Lewis acid induced reduction

The synthesis of the right-wing sector, compound 4, commences with the prochiral diol 26 (see Scheme 4). The latter substance is known and can be conveniently prepared in two steps from diethyl malonate via C-allylation, followed by reduction of the two ethoxy-carbonyl functions. Exposure of 26 to benzaldehyde and a catalytic amount of camphorsulfonic acid (CSA) under dehydrating conditions accomplishes the simultaneous protection of both hydroxyl groups in the form of a benzylidene acetal (see intermediate 32, Scheme 4). Interestingly, when benzylidene acetal 32 is treated with lithium aluminum hydride and aluminum trichloride (1 4) in ether at 25 °C, a Lewis acid induced reduction takes place to give... 

We invoke 7c-stacking of the alkene with a phenyl moiety on the silicon protecting group (since this high degree of selectivity was only observed for TBDPS and not with TBS), in the precursor to explain this remarkable selectivity (Figure 1). Lewis acid induced reduction of the epoxide with sodium cyanoborohydride led regioselectively to the 1,3-diol (11) the hydride attacks the more substituted position via an S 2 mechanism. ... 

Although 1,3,2-diazaphospholenium cations are usually prepared from neutral NHPs or 1,3,2-diazaphospholes via Lewis-acid induced substituent abstraction or A-alkylation, respectively (cf. Sect. 3.1.2), the group of Cowley was the first to describe a direct conversion of a-diimines into cationic heterocycles by means of a reaction that can be described as capture of a P(I) cation by diazabutadiene via [4+1] cycloaddition [31] (Scheme 4). The P(I) moiety is either generated by reduction of phosphorus trihalides with tin dichloride in the presence of the diimine [31] or, even more simply, by spontaneous disproportionation of phosphorus triiodide in the presence of the diimine [32], The reaction is of particular value as it provides a straightforward access to annulated heterocyclic ring systems. Thus, the tricyclic structure of 11 is readily assembled by addition of a P(I) moiety to an acenaphthene-diimine [31], and the pyrido-annulated cationic NHP 12 is generated by action of appropriate... 

Another Japanese group developed the Taxol synthesis shown in Scheme 13.47. The eight-membered B ring was closed early in the synthesis using a Lewis acid-induced Mukaiyama reaction (step B-l). Note that a trimethylsilyl dienol ether served as the nucleophile. The C-19 methyl group is introduced via a cyclopropanation in step C-5, followed by a reduction in step D-l. 

Treatment of 2,3 Cpoxy-l-amines with Lewis acid induces a rearrangement to aziridinium ions that react efficiently with a nucleophiles to give functionalized hydroxy sulfides or hydroxy amines (Equation 23) <1997SL11>. Under the influence of ethylaluminium chloride, an epoxide tethered to an azide undergoes Lewis acid-assisted cyclization followed by an intramolecular Schmidt reaction and subsequent in situ reduction of the intermediate iminium species upon addition of sodium borohydride (Scheme 8). This protocol was used as a key step in a novel synthesis of indolizidine alkaloids of pharmaceutical interest <20030L583, 2004JOC3093>. 

The Lewis acid-induced acylation of unsaturated fatty compounds combined with the following reactions allows the synthesis of natural products derived from fats. Acylation of 10-undecenoic acid [2a] with heptanoyl chloride gave 12-oxo-9-octadecenoic acid reduction with NaBHq afforded the racemate of ricinelaidic acid [6], a natural product (Fig. 2). 

Kakuda S, Rolle CJ, Ohkubo K, Siegler MA, Karlin KD, Pukuzumi S (2015) Lewis acid-induced change from four- to two-electron reduction of dioxygen catalyzed by copper complexes using scandium triflate. J Am Chem Soc 137 3330-3337... 

According to Ref [4], a radical cyclization process has to be envisaged, in which redoxactive Cp2TiCl participates Mn plays the role of a reductant regenerating the Ti(III) species in the catalytic cycle.By this cyclization method, tetrahydrofuran (TH F) derivatives have also been prepared. Compare the LEwis-acid-induced tandem oxacycUzation/oxepane formation in Ref [5]. 

A highly concise synthesis of ( )-epi-zephyrathine (310) was also achieved from the initially formed oxabicycUc intermediate (Scheme 13.69) [112]. Thus, the initially formed oxabicyclic compound 301a was subjected to a Lewis acid-induced ring opening in the presence of acetone to furnish acetonide 308. This was followed by f-Boc-depro-tection and coupling with benzoyl chloride. A subsequent radical cychzation, reduction, and acidic workup afforded the desired target molecule 310 in seven steps and in 15% overall yield. 

A second question involves the influence of ligands on the rate and selectivity of the Lewis-acid catalysed Diels-Alder reaction in water. In Chapter 3 we have demonstrated that nearly all the ligands studied induce a significant decrease in the affinity of the catalyst for the dienophile. This effect is accompanied by a modest reduction of the rate of the Diels-Alder reaction of the ternary dienophile -catalyst - ligand complex. 

The reaction processes shown in Scheme 8 not only accomplish the construction of an oxepane system but also furnish a valuable keto function. The realization that this function could, in an appropriate setting, be used to achieve the annulation of the second oxepane ring led to the development of a new strategy for the synthesis of cyclic ethers the reductive cyclization of hydroxy ketones (see Schemes 9 and 10).23 The development of this strategy was inspired by the elegant work of Olah 24 the scenario depicted in Scheme 9 captures its key features. It was anticipated that activation of the Lewis-basic keto function in 43 with a Lewis acid, perhaps trimethylsilyl triflate, would induce nucleophilic attack by the proximal hydroxyl group to give an intermediate of the type 44. 

A catalytic system consisting of cat. Sml2, Zn/Hg, Lil, and Me3SiOTf induces spirolactonization (Scheme 20) [56]. Me3SiOTf plays a similar role in converting the intermediary alkoxides to the silyl ethers. The efficacy of Lil depends on the formation of Sml3 from Sm OTf, which facilitates reduction by Zn/Hg. The Lewis acidity of Zn(II) is reduced by conversion to a non-Lewis-acidic species such as Li2Znl2(OTf)2. 

The above observations strongly indicate that O-protonation is an important step in this particular reaction for the reduction of coordinated CO. Recent studies in our laboratory provide other examples of proton induced reduction in metal cluster systems, and an example of proton induced CO reduction has recently been reported by Atwood (44). It thus appears that protons as well as Lewis acids are effective in the bifunctional activation of coordinated CO. 

Our research on CO activation is supported by the NSF. In addition to members of my group and colleagues who are mentioned in the text, I appreciate the contributions of former co-workers Dr. Hamdallah Hodali, who discovered the O-protonation of CO, and Dr. Norvell Nelson who discovered the Lewis acid attack of CO. Work on the proton induced reduction of CO has been aided by the exchange of information on iron butterfly compounds with Professor Earl Mutterties, Dr. John Bradly and Dr. Jack Williams. 

The depletion width can play a role in analyte-induced modulation of the semiconductor PL [4]. As molecules adsorb onto the surface of the semiconductor, the dead-layer thickness can change, resulting in what can be described as a luminescent litmus test When Lewis bases adsorb onto the semiconductor surface, they donate electron density to the solid, which decreases the electric field and thus decreases the dead-layer thickness. The reduction in D causes an enhancement in the PL intensity from the semiconductor. Figures 2a and 2b present typical PL enhancements observed from an etched n-CdSe substrate Relative to a nitrogen reference ambient, adsorption of the Lewis bases ammonia and trimethylamine cause a reversible increase in PL intensity. In contrast, when Lewis acids adsorb onto the surface, they can withdraw additional electron density, causing the electric field to increase and the PL intensity to decrease. Such effects have been observed with gases like sulfur dioxide [5]. 

Extension of the scope of the method by full control of the stereoselectivity failed because the use of Lewis acids as chelating agents to induce the syn addition of 25a to aldehydes produced instead a substantia] desilylation of the reagent. A remedy to this limitation was provided by the conversion of the anti adduct into the syn isomer by an oxidation-reduction sequence [43J (Scheme 10). 

---