title: Use of Silicon-Based Tethers to Control Diastereofacial Selectivity in Azomethine Ylide Cycloadditions(1).
abstract: A novel approach to controlling the diastereofacial selectivity of intramolecular dipolar cycloadditions of azomethine ylides (cf. 9 --> 8) by varying the structure a silicon-based tether is described. A correlation is found between the length of the tether dipolarophile conjugate (TDC) and the observed sense of diastereocontrol. Azomethine ylides incorporating longer [OSiPh(2)OCH(2)CH(2)OCOCH=CH(2)], [OSi(i-Pr)(2)OSi(i-Pr)(2)OCH(2)CH=CH(2)], and [OSiPh(2)OCH(2)CH=CH(2)] TDCs favor endo-si attack (14 --> 16, 19 --> 20, and 21 --> 22) while the shorter TDC [OSiR(2)CH(2)CH=CH(2)] leads to a reversal in selectivity favoring the endo-re product (23a,b --> 24a,b). Structures of the cycloadducts have been assigned on the basis of selected X-ray diffraction data in combination with chemical/spectral correlation experiments. The work described herein represents a conceptually new approach to stereocontrol and extends the use of silicon-based tethers in asymmetric synthesis.
authors.affiliation: Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078.
authors.forename: Philip
authors.initials: P
authors.lastname: Garner
authors.forename: Philip B.
authors.initials: PB
authors.lastname: Cox
authors.forename: James T.
authors.initials: JT
authors.lastname: Anderson
authors.forename: John
authors.initials: J
authors.lastname: Protasiewicz
authors.forename: Rebecca
authors.initials: R
authors.lastname: Zaniewski