Divergent Diastereoselectivity in the Addition of Nucleophiles to Five-Membered-Ring Oxonium Ions

Furanosyl Oxocarbenium Ion Conformational Energy Landscape Maps as a Tool to Study the Glycosylation Stereoselectivity of 2‐Azidofuranoses, 2‐Fluorofuranoses and Methyl Furanosyl Uronates

“…Woerpel and co‐workers have devised an empirical model to rationalise the stereoselectivity observed in addition reactions to furanosyl oxocarbenium ions. This model takes the two most relevant structures of the ions, that is, the E 3 and 3 E conformers, into account and describes that these will be preferentially attacked form the “inside” of the envelope (see Figure A) . Thus, the former ion is stereoselectively attacked from the bottom face, whereas the latter is substituted on the top face and the population of both conformational states determines the overall stereoselectivity.…”

“…This model takes the two mostr elevant structures of the ions, that is, the E 3 and 3 E conformers, into accounta nd describes that these will be preferentially attacked form the "inside" of the envelope (see Figure 4A). [32,[51][52][53][54] Thus, the former ion is stereoselectively attacked from the bottom face, whereas the latter is substituted on the top face and the population of both conformational states determines the overall stereoselectivity.T he stereoelectronic effects of the ring substituents dic-tate the relative stability of the conformers and the stabilising/ destabilising spatial positions are graphically presented in Figure 4B for the four tri-O-benzylfuranosyl oxocarbenium ions. The oxocarbenium ion conformers are most stable when the positive charge at the anomeric centre is stabilised by C2ÀH hyperconjugation, and by placing the alkoxy substituentsa t C3 and C5 in an orientation that brings the lone pairs of the oxygen atoms closestt ot he anomeric centre.…”

ZnBr2-Catalyzed Insertions of Carbonyl Compounds into Silacyclopropanes: Regiochemical Reversal Dependent on Metal Salt

“…Methyl formate inserted into the more substituted C−Si bond of silacyclopropane 1 a to afford 1,2‐regioisomer 6 (Scheme ) when copper salts were used, but a complete reversal of regiochemistry to give 1,3‐regioisomer 7 a was observed when ZnBr 2 was employed 8, 9. We have shown that oxasilacyclopentane acetals such as 6 and 7 a serve as precursors to oxocarbenium ions that undergo Lewis acid mediated nucleophilic substitution with excellent diastereoselectivities 10, 11…”

Silver‐Catalyzed Silylene Transfer

“…65 The observed diastereoselectivity was rationalized using the Woerpel model for the reaction of reagents with five-membered rings. 98 Fewer destabilizing steric interactions occur from addition of hydrogen to the face opposite the pseudoaxial tert-butyl group (TS-102). When the oxasilacyclopentene bears an olefin substituent (e.g., 100k), a [4 þ 2] cycloaddition dramatically increases the molecular complexity.…”