Acetylene cyclisation

K. P. C. Vollhardt, Transition Metal Catalysed Acetylene Cyclisations in Organic Synthesis , Accounts Chem. Res., 1977,10, 1. 

In adding two molecules of acetone to acetylenes, there are no problems of chemoselectlvity as the dl-anion (15) of the monoadduct reacts preferentially on carbon. Diol (13) cannot bo Isolated as it cyclises under the hydration conditions. 

The lactone (59) is made by the method described on page T 96. Addition of the protected acetylene (61) gives the hemlacetal (62) which can be hydrogenated directly to (63). Cyclisation occurs as soon as the protecting group is removed. 

Occurring Acetylene Compounds. XXVII. The Synthesis of a Mixture of Trideca-1.3,1l-triene-5,7,9-triyne (II) and trans-l-Phenylhepta-l,3-diyn-5-ene (V). A Novel Cyclisation Reaction. Acta Chem. Scand. 13, 2101 (1959). 

In attempts to cany out cyclisation from butadiene 1,3 in the gaseous phase and under photochemical conditions R Srinivasan (J.Amer.Chem.Soc. 88, 3765 (1966) showed that irradiation yields a complex mixture of ethylene, acetylene, but-1 ene, hydrogen and polymers. The quantum yields of acetylene and ethylene formation increase with pressure. 

The indirect cyclisation of bromoacetals via cobaloxime(I) complexes was first reported in 1985 [67], At that time the reactions were conducted in a divided cell in the presence of a base (40yo aqeous NaOH) and about 50% of chloropyridine cobaloximeflll) as catalyst precursor. It was recently found that the amount of catalyst can be reduced to 5% (turnover of ca. 50) and that the base is no longer necessary when the reactions are conducted in an undivided cell in the presence of a zinc anode [68, 69]. The method has now been applied with cobaloxime or Co[C2(DOXDOH)p ] to a variety of ethylenic and acetylenic compounds to prepare fused bicyclic derivatives (Table 7, entry 1). The cyclic product can be either saturated or unsaturated depending on the amount of catalyst used, the cathode potential, and the presence of a hydrogen donor, e.g., RSH (Table 7, entry 2). The electrochemical method was found with some model reactions to be more selective and more efficient than the chemical route using Zn as reductant [70]. 

Propose a mechanism for the samarium iodide promoted cyclisation of acetylene 6. What is the reactive intermediate responsible for this step Give reasons why the formation of cyclohexene 11 should be disfavoured. 

Not surprisingly, these rhodium and iridium carbene complexes were tested for their catalytic behaviour in the transfer hydrogenation of benzophenone and acetophenone (M +3), the hydrosilylation of alkynes (M +1) and also the catalytic cyclisation of acetylenic carboxylic acids (M +1). Hydrogenation works better for iridium than rhodium and for aromatic than for aliphatic ketones [40,43,44]. The iridium(I) complex is the first iridium catalyst showing activity for the cyclisation of acetylenic carboxylic acids [40]. The results for the hydrosilylation reactions were very moderate. 

These spacer-free Janus bis-carbene complexes were successfully employed in the intramolecular cyclisation of acetylenic carboxylic acids (4-pentynoic acid and 5-hexynoic acid) as well as the transfer hydrogenation of ketones and imines. 

The electrophilic 6-endo cyclisation of derivatives of 1-sulfanylpent-4-en-2-ol leads to thiopyrans when effected by I2, but to tetrahydrothiophenes by a 5-exo route when induced by Se reagents (Scheme 27) <03EJ0209>. Similarly, the eonditions used for the ICl-promoted cyclisation of bis(4-methoxybenzylthio)acetylene influence the competition between Ar2-6 ortho) and Ari-5 ipso) routes. In the strict absence of water, the exclusive product is the l//-2-benzothiopyran 30, whereas in the presence of nucleophiles ipso attack is favoured and... 

N. Hagihara Yuki Gosei Kagaku Kyokai Shi 19, 20 (1961) Cyclisation of acetylenic compounds in the presence of organo-metallic compounds 9 (45) Japan. 

Reactions and catalytic action of metal complexes. Complexes of allene and acetylene and tbeir catalytic polymerisation Tbe cyclisation of alkynes and tbeir derivatives... 

The stannylsilylation of terminal acetylenes occurs by cis addition, with silicon adding to the terminal carbon,66 and the differentially bifunctional alkenes so obtained are valuable reagents for further transformations, as shown in Scheme 19-7,67 68 1,6-Diynes69 and l-ene-6-ynes undergo bismetallative cyclisation.65... 

Intramolecular Pauson-Khand reactions are often regioselective because it is physically impossible for the molecule to cyclise any other way. Pauson-Khand disconnection of bicyclic 111 reveals an allyl ether 112 of the alcohol 113, easy to make from acetylene and cyclohexanone. In the cobalt-catalysed cyclisation, only one regioisomer is possible and this, the TM111, is formed in an excellent 80% yield.28... 

P-Aminoacrylates, for cyclisation to 4-quinolones, are also available via the addition of anihnes to acetylenic esters ° ° or by displacement of ethoxy from ethoxymethylenemalonate (Et0CH=C(C02Et)2. ... 

Generally speaking, unsymmetrical 1,3-dicarbonyl components produce mixtures of 1,2-azole products. This difficulty can be circumvented in a number of ways by the use of acetylenic aldehydes or ketones (as synthons), where a hydrazone or oxime can be formed first by reaction at the carbonyl group, and this can then be cyclised in a separate, second step. "" A nice example of this is the formation of... 

Methanol and propan-2-ol add non-stereospecifically to cholest-4- and -5-ene as do ethylene and tetrafluoroethylene to 3 -acetoxypregna-5,16-dien-20-one although only 16a,17a-addition products were isolated from allene, acetylene, and dichloroethylene the derived 20-anti-oxime isomerises to the syn-isomer on irradiation in THF and cyclises internally in benzene solution. Irradiation of the trans-acid (561) in methanol resulted only in a cis-trans equilibration, although the trans-acid (562) produced the unsaturated lactone (563). 

Synthesis. - Type A Syntheses (S-C H -N + C). The C-nucleoside (182) is formed by condensation of 2-amino-4-chlorobenzenethiol with an acetylated aldonic nitrile or glycosyl cyanide followed by deacetylation23. 2-Aminobenzenethiol reacts with RC0CsCPh(Rs2-thienyl) to give (183). The latter cyclises to 2-phenylbenzo-thiazole on attempted recrystallisation (MeOH). With terminal acetylenic ketones a mixture of benzothlazolines results232. 

From an investigation of various Michael acceptors, it has emerged that ethoxymethylidene malonates are suitable for addition-elimination reactions with acetylides to give diacids (426) after hydrolysis and elimination. Upon heating in a-dichlorobenzene, these acids cyclise to give ylidenebutenolides (427)Chromium carbonyl complexes of acetylenes have been used to prepare 2-methoxyfurans and... 

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