Anti addition reactions nucleophilic trapping

The Adg2 is the major addition pathway of alkenes and dienes. Electrophile addition to a pi bond or protonation of a pi bond, path Ag, produces the most stable cation, which is then trapped by a nucleophile, path An. (Section 4.4.2 discusses the various addition types.) If the electrophile is a proton, this reaction is the reverse of the El reaction. The reaction commonly produces a mixture of syn and anti addition. 

As was discussed in detail in Sections 4.4.1 through 4.4.4, here are two mechanisms by which acids add to alkenes, the AdE2 and AdgS processes. The reaction is carried out in the dark to minimize free radical side reactions (Chapter 11). In the Ad 2, the proton adds by pathway Ag to produce the more stable carbocation intermediate (Markovnikov s rule) in a second step the cation is trapped by a nucleophile, path An-The Adg2 often gives a mixture of syn and anti addition since the nucleophile can approach the carbocation from either top or bottom face. Adg2 example ... 

Preparation of the temporary connection was achieved by reaction of acetoxy lactam 316 with allyl(chloro)silane 317, generating the A -silyl species 318. Exposure to TMSOTf at 0 °C effected in situ formation of the /V-acyliminium species, which was trapped by the pendant allyl nucleophile, providing two out of the possible four dia-stereoisomeric cyclized products, 319 and 320 in a 4 1 ratio (Scheme 10-102). Formation of the two products may be accounted for by cyclic T. S.s where addition occurs onto the re face of the azetinone, anti to the sterically demanding silyl ether group through a chair-like or boat-like conformation. 

The reaction could be extended to )3-hydroxy esters (Scheme 10-115) [144], In this case, TBAF proved the most effective reagent for providing an almost quantitative conversion to the corresponding alkoxysiladioxanes. Activation of the acetal functionality was best achieved with the superacid [TfOH2B(OTf)4], and trapping with allyltrime-thylsilane provided the anti diol products in excellent yield and stereoselectivity. The outcome is in accord with chelation-controlled intermolecular addition with axial attack of the nucleophile onto the cationic oxonium intermediate. 

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