Stereospecific transformations eliminations

It may be of interest to note that the stereospecific transformation shown in equation 15 has been cited as the first reported observation of an 1 - 3 chirality transfer. It is evident that on rearrangement of optically active 6d to 7d, the chiral center at C-a is eliminated and a new one created at C-y. The term self-immolative asymmetric synthesis has also been used to describe syntheses of this kind. As pointed out by Hoffmann , quantitative 1 - 3 chirality transfer will follow from the suprafacial - course of rearrangement, provided the reactant has a uniform configuration at the j8, y-double bond. This stereochemical prediction has also been confirmed by the results obtained in several other [2,3]sigmatropic rearrangements, subsequently reported " . 

The oxidation of sulfides to sulfoxides is a facile transformation for which many reagents have been employed in the literature. These include hydrogen peroxide, ozone, nitric acid, chromic acid or tert-butylhypochlorite. Here, meto-chloroperbenzoic acid is used to oxidize a-phenylthio ketone 14, and successive elimination of the resulting a-phenylsulfinyl ketone 15 by heating at reflux in benzene provides the or,ji3-unsaturated ketone 16. Because of their thermal instability sulfoxides easily undergo elimination. The mechanism is explained by Cram as stereospecific cw-elimination. ... 

In addition to the synthetic applications related to the stereoselective or stereospecific syntheses of various systems, especially natural products, described in the previous subsection, a number of general synthetic uses of the reversible [2,3]-sigmatropic rearrangement of allylic sulfoxides are presented below. Several investigators110-113 have employed the allylic sulfenate-to-sulfoxide equilibrium in combination with the syn elimination of the latter as a method for the synthesis of conjugated dienes. For example, Reich and coworkers110,111 have reported a detailed study on the conversion of allylic alcohols to 1,3-dienes by sequential sulfenate sulfoxide rearrangement and syn elimination of the sulfoxide. This method of mild and efficient 1,4-dehydration of allylic alcohols has also been shown to proceed with overall cis stereochemistry in cyclic systems, as illustrated by equation 25. The reaction of trans-46 proceeds almost instantaneously at room temperature, while that of the cis-alcohol is much slower. This method has been subsequently applied for the synthesis of several natural products, such as the stereoselective transformation of the allylic alcohol 48 into the sex pheromone of the Red Bollworm Moth (49)112 and the conversion of isocodeine (50) into 6-demethoxythebaine (51)113. 

The series of reactions leading to the 5-silyl-l-pentene - epoxidation, ring expansion, and Peterson elimination -are all stereospecific. Therefore, epoxides with different geometry can be transformed into the corresponding (E)- or (Z)-olefinic silanols <1994BCJ1694, 1991TL4545>. Subsequent Tamao oxidation affords stereodefined pentenols. 

Isotopic 0 labeling experiments suggested that the formation of 5-HPETE occurs via radical trapping of oxygen. The 5-HPETE is then transformed into the (5S, 6S)-epoxide LTA4 by a loss of water, involving stereospecific elimination of a C-10 hydrogen from 5-HPETE. ... 

Stereospecificity is a measure of the mechanistic purity of a kinetically-controlled transformation (inversion vs. retention, syn- vs. anif-addition, anti- vs. syn-elimination). In all of the examples of Figures 17.37-17.40, the outcome of a syn addition to a transoid alkene is considered to be equivalent to an anti addition to a cisoid alkene. Stereoselectivity characterizes the relative proportions of the stereomeric products in any given transformation, regardless of the mechanistic purity of the transformations. 

The transformation was found to be stereospecific since [Dj]-trans-215a reacted with (2-naphthyl)magnesium bromide to give [Djj-216 with complete retention of the deuterium label. Under the same reaction conditions, the deuterium atom was completely eliminated from [Dj ]-ds-215b resulting in the unlabeled arylated alkene 216 and [Djj-naphthalene, which is consistent with a possible pathway outlined in Scheme 10.70. 

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