carbohydrates into oxepines is an attractive method for accessing diverse members of the septanoside family of carbohydrate mimetics. 2-Bromooxepines are obtained through silver(I)-promoted thermal ring expansion of a d-glucal-derived gem-dihalocyclopropanated sugar. In contrast, cyclopropane ring cleavage under basic conditions leads to 2-C-branched pyranosides, not the 2-bromooxepine structures assigned in an earlier report.
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TL;DR: An expedient one-step synthesis of 2-iodoglycals and 2-bromoglycal from glycals using NIS/AgNO3 and NBS/ AgNO3 as reagent systems has been developed and the utility of these 2-haloglycals has been demonstrated by converting them into 2C-branched glycals via the Heck coupling reaction.
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01 May 2012
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TL;DR: In this article, a new and efficient method was developed for the synthesis of highly functionalized 2-amino-4H-pyran derivatives through one-pot, three component tandem reactions of structurally diverse 1,3-diketones, dialkyl acetylenedicarboxylates, and malononitrile or ethyl cyanoacetate in the presence of a catalytic amount of Na2CO3 in ethanol.
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TL;DR: In this article, a cyclopropanation of glycals 1, 2, 3, 4, and 41 was carried out using either dihalocarbene addition or zinc-carbenoid addition to yield 1,2-cyclopropanated sugars.
Abstract: Diastereospecific cyclopropanation of glycals 1, 2, 3, 4, and 41 was carried out using either dihalocarbene addition or zinc-carbenoid addition to yield 1,2-cyclopropanated sugars. Dichloro- and dibromocarbenes added stereoselectively anti to the C3 benzyloxy group, whereas (under Simmons−Smith conditions) the cyclopropanes were formed syn to the same substituent. The reactions of these 1,2-cyclopropanated sugars to provide either ring expanded glycosides or C2-branched chain glycosides were explored. Solvolytic ring expansion of 1,2-dibromocyclopropanated sugars with K2CO3 in refluxing methanol yielded the corresponding chiral oxepins 20−22. Electrophilic ring openings of parent cyclopropanes (14 and 17 derived from glucal 1) were carried out with different electrophiles to give functionalized 2-deoxy-2-C-branched chain glycosides. The ring openings of 14 in different solvents resulted in both α- and β-anomers, whereas 17 gave exclusively the α-anomer.
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TL;DR: This novel transformation of alkynyldiols 1 to corresponding seven-membered cyclic enol ethers 2 under tungsten carbonyl catalysis proceeds with good yields and virtually complete regioselectivity for all diastereomers, favoring the product 2 resulting from endo-mode cyclization.
81 citations
TL;DR: In this article, a ring-closing metathesis approach to a family of carbohydrate-based oxepines is described, and a variety of readily accessible, protected monosaccharide derived dienes are used to demonstrate the utility of the synthetic sequence and to investigate how factors such as rigidification and deoxygenation mediate RCM using the Grubbs or Schrock catalyst.
68 citations