Stepwise acid-promoted

Considering the Woodward-Hoffinann rules, if the conrotatory six-electron ring closure is allowed under photochemical conditions, it must be forbidden under thermal conditions. In consequence, the cyclization of 1 to tra 5-dihydronaph-thalene 3 in the presence of triflic acid should not be a concerted process. Therefore, in this case, it is more reasonable to think of a stepwise acid-promoted mechanism for the cyclization, possibly involving carbocations as intermediates. 

According to Kiirti and Czako,36 there is an important interaction between the HOMO of the ene component CH2=CH-C-H and the LUMO of the enophile. In agreement with this interpretation, the reaction is favored by electron-withdrawing substituents on the enophile. However, the rate is also enhanced if electrons are withdrawn from the ene component. For example, according to the same authors, the mechanism of the Lewis acid promoted ene reaction is believed to involve both a concerted and a cationic pathway.37 Whether the mechanism is concerted or stepwise, a partial or full positive charge is developed at the ene component in Lewis acid promoted ene reaction.36 Explain this contradiction. 

Two general routes are available for the synthesis of calixarenes the base-and acid-promoted one-step synthesis and the convergent stepwise synthesis (fragment condensation).2,3... 

As described in the sections above, it is well established that reactions of Lewis acid-activated aldehydes and ketones with silyl enolates afford -hydroxy or /7-sil-oxy carbonyl compounds (Mukaiyama aldol reactions). Occasionally, however, ene-type adducts, that is /-siloxy homoallyl alcohols, are the main products. The first example of the carbonyl-ene reaction of silyl enolates was reported by Snider et al. in 1983 [176]. They found that the formaldehyde-MesAl complex reacted smoothly with ketone TMS enolates to give y-trimethylsiloxy homoallyl alcohols in good yield. Yamamoto et al. reported a similar reaction of formaldehyde complexed with methylaluminum bis(2,6-diphenylphenoxide) [177]. After these early reports, Kuwajima et al. have demonstrated that the aluminum Lewis acid-promoted system is valuable for the ene reactions of several aldehydes [178] and for-maldimine [179] with silyl enolates bearing a bulky silyl group. A stepwise mechanism including nucleophihc addition via an acyclic transition structure has been proposed for the Lewis acid-promoted ene reactions. 

Cerium(iv).— The acid-promoted redox decomposition and cerium(iv) oxidation of the tris(oxalato)cobaltate(in) ion have been studied in aqueous acid media. In IM sulphuric acid, in the absence of oxidant, there occurs an induction period prior to the internal redox decomposition of the anion. On addition of the cerium(iv), however, there results an increased rate of reduction of the cobalt(ni) centre in contrast to the behaviour of this oxidant to M(C20 ) complexes where M = Cr, Rh, or Ir. The induction in the add-catalysed decomposition is consistent with the formation of a unidentate oxalato-complex-ion which may be the main route towards the stepwise reduction to yield Co and COg. From spectral studies on the total expected absorbance values on mixing, it would appear that the cerium(iv) ion is involved in the pre-equilibrium formation of a dinuclear species which might undergo internal electron transfer with reduction to cerium(in). A possible mechanism in this system may then be written as shown in Scheme 5 (ox = C2O4 -). The variations in rate of the one-electron redox reactions of this type are dependent on the nature of the activated complex, which may differ from one metal centre to another in respect of the number of protons and sulphate anions incorporated. 

Lewis acid catalysis has been used to promote stepwise [2 + 2] cycloaddition of silyl enol ethers and unsaturated esters.178 The best catalyst is (C2H5)2A1C1 and polyfluoroalkyl esters give the highest stereoselectivity. The reactions give the more stable trans products. 

Normally, no catalyst is needed to promote the reactions. But Lewis acids may catalyze many forms of pericyclic reactions, either directly or by changing the mechanism of the reaction so that it becomes a stepwise process and hence no longer a true pericyclic reaction. 

Annulation to carbonyl functions has also been achieved with Trost s bifunctional reagents. Whereas the parent silyl acetate (97) yields only simple alkylation products with aldehydes under normal conditions, addition of only a few mole % of trimethyltin acetate to the reaction mixture results in facile formation of methylenetetrahydrofurans Furthermore, excellent diastereoselectivity is observed in the cycloaddition to a galactose-derived aldehyde (125) (equation 136). The tin acetate co-catalyst also promotes addition to relatively unreactive ketone carbonyls, such as in the case of benzofuran (126) and the acetylenic ketone (127) (equations 137, 138). It is remarkable that even the sterically hindered enone (128) reacts preferentially at the ketone function (equation 139). A tributyltin analog (129) of (97) has been used in the stepwise formation of a methylenetetrahydrofuran from aldehydes. Similarly, pyrrolidines can be prepared from the corresponding imines in two steps via a Lewis acid-catalyzed 1,2-addition of the tin reagent, which is then followed by a Pd-catalyzed cyclization (equation 140). Direct formation of pyrrolidine from the imine is possible if one uses a mesylate analog of (97) and a nickel(O) catalyst (equation 141). ... 

Similarly, Crespi-Perellino et al. (13,15), using cell cultures of A. altissima and providing L-, D-, and D,L-[w(7Av/cz c- C lryptophan as the precursor, carried out tracer experiments and proved the biosynthetic pathway to canthin-6-one alkaloids to be as follows (Scheme 7) tryptophan )8-carboline-l -propionic acid — 4,5-dihydrocanlhin-6-one (29) canthin-6-one (1)—> l-hydroxycanthin-6-one (10) l-methoxycanthin-6-one (11) —> l-methoxycanthin-6-one 3-oxide (12). In the biosynthetic pathway to canthin-6-one alkaloids, oxidation proceeds stepwise. The hydroxyl group at position 1 of canthin-6-one is methylated, and 11 is readily formed this formation is considered to be a transmethylation promoted by a specific enzyme. 

The reaction of succinimide derivatives wifh catalytic amounts of Al(OTf)3 proceeded via several stepwise reactions, resulting in ring-enlargement (Scheme 6.157) [201]. Both bond-breaking and bond-making were assumed to be readily promoted by the strong Lewis acidity of Al(OTf)3. These involve intramolecular rearrangement of fhe C-N bond consistent with attack at fhe central carbon atom of the allenyl intermediate. 

The identification of the cis-acting sequences and protein factors involved in ribosomal DNA transcription strongly suggested that the network of protein-protein and protein—DNA interactions among UBF, SL1, and the promoter elements plays a major role in the assembly of a stable preinitiation complex. A number of studies have established that UBF and SL1 play a key role in this process and are necessary to direct a high level of RNA Pol I transcription in vitro. The current stepwise model of factors assembly predicts that the binding of UBF dimer to the UCE and CORE elements is a prerequisite for the recruitment of the selectivity factor SL1 to the rDNA promoter (Fig. 4, step I). Biochemical studies have indicated that two subunits of SL1, TAFi-18 and TBP, interact directly with the carboxy-terminal acidic domain of UBF (Beckmann et al., 1995 Tuan et al., 1999 step II). This finding demonstrates that the function of the carboxyl-terminal activation domain of UBF is to recruit the essential... 

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