Calyculin synthetic studies. 4. remarkable reversal of diastereo-selectivity in payne epoxidation of vinyl spiroketal intermediates
作者:Amos B. Smith、Makoto Iwashima
DOI:10.1016/0040-4039(94)88072-7
日期:1994.8
Differential protection of two secondary hydroxyl groups led to a dramatic reversal of diastereofacial selectivity in Payneepoxidation of vinylspiroketals 2a–g, key building blocks in a proposed total synthesis of calyculins A–H.
Iodine monobromide (IBr) at low temperature: enhanced diastereoselectivity in electrophilic cyclizations of homoallylic carbonates
作者:James J. W. Duan、Amos B. Smith
DOI:10.1021/jo00066a024
日期:1993.7
Iodine monobromide affords superior diastereoselectivity in low-temperature electrophilic cyclizations of homoallylic carbonates. Solvent and temperature effects and the scope and limitations of the method are discussed; optimal selectivity is obtained in toluene at -80 to -85-degrees-C. The latter protocol generally furnishes significantly enhanced selectivity, vis-a-vis the original procedure employing 12 in acetonitrile at -20-degrees-C; for example, the IBr-induced cyclization of 14 affords a 25.8:1 mixture of 15 and 16, whereas I2 gives an 8.4:1 ratio. An equilibration experiment established that the diastereoselectivity derives primarily or exclusively from kinetic control of the cyclization process.
A caveat on the Sharpless asymmetric dihydroxylation
作者:Makoto Iwashima、Takeshi Kinsho、Amos B. Smith
DOI:10.1016/0040-4039(95)00274-g
日期:1995.3
Sharpless asymmetric dihydroxylation (AD) of the homochiral synthetic intermediates 2a-c gave anomalous results: pairs of pseudoenantiomeric reagents, expected to generate complementary diastereomer ratios characteristic of double diastereoselection, instead generally furnished indistinguishable product mixtures. AD reactions of related monosubstituted olefins failed to pinpoint the structural features responsible for the unexpected behavior.