Browsing by Author "Prabhakar, Sundaresan"
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- Asymmetric synthesis of N-aryl aziridinesPublication . Aires-de-Sousa, João; Prabhakar, Sundaresan; Lobo, Ana M.; Rosa, Ana M.; Gomes, Mário; Corvo, Marta C.; Williams, David J.; White, Andrew J.The reactions of a variety of N-arylhydroxamates as nitrogen transfer reagents to acryloyl derivatives of (−)-8-phenylmenthol, (−)-quinine and (−)-Oppolzer’s sultam acting as Michael acceptors was studied. Poor to modest diastereoselection was observed in the formation of aziridines. The absolute structure of one of the pure diastereomers secured from Oppolzer’s auxiliary was established by X-ray crystallography and hence the absolute configuration of the derived methyl-N-phenylaziridine-2-carboxylate could be assigned. Whilst only poor facial selectivity was observed for chiral hydroxamic acid prepared from dehydroabietic acid, moderate to good enantioselection of aziridines could be achieved with the chiral quaternary salts based on cinchona alkaloids, especially with that of cinchonine. A model is presented to explain the origin of enantioselection and a mechanism is proposed for the aziridination reaction.
- A dramatic effect of double bond configuration in N-oxy-3-aza Cope rearrangements: a simple synthesis of functionalised allenesPublication . Pinto, Luís; Glória, Paulo; Gomes, Mário; Rzepa, Henry S.; Prabhakar, Sundaresan; Lobo, Ana M.The first examples of low temperature N-oxy-3-aza Cope rearrangements, leading to functionalised allenes are described, where the Z-configuration of the enaminic double bond in the rearranging system proves critical.
- Intramolecular addition of acyldiazenecarboxylates onto double bonds in the synthesis of heterocyclesPublication . Prata, José V.; Clemente, Dina-Telma S.; Prabhakar, Sundaresan; Lobo, Ana M.; Mourato, Isabel; Branco, Paula SérioAppropriate aryl-substituted unsymmetrical azodicarbonyl compounds, generated from bishydrazides by oxidation, undergo intramolecular cyclisations to furnish a variety of useful heterocycles such as N-substituted oxindoles, carbostyrils, benzazepinones, benzazocinones, benzimidazolones, benzoxazinones and pyrazolones in varying degrees of efficiency. Methods are described to remove the N-acyl groups from the heteroaromatic compounds. Under mildly acidic conditions where equal opportunities are available for an ipso or a normal cyclisation it is the former process that occurs preferentially. Evidence is presented in favour of a C-to-C migration in the ipso product for the formation of a methoxy-substituted carbostyril derivative. One of the spiro substances is shown to participate in dienone–phenol rearrangement to provide the corresponding quinolone–phenol in high yield.
- SbCl5—wet acetonitrile: a new system for chemoselective O-desilylationPublication . Glória, Paulo M. C.; Prabhakar, Sundaresan; Lobo, Ana M.; Gomes, MárioAbstract—A new efficient method for deprotection of TBDMS derivatives of phenols, primary alcohols, carboxylic acids and secondary amines, consisting of SbCl5 and MeCN with 0.1% water (w/v), is reported. It effects inter alia desilylation of a CH2OTBDMS group in the presence of a ketal function.
- Studies in 3-oxy-assisted 3-aza Cope rearrangementsPublication . Gomes, Mário; Sharma, Lalit; Prabhakar, Sundaresan; Lobo, Ana M.; Glória, PauloOn thermolysis appropriately substituted N-silyloxy-N-allyl enamines undergo smooth 3,3-sigmatropic rearrangments to the corresponding N-silyloxy imino ethers.