Addition inverse

For the same reason inverse addition was applied introduction of carbon dioxide into the solution of RCH=C=CHLi has been shown to give bad results note 2. The actual yields (based on MNR spectroscopy) are between 75 and 85%. 

Note 2. Propargyl bromide itself also reacts with ammonia and therefore the interval between the addition of this compound and its conversion with the enethiolate should be kept as short as possible. Inverse addition was applied if propargyl bromide is added to the enthiolate solution, the primary product partly isomerizes into H2C=C(5C2Hs)SCH=C=CH2 under the catalytic influence of the enethiolate. 

Direct oxidation of ethylpyrazines to the corresponding acetylpyrazines may also be carried out in favourable circumstances using hot chromic acid (75JOC1178). Treatment of 2-ethyl-3-alkylpyrazines with chromic acid yields the corresponding 2-acetyl-3-alkyl-pyrazines in yields of 50-70%. In the absence of the 3-alkyl substituent the yields fall dramatically to less than 10%. Acetylpyrazines are more generally prepared by the inverse addition of a Grignard reagent to a cyanopyrazine. 

A variation of the solution technique uses inverse addition of substrate to a well-stirred, fluorine saturated solvent and ultraviolet light to hasten reaction at lower temperatures [2 7]... 

When compound 7 is treated in a 1 1 ratio with phenyhnagnesium bromide using inverse addition, the reaction is sluggish and stops at the ketone stage giving a-benzamidobenzalacetophenone (29) in low yield as the only isolable product. 

An inverse addition of sulfinic acid to a thiocarbonyl group could have taken place with the reactive intermediate 8, which should arise from thiophosgene and methanesulfmic acid (sodium salt)106 (equation 28). The first step of this reaction represents an S-acylation... 

Ziegler and Saprong described a stoichiometric cyclization onto an alkyne for the synthesis of the carbocyclic core of entecavir from diacetone glucose. Inverse addition was required to minimize deoxygenation. The highly diastereoselective reaction is tolerant to silylethers [101]. 

In cases b)-d) it was necessary to add the imidazole-AT-thionocarboxylate to the stannane (inverse addition). 

Variables in the condensation reaction include the temperature, nature of the solvent, order of addition (either chlorosilane to excess sodium or "inverse" addition, sodium to excess chlorosilane), and the rate of addition. A careful study of reaction variables by the Sandia group of Dr. John Zeigler(15) will be described in detail elsewhere in this volume. 

Our initial studies (23) were performed in toluene, and Table I shows the results from the polymerization of a number of representative monomers. The data reported in Table I are for direct addition of the monomer to the sodium dispersion. Inverse addition often leads to higher molecular weights, although the overall polymer yields are usually lower (15,23). The results in Table I show that, under these reaction conditions, a bimodal molecular molecular weight distribution is normally obtained. Furthermore, it is obvious that the crude polymer yields drop precipitously as the steric hindrance in the monomer increases. 

Two examples of pyrimidines 57, 58 were included in a study of direct magnesiation of heterocycles. Both of these metallated selectively at C-4 using inverse addition i.e. substrate added to the metallating agent <06AG(I)2958>. 

The inverse addition of the Grignard reagent to an ethereal solution of the [Pg.50]

If the temperature falls below -60°C, the reaction mixture solidifies but returns to a liquid upon warming. The inverse addition, i.e., addition of chlorotrimethylsilane to bromotrifluoromethane, causes formation of a solid block that impedes stirring. 

Chlorides of sulfonic acids can be reduced either partially to sulfinic acids, or completely to thiols. Both reductions are accomplished in high yields with lithium aluminum hydride. An inverse addition technique at a temperature of —20° is used for the preparation of sulfinic acids, while the preparation of thiols is carried out at the boiling point of ether [69S]. 

The reaction of 3-aminocinnamates 9 with sulfur monochloride leading to tetrasubstituted pyrroles 10 (see Section 2) can be stopped at the formation of 2H-l,4-thiazines 12 by the inverse addition of sulfur monochloride to compound 9. Yields of thiazines 12 may reach 58-77% (1984JOC4780 see Scheme 6). 

Treatment of "skipped" enynes H CCH2CH=CHR with alkali amides presumably will lead to extensive isomerization into C>CCH=CHCH2R. With alkyllithium bases inverse addition has to be applied to avoid dimetallation. This isomerization reaction is likely to be even more serious in the case of the diynes HC=CCH2OCR, since the CH2-protons have kinetic acidities comparable with that of the ethynyl proton. 

---