Vicinal stereochemistry rearrangement

Let s begin with 59 (R=Me). Treatment of this rearrangement substrate with KH provided a 96 4 ratio of 67 + 68. These diastereomers differ in terms of vicinal stereochemistry and enol ether geometry. The major product (67) presumably was formed via rearrangement of the intermediate alkoxide via a chairlike transition state as anticipated. The minor product (68) was presumed to arise from rearrangement via a competing boat-like transition state. This nicely explains the differences in stereochemistry between the observed products (vicinal stereochemistry and olefin geometry). 

This study provided insight into mechanistic aspects of the anion accelerated oxy-Cope rearrangment. It also illustrates how control of terminal olefin stereochemistry in [3.3]-sigmatropic rearrangements can be used to establish vicinal stereochemistry. Note that both 59 (R=Me) and 71 give superb control. Only when there are overriding steric factors (70 and 72) does erosion of stereocontrol begin to occur. 

The stereochemistry of formation and rearrangement of vicinal dihaUdes has been elucidated, chiefly by Barton s group. Trans diaxial addition occurs but the product may then equilibrate with the more stable... 

CHAPTER 7 Inversion of Configuration in the Sn2 Reaction 244 Racemization in the Sn1 Reaction 252 Hydride Shift in an Sn1 Reaction 253 Methyl Shift in an Sn1 Reaction 254 Rearrangement in an E1 Reaction 261 Dehydrohalogenation by the E2 Mechanism 304 Stereochemistry of the E2 Reaction 306 E2 Debromination of a Vicinal Dibromide 310... 

Since 3,6-dihydrothiazine 1-imines exist in conformations having a quasi-axial S—N bond, these imines are properly disposed stereoelectronically for such a pericyclic process. This reaction is essentially irreversible, similar to the equilibrium in a [2,3]-sigmatropic rearrangement of allylic sulfilimines (55), which lies to the side of the sulfenamide (56) (Scheme 11). There is a clean transfer of stereochemistry from C-6 of the dihydrothiazine (52) to C-4 of the thiadiazolidine (53), leading to the (E)-erythro-vicinal diamine derivative (54). In a similar fashion, thermolysis of the epimeric imine (57) gives the thiadiazolidine (58), which can be converted into the Z)-threo-vicinal diamine... 

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