1,4-Dioxaspiro[2.2]pentanes. Synthesis, spectroscopic properties, and reactions with nucleophiles
摘要:
A number of simple allenes have been converted cleanly to the corresponding 1,4-dioxaspiro[2.2]pentanes in good yields by oxidation with dimethyldioxirane. Mono- and trisubstituted allenes give the anti diastereomers with good stereoselectivity, whereas 1,3-disubstituted allenes show only a slight preference for the anti,anti isomers, unless steric effects are extreme. Stereochemical assignments are based on steric considerations and a consistent set of NMR characteristics that permit structural attributions. The addition of a range of nucleophiles (water, alcohols, amines, thiophenol, acetate, chloride, and fluoride) to these intermediates proceeded smoothly under buffered conditions to give highly functionalized products of general type 5. These reactions were shown to take place with the appropriate selectivities for S(N)2 substitutions in instances where these features were observable, namely, regioselective attack at the less-substituted epoxide carbon and inversion of configuration at the site of substitution. Under acidic conditions dioxaspiropentanes gave mixtures of other types of products, which appear to arise from carbocationic processes.
Synthesis of naphthoquinone antibiotics by intramolecular alkyne cycloaddition to carbene-chromium complexes
作者:M.F. Semmelhack、Joseph J. Bozell、Leonard Keller、Tadahisa Sato、E.J. Spiess、W. Wulff、A. Zask
DOI:10.1016/s0040-4020(01)91420-6
日期:1985.1
The reaction of carbene-chromium complexes with alkynes provides a direct route to naphthoquinone derivatives and is the key step in a new approach to the isochromanone antibiotics exemplified by deoxyfrenolicin (1) and nanaomycin A (2). While the regioselectivity of intermolecular addition of the appropriate unsymmetrical disubstituted alkyne is unfavourable, two successful approaches have been developed
Chemoselective tetrahydropyranylation of alcohols and phenols using polystyrene supported aluminium chloride as a catalyst
作者:Bahman Tamami、Kaveh Parvanak Borujeny
DOI:10.1016/j.tetlet.2003.11.056
日期:2004.1
A simple, efficient and highly chemoselective method for tetrahydropyranylation of alcohols and phenols has been developed by their reaction with 3,4-dihydro-2H-pyran at room temperature in the presence of a catalytic amount of polystyrenesupported AlCl3. The method is also highly selective for monoprotection of symmetrical diols.