Cycloisomerization cyclic

It should also be mentioned that very recently, a new cycloisomerization of enynes has been shown to proceed via a rhodium-vinylidene complex,187 which, after [2 + 2]-cycloaddition and ring opening of a rhodacyclobutane, furnishes versatile cyclic dienes (Scheme 47).188 Not only does this constitute a fifth mechanistic pathway, but it also opens new opportunites for C-C bond constructions. 

Cycloisomerization represents another approach for the construction of cyclic compounds from acyclic substrates, with iridium complexes functioning as efficient catalysts. The reaction of enynes has been widely studied for example, Chatani et al. reported the transformation of 1,6-enynes into 1-vinylcyclopentenes using [lrCl(CO)3]n (Scheme 11.26) [39]. In contrast, when 1,6-enynes were submitted in the presence of [lrCl(cod)]2 and AcOH, cyclopentanes with two exo-olefin moieties were obtained (Scheme 11.27) [39]. Interestingly, however, when the Ir-DPPF complex was used, the geometry of olefinic moiety in the product was opposite (Scheme 11.28) [17]. The Ir-catalyzed cycloisomerization was efficiently utilized in a tandem reaction along with a Cu(l)-catalyzed three-component coupling, Diels-Alder reaction, and dehydrogenation for the synthesis of polycyclic pyrroles [40]. 

In the presence of [lrCl(cod)]2, heteroatom-tethered enynes with ( )-olefinic moieties were transformed into cyclic 1,4-dienes. The ene-type reaction was achieved previously with a Rh catalyst, but only enynes with (Z)-olefinic moieties were used. It is worthy of note here that the cycloisomerization showed a clear acceleration when an ionic liquid was used as the solvent (Scheme 11.29) [41]. 

Other catalytic reactions involving a transition-metal allenylidene complex, as catalyst precursor or intermediate, include (1) the dehydrogenative dimerization of tributyltin hydride [116], (2) the controlled atom-transfer radical polymerization of vinyl monomers [144], (3) the selective transetherification of linear and cyclic vinyl ethers under non acidic conditions [353], (4) the cycloisomerization of (V2V-dia-llyltosylamide into 3-methyl-4-methylene-(V-tosylpyrrolidine [354, 355], and (5) the reduction of protons from HBF4 into dihydrogen [238]. 

A complementary route to carbohydrate-based oxepines was developed by the McDonald group.67 It is based on the endo-selective cycloisomerization of alkynyl alcohols in the presence of molybdenum or tungsten catalysts to give the cyclic enol... 

Mild Ni(0)-catalysed rearrangements of l-acyl-2-vinylcyclopropanes to substituted dihydrofurans have been developed.86 The room temperature isomerizations afford dihydrofuran products in high yield. A highly substituted, stereochemically defined cyclopropane has been employed in the rearrangement to evaluate the reaction mechanism. The Cu(II)-catalysed cycloisomerization of tertiary 5-en-l-yn-3-ols with a 1,2-alkyl shift affords stereoselectively tri- and tetra-cyclic compounds of high molecular complexity (Scheme 29).87 A proposed mechanism has been outlined in which... 

Intramolecular 2 + 2 + 2-cycloisomerizations of cyclic triynes and enediynes have been reported with RhCl(CO)(PPh3)2.126 The transition metal-catalysed rearrangement of alk-5-ynals to /-alkynyl ketones and cyclopent-l-enyl ketones was developed using [Rh(P(OPh)3)2]BF4 or Cu(OTf)2 as a catalyst and the effect of substituents on the partition to products was elaborated (Scheme 84).127... 

The photocycloaddition of (cyclic) a,(B-unsaturated ketones to alkenes affording cyclobutanes as products comprises the four reaction types shown in Sch. 1, i.e., (a) intermolecular enone + alkene cycloaddition (b) cycloisomerization of alkenylsubstituted enones (c) photocyclodimerization of enones, one ground state enone molecule acting as alkene and (d) cycloisomerization of fe-enones. 

The ruthenium-catalyzed cycloisomerization of a variety of <5-enallenes was also achieved, forming cyclic 1,3-dienes or 1,4-dienes depending on the substrates and reaction conditions [32] (Eq. 22). This intramolecular coupling of the C=C bond and allenes can be envisioned by the initial hydrometallation of the allene moiety followed by intramolecular olefin insertion and isomerization. 

Yet another palladium-catalyzed transformation leading to 1,2-dialkyl-idenecycloalkanes was established by Trost et al. when investigating a catalytic Alder-ene reaction (path D in Scheme 12). They showed that two different catalyst systems are capable of cycloisomerizing enynes 92 to either cyclic 1,4-dienes 96—the products of regular Alder-ene reactions— or the 1,3-dienes 95 (Scheme 15) [66-68]. Starting from palladium acetate, the reaction presumably occurs by coordination of both unsaturated moieties (intermediate 93) and subsequent cycloisomerization to the ring-... 

Of the two mechanistic pathways, i.e., via palladacyclization or via hydropalladation-cyclic carbopalladation, the latter seems to be more suitable for the development of sequentially catalyzed processes. Considering cycloisomerizations via the hydropalladation-cyclic carbopalladation route the catalytic reaction can terminate by /1-hydride elimination giving rise to the formation of dienes and derivatives thereof (Scheme 79). Alternatively, the alkyl-Pd species formed in the cyclic carbopalladation can be susceptible to subsequent transmetallation with organometallic substrates. Then, a reductive elimination could conclude this second Pd-mediated step releasing the Pd(0) species for a new catalytic cycle. 

Scheme 82 Cycloisomerization of alkynyl allyl alcohols to cyclic enals [148]...

Examples for the transformation of 2-alkynyl-2-hydroxyalkanones to 3(2//)-futanones using either Pt(ll) or Au(lll) catalysts have been reported. This cycloisomerization process involves a 1,2-migration of an alkyl group. If cyclic substrates are employed, spirocyclic 3(2//)-furanones are obtained under ring contraction (Equation 51) <2006AGE5878>. 

In the case of the cyclic substrate 51, the intervention of a C-H insertion pathway reveals itself in terms of the diastereoselectivity, not regioselectivity. Thus, exposure of enyne 51 to the standard Ru catalyst at ambient temperature produced the transfused bicyclo[5.4.0]undecene 52 (Equation 1.60, path a) [55]. If a metallacycle mechanism was operative, coordination of the metal with both the alkene and alkyne must occur to form the cis-fused product. On the other hand, coordination of the Ru with the Lewis basic bridgehead substituent directs it to abstract an allylic C-H on the same face as the substituent, which then leads to the trans-fused product as observed. On the other hand, cycloisomerization using a Pd(0) precatalyst does indeed lead to the Z-fused bicycle (Equation 1.60, path b). 

Shortly after the discovery of enyne metathesis, Trost began developing cycloisomerization reactions of enynes using Pd(ll) and Pt(ll) metallacyclic catalysts (429-433), which are mechanistically divergent from the metal-carbene reactions. The first of these metal catalyzed cycloisomerization reactions of 1,6-enynes appeared in 1985 (434). The reaction mechanism is proposed to involve initial enyne n complexation of the metal catalyst, which in this case is a cyclometalated Pd(II) cyclopentadiene, followed by oxidative cyclometala-tion of the enyne to form a tetradentate, putative Pd(IV) intermediate [Scheme 42(a)]. Subsequent reductive elimination of the cyclometalated catalyst releases a cyclobutene that rings opens to the 1,3-diene product. Although this scheme represents the fundamental mechanism for enyne metathesis and is useful in the synthesis of complex 1,3-cyclic dienes [Scheme 42(fe)], variations in the reaction pathway due to selective n complexation or alternative cyclobutene reactivity (e.g., isomerization, p-hydride elimination, path 2, Scheme 40) leads to variability in the reaction products. Strong evidence for intermediacy of cyclobutene species derives from the stereospecificity of the reaction. Alkene... 

Scheme 55. Formation of cyclic carboxylic acid derivatives by enyne cycloisomerization and CO2 insertion at Ni(0).

Iridium-catalyzed intramolecular l,n-enyne metathesis has been studied as a unique tool for the synthesis of various types of cyclic compounds. Reactions of this type depend on both the structure of substrates and the nature of catalyst systems used (411). Recently, the cycloisomerization of various 1,6-enynes have been shown to be catalyzed by [Ir(cod)Cl]2/dppf (494). These reactions are highly stereoselective, and generate the (Z)-isomer preferentially over the ( )-isomer (Scheme 63). The proposed mechanism (Scheme 64) involves oxidative cyclization of the enyne at Ir(I) to give the trivalent iridacyclopentene. The intermediate undergoes (3-hydride elimination to give the irida-1,3-diene, which experiences steric repulsion between the metal fragment and the cis substituent on the... 

Scheme 61. The Ti(II)-enyne cycloisomerization as a route to cyclic siloxane.

Cycloisomerization. Activation of a C—H bond by the Rh complex for intramolecular hydrometallation of a proximal double bond can lead to valuable cyclic products. Examples for such reactions include elaboration of dehydrobenzosuberones from o-formylbenzylide-necyclopropanes and of cyclopentane derivatives through addition of an azadiene." ... 

Trost and co-workers have made great strides in developing the palladium-catalyzed cycloisomerization of enynes into a powerful ring-forming method [39]. In most cases, the intimate details of these reactions are unknown. They are considered here, since a Heck cyclization is a potential step of one possible mechanistic sequence [40]. Two plausible mechanisms for palladium-catalyzed cycloisomerization of enynes are depicted in Scheme 6-17. In the Heck pathway (101 102 -> 103 - 104), hydropalladation of the alkyne is followed by alkene insertion and /3-hydride elimination to provide cyclic diene 104. 

Cycloisomerization of enynesJ Cyclization of 1,6- and 1,7-enynes under CO to afford the cyclic isomers proceeds in excellent yields in the presence of... 

Preparation of an useful synthon, the cyclic carbamate of L-daunosaminal (104), was achieved by sequential catalytic transformations of a key intermediate. Suitably protected propargyl diol 101, which was obtained by diastereoselective addition of allenyl stannate 99 to 0-benzyl (S)-lactyl aldehyde 100, was transformed as depicted in Scheme 18. Cycloisomerization to 3-deoxyglycal 103 was achieved by irradiation in the presence of tungsten hexacarbonyl, and subsequent nitrene insertion was catalyzed by rhodium acetate. Overall yield of the bicyclic daunosaminal (104) derivative from lactic acid derivative amounted to 44% [78]. 

Another cycloisomerization pathway of 1,6-enynes involves intramolecular cyclopropanation of the alkene by the alkyne (Scheme 4-24). This reaction is favored for enynes bearing a cyclic olefin and probably proceeds by 6-encto-dig-cyclization, followed by proton loss and protodeauration of the gold carbene... 

The corresponding reaction of but-3-yn-l-ols or pent-4-yn-l-ols with primary or secondary alcohols in the presence of catalytic amounts of Ph3PAuBF4 and p-TsOH afforded tetrahydrofuranyl ethers (Scheme 4-76). This tandem 5-endo-cycloisomerization/hydroalkoxylation proceeds via 2,3-dihydrofurans, which then undergo an intermolecular Bronsted acid-catalyzed addition of the external alcohol. The transformation is not restricted to internal alkynols but can be applied to terminal acetylenes as well. Application of the method to the s thesis of bicyclic heterocycles with a P-lactam structure was reported recently.Under the same conditions, epoxyalkynes undergo a sequence of epoxide opening, 6-exo-cycloisomerization, and nucleophilic addition to afford tetrahydropyranyl ethers. In a closely related transformation, cyclic acetals were obtained from alk-2-ynoates bearing a hydroxy group in 6- or 7-position by treatment with AuCU and MeOH. ... 

MpUer, B. and Undheim, K. (1998) Cyclic a-amino acids by Pd-mediated cycloisomerization and couphng reactions. Tetrahedron, 54, 5789-804. 

When 1,6-diene 46 was catalyzed by Grubbs carbene complex/trimethylsilyl vinyl ether or NiBr2(PPh3)2/Et2AlCl, exo-methylene cyclic compound 47 was obtained as the major product of the cycloisomerization (Scheme 27) (64,65). 

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