Chelation and Stereodirecting Group Effects on Regio- and Diastereoselective Samarium(II)-Water Allylic Benzoate Reductions

Abstract

SmI2(H2O)n reductions of allylic benzoates adjacent to a trisubstituted alkene occur in high yields with complete regioselectivity and good diastereoselectivity (up to 90:10) for substrates containing properly positioned stereodirecting- and chelating groups. The outcome of these reactions can be rationalized by ring conformation considerations of a proposed chelated organosamarium intermediate, and a mechanism involving intramolecular protonation by a samarium-bound water.

Hydroxyl-Directed Regio- and Diastereoselective Allylic Sulfone Reductions with [Sm(H2O)n]I2

“…The existing stereocenter directs the facial selectivity of intramolecular protonation by a samarium-bound water. This model is consistent with the stereochemistry of the newly formed asymmetric carbon atom (*) for the major diastereomers obtained. − …”

“…Interestingly, 3a was obtained from the OBz analogue in nearly identical d.r. (6.1:1) and with the same sense of diastereoselection, supporting the formation of a common intermediate from the two substrate classes. The stereochemistry of the major diastereomer of 3a was previously determined when produced from the OBz analogue .…”

“…(6.1:1) and with the same sense of diastereoselection, supporting the formation of a common intermediate from the two substrate classes. The stereochemistry of the major diastereomer of 3a was previously determined when produced from the OBz analogue . By analogy, we assume that the stereochemistry of the major diastereomer for the products contained in Table is as drawn.…”

“…Reduction of 6 proceeded with the same diastereoselectivity as its OBz analogue 13 5 (Scheme 5), giving 9 in both cases as an approximately 70:30 mixture of diastereomers. 5 Previously the major diastereomer produced in these reactions was identified as the 2R,6S-isomer, 5 explained by a mechanism involving formation of intermediate Sm−IV where the methyl group (Me) assumes a preferred equatorial position. 26 Formation of 9-d when D 2 O was used further supports this proposed mechanism.…”

“…Our group has been exploring a samarium diiodide (SmI 2 ) mediated elimination/isomerization process for asymmetric carbon atom synthesis. − We have found that upon treatment with SmI 2 and water ([Sm­(H 2 O) n ]­I 2 ), , substrates of type A are transformed into alkene products B in high yield and in some cases high diastereoselectivity (up to 90:10; Scheme ). The reaction is thought to proceed through a chelated organosamarium intermediate ( Sm–I ), formed upon the reduction and loss of the benzoyl ester (OBz).…”