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The feasibility of sequential vinylogous aldol (intermolecular)/silylative aldol (intramolecular) addition reactions involving furano and pyrrole-based dienoxysilanes, 6 and 12, in the synthesis of carbasugar frameworks is illustrated by the preparation of the scantily investigated carbaseptanose and carbaoctanose representatives of this class of compounds. The target compounds, 1, 2, 3, ent2, ent-3, and 4, were obtained from readily available carbohydrate precursors (5 and 19) in yields of 21-30% over 8-12 steps. The irreversible silylative ring-closing aldolisation of γ-substituted dihydro-5H-furan-2-one and pyrrolidin-2-one aldehydes (9, 16, ent-16, and 22) driven by the TBSOTf/ Pr[sup i, sub 2]EtN Lewis acid-Lewis base couple was shown to be a practical, diastereoselective maneuver to forge the densely functionalized, medium-sized core carbocycles. [ABSTRACT FROM AUTHOR]

A diversity-oriented protocol has been developed for the assembly of densely hydroxylated cycloheptane amino acids via succession of a vinylogous Mukaiyama aldol reaction (VMAR), a Morita-Baylis-Hillman reaction (MBHR), and an intramolecular pinacol coupling reaction (IPCR). The plan utilizes D- or L-configured glyceraldehyde derivatives as ‘chirar’ surrogates of glyoxal and N-[(tert-butoxycarbonyl)-2-(tert-butyldimethylsilyl)oxy]pyrrole as the synthetic equivalent of the α,γ-dianion of γ-aminobutanoic acid. The parallel, asymmetric syntheses of four cycloheptane representatives proceed with high diastereocontrol and virtually complete enantioselectivity in ten steps and overall yields of 15–37%. [ABSTRACT FROM AUTHOR]