Ring-opened by-products

The reaction of a,/3-unsaturated carbonyl compounds with enamines also leads to dihydropyrans, although it is not always possible to isolate these since they react further to give either ring-opened by-products or bicyclic derivatives arising from a Stork annelation. There has been considerable discussion on the mechanism of this reaction, although the initial nucleophilic attack of the enamine on the /3-carbon of the diene is not in doubt (63JA207). It is possible that a zwitterionic species is involved, either as an intermediate or merely in equilibrium with the dihydropyran (67JCS(C)226). 

Solvent system° Catalyst mass (g) Propylene conversion (%) PO (%) Propylene C3H8 (%) selectivity Ring-opened by-products (%)... 

No PO ring-opened by-products were detected by gas chromatography (GC). 

In our systems the nitration gave good conversions and by avoiding the ring-opened by-products the selectivity was superior to the batch process. 

Cooxidation of the ketone product via abstraction of a weakly bound aH-atom was, in the case of cyclohexane oxidation, assumed to be the predominant source of ring-opened by-products such as adipic acid, glutaric acid, and 6-hydroxy caproic acid [4, 5]. This hypothesis was based on two wrong assumptions. 

Formation of Ring-Opened By-Products in the Case of Cyclohexane Oxidation... 

Figure 1.5 Competing reactions for the cyclohexoxyl radical formation of the primary ring-opened by-products [22].

Y 95%. The reaction is fast, high-yielding, and especially suitable for acid-sensitive compds. in the absence of NaHCOj some ring-opened by-products formed. Site- and face-selectivity under these heterogeneous conditions aie the same as in homogeneous media. F.e.s. F. Fringuelli et al., Tetrahedron Letters 30, 1427-8 (1989). 

Birch reduction of more heavily substituted thiophene carboxylic acid salts proceeded readily to afford the corresponding substituted 2,5-dihydrothiophenes however, the ring-opened by-product (Z)-5-mercapto-3-pentenoic acid was still detectable. Reduction of dilithium 3,4-dimethylthiophene-2,5-dicarboxylate also... 

When 6-diazopenicillanates are irradiated in the presence of sulfur nucleophiles, predominantly 6/3-substitution products are obtained (77JOC2224). When BFs-EtiO is used to catalyze the reaction with nucleophiles, however, the products are primarily the 6a-isomers (78TL995). The use of rhodium or copper catalysis led primarily to ring-opened thiazepine products, presumably by way of the intermediate (56 Scheme 39) (80CC798). 

Sasidharan et al. (258) reported the formation of pinacols from alkenes catalyzed by various titanosilicates. Aluminum-free Ti-beta exhibited better activity and pinacol selectivity than TS-1, TS-2, Ti-MCM-22, or mesoporous Ti-MCM-41 (Table XLVIII). The side products included the pinacolone, alcohol, and oligomers. The epoxide was the initial product, which underwent acid-catalyzed nucleophilic ring-opening by H2Q molecules to give the pinacol (Scheme 25). 

Ring-opening by azide and the subsequent work-up is not ideal from a point of view of safety and costs and ring-opening by other nucleophiles is attractive. Furthermore, the use of meso compounds restricts the use of the catalyst to a small number of substrates and useful products. 

Treatment of the /3-keto ester 220 with sodium ethoxide at elevated temperature triggered off an epoxide ring opening by / -elimination that was followed by the desired Knoevenagel condensation to afford the tricyclic product 206 (Scheme 34). The enone moiety in the intermediate 221 did not show a propensity for deprotonation and, therefore, the ketone carbonyl function of the enone moiety was available for a Knoevenagel condensation. The reduction of the p-keto ester (206) to the corresponding diol was the next objective. Treatment of the TES-protected -keto ester (TES-206) with DIBAH afforded the diastereomeric diols 222 and 223 in a moderate diastereoselec-tivity in favour of the undesired diastereomer 222. The diastereomers were separated and the undesired diastereomer 222 was epimerized to 223 by a sequence that consists of Mitsunobu inversion and benzoate ester reduction [98, 99]. 

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