Brønsted acid-promoted azide–olefin [3 + 2] cycloadditions for the preparation of contiguous aminopolyols: The importance of disiloxane ring size to a diastereoselective, bidirectional approach to zwittermicin A

• Hubert Muchalski,
• Ki Bum Hong and
• Jeffrey N. Johnston

Beilstein J. Org. Chem. 2010, 6, 1206–1210, doi:10.3762/bjoc.6.138

• benzyl azide with α,β-unsaturated imides. Using a strong Brønsted acid (triflic acid) to activate the electron deficient imide π-bond, high diastereoselection was observed provided that a 1,1,3,3-tetraisopropoxydisiloxanylidene group (TIPDS) is used to restrict the conformation of the central 1,3-anti

• diol. This development provides a basis for a stereocontrolled approach to the aminopolyol core of (−)-zwittermicin A using a bidirectional synthesis strategy. Keywords: azide; bidirectional synthesis; cycloaddition; dialkoxydisiloxane; TIPDS; triazoline; triflic acid; zwittermicin A; Introduction

• aminohydroxylations [6][7][8]. Triflic acid-promoted reaction of an alkyl azide with an α,β-unsaturated imide delivers a formal anti-aminohydroxylation product. We wondered whether azide–olefin functionalization could be used to prepare the complex aminopolyol core of zwittermicin A [9][10][11][12]. We were

Shelf-stable electrophilic trifluoromethylating reagents: A brief historical perspective

• Norio Shibata,
• Andrej Matsnev and
• Dominique Cahard

Beilstein J. Org. Chem. 2010, 6, No. 65, doi:10.3762/bjoc.6.65

• dicyanoalkylidenes and is now commercially available. We also disclosed an easy-access to extended Yagupolskii-Umemoto type reagents, S-(trifluoromethyl)thiophenium salts, through triflic acid-catalyzed intramolecular cyclization of o-ethynylaryltrifluoromethylsulfanes. A series of S-(trifluoromethyl)benzo[b

• (or selenides) with m-chloroperbenzoic acid followed by cyclization of the corresponding sulfoxides (or selenoxides) either with triflic anhydride or by direct fluorination with 10% F2/N2 in the presence of one equivalent of triflic acid or HBF4 (Scheme 2). The tellurophenium salt 7 was synthesized in

• compounds that are benzothiophenium salts rather than dibenzo analogs. Ortho-ethynylaryl- and alkyl-trifluoromethylsulfanes were cyclized under strong acidic conditions with triflic acid to give the corresponding sulfonium salts in 64–94% yields (Scheme 12). It should be noted, that in the presence of gold

Chemical synthesis using enzymatically generated building units for construction of the human milk pentasaccharides sialyllacto-N-tetraose and sialyllacto-N-neotetraose epimer

• Dirk Schmidt and
• Joachim Thiem

Beilstein J. Org. Chem. 2010, 6, No. 18, doi:10.3762/bjoc.6.18

• glycosylations employing triflic acid/N-iodosuccinimide. Easily accessible selectively protected lactoside derivatives served as acceptor disaccharides to give the corresponding human milk pentasaccharides in good yields. These were characterized by spectroscopic means in the form of their peracetylated

• target pentasaccharide. Conclusion In this contribution chemoenzymatically generated sialyl α,2-3- and sialyl α,2-6-glycosylated thiophenol 2-azido-lactose derivatives were employed as precursors for sialylated lactosaminide donor substituents in triflic acid/N-iodosuccinimide glycosylations. With easily

Convenient method for preparing benzyl ethers and esters using 2-benzyloxypyridine

• Susana S. Lopez and
• Gregory B. Dudley

Beilstein J. Org. Chem. 2008, 4, No. 44, doi:10.3762/bjoc.4.44

• of benzyl trichloroacetimidate using triflic acid, except that alkylation under neutral conditions is compatible with acid- (and base-) sensitive substrates. Results and Discussion 2-Benzyloxypyridine was prepared in 97% yield by heating a mixture of benzyl alcohol, 2-chloropyridine (1.1 equiv), and

• benzyl ethers is described using 2-benzyloxypyridine and methyl triflate in lieu of benzyl trichloroacetimidate and triflic acid. N-Methylation of 2-benzyloxypyridine gives rise to an active benzyl transfer reagent (1) in situ, presumably in much the same way as N-protonation activates benzyl

Synthesis of crispine A analogues via an intramolecular Schmidt reaction

• Ajoy Kapat,
• Ponminor Senthil Kumar and
• Sundarababu Baskaran

Beilstein J. Org. Chem. 2007, 3, No. 49, doi:10.1186/1860-5397-3-49

• indolizidine skeleton using azido-olefin as a key intermediate (Scheme 3). In this reaction, in addition to benzo[e]indolizidine A, a minor product B having the basic skeleton of crispine A was isolated in 28% yield. The intramolecular Schmidt reaction of azido-olefin in the presence of triflic acid proceeds