4-A molecular sieves

Bu4N F, DMPU, 4 A molecular sieves, 45-80°, 80-95% yield. These conditions were especially effective in cleaving tertiary SEM derivatives and avoid the use of the toxic HMPA. 

Reaction of (25,55)- and (2i ,55)-2-[5-(terc-butyldimethylsilyloxy)piper-idin-2-yl]ethanols with phenylglyoxal monohydrate in the presence of 4 A molecular sieves in boiling CH2CI2 overnight gave (15,4n5,7i )- and (2i ,4ni ,7i )-l-benzoyl-7-(terc-butyldimethylsilyloxy)perhydropyrido[l,2-c][l,3]oxazines 97 and 100, respectively, in good yield (99MI19). Both products were accompanied by unidentified minor isomers. 

Chiral salen chromium and cobalt complexes have been shown by Jacobsen et al. to catalyze an enantioselective cycloaddition reaction of carbonyl compounds with dienes [22]. The cycloaddition reaction of different aldehydes 1 containing aromatic, aliphatic, and conjugated substituents with Danishefsky s diene 2a catalyzed by the chiral salen-chromium(III) complexes 14a,b proceeds in up to 98% yield and with moderate to high ee (Scheme 4.14). It was found that the presence of oven-dried powdered 4 A molecular sieves led to increased yield and enantioselectivity. The lowest ee (62% ee, catalyst 14b) was obtained for hexanal and the highest (93% ee, catalyst 14a) was obtained for cyclohexyl aldehyde. The mechanism of the cycloaddition reaction was investigated in terms of a traditional cycloaddition, or formation of the cycloaddition product via a Mukaiyama aldol-reaction path. In the presence of the chiral salen-chromium(III) catalyst system NMR spectroscopy of the crude reaction mixture of the reaction of benzaldehyde with Danishefsky s diene revealed the exclusive presence of the cycloaddition-pathway product. The Mukaiyama aldol condensation product was prepared independently and subjected to the conditions of the chiral salen-chromium(III)-catalyzed reactions. No detectable cycloaddition product could be observed. These results point towards a [2-i-4]-cydoaddition mechanism. 

Although trityl perchlorate is used to accomplish the glycosidation of the C-8 hydroxyl in 44 with acetoxy glycoside 49, control experiments have demonstrated that no reaction takes place in the presence of 4 A molecular sieves or 2,6-di-terf-butylpyridine. This observation suggests that the actual catalyst is not trityl perchlorate, but perchloric acid. Consistent with this conclusion is the observation that catalytic amounts of a strong Brpnsted acid such as triflic or perchloric acid can catalyze the glycosidation of 44 with 49 in the absence of trityl perchlorate. 

The checkers purchased [(COD)RuCl2]n from Fluka (purum quality), BINAP from Aldrich (97 %), and toluene (HPLC grade) and triethylamine (reagent grade) from Fisher Scientific the latter was distilled from CaH2 under Ar prior to use. The submitters dried toluene and triethylamine over 4 A molecular sieves. Karl-Fischer titration indicated <200 pg/mL water. 

A very mild oxidative transformation of nitro compounds into ketones using tetrapropylam-monium perruthenate (TPAP) has been developed. A stoichiometric amount of TPAP in the presence of A-methylmorpholine A-oxide (NMO) and 4 A molecular sieves (MS).18a As the reaction conditions are neutral and mild, this method is compatible with the presence of other sensitive functionalities (Eq. 6.11). This transformation can be carried out with 10 mol% of TPAP and 1.5 equiv of NMO in the presence of potassium carbonate, 4 A MS, and silver acetate (Eq. 6.12).18b... 

Hexane is freshly distilled from calcium hydride (CaH2) or dried over 4 A molecular sieves. 

Dichloromethane was dried by distilling from calcium hydride. The checkers used dichloromethane (reagent grade) kept over 4 A molecular sieves for 2 days to obtain the same results. 

Anhydrous tetrahydrofuran (THF) was distilled from benzophenone ketyl under nitrogen. The checkers used THF dried over 4 A molecular sieves, under nitrogen, for 2 days and obtained the same results. 

A multicomponent assembly of pyrido-fused tetrahydroquinolines has been accomplished by Lavilla and coworkers in a one-pot process by the interaction of dihydroazines, aldehydes, and anilines (Scheme 6.242) [425], The reactions were conducted with 20 mol% of scandium(III) triflate as a catalyst in dry acetonitrile in the presence of 4 A molecular sieves, employing equimolar amounts of the building blocks. This protocol provided the cycloadducts shown in Scheme 6.242 in 80% yield as a 2 1 mixture of diastereoisomers following microwave irradiation at 80 °C for 5 min. The same reaction at room temperature required 12 h to reach completion. 

Mejla-Oneto and Padwa have explored intramolecular [3+2] cycloaddition reactions of push-pull dipoles across heteroaromatic jr-systems induced by microwave irradiation [465]. The push-pull dipoles were generated from the rhodium(II)-cata-lyzed reaction of a diazo imide precursor containing a tethered heteroaromatic ring. In the example shown in Scheme 6.276, microwave heating of a solution of the diazo imide precursor in dry benzene in the presence of a catalytic amount of rhodium I) pivalate and 4 A molecular sieves for 2 h at 70 °C produced a transient cyclic carbonyl ylide dipole, which spontaneously underwent cydoaddition across the tethered benzofuran Jt-system to form a pentacyclic structure related to alkaloids of the vindoline type. 

A carbonyl-ene reaction between a variety of a-methyl styrenes and paraformaldehyde was effected using the combined boron trifluoride and 4 A molecular sieves (Equation (5)). The reaction worked best when electron-withdrawing groups (Cl or F) were present on the aromatic ring. 

The 4 A Molecular Sieves System. The initial procedure for the Sharpless reaction required a stoichiometric amount of the tartrate Ti complex promoter. In the presence of 4 A molecular sieves, the asymmetric reaction can be achieved with a catalytic amount of titanium tetraisopropoxide and DET (Table 4-2).15 This can be explained by the fact that the molecular sieves may remove the co-existing water in the reaction system and thus avoid catalyst deactivation. Similar results may be observed in kinetic resolution (Table 4-3).15... 

Narasaka et al.16 reported that 53 catalyzes Diels-Alder reactions of 54-type substrates with diene in the presence of 4 A molecular sieves (Scheme 5-18). A remarkable solvent effect on the enantioselectivity is observed. High enantio-selectivity is attained using mesitylene as the solvent. As shown in Scheme 5-18, the reaction of 54a with isoprene proceeds smoothly in this solvent, affording product 55a with 92% ee. Other 3-(3-substituted acryloyl)-l,3-oxazolidin-2-ones 54b-d also give good results (75-91% ee) when reacted with cyclopentadiene. 

Bis(oxazoline)-type complexes, which have been found useful for asymmetric aldol reactions, Diels-Alder, and hetero Diels-Alder reactions can also be used for inducing 1,3-dipolar reactions. Chiral nickel complex 180, which can be prepared by reacting equimolar amounts of Ni(C10)4 6H20 and the corresponding (J ,J )-4,6-dibenzofurandiyl-2,2 -bis(4-phenyloxazoline) (DBFOX/Ph) in dichloromethane, can be used for highly endo-selective and enantioselective asymmetric nitrone cycloaddition. The presence of 4 A molecular sieves is essential to attain high selectivities.88 In the absence of molecular sieves, both the diastereoselectivity and enantioselectivity will be lower. Representative results are shown in Scheme 5-55. 

A case of the addition of an allylstannane to aldehydes has been reported by Tagliavini to proceed with appreciable enantioselectivity (Scheme 6.15) [40]. A notable feature of the Zr-catalyzed transformations is that they proceed more rapidly than the corresponding Ti-catalyzed processes reported by the same research team (see Scheme 6.16). Furthermore, C—C bond formation is significantly more efficient when the reactions are carried out in the presence of 4 A molecular sieves the mechanistic rationale for this effect is not known. It should be noted that alkylations involving aliphatic aldehydes are relatively low-yielding, presumably as the result of competitive hydride transfer and formation of the reduced primary alcohol. 

Maruoka and co-workers recently reported an example of a Zr-catalyzed cyanide addition to an aldehyde [64]. As is also illustrated in Scheme 6.20, the reaction does not proceed at all if 4 A molecular sieves are omitted from the reaction mixture. It has been proposed that the catalytic addition proceeds through a Meerwein—Ponndorf—Verley-type process (cf. the transition structure drawn) and that the crucial role of molecular sieves is related to facilitating the exchange of the product cyanohydrin oxygen with that of a reagent acetone cyanohydrin. The example shown is the only catalytic example reported to date the other reported transformations require stoichiometric amounts of the chiral ligand and Zr alkoxide. 

Scheme 6.20. Zr-catalyzed cyanide addition to an aldehyde is facilitated by the presence of 4 A molecular sieves.

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