Ag-catalyzed cyclization

Scheme 62 Ag-catalyzed cyclization to form isoquinoline A/-oxides.

As a result of Ag (I) catalyzed cyclization of allenic oximes (124) and (125), stable five- (126) and six-membered (127) cyclic nitrones were obtained (Scheme 2.46) (298a). Recently, a novel method of pyrroline-type nitrone formation via P-allenyl-oxime cyclization has been described (298b). 

Azadiene 132, containing electron-donor and -acceptor substituents, has been shown by Gomper and Heinemann [81 AG(E)296] to be a suitable precursor of several five-membered rings by acid-catalyzed cyclization reactions (Scheme 33). Thus, oxazole 133 (86%), thiazole 134 (85%), and imidazoles 135 (22-38%) were obtained upon reaction of 132 with aqueous HC1, HC1/H2S, and RNH2 HC1, respectively. 

The synthesis of 5-substituted 2-carbomethoxy-methylfurans can be achieved by base catalyzed cyclization of 4-pentynones <95TL4405> It is well known that allenylketones 53 on treatment with Rh(I) or Ag(I) catalysts may yield furans 54. 

Cyclization of quinone methides.2 p-Quinone methides, particularly those substituted at the 2- and 6-positions, are stable enough to be characterized by IR and NMR, and can be generated in fairly high yield by oxidation of a phenol with Ag,0 (10 equiv.) in CH2C12. When substituted by a suitable terminator, these quinonemethides can undergo Lewis-acid-catalyzed cyclization. Suitable terminators that can survive the initial oxidation include allylsilanes and 3-keto esters. In the latter case, the initial product undergoes oxidation to afford a cyclohexenone. 

An interesting extension of palladium-catalyzed arylation of directing-group containing arenes by aryl iodides has been recently described by Cheng [74], The arylation of aldoxime ethers under standard conditions (Pd(OAc)2 catalyst, CF3C02H solvent, stoichiometric Ag+ source at elevated T) affords fluorenones by tandem o-ary I at ion/pal ladium-catalyzed cyclization pathway (Scheme 20). 

Dihydrofurans can also be prepared by a Ag(I)-catalyzed cyclization of allenols (Equation (91)) <741289, 79S743, 84CC804, 90JOC2995, 94JOC324>. 

The same group demonstrated that a variety of mono- and disubstituted 1-oxyindolizine derivatives 3S2 could be readily synthesized via a facile Ag-catalyzed cydoisomerization of skipped propargylpyridines 3S1 (Scheme 9.121) [300, 301]. It was suggested that this Au-catalyzed reaction involved a 5 -endo-d cyclization of the alkyne 351 activated by a jt-philic metal. Formation of the indolizine product 352 was accomplished via a subsequent proton transfer in cyclic vinylmetal zwitterion 354 (Scheme 9.122). It should be noted that a variety of transition metals, such as Au(I), Au(III), Cu(I), Pt(II), and Pd(II), were shown to catalyze this transformation with variable degrees of efficiency. 

In 2000, Hashmi reported the gold(III)-catalyzed cyclization of a-allenyl ketones to form substituted furans [15]. As an example, treatment of allenyl p-methoxybenzyl ketone 70 with a catalytic amount of AuCls formed a 2 1 mixture of the cycloisomerization product 71 and the cycloisomerization/hydroarylation product 72 in 91% combined yield (Eq. (12.40)). Marshall had previously demonstrated the Ag(I)-catalyzed conversion of allenyl ketones to furans [111]. The Ag(I)-catalyzed transformation was four orders of magnitude slower than was the gold(III)-catalyzed transformation, but the silver-catalyzed process forms the 2-substituted furan as the exclusive product without the competing secondary hydroarylation. The selective conversion of a-allenyl ketones to substituted furans is also catalyzed by gold(III)-porphyrin complexes [112]. 

Scheme 9.38 The tandem Ag-catalyzed [3, 3]-sigmatropic rearrangement/Myers-Saito cyclization as reported by Toste and coworkers [113].

Nitrogen heterocycles can also b e prepared by silver(I)-catalyzed cyclization reaction. Asao, Yamamoto, and their colleagues have shown synthesis of 1,2-dihydro-isoquinoline derivatives by addition of pronucleophiles to ortho-alkynylaryl ald-imines employing AgOTf as a catalyst [29]. For example, treatment of imine (19) with 2 equiv of nitromethane (20) in the presence of 3 mol% of the catalyst at 80 °C, 1,2-dihydroisoquinoline derivative (21) is produced in 85% yield (Scheme 18.7). Terminal alkynes and carbonyl compounds possessing activated methylene groups are also usable as the pronucleophiles [29]. 2-Alkoxyazetidines are efficiently synthesized by Ag(fod)-catalyzed [2-F2] cycloadditions of imines to (alkoxymethylene)... 

Scheme 19 Ag-catalyzed electrophihc cyclization of 2-alkynylbenzaldehyde O-alkyl oxime for S-fluoroisoquinohne synthesis...

Scheme 21 Ag-catalyzed electrophilic cyclization of 2-alkynylbenzoaldoxime for 6-fluoroisoquinoline synthesis...

Hydroamination has been found to be useful in racemic synthesis of various natural products. Helquist and coworkers recently synthesized ( ) momomorine I using Ag-catalyzed hydroamination/cyclization of amino-diyne as the key step. Monomorine I is an indolizidine alkaloid isolated from ant Monomorium pharaoins. Beller and coworkers found hydroamination useful in the synthesis of amphetamines. It is worth mentioning that amphetamine is a class of compounds having several biological activities such as a stimulant to the central nervous system, an antiinflammatory agent, and inhibitors of enzymes. ... 

In 1979, Claesson et al. observed the formation of the dihydropyrrole 125 and the pyrrole 126 when trying to purify the amine 124 by GLC [85]. They suspected that an initial cycloisomerization first leads to 125 and a subsequent dehydrogenation then delivers 126. Guided by other intramolecular nucleophilic additions to alkynes that are catalyzed by AgBF4, they discovered that this catalyst efficiently allowed the transformation of 124 to 125 (Scheme 15.38). Reissig et al. found that with enantio-merically pure substrates of that kind a cyclization without racemization is possible with Ag(I) catalysts [86],... 

Stereocontrolled conjugate addition of lithium dimethylcuprate to the electron deficient 2,3-double bond of allenes 851 leads to 5,6-dihydropyranM-oncs 852 in moderate yield (Equation 343) <2000J(P1)3188>. Similarly, the Ag(l)-catalyzed intramolecular cyclization of the allenic acid 853 is accelerated upon addition of diisopropylethyl-amine to afford the 3,6-dihydropyran-2-one 854, an intermediate during the total synthesis of (—)-malyngolide (Equation 344) <2000JA10470>. 

Giese and coworkers investigated radical 5-exo cyclization reactions of bromoacetaldehyde allyl acetals 264 (Y=0, R1=OBu) catalyzed by 251 and found that either product 265 or 266 results selectively, depending on the amount of catalyst (1-40 mol%) and the reducing conditions (entry 8) [304]. Mild reducing conditions, such as Zn in DMF or a potential of —0.8 V vs Ag/Ag+, and higher... 

Intramolecular allylation of alkynes with allylsilanes is catalyzed by a variety of electrophilic transition metal halides and complexes (e.g. Pt(II), Pd(II), Ru(II), Au(III), and Ag(I)) (Scheme 10.196) [518]. Unlike the EtAlCl2- or HfCL,-catalyzed reaction, alkyriylaled allylsilanes are cyclized in an exo mode by these catalysts. The proposed reaction mechanism involves nucleophihc addition of an allylsilane to a metal-coordinated alkyne. 

Cyclization. The modes of Ag(I)-catalyzed cycUzation of 2-alkynyl-3-formyl-quinolines in MeOH have been correlated with tbe pKa values of tbe silver salts used. Both 5-exo-dig and 6-endo-dig pathways are equally favored in the presence of AgOAc, products arising from the former pathway predominate in using more basic silver salts (e.g., Ag20, AgO). 

The first carbon addition to allenes catalyzed by gold was described by Hashmi and was the addition of furans to allenones, a reaction that is discussed as a conjugate addition to carbonyls. Then the annulation of 2-allenyl indoles was described (equation 111)and an enantioselective version was later developed for terminal allenes. Then Nelson described the cyclization of an N-substituted pyrrol in the synthesis of (—)-Rhazinilam (equation 112). In that case, gold notably improved the yields and selectivities obtained with Pd(II) or Ag(l), and gave complete retention of the chiral information of the allene. The most recent report is on the hydroarylations of allenes with gold-covered substituted benzenes as substrates. ""... 

The cyclization-dimerisation of allenylketones 29 to furans 28 and/or 2,4-disubstituted furans 30 is well known. By making the choice of either PdCl2(MeCN)2 or rac-31 as catalyst, the allenylketones 29 could be preferentially converted into 28 or 30. Since the catalyst tolerates several functional groups (terminal alkynes, a-halogenketones, alkyl halides) this catalyst is an important improvement of Marshall s Ag(l)-catalyzed isomerisation of 29 to 28 <97CB1449 97CB1457>. 

The Ag- or Pd-mediated intramolecular cyclization of urethanes onto allylic substituents is used in the synthesis of piperidine alkaloids <97JOC776, 97CL221>. Similarly, the tungsten-mediated cyclization of urethanes on alkynes via the metal vinylidene carbene has been reported <97TL7687>. Ni-catalyzed intramolecular cyclization of a 1,3-diene side chain of 5 8 onto an aldehyde affords a precursor to the indolizidine alkaloid (-)-elaeokanine C <97TL3931>. 

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