ISSN:
0009-2940
Keywords:
Nucleophilic acylation
;
Umpolung
;
Regioselectivity
;
Oxy-;Cope rearrangement
;
Chemistry
;
Inorganic Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
Trimethylsilyl Cyanide - A Reagent for Umpolung, XXI. - Regioselectivity of the Reaction between Substituted α-Siloxy-α-cyanoallylic Anions and α,β-Unsaturated KetonesAmbident allylic anions A and ambident α,β-unsaturated ketones B combine to the four isomeric products D - G (Scheme 1). Systematic variation of substituents in A, obtained by deprotonation of 1a - c with LDA, and in B (→ 2a - d) as well as variation of the reaction medium and temperature reveals the following results: In diethyl ether (DE) at -78°C the kinetic products 3 (1,2/α) prevail, except with 2d. On warming rearrangement occurs to the thermodynamic products 4 (1,4/α) and/or 6 (1,4/γ) depending strongly on the substituents in both 1 and 2 (Tables 1 - 3). Adducts 6 (1,4/γ) are supposed to be formed by anionic oxy-Cope rearrangement, since the isolated products 4 rearrange thermally to 6 in rates which reflect the yields of 6 isolated (Table 7). Substitution of DE by THF (and addition of HMPT) strongly favours adducts 4 and 6 (Table 4). With 1a + 2c a series of Lewis acids was also tested (Table 5). With MgBr2 the so far missing 1,2/γ product 5ac becomes the major one. In contrast, ClTi(NEt2)3 preserves the kinetic 1,2/α product 3ac totally even at 0°C. This behaviour is found also with other combinations of 1 + 2 (Table 6). By proper choice of the conditions the reaction between a given pair of 1 and 2 can be steered to produce one of the four possible adducts predominantly.
Additional Material:
7 Tab.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1002/cber.19931260127
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