- 'Conformationally restricted β-amino acid isosteres prepared through regioselectively controlled aza-annulation'
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A variety of electron withdrawing substituents were used to enhance the aza-annulation of enamines with acryloyl chloride, to direct the regioselectivity of alkene formation, and to facilitate hydrogenation of the unsaturated annulation product. The resul
- Paulvannan,Stille
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- Heterocycle Formation through Aza-Annulation: Stereochemically Controlled Syntheses of (+/-)-5-Epitashiromine and (+/-)-Tashiromine
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N-alkenylamines, stabilized through conjugation with an electron-withdrawing group, undergo aza-annulation with acryloyl chloride to provide a convergent route for the construction of six-membered nitrogen heterocycles.In addition to enhancing the C-alkylation process of annulation relative to the competing N-acylation process, the electron withdrawing substituent controlled the regioselectivity of alkene formation in both the intermediate enamine and in the unsaturated lactam product.A variety of functional groups, which include -COMe, -COPh, -CO2R, -CONHPh, -CN, -P(O)(OEt)2, and -SO2Ph, were used to determine the effect of the electron-withdrawing substituents upon both the annulation reaction with acryloyl chloride and the subsequent hydrogenation process.When the enamide annulation product was stabilized through conjugation with ester or amide substituents, catalytic hydrogenation of the aza-annulation product resulted in the formation of vicinal stereocenters with high cis selectivity.The utility of this methodology was demonstrated by application of the condensation/aza-annulation/hydrogenation sequence as the key for construction and stereochemical control of the indolizidine ring system of (+/-)-tashiromine.
- Paulvannan, K.,Stille, John R.
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p. 1613 - 1620
(2007/10/02)
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- Construction of Hydroxylated Alkaloids (+/-)-Mannonolactam, (+/-)-Deoxymannojirimycin, and (+/-)-Prosopinine through Aza-Annulation
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The aza-annulation of β-enamino carbonyl substrates with acrylate derivatives provides an efficient and convenient route for the regioselective construction of δ-lactams.This two-step ring-forming sequence involved initial generation of the benzyl enamine through either a condensation or conjugate addition reaction with BnNH2, followed by aza-annulation with acryloyl chloride or acrylic anhydride.Controlled by the rigid framework of the intermediate lactam, introduction of ring substituents was accomplished with high relative stereoselectivity.The carbonyl functionality, which was necessary to direct the regioselectivity of the aza-annulation reaction, was then transformed into a protected hydroxyl substituent through Baeyer-Villiger oxidation.The resultant δ-lactam product was used as a valuable intermediate in the synthesis of three natural products.Subsequent modification of this δ-lactam gave the naturally occurring α-mannosidase inhibitors (+/-)-mannonolactam and (+/-)-deoxymannojirimycin, while synthesis of the alkaloid (+/-)-prosopinine was accomplished through homologation of the lactam carbonyl.
- Cook, Gregory R.,Beholz, Lars G.,Stille, John R.
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p. 3575 - 3584
(2007/10/02)
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