Bicyclization of Diynes

Scheme 53 Bicyclization of diynes with group 6 metal-carbyne complexes.

Although mechanistic details are unclear, a very interesting Mn-induced bicyclization of diynes shown in Scheme 62 has been reported.270 Unfortunately, this reaction gives mixtures of three isomeric products. 

Scheme 37 Zr-promoted bicyclization of enynes, dienes, and diynes. Some natural products synthesized by these reactions.

Vaska s complex ([IrCl(CO)(PPh3)2]) also catalyzed the carbonylative coupling of diynes, which provided bicyclic cyclopentadienones (Scheme 11.23) [35]. Due to the instability of the products, the substrates are limited to symmetrical diynes with aromatic groups on their termini nonetheless, this reaction still serves as the catalytic and practical procedure for the synthesis of cyclopentadienones, which are anti-aromatic with a 47t system and serve as active synthetic intermediates. 

In contrast to isocyanates, isothiocyanates have hardly been examined as cycloaddition components, because the strong coordination of organosulfur compounds frequently deactivates a catalytic species [21]. Some organoruthe-nium complexes, however, recently proved to be efficient catalysts for the formation of carbon-sulfur bonds [21]. The catalytic cycloaddition of diynes with isothiocyanates was also successfully achieved using Cp RuCl(cod) as a precatalyst [22]. Importantly, the cycloaddition took place at the C=S double bonds of the isothiocyanates to afford thiopyranimines 26 (Eq. 13). This reaction requires 10 mol % of the precatalyst as well as the diynes possessing a quarternary carbon center at the 4-position. When excess amounts of carbon disulfide were also employed in place of the isothiocyanates, a bicyclic dithiopyrone 26 [X is C(C02Me)2, Z is S] was obtained in 50% yield. 

The reactions of Fischer carbene complexes with alkynes can under certain conditions lead to products that result from the incorporation of two alkynes, the carbene ligand and a carbon monoxide. In inter-molecular reactions, this is most commonly observed for acetylene itself or for sterically unhindered al-kynes. °2 As can be anticipated by the mechanism in Scheme 36, two-alkyne incorporated products of the type (258) are also favored for high alkyne concentration. Synthetically, the two-alkyne reactions are most useful in intramolecular reactions, two of which have been reported and are exemplified by the reactions in Scheme 43. The typical product from the reaction of a Fischer carbene complex with a diyne, such as (308), is a bicyclic phenol of the type (309). ° These products are apparently the result of the assembly of pieces indicated by (311). Under some conditions, dienones of the type (310) and (314) can be isolated, and it is thought they are the immediate precursors of the phenol products via an in situ reduction by a chromium(O) species. This reaction is completely regioselective with diyne (308) and the phenol (309) results from incorporation of the terminal alkyne of (308) before the disubstituted alkyne. Phenols of the type (309) have also been observed from the reaction of diynes with carbyne complexes. ... 

A novel cycloaddition of diynes with aldehydes is catalyzed by the same metal system, giving rise to bicyclic a-pyrans after 1,5-hydrogen shifts. Yields of these sensitive compounds are remaritably high (equation 52). ... 

Bicyclization of linear diynes (30) to produce bicyclic metallacyclopentadienes (31) followed by their conversion into tra s,trans-l,2-bis(alkylidene)cycloalkanes (32) is a highly attractive synthetic operation primarily because (32) can now serve as the diene component in the Diels-Alder reaction (equation 26). 

The cooligomerization of diynes and simple alkynes provides a convenient route to bicyclic derivatives, many of which would be very difficult to obtain by other routes--" In the presence of CpCo(CO), 1,5-hexadiyne reacts with simple... 

Sato, F., Urabe, H., Okamoto, S. Bicyclization of dienes, enynes, and diynes with Ti " reagent. New developments towards asymmetric synthesis. PureAppi. Chem. 1999, 71,1511-1519. 

Pd-catalysed bicyclizations of l-ene-6,ll-diynes with bis(pinacolato)diboron give homoallylic alkylboronates. One C B bond and two new C C bonds are created, and two new stereogenic centres are stereospecificaUy formed in a single tandem operation involving a cascade reaction in which the last step is the transmetalation of bis(pinacolato)diboron (Scheme 143). " ... 

The chemo- and regioselectivity problems can be solved by the [2+2+2] cycloaddition of diynes with monoynes, although the accessible products are limited to bicyclic compounds. The [2+2+2] cycloaddition of diynes with monoynes has been applied to the synthesis of biologically active molecules and functional materials. The first application in the natural product synthesis was the dl-estrone synthesis using CpCo(CO)j as a catalyst. The [2+2+2] cycloaddition followed by the benzocyclobutane to ort/jo-quinodimethane rearrangement and intramolecular Diels-Alder reaction afforded a dZ-estrone precursor (Scheme 21.8) [11]. 

The cationic rhodium(I)/BINAP complex catalyzed decarboxylative [2 - - 2 - - 2] cycloaddition of diynes with commercially available vinylene carbonate to give bicyclic phenols in good yields (Scheme 4.47) [50]. In this reaction, vinylene... 

Subsequently in 2011, Wan et al. achieved the [2 + 2 + 2] cycloaddition of diynes 110 with nitriles 111, leading to bicyclic pyridines 112, by using an in situ-generated low-valent iron/diphenylphosphinopropane (DPPP) complex (Scheme 6.31) [45]. Importantly, this cycloaddition proceeded at room temperature. 

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