Regioisomeric alternative

Dimethylborane+propene C2 and 2-propyldimethyl borane depict the regioisomeric transition state and addition product. Calculate the energies of these species relative to those of the alternative transition state and product. Given these energy differences, and the experimental observation that this addition is almost completely selective for the anti-Markovnikov product, does it appear that this reaction is under kinetic or thermodynamic control Explain. 

We have found in our studies that the use of the lithium counter ion in the base is essential for successful reaction with regiochemical control and allows cyclization to proceed under mild thermal conditions. In contrast, use of sodium (NaH, NaN(SiMe3)2) or potassium (KH, KN(SiMe3)2) bases require elevated temperatures, and results in poor yields of the desired product, and a mixture of several by-products, including the regioisomeric 4-hydroxymethy-2-oxazolidinone,6 resulting from alternate processes. Thus, the lithium ion plays a very important role in the mechanism of this reaction. 

An alternative explanation is that the regioselectivity of insertion (180 vs. 181) is determined by the rate of 1,2-metallate rearrangement, the formation of the regioisomeric ate complexes 176 and 177 being fast and reversible (Scheme 3.39). Interconversion of 176... 

The enedione (283) is a useful starting material for a two-step synthesis of 2,3,4,5-tetrasubstituted furans which are not otherwise readily accessible (81JCS(P1)2398). Michael addition of an active methylene compound, e.g. (284), to the enedione (283) led to the two regioisomeric adducts (285) and (286) which could then be cyclized to furans (287) and (288) under mild conditions (Scheme 75). The formation of Michael adducts was successful with both j8-ketoesters (284) and cyclic 1,3-diones. Normal routes to furans require much more drastic conditions the Michael addition allowed the preparation of 1,4-diones particularly activated by the presence of an easily enolizable group. This is a useful synthetic pathway because alternative routes for the preparation of complex furans are not at present available. 

Alternatively, the (two) regioisomeric elimination products of H/Het from —Het could... 

Alternatively, the (two) regioisomeric elimination products of H/Het from Rsec—Het could also have the same number of alkyl substituents on their C=C double bonds. In such a case the regioselective production of one olefin at the expense of the other... 

The detethering of one such trans cycloadduct (208), containing a tertiary ether linkage, was carried out. An El process, mediated by a Lewis acid, effected selective C-0 bond cleavage to give the most stable of the alternative regioisomeric carbo-cations, and finally olefin (209) (Scheme 54). 

Epoxide ring opening is also an alternative route for the introduction of a phosphorus atom into carbohydrates. Thus, the U-allo epoxide 72 reacts with LiPPh2, giving an intermediate phosphine which is spontaneously oxidized to the 2-phosphonate 187 [125], whereas the isomeric D-manno epoxide 188 affords the regioisomeric 3-phosphonate 189 (O Scheme 80) [124]. 

The alternative compound 17, with the nucleophile placed at the C-4 position, was prepared hy the same method [19] from regioisomeric (to Cemy epoxide) oxirane 18, which was obtained from D-glucal by iodocyclization followed by standard reactions (O Scheme 7). 

Reaction of dienoaldehyde with the stabilized Wittig reagent(s) provided a triene 88, which upon high-pressure Diels-Alder cyclization furnished bicyclo[4.3.0]nonene [78]. Alternatively the aldehyde S7-E was converted into the methyl ester 89, which reacted with the dimethyl methylphosphonate anion affording the phosphonate 90. Reaction of the latter with aldehydes under mild PTC conditions yielded a (regioisomeric to the previous one) triene 91, which cyclized spontaneously to bicyclo[4.4.0]decene (O Scheme 37) [79]. 

An alternative regioisomeric structure is possible to the structure shown for 9 for each reactant. 

A dihydropyran was formed as a minor product. Photolysis leads to the cleavage of the alternative cyclopropene single bond and cyclization to regioisomeric indenes. " ... 

These various approaches, however, require vigorous experimental conditions which may not be appropriate for the synthesis of polyfunctional oxanthrenes. Moreover, the formation of a mixture of regioisomeric products (often very difficult to separate) when each of the coupling partners is unsymmetrical presents an additional problem. Activation of 1,2-dichlorobenzene by complexation with tricarbonylchromium solves the former problem. Thus, the reaction of this activated dichlorobenzene with an excess of the potassium salt of the catechol gave a good yield of the oxanthrene at relatively low temperature <86ZORi57>. An alternative method, which uses a cationic (cyclo-... 

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